|
80m Inverted L Initial measurements
45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12
THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ....hasan, N0AN |
Starting point for the inverted L with sloping top loading wire.
45' high, 25.8' sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Mounted 7" above the earth, directly to a 3' ground rod (strictly for mechanical support, as I had a prefab mount with 3/8x24 on one end and SO-29 on the other) Predicted Radiation Resistance (Devoldere): 25.8 ohms, very good ground (rich pastoral, midwest). I see I had confused feedpoint impedance and radiation resistance (after reading some more in Devoldere's "Low-Band DX'ing") According to Figure 9-94, the radiation resistance for my inverted L is approximately 25 ohms. According to EZnec 4.1, source data says: Impedance = 25.58 + J 1.872 ohms at 3600 khz. When I measure the antenna with the MFJ-269, I get: R=37 and X=0 If Devoldere and EZNEC are correct (although EZNEC may or may not be measuring radiation resistance), then my radial-less inverted L is showing ground losses of 37-25=12 ohms. If so, then my efficiency, without radials is: 25/(25+12) or 25/37 or approximately 67%. Before installing radials this morning, I just want to make sure I'm looking at the right variables and interpreting them properly. 4 radials, laying on the ground, will be ready to install when the sun comes up. 4 more later in the day, etc., up to a total of 16 (the amount of wire I have ready to go). Each time I put down 4, I'll take another measurement with the 269. For kicks, I might take a measurement with just one or two first. ....hasan, N0AN |
At what point in the system are you making your measurements?
hasan schiers wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ...hasan, N0AN |
Question: Where was I taking my measurements.
I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 0 3340 3522 3920 580 36,0 1.3 2 3354 3524 3774 420 45,0 1.0 4 3419 3533 3741 322 60,1 1.2 8 3445 3550 3742 297 65,4 1.3 The formatting is bad, but in order, left to right: Number of Radials Lower 2:1 vswr point Resonant Freq point Upper 2:1 point Bandwidth in kilohertz Impedance as shown on the MFJ 269 at resonance. I'm a bit confused by the rising feedpoint impedance with increasing number of radials. That seems backwards to me. 73, ....hasan, N0AN VSWR shown by MFJ 269 at resonance "Ham op" wrote in message ... At what point in the system are you making your measurements? hasan schiers wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ...hasan, N0AN |
OK, here is my summary table for up to 8 radials, 66 feet long, insulated
wire, laying on the ground: (all measurements at the shack end of 55' of LMR-400 coax, buried) Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 0 3340 3522 3920 580 36,0 1.3 2 3354 3524 3774 420 45,0 1.0 4 3419 3533 3741 322 60,1 1.2 8 3445 3550 3742 297 65,4 1.3 The formatting is bad, but in order, left to right: Number of Radials Lower 2:1 vswr point Resonant Freq point Upper 2:1 point Bandwidth in kilohertz Impedance as shown on the MFJ 269 at resonance. I'm a bit confused by the rising feedpoint impedance with increased radial numbers, but the narrowing down of the bandwidth indicates decreased losses. It also indicates (since it continues to decrease), that I should add another 8 radials to see if I begin approaching the asymptote (sp?). The rate of change is slowing quite a bit. Any input as to explanations and the data are most welcome. ....hasan, N0AN p.s. Performance seems quite good for a VERY limited sample. A w6 this morning was 2 to 3 S-units stronger on the inverted L than on the 45' high Carolina Windom 80. An LU6 was coming in S9 with an S5 noise level, and responded with a 59+ signal report on the first call. A sampling of other signals prior to or shortly after sunrise is showing a clear superiority of the inverted L with 8 radials over the CW 80, at distances 1500 km. Less than that distance and either antenna could have the upper hand until one gets to within the state of Iowa. I'll have to do more measurements to get a firm idea as to where the cross-over point is between the two antennas. "hasan schiers" wrote in message ... 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ...hasan, N0AN |
On Fri, 2 Sep 2005 23:29:17 -0500, "hasan schiers"
wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! Indeed. Unfortunately, I believe that you are at the mercy of your instrumentation. I doubt that you can make meaningful measurements of the differences resulting from the addition of a few radials at a time. The fact that you can't get correlation between measurements made on the -same- antenna with different instruments is a problem. Until you have two clocks that read the same, you don't know what time it is. |
I do have correlation now. The vswr meters and the mfj all read the same
resonant frequency now, Wes. See my post that shows the data for 0,2,4,8 radials. While a given absolute value might be in error (a certainty due to inexpensive instruments), the trend is sound for bandwidth, but a bit strange for feedpoint impedance. (BW is narrowing but input Z is going up, with increasing numbers of radials.) Ideas? 73, ....hasan, N0AN "Wes Stewart" wrote in message ... On Fri, 2 Sep 2005 23:29:17 -0500, "hasan schiers" wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! Indeed. Unfortunately, I believe that you are at the mercy of your instrumentation. I doubt that you can make meaningful measurements of the differences resulting from the addition of a few radials at a time. The fact that you can't get correlation between measurements made on the -same- antenna with different instruments is a problem. Until you have two clocks that read the same, you don't know what time it is. |
If your antenna is over a PERFECT ground, an EM mirror, then the
