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Are all RG8s created equal?
I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past
for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? -- Bob Donahue ND9B |
Are all RG8s created equal?
i don't know what 'RG' itself stands for, but the numbers describe different
'types' of cables... not necessarily construction or size. and no, all rg8's are definately not the same. what type of cable you want to use depends on the impedance, length needed, frequencies to be used, acceptable loss, and also flexibility, weather resistance, and power handling capability for transmitting... and then of course the big factor, cost. "Bob D." wrote in message m... I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? -- Bob Donahue ND9B |
Are all RG8s created equal?
On Sun, 3 Sep 2006 10:32:41 -0500, "Bob D."
wrote: I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? Cables advertised as RG8 should have similar characteristics, though the market contains sub standard products, and variants (eg foam dielectric). RG8 (or more commonly used RG213 which is similar in characteristics) is quite lossy at 70cm, and you might not find its loss acceptable, depending on the length of your run and your own requirements. For example, the loss on 30m of Belden 8267 (RG213) at 440MHz with a load end VSWR of 1.2 is 4.8dB, about two thirds of your transmitter power is lost in the cable. 9913 is a little better at 2.7dB, LDF4-50A at 1.4dB. Try your own scenario at http://www.vk1od.net/tl/tllc.php . If you think RG8 is stiff and expensive, you don't seem to have much experience of UHF. If you are more familiar with RG58C/U, it has a loss in the above scenario of 12dB, 6% efficiency, but it works for some. Re what is RG anyway? Google for info on the RG scheme, try searhing for "Radio Guide". Owen -- |
Are all RG8s created equal?
I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50
foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. Greg ki4bbl "Bob D." wrote in message m... I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? -- Bob Donahue ND9B |
Are all RG8s created equal?
On Sun, 3 Sep 2006 21:09:06 -0400, "G. Doughty"
wrote: I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. Greg ki4bbl You really don't know for sure unless you measure it. Not a big deal if you've got a dummy load and a in-line watt meter. RG-8 is a type not a specification. Danny, K6MHE |
Are all RG8s created equal?
"G. Doughty" wrote in message news:T0LKg.9999$xk3.7812@dukeread07... I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. Greg ki4bbl I don't know what kind of cable Radio Shack is selling for that price, but it must be very poor quality. Probably 5 times the loss of any quality rg-8 size cable at 2 meters and above. RG-8 size cable of any type should be atleast $ .50 per foot and more like $ .75 per foot or more. There should not be any tables for SWR for differant cables based on the band. Maybe you mean the loss in dB instead of SWR. |
Are all RG8s created equal?
G. Doughty wrote:
I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. What's an SWR rating? The cable with the most loss will have the best SWR when feeding a mismatched load. Roy Lewallen, W7EL |
Are all RG8s created equal?
I don't know what kind of cable Radio Shack is selling for that price, but it must be very poor quality. Probably 5 times the loss of any quality rg-8 size cable at 2 meters and above. RG-8 size cable of any type should be atleast $ .50 per foot and more like $ .75 per foot or more. There should not be any tables for SWR for differant cables based on the band. Maybe you mean the loss in dB instead of SWR. It was a close out special at $10 per 50 feet. I had to hit two stores before I found any, a Ham who was working at the first RS store had bought out all that his store had in stock. I bought a 100 feet at that price at the second store. I also have a 50 foot section of some of their RG-8 stuff I bought 20 years ago for SSB CB use (yes, I once was wooed by the dark side). Can't tell any real difference on HF frequencies when using the old RS RG-8 or some new RG-213 I got from Texas Towers. Harold KD5SAK |
Are all RG8s created equal?
Loss is right. Sorry about that. RS is trying to get rid of everything
radio. These were on clearance. Normally sold for much more. "Ralph Mowery" wrote in message ink.net... "G. Doughty" wrote in message news:T0LKg.9999$xk3.7812@dukeread07... I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. Greg ki4bbl I don't know what kind of cable Radio Shack is selling for that price, but it must be very poor quality. Probably 5 times the loss of any quality rg-8 size cable at 2 meters and above. RG-8 size cable of any type should be atleast $ .50 per foot and more like $ .75 per foot or more. There should not be any tables for SWR for differant cables based on the band. Maybe you mean the loss in dB instead of SWR. |
Are all RG8s created equal?
Ralph Mowery wrote:
SNIPPED I don't know what kind of cable Radio Shack is selling for that price, but it must be very poor quality. Probably 5 times the loss of any quality rg-8 size cable at 2 meters and above. RG-8 size cable of any type should be atleast $ .50 per foot and more like $ .75 per foot or more. There should not be any tables for SWR for differant cables based on the band. Maybe you mean the loss in dB instead of SWR. One of the basic components of coax cable is the design of the outer shield. The measure of that shield is optical coverage. Cable is available with optical coverage that ranges from 80% to 95%. The optical coverage contributes to the shielding effectiveness and losses through the cable shield, leakage inductance. A quick test of shield design is the remove about 1 inch of jacket and inspect the shield . A tightly woven shield with a lay angle of close to 45 degrees will have excellent optical coverage. In the past, RS cable had optical coverage in the 75 to 80 percent range. /s/ DD |
Are all RG8s created equal?
