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Matching Issue ........... I think!
I recently built a modified vertical dipole
from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
On Jan 30, 10:19 pm, wrote:
I recently built a modified vertical dipole from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! What do you mean by "modified"? We will need antenna dimensions, where the feed point is, overall height, height above ground, is it straight or bent, etc., in order to give you any advice... The only immediate comment I can make is that any 'dipole' that is near 50 ohms at the feed point is going to be several thousand ohms at double the frequency... A matching section of coax will not help... denny / k8do |
Matching Issue ........... I think!
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Matching Issue ........... I think!
wrote in
news:DoidnRiueftdl13YnZ2dnUVZ_rXinZ2d@hawaiiantel. net: I recently built a modified vertical dipole from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. The feedpoint impedance of a full wave dipole is likely to be in the region of 4000 ohms. A quarter wave transformer needs to be of Zo (50*4000)^0.5 or 447 ohms. You will not find a practical coax line with such Zo. If you for instance used a quarter wave of RG58C/U for a transformer, the input impedance would be 2 ohms (VSWR=25) (taking into account the 5+dB of loss in the transformer. Both the length and the Zo of a quarter wave transformer are important to its operation. Does this help to explain the results you observed? Owen I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. |
Matching Issue ........... I think!
Jim Kelley wrote:
wrote: I recently built a modified vertical dipole from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. Try a half wavelength of coax. ac6xg why a half wave? -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
Owen Duffy wrote:
wrote in news:DoidnRiueftdl13YnZ2dnUVZ_rXinZ2d@hawaiiantel. net: I recently built a modified vertical dipole from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. The feedpoint impedance of a full wave dipole is likely to be in the region of 4000 ohms. its half wave A quarter wave transformer needs to be of Zo (50*4000)^0.5 or 447 ohms. You will not find a practical coax line with such Zo. If you for instance used a quarter wave of RG58C/U for a transformer, the input impedance would be 2 ohms (VSWR=25) (taking into account the 5+dB of loss in the transformer. Both the length and the Zo of a quarter wave transformer are important to its operation. Does this help to explain the results you observed? maybe I should have asked does my coax tune my antenna??????? Owen I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
Denny wrote:
On Jan 30, 10:19 pm, wrote: I recently built a modified vertical dipole from wire and using a pole and trees as supports. I had hoped it would work on 2 bands 75 and 40 but it only works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax matching section 50 ohms but this doesnt solve the problem either as the VSWR doesnt change much with the extra coax section. I did cut the section properly as the velocity factor for the coax is .78% which gave me 26.825 ft. Not sure what I am doing wrong or assuming - but looks like I could use some input. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! What do you mean by "modified"? looks like the letter "H" on its side "normally" but modified the feed is asymmertical about 5 ft off the gound. similar to Force 12 Sigma 80 it looks like this | | | abot 56 ft high | | | | feed here has 2 loading coils | ___________|____________ both sides approx 17 ft | base insulator here _|_ base mount ground feed point impedance is supposed to be very low - maybe 12 ohms We will need antenna dimensions, where the feed point is, overall height, height above ground, is it straight or bent, etc., in order to give you any advice... The only immediate comment I can make is that any 'dipole' that is near 50 ohms at the feed point is going to be several thousand ohms at double the frequency... A matching section of coax will not help... denny / k8do -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
On Wed, 31 Jan 2007 18:40:56 -1000, wrote:
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 it looks like this | | | abot 56 ft high | | | | feed here has 2 loading coils | ___________|____________ both sides approx 17 ft | base insulator here _|_ base mount ground feed point impedance is supposed to be very low - maybe 12 ohms Which gives rise to the question, where did the match at 75 meters come from? I suspect it comes from the added loading. Next question, why does it have the high mismatch for 40 meters. The same suspicion follows logically (as the harmonic relationships follow logically). With loading resonance is not length, it is the combination of all elements. What appears to be a normal (good) 75 meter operation should also appear to be normal (poor) 40 meter operation. At 75 meters, the antenna is normally Low-Z. At 40 meters, the antenna is normally Hi-Z. Your SWR measurement confirms the sense of "normal" in both bands. 73's Richard Clark, KB7QHC |
Matching Issue ........... I think!