mutually coupled impedance between the antenna and it's image antenna gets to be a bit difficult to calculate or estimate. The Apex and it's image are 80 feet apart. That is 107 degree spacing, and the ends are 77 degrees apart. The mutual impedance along the length of the antenna changes due to the difference in phase shift. Change from perfect reflecting ground to 'real' ground with varying conductivity and permittivity as a function of length, width and depth and the problem becomes much more complicated. Your measurements, in your location, with your installation indicate, to me, that your 8 radial solution is starting to converge to best solution. I say this for two reasons. Your 2:1 VSWR bandwidth is narrow and your Z is approaching 73 ohms. I would use the eight radial solution and trim the antenna length to your desired center frequency. Remember, a 2:1 VSWR indicates that 90% of your energy is going into the antenna. For the best analysis of your installation, the soil characteristics would have to be known and controlled for several wavelengths square and approximately 1/4 wavelength deep. Your radials [counterpoise] are simplifying and stabilizing your solution. Roy Llewellen is much better qualified than I to proceed deeper into the EM Physics regarding all the possible interactions. If the science of what's happening is your interest then contact Roy off-line [Roy, sorry to volunteer you without your consent]. If a brief understanding of possible contributing EM effects is your desire, then I hope this contributes to your understanding. Ham Op hasan schiers wrote: Question: Where was I taking my measurements. I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 0 3340 3522 3920 580 36,0 1.3 2 3354 3524 3774 420 45,0 1.0 4 3419 3533 3741 322 60,1 1.2 8 3445 3550 3742 297 65,4 1.3 The formatting is bad, but in order, left to right: Number of Radials Lower 2:1 vswr point Resonant Freq point Upper 2:1 point Bandwidth in kilohertz Impedance as shown on the MFJ 269 at resonance. I'm a bit confused by the rising feedpoint impedance with increasing number of radials. That seems backwards to me. 73, ...hasan, N0AN VSWR shown by MFJ 269 at resonance "Ham op" wrote in message ... At what point in the system are you making your measurements? hasan schiers wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ...hasan, N0AN |
I'm a bit confused by the rising feedpoint impedance with increasing
number of radials. That seems backwards to me. ============================= SWR increases the further the impedance, either HIGHER or LOWER, departs from 50 ohms. The only way to measure antenna input impedance is by means of an impedance bridge. Try one of the small antenna analysers. ---- Reg. |
That is exactly what I'm using Reg, and why I"m puzzled. I'm getting rising
feedpoint impedance with increasing numbers of radials. The 2:1 bandwidth is getting narrower, as it should, but what accounts for the increasing feedpoint impedance? MFJ-269 antenna analyzer is what generated the table I posted earlier. Very strange? ....hasan, N0AN "Reg Edwards" wrote in message ... I'm a bit confused by the rising feedpoint impedance with increasing number of radials. That seems backwards to me. ============================= SWR increases the further the impedance, either HIGHER or LOWER, departs from 50 ohms. The only way to measure antenna input impedance is by means of an impedance bridge. Try one of the small antenna analysers. ---- Reg. |
Interesting observations.
The 2:1 swr points are only being mentioned to show that the bandwidth is narrowing with increasing number of radials. That makes sense. What doesn't make sense is the increasing feedpoint Z at resonance in response to increasing radial numbers. There could be some minor trimming, but 3 or 4 ohms reactance is probably close enough...the antenna is near resonance and the resistive component is not likely to change much by trimming the last 3 or 4 ohms of reactance out. The problem is, I can't explain the the 65 ohm feedpoint impedance. Most installations, start with a Z too high (because it includes ground losses), and as radials are added, the feedpoint Z drops (as the loss resistance disappears). This is the classical 1/4w ground mounted vertical case. You know when to stop adding radials when the measured feedpoint impedance at resonance is equal to the Z of the antenna over a "perfect" ground. Another way to accomplish the same thing is to keep adding radials until the 2:1 vswr bandwidth no longer narrows. I think I'm approaching that right now. The anomaly I'm trying to deal with is why the feedpoint Z has increased as I went from 0 to 2, to 4, to 8 radials, when it should have decreased. I wouldn't care a whit, if the terminal (end point) feedpoint Z was 65 ohms resistive with no reactive component. This is a technical element that I want to understand...no further changes are likely to make any real difference in performance. What is it Johnny Cochran said, "If the data don't fit, I just won't quit."? vbg 73, ....hasan, N0AN "Ham op" wrote in message ... If your antenna is over a PERFECT ground, an EM mirror, then the mutually coupled impedance between the antenna and it's image antenna gets to be a bit difficult to calculate or estimate. The Apex and it's image are 80 feet apart. That is 107 degree spacing, and the ends are 77 degrees apart. The mutual impedance along the length of the antenna changes due to the difference in phase shift. Change from perfect reflecting ground to 'real' ground with varying conductivity and permittivity as a function of length, width and depth and the problem becomes much more complicated. Your measurements, in your location, with your installation indicate, to me, that your 8 radial solution is starting to converge to best solution. I say this for two reasons. Your 2:1 VSWR bandwidth is narrow and your Z is approaching 73 ohms. I would use the eight radial solution and trim the antenna length to your desired center frequency. Remember, a 2:1 VSWR indicates that 90% of your energy is going into the antenna. For the best analysis of your installation, the soil characteristics would have to be known and controlled for several wavelengths square and approximately 1/4 wavelength deep. Your radials [counterpoise] are simplifying and stabilizing your solution. Roy Llewellen is much better qualified than I to proceed deeper into the EM Physics regarding all the possible interactions. If the science of what's happening is your interest then contact Roy off-line [Roy, sorry to volunteer