On Sun, 3 Sep 2006 10:32:41 -0500, "Bob D."
wrote: I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? The Wireman's Wirebook IV gives a pretty good history of coax: Going back to the 1940s, R = Radio Frequency; G = Government; 8 is a sequential number assigned for government approval, thus RG8/U with U = Universal Specification. That said, all of these terminologies are fairly obsolete to the modern manufacture of coax, according to the Wireman. bob k5qwg |
Are all RG8s created equal?
Yeh, BUT, some coax has a smaller amount of copper
in the shield, and is therefore lossier at VHF/UHF freqs. tho probably ok at HF. Also, look at the RG number on the outer shieth of the coax. IF it says " RG- 8 TYPE" instead of RG8, avoid it like the plague , if you want to use it at VHF-UHF , and "RG-8X" is another horse of a different color, too, at vhf. Also, remember, that if you are going to use for impedence transformation, that the velocity factor of Poly center insulator is .66 X the desired wavelength, while , Foam dielectric, is .8 X the desired wavelength. (No insulation, gives a velocity factor of 1.0). In short: you get what you pay for (no free lunch) Jim NN7K G. Doughty wrote: I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. I searched for rg8 and the site popped up. It was complete with charts, etc. Very nice. It only takes a few minutes to find if you don't mind trudging through the sites. Greg ki4bbl |
Are all RG8s created equal?
Roy Lewallen wrote: G. Doughty wrote: I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50 foot run. Anyways, there was a website I came across recently that showed the swr rating of different size rg cables based on band. What's an SWR rating? The cable with the most loss will have the best SWR when feeding a mismatched load. Roy Lewallen, W7EL Not to mention that, at VHF (and other freqs, if you are so unfortunate, as to have cut the coax length just right, you can show a GREAT SWR, and still be in for problems (this like pruneing a coax for the "Best Match")! A way to Double Check for this, is to use a 1/4 wave (electrical) length of coax , OF the SAME impedance, in series with your feed line to the antenna. IF the SWR stays the same, Should have a match, BUT IF the SWR goes up, better check for match problems! As info, Jim NN7K |
Are all RG8s created equal?
On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote:
a coax for the "Best Match")! A way to Double Check for this, is to use a 1/4 wave (electrical) length of coax , OF the SAME impedance, in series with your feed line to the antenna. IF the SWR stays the same, Should have a match, BUT IF the SWR goes up, better check for match problems! As info, Jim NN7K Jim, can you explain what is going on here, why what happens happens. It has more to do with what is happening on the outside of the coax than on the inside of the coax. Owen -- |
Are all RG8s created equal?
Quite easily. Impedence matches repeat, every 1/2
wavelength. This is the basis for those old "CB" shops , pruning the coax for the best match (rather then the antenna)! At ODD multiples of a 1/4 wavelength. the coax, becomes a MATCHING Transformer (like stacking 2- 50 ohm antennas, you use 1/4 wavelength sections of 75 ohm coax) In this case, we are matching 2 -50 ohm loads to a 25 ohm source)-- Coax has a characteristic impedence that is constant along its length. as long as the Source Impedence = the Load Impedence. IF the source is DIFFERENT from the load impedence, or the Coax has a different impedence, from the source and/or load, however the source/coax/and load will show 1:1 SWR, AT the 1/2 wave points of the line, repeating EVERY 1/2 wavelength! However, as stated, ODD 1/4 wavelengths become a impedence XFMR. (the formula being : X Zo (1/4 wave) = the SQUARE ROOT of R1 x R2! Was well documented in an a old VHF'er article from the '60's! Ill post it (in text format, when I find it-- Jim Owen Duffy wrote: On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote: a coax for the "Best Match")! A way to Double Check for this, is to use a 1/4 wave (electrical) length of coax , OF the SAME impedance, in series with your feed line to the antenna. IF the SWR stays the same, Should have a match, BUT IF the SWR goes up, better check for match problems! As info, Jim NN7K Jim, can you explain what is going on here, why what happens happens. It has more to do with what is happening on the outside of the coax than on the inside of the coax. Owen -- |
Are all RG8s created equal?