Richard Clark wrote:
On Wed, 31 Jan 2007 18:40:56 -1000, wrote: works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz for the 2:1 points. On 40 meters the VSWR is over 10:1 it looks like this | | | abot 56 ft high | | | | feed here has 2 loading coils | ___________|____________ both sides approx 17 ft | base insulator here _|_ base mount ground feed point impedance is supposed to be very low - maybe 12 ohms Which gives rise to the question, where did the match at 75 meters come from? I suspect it comes from the added loading. Next question, why does it have the high mismatch for 40 meters. The same suspicion follows logically (as the harmonic relationships follow logically). With loading resonance is not length, it is the combination of all elements. What appears to be a normal (good) 75 meter operation should also appear to be normal (poor) 40 meter operation. At 75 meters, the antenna is normally Low-Z. At 40 meters, the antenna is normally Hi-Z. Your SWR measurement confirms the sense of "normal" in both bands. 73's Richard Clark, KB7QHC so whats the solution to making it work on 40? a tuner? some kind of tuned piece of coax????????????? -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
On Wed, 31 Jan 2007 20:42:51 -1000, wrote:
so whats the solution to making it work on 40? a tuner? some kind of tuned piece of coax????????????? Read Owen's reply. 73's Richard Clark, KB7QHC |
Matching Issue ........... I think!
wrote:
maybe I should have asked does my coax tune my antenna??????? When the load impedance is not 50 ohms and the target impedance is 50 ohms, 50 ohm coax cannot tune your antenna system to a 50 ohm match. In fact, using 50 ohm coax with an SWR not equal to 1:1 is one way of guaranteeing that the transmitter will *never* see 50 ohms without some help from a tuner or other matching method. (Extensive losses will also bring the SWR closer to 1:1). If the target impedance is 50 ohms, only a characteristic impedance different from 50 ohms can transform the non-50 ohm impedance to 50 ohms. And that can only be done when the SWR is not 1:1. Note that in Owen's 1/4WL matching equation, only if the load and source are both 50 ohms does the equation result in Z0 = 50 ohms. Such a system is called a "flat" system with a trivial default SWR of 1:1. An autotuner at the feedpoint will solve the problem or a parallel feedline with a Z0 of 300-600 ohms will solve the problem. I have solved the problem at my QTH with a 148 foot dipole and 90 feet of 450 ohm ladder-line. That antenna system is simultaneously resonant at my QTH on both 7.15 MHz and 3.8 MHz at the same time. And it is also close to resonance on 17m. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
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Matching Issue ........... I think!
Hmmm, I can't make much of this diagram and the group has decided to
use passive aggressive behavior to "teach you"... You are asking for help and getting didactics thrown in your face... Shame on them! I doubt you can feed this antenna on 40 with coax... You will need to either put a tuner at the feed point, or resort to open wire line and a tuner at the radio... My first impulse would be to turn this into a top loaded vertical with an elevated counterpoise, the vertical self resonant on 40 and loaded for 80... Feed it at the bottom of the vertical section with the braid going to the 17 foot horizontals (some tuning needed)... Use a loading coil about half way up the vertical for 80 and short it out for 40... The counterpoise (the 17 foot horizontals as best I can make out) would be coil loaded at at the center connection of the 2 "radials" so they are resonant on 80 with a shroting clip for 40... A relay will select for you, or you can walk out and manually move the clips... Go to the library there and get an interloan copy of Moxon's "HF Antennas for all locations"... He has an excellent discussion of loaded radials/counterpoises, etc... With his help you should see how to make your antenna play... Any problems drop me an email through and I will give you a fax number where you can shoot me a diagram of your antenna and we can discuss it... cheers ... denny / k8do |
Matching Issue ........... I think!
wrote in
: its half wave I would not have guessed that from the meagre information you supplied. Owen |
Matching Issue ........... I think!
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Matching Issue ........... I think!
Denny wrote:
Hmmm, I can't make much of this diagram and the group has decided to use passive aggressive behavior to "teach you"... You are asking for help and getting didactics thrown in your face... Shame on them! I doubt you can feed this antenna on 40 with coax... You will need to either put a tuner at the feed point, or resort to open wire line and a tuner at the radio... My first impulse would be to turn this into a top loaded vertical with an elevated counterpoise, the vertical self resonant on 40 and loaded for 80... Feed it at the bottom of the vertical section with the braid going to the 17 foot horizontals (some tuning needed)... Use a loading coil about half way up the vertical for 80 and short it out for 40... The counterpoise (the 17 foot horizontals as best I can make out) would be coil loaded at at the center connection of the 2 "radials" so they are resonant on 80 with a shroting clip for 40... A relay will select for you, or you can walk out and manually move the clips... Go to the library there and get an interloan copy of Moxon's "HF Antennas for all locations"... He has an excellent discussion of loaded radials/counterpoises, etc... With his help you should see how to make your antenna play... Any problems drop me an email through and I will give you a fax number where you can shoot me a diagram of your antenna and we can discuss it... cheers ... denny / k8do the physical antenna stays as is and works extremely well on 75. What has me asking the questions is that I was told a 1/4 wave piece of coax would get it to work on 40. Maybe I need to run it thru some modeling software and see what actually does need to happen to get 40 to work. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
Cecil Moore wrote:
wrote: so whats the solution to making it work on 40? a tuner? some kind of tuned piece of coax????????????? A better description would help. That looks more like a ground plane antenna than it does a dipole. Please describe the antenna in more detail. go to Force 12 website and check out the sigma 80 this is similar in that instead of having horizontal elements on the top its straight vertical. signal reports on 80 have been very impressive. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
On Thu, 01 Feb 2007 19:32:05 -1000, wrote:
signal reports on 80 have been very impressive. You sound like a very good customer prospect weighed against the testimonials offered to a German inventor of a 9 foot tall 160M radiator. Signal reports on 80 still doesn't make it work on 40M however. The reasons have been explained for your difficulties, and solutions offered. Are you looking for something different? 73's Richard Clark, KB7QHC |
Matching Issue ........... I think!