you without your consent]. If a brief understanding of possible contributing EM effects is your desire, then I hope this contributes to your understanding. Ham Op hasan schiers wrote: Question: Where was I taking my measurements. I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 0 3340 3522 3920 580 36,0 1.3 2 3354 3524 3774 420 45,0 1.0 4 3419 3533 3741 322 60,1 1.2 8 3445 3550 3742 297 65,4 1.3 The formatting is bad, but in order, left to right: Number of Radials Lower 2:1 vswr point Resonant Freq point Upper 2:1 point Bandwidth in kilohertz Impedance as shown on the MFJ 269 at resonance. I'm a bit confused by the rising feedpoint impedance with increasing number of radials. That seems backwards to me. 73, ...hasan, N0AN VSWR shown by MFJ 269 at resonance "Ham op" wrote in message ... At what point in the system are you making your measurements? hasan schiers wrote: 45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12 THHN Insulated copper wire, stranded. Predicted Radiation Resistance: 25.8 ohms, very good ground (rich pastoral, midwest). With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. Also, predicted efficiency would then be 50 % (25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region losses, this seems to be better than I had expected. 2:1 vswr bandwidth is very broad....broader than what I get with EZnec 4.x with a 25 ohm load in the base. This means one of two things to me: Either my ground losses are much higher than the 25 ohms I'm indirectly calculating, or I have made some sort of conceptual error in thinking about what the implications are of a 25 or 26 ohm feedpoint. All my references point toward a 25 ohm radiation resistance for my 42' vertical x 25.8 ft inverted L (with sloping top wire instead of flat top wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I measure a flat VSWR (on two other meters) at 3600 khz, then isn't the remaining 25 ohms, ground loss? I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz with my MFJ-269, which is really confusing, in that if I have 25 ohms for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600, and the MFJ showing 37 ohms. This is a pretty large percentage difference. I would be inclined to believe the 25.8 ohms predicted by both EZnec 4 and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials tomorrow morning and repeat all my measurements, looking for narrowing bandwidth and lowered input Z as my ground losses decrease. Ultimately, I'm going to put down 16, 66' radials, in steps of four, taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at 0,4,8 and 16 radials. I'll report what happens as I go along. Anywho, without any radials at all there are quite a few distant signals on 80m this evening, that are consistently louder on the newly installed inverted L, than on my Carolina Windom at 45'. Most signals as one pans the band, are louder (at 2 hours after sunset) on the C. Windom than on the radial-less inverted L, but ones from several states away are equal or better on the radial-less inverted L. Both seem to make good sense at this point. I sure will be interested to see the effects of 4 and then 8 and on up radials, but that is going to take a few days, because I don't want to make radial changes unless I have access to low angle signals, which only happens at night or just before sunrise. Tomorrow is radial day. I will be laying out 4 radials 65' long to begin with. I have resistance and reactance measurements every 50 khz as a baseline, before installing radials. 4 in the early morning, 4 more just before sunset, then 4 more the next morning, and the final 4 the next evening. #14 THHN stranded insulated copper wire for the radials, btw. Depending on what I end up seeing for "effect" I'll go to 24 or 32 radials by winter....but only if the improvement is both measurable and "observable on the air"...radials are a pain in the rear (or more accurately, the knees) to put in. What fun! ...hasan, N0AN |
Hasan,
I think you are measuring the input impedance of an Inverted-L against a system of ground radials. You are trying to estimate the input resistance of the ground radials by subtracting the CALCULATED radiation resistance of the Inverted-L from the measured antenna input resistance. Excellent, there is no better way of doing it! First of all, the overall length of the antenna must be 1/4-wavelength resonant at the testing frequency such that its input impedance is PURELY RESISTIVE. The measured input resistance, of course, will be greater than the calculated radiation resistance referred to its base. The difference between them is the required input resistance of the ground radials. The hard part of the exercise is calculating the radiation resistance referred to the base of the Inverted-L. The radiation resistance is a very complicated function of the dimensions, overall length and height, of the antenna. However, for the purposes of estimating ground loss resistance, (it changes with rainfall and temperature of the season), the following approximation for radiation resistance is good enough. RadRes = 18 * ( 1 - Cos( Theta ) ) ohms, where Theta is an angle = 180 * H / ( H + L ) / Lambda degrees, H = height of vertical portion of Inverted-L, L = length of horizontal portion of Inverted-L and Lambda is the free-space wavelength. This formula applies ONLY when L+H is 1/4-wave resonant. Which is the condition under which you are working if you are doing the job correctly. You will not find the formula in the books of bible-writer Terman. Nor in any of the works of the other regular gurus. If you ask from where it came from, it came from one of my old notebooks and I worked it out for myself, years back. Bear in mind it is only an approximation. It would take 6 months to work out how precisely accurate it is and I don't have the time. But it's as least as accurate as you can make impedance measurements. I do hope I have copied it out correctly. By the way, as the number of your radials increases and the ground loss resistance gets very low, don't be surprised if you calculate negative values of ground loss resistance. ---- Reg, G4FGQ |
I'm calculating as we speak, thanks tip, I'll see if the numbers line up.