wrote:
I have solved the problem at my QTH with a 148 foot dipole and 90 feet of 450 ohm ladder-line. That antenna system is simultaneously resonant at my QTH on both 7.15 MHz and 3.8 MHz at the same time. And it is also close to resonance on 17m. can this be done with one feedline? Yes, I have done it with one feedline and written an article for "Worldradio" about it. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
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Matching Issue ........... I think!
Cecil Moore wrote:
wrote: the physical antenna stays as is and works extremely well on 75. What has me asking the questions is that I was told a 1/4 wave piece of coax would get it to work on 40. Was that a parallel stub by any chance? hmmmm, thats not what I recall being told but would it work? one feed line? -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
Richard Clark wrote:
On Thu, 01 Feb 2007 19:32:05 -1000, wrote: signal reports on 80 have been very impressive. You sound like a very good customer prospect weighed against the testimonials offered to a German inventor of a 9 foot tall 160M radiator. Signal reports on 80 still doesn't make it work on 40M however. The reasons have been explained for your difficulties, and solutions offered. Are you looking for something different? 73's Richard Clark, KB7QHC Richard, I am not trying to be an ass here, but I was told BEFORE the antenna was built it would work on 40. So it got built and doesnt work on 40 as is. It was suggested at the time to use a 1/4 wave section of coax to get it to work on 40, which as you know it doesnt. All I want to do is figure out a way to get it to work on 40 now. Believe me this antenna works extremely well on 80. 20 over 9 reports from ZL when the VK's there are only 10 over 9. This configuration works well, I am sure someone could build a similar version just for 40 and be a big gun on 40. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
wrote:
hmmmm, thats not what I recall being told but would it work? one feed line? A shorted 1/4WL 80m parallel stub will yield a high impedance on 80m and a low impedance on 40m. This is sometimes used with open-wire line to achieve a match on both bands for a dipole that is 1/2WL on 80m and 1WL on 40m. Also to open up a 1WL 40m loop for use on 80m as a 1/2WL dipole. -- 73, Cecil, http://www.qsl.net/w5dxp |
Matching Issue ........... I think!
On Fri, 02 Feb 2007 05:59:56 -1000, wrote:
I was told BEFORE the antenna was built it would work on 40. You are a victim of poor information. That is evident in the false assurance (it simply doesn't make sense) and the positive proof of your measurements. So it got built and doesnt work on 40 as is. No one here is surprised by that outcome. It was suggested at the time to use a 1/4 wave section of coax to get it to work on 40, which as you know it doesnt. A quarter wave solution was offered by Owen which was the correct solution. This does not mean it would turn a dog into a performer, it simply meant it would match. Now, as to the matter of making it match using coax, THAT is a hash pipe dream. You may eventually succeed through luck and happenstance, but chances are that coax will glow at night. All I want to do is figure out a way to get it to work on 40 now. As I offered, Owen has already done the work, but only you can build it and discover it works. Believe me this antenna works extremely well on 80. 20 over 9 reports from ZL when the VK's there are only 10 over 9. That's nice, but it doesn't solve 40M. 73's Richard Clark, KB7QHC |
Matching Issue ........... I think!