Also, I'm going out to the feedpoint to do some measurements as the 55' of coax is transforming the impedance at the feedpoint. Gotta beat the misquitoes, the come in swarms at local sunset. 73 ....hasan, N0AN 45 high by 25.8 feet are the antenna dimensions. BTW is lambda in meters (free space wavelength)? I got something on the order of 11.28 ohms, but that was with a new calculator that I'm not sure of the precedence of operations in. Very fancy, but completely backwards from all the other calcs I had previously owned. No wonder it was cheap. "Reg Edwards" wrote in message ... Hasan, I think you are measuring the input impedance of an Inverted-L against a system of ground radials. You are trying to estimate the input resistance of the ground radials by subtracting the CALCULATED radiation resistance of the Inverted-L from the measured antenna input resistance. Excellent, there is no better way of doing it! First of all, the overall length of the antenna must be 1/4-wavelength resonant at the testing frequency such that its input impedance is PURELY RESISTIVE. The measured input resistance, of course, will be greater than the calculated radiation resistance referred to its base. The difference between them is the required input resistance of the ground radials. The hard part of the exercise is calculating the radiation resistance referred to the base of the Inverted-L. The radiation resistance is a very complicated function of the dimensions, overall length and height, of the antenna. However, for the purposes of estimating ground loss resistance, (it changes with rainfall and temperature of the season), the following approximation for radiation resistance is good enough. RadRes = 18 * ( 1 - Cos( Theta ) ) ohms, where Theta is an angle = 180 * H / ( H + L ) / Lambda degrees, H = height of vertical portion of Inverted-L, L = length of horizontal portion of Inverted-L and Lambda is the free-space wavelength. This formula applies ONLY when L+H is 1/4-wave resonant. Which is the condition under which you are working if you are doing the job correctly. You will not find the formula in the books of bible-writer Terman. Nor in any of the works of the other regular gurus. If you ask from where it came from, it came from one of my old notebooks and I worked it out for myself, years back. Bear in mind it is only an approximation. It would take 6 months to work out how precisely accurate it is and I don't have the time. But it's as least as accurate as you can make impedance measurements. I do hope I have copied it out correctly. By the way, as the number of your radials increases and the ground loss resistance gets very low, don't be surprised if you calculate negative values of ground loss resistance. ---- Reg, G4FGQ |
On Sat, 3 Sep 2005 09:27:52 -0500, "hasan schiers"
wrote: Question: Where was I taking my measurements. I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo Hassan, Where are the radials (above ground, buried, how far)? I think you told us that they radial wire is insulated, is that correct? Why did you choose 66' long radials? Owen -- |
Owen,
Wire is laying on the ground, insulated, as it doesn't make any difference and lasts longer. 66' is 1/4 wave at 80m which is always a safe starting point. I probably could shorten them and do more. I intend to put in at least 8 more for a total of 16. I found out why the input Z was going up as I put more radials in...I was measuring at the end of 55' of LMR-400 (as opposed to the measurements I took when I was outside originally tuning the antenna.), and that was acting as a 1/4 wave transformer. I just went outside and connected through an 18 inch jumper to the MFJ-269 and got the following measurements: 2:1 VSWR Low freq Point: 3460 khz 2:1 VSWR Hi freq Point: 3801 khz 2:1 VSWR Bandwidth: 341 khz Fo (Resonant freq) = 3560 khz, 40 ohms resistive, 0 ohms reactance 3500 32,9 1.6 3550 38,0 1.0 3600 44,9 1.2 3650 52,19 1.4 3700 57,30 1.6 3750 63,35 1.8 3800 69,39 2.0 Above table: freq, R,+/- j, VSWR, all taken from the MFJ-269 If the radiation resistance is 26 ohms and I measure 38 ohms feed impedance at resonance, then apparently I have 12 ohms of ground loss, for an efficiency of: 26/(26+12) or 26/38 = 68% At least, that's where I'm at for the moment. Time for another 8 radials. That 1/4 wave transformer (55' of LMR-400 between feedpoint and shack) that Tom, W8JI, pointed out, makes a big difference in my confusion...at least it all makes sense now. 73, ....hasan, N0AN "Owen Duffy" wrote in message ... On Sat, 3 Sep 2005 09:27:52 -0500, "hasan schiers" wrote: Question: Where was I taking my measurements. I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo Hassan, Where are the radials (above ground, buried, how far)? I think you told us that they radial wire is insulated, is that correct? Why did you choose 66' long radials? Owen -- |
Reg, et al, yes, you have what I'm doing correctly. I just went out to the