Cecil Moore wrote:
wrote: hmmmm, thats not what I recall being told but would it work? one feed line? A shorted 1/4WL 80m parallel stub will yield a high impedance on 80m and a low impedance on 40m. This is sometimes used with open-wire line to achieve a match on both bands for a dipole that is 1/2WL on 80m and 1WL on 40m. Also to open up a 1WL 40m loop for use on 80m as a 1/2WL dipole. I have seen this before, but somehow I would have to be able to connect and unconnect the stub. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
wrote:
Cecil Moore wrote: wrote: hmmmm, thats not what I recall being told but would it work? one feed line? A shorted 1/4WL 80m parallel stub will yield a high impedance on 80m and a low impedance on 40m. This is sometimes used with open-wire line to achieve a match on both bands for a dipole that is 1/2WL on 80m and 1WL on 40m. Also to open up a 1WL 40m loop for use on 80m as a 1/2WL dipole. I have seen this before, but somehow I would have to be able to connect and unconnect the stub. No you don't. The stub is a high impedance on 80m in parallel with a low feedpoint impedance and therefore has a negligible effect on 80m. It can be left in the circuit. The stub is a low impedance on 40m in parallel with a high feedpoint impedance which lowers the SWR and makes your transmitter happy. On 40m, the stub acts as a low impedance load that eventually delivers most of its stored energy to the high impedance antenna (assuming the stub is made from low-loss transmission line). -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
In message , Cecil Moore
writes I have seen this before, but somehow I would have to be able to connect and unconnect the stub. No you don't. The stub is a high impedance on 80m in parallel with a low feedpoint impedance and therefore has a negligible effect on 80m. It can be left in the circuit. The stub is a low impedance on 40m in parallel with a high feedpoint impedance which lowers the SWR and makes your transmitter happy. On 40m, the stub acts as a low impedance load that eventually delivers most of its stored energy to the high impedance antenna (assuming the stub is made from low-loss transmission line). I don't understand that, Cecil. Yes, the stub will be a halfwave on 40m, and therefore add a low impedance in parallel with the antenna feedpoint. Depending on the exact resonant length of the stub, the impedance will be either a very low value resistor, a low value inductor, or a large value capacitor. None of these is going to help to convert the resistive part of the input impedance of the antenna to something around 50 ohms. However, I suspect that what the matching arrangement actually consists of a stub which is connected across the feeder AT SOME DISTANCE from the antenna feedpoint. If the antenna feedpoint presents a high impedance on 40m, there will be a point at some distance (less than a quarterwave) back along the feeder (towards the TX) where the resistive part of the impedance (at that point) is 50 ohm, in parallel with a considerable capacitive reactance. By choosing the correct length of stub and the correct distance between the connection of the stub and the antenna feedpoint, you can probably get a 'reasonable' match for both 80m and 40m. If this IS the actual physical arrangement, it's time to get the Smith Charts out! Ian. -- |
Matching Issue ........... I think!
Ian Jackson wrote:
I don't understand that, Cecil. Yes, the stub will be a halfwave on 40m, and therefore add a low impedance in parallel with the antenna feedpoint. Depending on the exact resonant length of the stub, the impedance will be either a very low value resistor, a low value inductor, or a large value capacitor. None of these is going to help to convert the resistive part of the input impedance of the antenna to something around 50 ohms. You're right, Ian. I was trying to apply the same principles that causes the stub to turn a full-wave 40m loop into a 1/2WL 75m dipole and had a senior moment. But that idea would work with RG-174. With a 3 dB loss in the stub, the impedance looking into the stub would be around 50 ohms. :-) -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
In message , Cecil
Moore writes Ian Jackson wrote: I don't understand that, Cecil. Yes, the stub will be a halfwave on 40m, and therefore add a low impedance in parallel with the antenna feedpoint. Depending on the exact resonant length of the stub, the impedance will be either a very low value resistor, a low value inductor, or a large value capacitor. None of these is going to help to convert the resistive part of the input impedance of the antenna to something around 50 ohms. You're right, Ian. I was trying to apply the same principles that causes the stub to turn a full-wave 40m loop into a 1/2WL 75m dipole and had a senior moment. But that idea would work with RG-174. With a 3 dB loss in the stub, the impedance looking into the stub would be around 50 ohms. :-) I'm not an antenna person, Cecil. Are you going to do the hono(u)rable thing and calculate the dimensions? Ian -- |
Matching Issue ........... I think!