feedpoint and made measurements. Here's what I found. (I'm still waiting for clarification on your formula, btw) I found out why the input Z was going up as I put more radials in...I was measuring at the end of 55' of LMR-400 (as opposed to the measurements I took when I was outside originally tuning the antenna.), and that was acting as a 1/4 wave transformer. I just went outside and connected through an 18 inch jumper to the MFJ-269 and got the following measurements: 2:1 VSWR Low freq Point: 3460 khz 2:1 VSWR Hi freq Point: 3801 khz 2:1 VSWR Bandwidth: 341 khz Fo (Resonant freq) = 3560 khz, 40 ohms resistive, 0 ohms reactance 3500 32,9 1.6 3550 38,0 1.0 3600 44,9 1.2 3650 52,19 1.4 3700 57,30 1.6 3750 63,35 1.8 3800 69,39 2.0 Above table: freq, R,+/- j, VSWR, all taken from the MFJ-269 If the radiation resistance is 26 ohms and I measure 38 ohms feed impedance at resonance, then apparently I have 12 ohms of ground loss, for an efficiency of: 26/(26+12) or 26/38 = 68% At least, that's where I'm at for the moment. Time for another 8 radials. That 1/4 wave transformer (55' of LMR-400 between feedpoint and shack) that Tom, W8JI, pointed out, makes a big difference in my confusion...at least it all makes sense now. 73, ....hasan, N0AN "Reg Edwards" wrote in message ... Hasan, I think you are measuring the input impedance of an Inverted-L against a system of ground radials. You are trying to estimate the input resistance of the ground radials by subtracting the CALCULATED radiation resistance of the Inverted-L from the measured antenna input resistance. Excellent, there is no better way of doing it! First of all, the overall length of the antenna must be 1/4-wavelength resonant at the testing frequency such that its input impedance is PURELY RESISTIVE. The measured input resistance, of course, will be greater than the calculated radiation resistance referred to its base. The difference between them is the required input resistance of the ground radials. The hard part of the exercise is calculating the radiation resistance referred to the base of the Inverted-L. The radiation resistance is a very complicated function of the dimensions, overall length and height, of the antenna. However, for the purposes of estimating ground loss resistance, (it changes with rainfall and temperature of the season), the following approximation for radiation resistance is good enough. RadRes = 18 * ( 1 - Cos( Theta ) ) ohms, where Theta is an angle = 180 * H / ( H + L ) / Lambda degrees, H = height of vertical portion of Inverted-L, L = length of horizontal portion of Inverted-L and Lambda is the free-space wavelength. This formula applies ONLY when L+H is 1/4-wave resonant. Which is the condition under which you are working if you are doing the job correctly. You will not find the formula in the books of bible-writer Terman. Nor in any of the works of the other regular gurus. If you ask from where it came from, it came from one of my old notebooks and I worked it out for myself, years back. Bear in mind it is only an approximation. It would take 6 months to work out how precisely accurate it is and I don't have the time. But it's as least as accurate as you can make impedance measurements. I do hope I have copied it out correctly. By the way, as the number of your radials increases and the ground loss resistance gets very low, don't be surprised if you calculate negative values of ground loss resistance. ---- Reg, G4FGQ |
On Sat, 3 Sep 2005 09:27:52 -0500, "hasan schiers"
wrote: Question: Where was I taking my measurements. I started out at the actual feedpoint...then put in about 55' of LMR-400, and started a new set of measurements in the shack. Here is the latest data: Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 0 3340 3522 3920 580 36,0 1.3 Ok, you have figured out the coax transforms the impedance. If indeed the Z at the generator end of the coax was 36+j0 (ie it was resonant at that point), then I make the load Z ~ 62-j5. This will confuse your efforts to infer an effective ground system impedance, and losses. Another point to consider, instruments like you are using (in my experience) are more accurate near to 50+j0, and you may get better results by bulking up the feedpoint Z with a known fixed resistor to exploit the better measurement accuracy around 50+j0. Owen -- |
On Sat, 03 Sep 2005 23:49:14 GMT, Owen Duffy wrote:
Ok, you have figured out the coax transforms the impedance. If indeed the Z at the generator end of the coax was 36+j0 (ie it was resonant at that point), then I make the load Z ~ 62-j5. Sorry that should have been 69-j7 (above calc was for one of your other figures, 40+j0). Owen -- |
On Sat, 3 Sep 2005 09:50:43 -0500, "hasan schiers"
wrote: I do have correlation now. The vswr meters and the mfj all read the same resonant frequency now, Wes. See my post that shows the data for 0,2,4,8 radials. While a given absolute value might be in error (a certainty due to inexpensive instruments), the trend is sound for bandwidth, but a bit strange for feedpoint impedance. (BW is narrowing but input Z is going up, with increasing numbers of radials.) Ideas? Well, I think that you're expecting the thing to behave just like a monopole but it's not a monopole. When I model your configuration (except for insulation) I get a resonant freq of 3.573 MHz and R = 23.3 ohm over a Mininec Pastoral ground. Making it a full height monopole it must be 67' high and the feedpoint R = 37. I believe that I read elsewhere that you are taking data at the input end of a coax line. Unless you have characterized the line and are backing out its effects you are going to remain mystified. Also it you have any other antennas in the area that can be an issue. I'm a believer in modeling but unless -everything- is accounted for, reality and the model won't correlate. For example, I have a Cushcraft AV-80 vertical that I've used as a test bed. This is a 2" diameter, 36' high pipe with four 3' top hat wires. Cushcraft supplied a base-loading coil, which is not used. The antenna is ground-mounted on a 16" square aluminum plate with sixteen 50' radials. Installing radials at this location is a "sticky" proposition and the soil is best characterized as sand (decomposed granite actually). I have measured the base impedance with both an HP8405 Vector Voltmeter and coax bridge that was calibrated with an open-short-load (OSL) method as well as a much faster N2PK vector network analyzer, calibrated the same way. All measurements were taken at the base of the antenna. This is an on again off again project (currently off) with the goal of determining the effectiveness of the radial field. The data are very repeatable and believed to be correct; however, I cannot construct a model that replicates the physical antenna. For example the curve on the Smith chart of the measured data has a little bump in it and the resonant frequency is slightly off from the model. The "bump" isn't a full-blown resonance loop, but it looks like it's trying. Here is a plot from 1.5 to 15 MHz. The resonant freq is 5.5 MHz. http://users.triconet.org/wesandlinda/Vertical_Z.gif As you can see from the photo, there several other "verticals" in the vicinity as well as my 45' foot tower with beam and wire dipoles 90' away. http://users.triconet.org/wesandlinda/Vertical_4.jpg |
Yep! A question remains for modelling. Is the value for Source Data in EZnec
4.x directly correlated to the Rrad? My EZnec model (a modified ARRL Inverted L with radials), shows: Frequency = 3.6 MHz Source 1 Voltage = 25.65 V. at 4.18 deg. Current = 1 A. at 0.0 deg. Impedance = 25.58 + J 1.872 ohms Power = 25.58 watts SWR (50 ohm system) = 1.958 (25 ohm system) = 1.081 Is that 25.58 ohms the same 25 ohms from the graph of Devoldere's book, or is it simply a coincidence? I have attached my *.ez file, if it makes it through the usenet group. From my other more recent posts on the topic, you can see that I went back out to the feedpoint with an 18" jumper and remeasured and the results are quite "traditional". If one can believe the graph in the latest version of Devoldere's "Low-Band DXing", an inverted L like mine withe a .16 wavelength vertical portion and the horizontal or vee'd portion making of the remainder to achieve resonance, the Rrad should be about 25 ohms. Since the Z at resonance at the feedpoint with 8 radials 66 feet long shows 38 ohms, it would appear I have something like 13 ohms of ground return loss. This would represent an efficiency of 67% or a loss of 1.7 dB. I'm thinking another 8 radials should get me closer to about 1 dB of loss, at which point, I think I'll be tired of cutting slits in the lawn. vbg I did use an on-line TLE calculator to correct for the 55' of LMR-400, and doing that yielded readings virtually identical to those I measured at the feedpoint through my 18" jumper. So, all in all, things worked as they should What got me confused was not considering that I had a quarter wave transformer after I moved into the shack to measure, and it was inverting my readings. Made me real queasy there for a while. Thanks to all for taking the time to provide ideas and answers. Tom and a few others caught the 1/4 wave transformer problem...and after all, that was my real question. I had no doubt that I was seeing an improvment in efficiency by adding radials (as the 2:1 BW was steadily decreasing with increasing radials)...I just couldn't get my head around the 60 ohm, ever increasing values of Z in the shack. All I can say now is, "DUH!" The sun has set....it's time to make instant switching comparisons between my 45' high dipole and the new Inverted L. Last night and this morning it appeared (although I didn't have a lot of data points yet), that stations 1500 km were much stronger on the inverted L than they were on the dipole, including a 59+ report from an LU early this morning. Thanks again, Wes (and all the others who commented). It has been fun working through the process. Now it's time to chase DX. ....hasan, N0AN "Wes Stewart" wrote in message ... On Sat, 3 Sep 2005 09:50:43 -0500, "hasan schiers" wrote: I do have correlation now. The vswr meters and the mfj all read the same resonant frequency now, Wes. See my post that shows the data for 0,2,4,8 radials. While a given absolute value might be in error (a certainty due to inexpensive instruments), the trend is sound for bandwidth, but a bit strange for feedpoint impedance. (BW is narrowing but input Z is going up, with increasing numbers of radials.) Ideas? Well, I think that you're expecting the thing to behave just like a monopole but it's not a monopole. When I model your configuration (except for insulation) I get a resonant freq of 3.573 MHz and R = 23.3 ohm over a Mininec Pastoral ground. Making it a full height monopole it must be 67' high and the feedpoint R = 37. I believe that I read elsewhere that you are taking data at the input end of a coax line. Unless you have characterized the line and are backing out its effects you are going to remain mystified. Also it you have any other antennas in the area that can be an issue. I'm a believer in modeling but unless -everything- is accounted for, reality and the model won't correlate. For example, I have a Cushcraft AV-80 vertical that I've used as a test bed. This is a 2" diameter, 36' high pipe with four 3' top hat wires. Cushcraft supplied a base-loading coil, which is not used. The antenna is ground-mounted on a 16" square aluminum plate with sixteen 50' radials. Installing radials at this location is a "sticky" proposition and the soil is best characterized as sand (decomposed granite actually). I have measured the base impedance with both an HP8405 Vector Voltmeter and coax bridge that was calibrated with an open-short-load (OSL) method as well as a much faster N2PK vector network analyzer, calibrated the same way. All measurements were taken at the base of the antenna. This is an on again off again project (currently off) with the goal of determining the effectiveness of the radial field. The data are very repeatable and believed to be correct; however, I cannot construct a model that replicates the physical antenna. For example the curve on the Smith chart of the measured data has a little bump in it and the resonant frequency is slightly off from the model. The "bump" isn't a full-blown resonance loop, but it looks like it's trying. Here is a plot from 1.5 to 15 MHz. The resonant freq is 5.5 MHz. http://users.triconet.org/wesandlinda/Vertical_Z.gif As you can see from the photo, there several other "verticals" in the vicinity as well as my 45' foot tower with beam and wire dipoles 90' away. http://users.triconet.org/wesandlinda/Vertical_4.jpg |