Ian Jackson wrote:
I'm not an antenna person, Cecil. Are you going to do the hono(u)rable thing and calculate the dimensions? What I am lacking is the feedpoint impedance on 40m. I'll try to get time to model the antenna. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
Ian Jackson wrote:
I'm not an antenna person, Cecil. Are you going to do the hono(u)rable thing and calculate the dimensions? I took a look at the Force 12 Sigma-80 and Sigma-40. Nowhere do they say a Sigma-80 will work on 40m. The Sigma antennas appear to be center-loaded vertical dipoles with end hats as in the following fixed font. FP is the feedpoint. | | | coil coil | +-------------/////FP/////-------------+ | || | | || | I estimated the dimensions of the Sigma-80 from pictures on the Force 12 web page. The center section is about 36 feet long and the end caps are about 22 feet long. Plugging those values into EZNEC says that simply shorting out the loading coils will make the antenna resonant very close to 40m. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
In message , Cecil
Moore writes Ian Jackson wrote: I'm not an antenna person, Cecil. Are you going to do the hono(u)rable thing and calculate the dimensions? I took a look at the Force 12 Sigma-80 and Sigma-40. Nowhere do they say a Sigma-80 will work on 40m. The Sigma antennas appear to be center-loaded vertical dipoles with end hats as in the following fixed font. FP is the feedpoint. | | | coil coil | +-------------/////FP/////-------------+ | || | | || | I estimated the dimensions of the Sigma-80 from pictures on the Force 12 web page. The center section is about 36 feet long and the end caps are about 22 feet long. Plugging those values into EZNEC says that simply shorting out the loading coils will make the antenna resonant very close to 40m. That's it then. Connect a 80m quarterwave S/C stub (preferably twin feeder) across both coils (ie from the outer end to the outer end). That will leave the coils O/C on 80m and S/C the coils on 40m. Should work! Ian. -- |
Matching Issue ........... I think!
Ian Jackson wrote:
Cecil Moore writes | | | coil coil | +-------------/////FP/////-------------+ | || | | || | Connect a 80m quarterwave S/C stub (preferably twin feeder) across both coils (ie from the outer end to the outer end). That will leave the coils O/C on 80m and S/C the coils on 40m. A somewhat weird looking modification for a center-fed vertical antenna. I suppose the two stubs could double as guy wires. I might be tempted to accomplish the switching with a large open frame relay or two. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
In message , Cecil Moore
writes Ian Jackson wrote: Cecil Moore writes | | | coil coil | +-------------/////FP/////-------------+ | || | | || | Connect a 80m quarterwave S/C stub (preferably twin feeder) across both coils (ie from the outer end to the outer end). That will leave the coils O/C on 80m and S/C the coils on 40m. A somewhat weird looking modification for a center-fed vertical antenna. I suppose the two stubs could double as guy wires. I might be tempted to accomplish the switching with a large open frame relay or two. From the original ASCII diagram, it isn't too clear what the configuration is. Is it actually a 'single-ended' vertical version of your diagram above of the horizontal balanced antenna? The two loading coils (original diagram) would in series on 80m, with the stub shorting out the lower one on 40m. This would enable freer adjustments of the resonances of the antenna on the two bands. It would also help explain the original description that the stub was connected 'across the feedpoint' (although it isn't). Ian. -- |
Matching Issue ........... I think!
Cecil Moore wrote:
Ian Jackson wrote: I'm not an antenna person, Cecil. Are you going to do the hono(u)rable thing and calculate the dimensions? I took a look at the Force 12 Sigma-80 and Sigma-40. Nowhere do they say a Sigma-80 will work on 40m. I did say modified and similar. I also stated that the top horizontal portion was now vertical. The Sigma antennas appear to be center-loaded vertical dipoles with end hats as in the following fixed font. FP is the feedpoint. this one ia asymmetrical feed about 6 ft off the ground which I also mentioned previously. | | | coil coil | +-------------/////FP/////-------------+ | || | | || | I estimated the dimensions of the Sigma-80 from pictures on the Force 12 web page. The center section is about 36 feet long and the end caps are about 22 feet long. Plugging those values into EZNEC says that simply shorting out the loading coils will make the antenna resonant very close to 40m. -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
Matching Issue ........... I think!
Ian Jackson wrote:
From the original ASCII diagram, it isn't too clear what the configuration is. As I understand it, the antenna under discussion is like the Sigma-80 except the top hat is omitted and the upper element is instead extended upward - like this. | coil coil | ---------------------/////FP/////-------------+ || | || | So I'm assuming whatever works for the Sigma-80 will work for this antenna. -- 73, Cecil http://www.w5dxp.com |
Matching Issue ........... I think!
Cecil Moore wrote:
Ian Jackson wrote: From the original ASCII diagram, it isn't too clear what the configuration is. As I understand it, the antenna under discussion is like the Sigma-80 except the top hat is omitted and the upper element is instead extended upward - like this. | coil coil | ---------------------/////FP/////-------------+ || | || | So I'm assuming whatever works for the Sigma-80 will work for this antenna. it does work and very well! -- I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!! |
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