"hasan schiers" wrote (I'm still waiting for clarification on your formula, btw) ==================================== There's a mistake in the formula. I copied it incorrectly from my old notebook. The wavelength Lambda doesn't come into it. No wonder you asked what units Lambda is in. The correct, more simple, formula is - RadRes = 18 * ( 1 - Cos( 180 * H / ( H + L ) ) ohms, Where H = Height, L = Length of horizontal section, and the angle is in degrees. Your antenna is 45 feet high and 70.8 feet overall length. (It doesn't matter what the measurement units are. It's just a ratio.) And so your radiation resistance, at 1/4-wave resonance, is 25.4 ohms, give or take a few ohms. The only thing I'm unhappy about is making impedance measurements at the other end of 55 feet of coax. You need to know the exact Zo and velocity factor and length of this cable, plus some accurate calculations. The technique is fraught with error. Get your hand-held antenna analyser right to the bottom end of the antenna wire, on the R + jX range, and immediately adjacent to the focal point of the radials. And hope you don't get interference from the local, high power, MF broadcast station. But you are already aware of this and I mention it for the benefit of the lurkers. I assume you measure SWR only to estimate antenna bandwidth. At the other end of 55 feet of coax anything can happen to SWR. But if bandwidth decreases as the number of radials increases then at least it is going in the right direction. I don't think you will squeeze any other information out of it. ---- Reg, G4FGQ |
Hasan,
If you download program ENDFEED and insert details of your antenna and ground radials system, you will find the impedances closely agee with what you actually get. (I'm always very happy when this happens.) Enter the exact 80-meter band frequency at which you have found your own antenna to be 1/4-wave resonant. Then slightly prune antenna height until the program says the antenna is 1/4-wavelength long. Very likely your antenna horizontal section is sloping. As a final check, do with the program what you have done with your own antenna, and subtract the RF ground resistance from the antenna input resistance to give the radiation resistance. (The radiation resistance is not displayed by the program although, obviously, it is used internally.) The primary purpose of the program is to calculate the L and C values of three different impedance-matching networks (tuners), which you may find useful. Radiating efficiency is also calculated and displayed. Download ENDFEED, an old program of mine, from website below. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
Outstanding, Reg. Your formula for Rrad agrees completely with the graph in
Devoldere's book. I will make a special effort to copy the formula down correctly and put it in the subdirectory of my hard disk that has all your other programs. I already figured out the 1/4 wave transformer problem, so I went out in measured again directly (18" jumper) the feedpoint impedance and got much more realistic readings. I recall from reading some of your other posts when people were wringing their hands about what antenna they could put up in a given circumstance. You advised the inverted L in the garden. When I started work on this project (whose goal was a decent DX antenna for 80m), I thought of your comments on many occasions. As you noted, I'm doing the 2:1 VSWR bandwidth measurements strictly to look for the point of diminishing returns on laying the radial field. It is quite clear at this point that 8 is not quite enough, but 16 should be more than adequate to get me within 1 dB of the idea. It has been a fun experiment so far, and an enlightening discussion. Thanks so much for your formula and other comments. Most helpful. ....hasan, N0AN "Reg Edwards" wrote in message ... "hasan schiers" wrote (I'm still waiting for clarification on your formula, btw) ==================================== There's a mistake in the formula. I copied it incorrectly from my old notebook. The wavelength Lambda doesn't come into it. No wonder you asked what units Lambda is in. The correct, more simple, formula is - RadRes = 18 * ( 1 - Cos( 180 * H / ( H + L ) ) ohms, Where H = Height, L = Length of horizontal section, and the angle is in degrees. Your antenna is 45 feet high and 70.8 feet overall length. (It doesn't matter what the measurement units are. It's just a ratio.) And so your radiation resistance, at 1/4-wave resonance, is 25.4 ohms, give or take a few ohms. The only thing I'm unhappy about is making impedance measurements at the other end of 55 feet of coax. You need to know the exact Zo and velocity factor and length of this cable, plus some accurate calculations. The technique is fraught with error. Get your hand-held antenna analyser right to the bottom end of the antenna wire, on the R + jX range, and immediately adjacent to the focal point of the radials. And hope you don't get interference from the local, high power, MF broadcast station. But you are already aware of this and I mention it for the benefit of the lurkers. I assume you measure SWR only to estimate antenna bandwidth. At the other end of 55 feet of coax anything can happen to SWR. But if bandwidth decreases as the number of radials increases then at least it is going in the right direction. I don't think you will squeeze any other information out of it. ---- Reg, G4FGQ |
"Reg Edwards" wrote in message ... "I assume you measure SWR only to estimate antenna bandwidth. At the other end of 55 feet of coax anything can happen to SWR. But if bandwidth decreases as the number of radials increases then at least it is going in the right direction. I don't think you will squeeze any other information out of it." Boy, Reg, that last sentence describes EXACTLY what I have been attempting to do, squeeze the last bit of information or inference that I can make of the data collected. Your formula works perfectly for me, giving the same results on this fancy new calculator. It is formula based (which I am not used to), so the data entry is backwards from what I'm used to (RPN). Once I entered your formula into the calculator just as you show it, left to right, it produced the expected 25.4 ohms. That is one handy formula indeed! Who knows the boundary limits for its accuracy, (only the creator would know that), but in the case of my particular antenna, it is right on the money and agrees with the "book" based graph that gave me the original value of 25 ohms for a 0.16 wavelength high quarter wave inverted L. It's nice to have convergence! ....hasan, N0AN |
On Sat, 3 Sep 2005 20:19:04 -0500, "hasan schiers"
wrote: Yep! A question remains for modelling. Is the value for Source Data in EZnec 4.x directly correlated to the Rrad? My EZnec model (a modified ARRL Inverted L with radials), shows: Frequency = 3.6 MHz Source 1 Voltage = 25.65 V. at 4.18 deg. Current = 1 A. at 0.0 deg. Impedance = 25.58 + J 1.872 ohms Power = 25.58 watts SWR (50 ohm system) = 1.958 (25 ohm system) = 1.081 Is that 25.58 ohms the same 25 ohms from the graph of Devoldere's book, or is it simply a coincidence? Without looking at the book I would say yes. If you put 1 A into a Z and 25.65 V @ 4.18 degree develops then that Z = 25.58 +j1.872 I have attached my *.ez file, if it makes it through the usenet group. It did. |
On Sat, 3 Sep 2005 17:55:17 -0500, "hasan schiers"
wrote: Owen, Wire is laying on the ground, insulated, as it doesn't make any difference and lasts longer. 66' is 1/4 wave at 80m which is always a safe starting point. It is in free space. Lying on the ground isn't free space. Were the radials buried, depending on the ground, 66' might be more in the vicinity of an electrical half wave. Suspended a short distance above the ground might be close to an electrical quarter wave. Yours will be somewhere between depending on the installation variables. It would be interesting to measure the impedance and find the resonance of a pair of opposite radials. Owen -- |
Final Measurements:
I added 8 more radials this evening for a total of 16. Radials 2:1 Fo 2:1 BW Z VSWR @ Fo 8 3460 3564 3801 341 40,0 1.2 (at antenna) 16 3524 3580 3800 276 31,0 1.7 (at antenna) Fo is the resonant freq, 2:1 are the lower and upper 2:1 vswr frequency points. BW is the 2:1 SWR Bandwidth in Khz. Z is the R +/- j impedance read from an MFJ-269 antenna analyzer at the feedpoint. Since the radiation resistance of the antenna is known to be 25.4 ohms: Efficiency with 8 radials: 25.4/40 = 63.5% or ground loss = 1.97 dB Efficiency with 16 radials: 25.4/31 =81.9% or ground loss = 0.86 dB Additional info, 2:1 VSWR Bandwidth in Khz: 0 Radials = 580 khz 8 Radials = 341 khz 16 Radials = 275 khz So, both "traditional" considerations of a 1/4 wave vertical have now been satisfied: Increasing the number of radials decreased the feedpoint Z, approaching the nominal 25.4 ohms radiation resistance of the antenna. Increasing the number of radials from 0 to 8 to 16, narrowed the bandwidth from 580 to 341 to 276 khz respectively. The SWR at resonance is worse with 16 radials than with 0 radials (higher ground losses mask the reactance at the feedpoint) 16 Radials over very good Iowa black loam yields an efficiency of approximately 82%, and I see no reason to further increase the number of radials to recover .86 dB...and of course, it would probably require doubling the radial number yet again, to 32 to get half way there. I'm now satisfied that the antenna behaves as expected and that the numbers are credible, but not absolute. Time to play on the air some more. So far it is getting out very well. Thanks to all of you for your input, I've learned a lot by playing. The TLE on-line calculator is a jewel! EZNEC 4.1 got me in the ballpark. The three most valuable lessons learned: beware of an unintentional 1/4 wave transformer (coax); Reg's clever formula for calculating radiation resistance of an inverted L, using simple trig functions, and 16 radials on 80m are quite sufficient OVER MY TYPE OF SOIL. ....hasan, N0AN "Reg Edwards" wrote in message ... Hasan, If you download program ENDFEED and insert details of your antenna and ground radials system, you will find the impedances closely agee with what you actually get. (I'm always very happy when this happens.) |
16 radials on
80m are quite sufficient OVER MY TYPE OF SOIL. ...hasan, N0AN ============================ So much for B.L & E ! ---- Reg. |
OUCH!
(Flame suit on) "Reg Edwards" wrote in message ... 16 radials on 80m are quite sufficient OVER MY TYPE OF SOIL. ...hasan, N0AN ============================ So much for B.L & E ! ---- Reg. |
Hassan Schiers wrote:
"--and 16 radials on 80m are quite sufficient over my type of soil." Glancing at Fig. 2.17 on page 119 of Laport`s "Radio Antenna Engineering" shows that BL&E would agree. 1000 watts into a 90-degree vertical with 16 radials will produce about 160 mv/m at one mile. Perfection is only about 190 mv/m. 85% should be close enough to perfection, unless you are a broadcaster or the FCC enforcing its rules on a broadcaster. Best regards, Richard Harrison, KB5WZI |
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