On Mon, 04 Sep 2006 23:31:00 GMT, Jim - NN7K wrote:
Quite easily. Impedence matches repeat, every 1/2 wavelength. This is the basis for those old "CB" shops , pruning the coax for the best match (rather then the antenna)! At ODD multiples of a 1/4 wavelength. the coax, becomes a MATCHING Transformer (like stacking 2- 50 ohm antennas, you use 1/4 wavelength sections of 75 ohm coax) In this case, we are matching 2 -50 ohm loads to a 25 ohm source)-- Coax has a characteristic impedence that is constant along its length. as long as the Source Impedence = the Load Impedence. IF the source is DIFFERENT from the load impedence, or the Coax has a different impedence, from the source and/or load, however the source/coax/and load will show 1:1 SWR, AT the 1/2 wave points of the line, repeating If the SWR on a line is 1:1 at any point, it is 1:1 at ALL points on the line. If the SWR on a low loss line is close to 1:1 at any point, it is approximately the same at ALL points on the line. On practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. If lengthening the line increases the SWR at the tx, or decreases it by more than accounted for by the extra line loss, then something else is happening. Owen EVERY 1/2 wavelength! However, as stated, ODD 1/4 wavelengths become a impedence XFMR. (the formula being : X Zo (1/4 wave) = the SQUARE ROOT of R1 x R2! Was well documented in an a old VHF'er article from the '60's! Ill post it (in text format, when I find it-- Jim Owen Duffy wrote: On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote: a coax for the "Best Match")! A way to Double Check for this, is to use a 1/4 wave (electrical) length of coax , OF the SAME impedance, in series with your feed line to the antenna. IF the SWR stays the same, Should have a match, BUT IF the SWR goes up, better check for match problems! As info, Jim NN7K Jim, can you explain what is going on here, why what happens happens. It has more to do with what is happening on the outside of the coax than on the inside of the coax. Owen -- -- |
Are all RG8s created equal?
The answer to the subject question is: NO!
Bob D. wrote: I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8 or a variant of RG8. Are they all the same? I want to minimize loss of course. Recommendations? BTW, what does the RG number signify anyway? The size? The construction? |
Are all RG8s created equal? (as promised)
Owen
Keep in mind that I'am (primarily) VHF/UHF op. You might find this interesting (- the drawings)! This from the VHF'er, Nov, 1964-- Jim ACID TEST FOR A FLAT TRANSMISSION LINE Loren Parks K7AAD (BTW, Loren runs Parks Electronics, in Portland, Or. and has given up ham radio long ago!) The mark of a beginner is blind faith in meters. Most hams get over believing an "S" meter after a few years, but very few get to the point of doubting that SWR or power meter. You ought to try hooking half a dozen SWR bridges and power meters in series with your transmission line--all at the same time. If the line is not pro*perly terminated you'll probably get as many different readings as you have meters. An SWR bridge or wattmeter can show zero reflected "power" or nearly so on a badly mis-matched transmission line. This is why the beginners "prune" the transmission line to " get the SWR down." Pruning can help you load into a badly mismatched line but the seat of the trouble is in the termination--the antenna. The proof of a properly terminated transmission line is that changing its length has no effect on transmitter loading or on an indicator in the line ahead of the length that is changed. If your bridge now shows little or no reflected power or voltage, it may be that it is inserted at a null. Therefore you have to move the null, if any, by chang*ing the distance from the meter to the load (not the transmitter.) Inserting 1/4 wave*length (electrically) of coax will give a maximum change in meter reading. Insert lengths as shown in the table below. Make some allowance for the additional length of fittings or adapters at 432 Mc. Dimensions are not very critical. Added length of Original length of transmission line transmission line Transmitter SWR Bridge or Power Meter Changing the length of the line to the right of the bridge or power meter should have no effect on the meter reading, regardless of the quality of the bridge or power meter. Plate current in the transmitter should not be affected either. If changing line length affects any meter reading, the transmission line is not properly terminated which means the load is not matched to the line. The degree of mismatch will be indicated in a rel*ative manner by the amount of change in meter readings. M Antenna 236" x .66 = 39" 4 144 Mc. 82" x .66 = 13.5" 4 220 Mc. 54" x .66 = 8.8" 4 432 Mc. 27.4" x .66 = 4.5" 4 50 Mc. It is possible that you will get a reflected energy reading that is approximately the same when you insert the quarter wave length line. This is because you are shifting phase 90 degrees and perhaps just moving to the other side of the "hump" in the line. Try different lengths of line, shorter or longer. Put most faith in whatever readings make your line look the worst. You cannot believe SWR values obtained from trans*mission line type instruments (monimatch) in which the line is an appreciable frac*tion of a wavelength. At 6 & 2 this is not a problem, but at 432 it is. knurled lock-nut -p A commercial "line stretcher." This one telescopes 20 cm. while main*taining a constant characteristic impedance of 50 ohms. It is most useful at 432 Mc. and above where 1/4 wavelength is about 17 cm. in air. SWR bridges and power meters are very valuable instruments, properly used. End. Also, found a article by the same, useing a 400 foot run of RG-58 U for a dummy load, terminated with couple 2 Watt 100 ohm resistors (truth be known, at 6 meters, and up you dont even need the resistors! The effect, tho is the same, on ANY freq, as long as the 1/4 (or the 1/2) wavelengths apply! Jim NN7K Owen Duffy wrote: If the SWR on a line is 1:1 at any point, it is 1:1 at ALL points on the line. If the SWR on a low loss line is close to 1:1 at any point, it is approximately the same at ALL points on the line. On practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. If lengthening the line increases the SWR at the tx, or decreases it by more than accounted for by the extra line loss, then something else is happening. Owen an a old VHF'er article from the '60's! Ill post it (in text format, when I find it-- Jim |
Are all RG8s created equal? (as promised)
On Tue, 05 Sep 2006 02:28:41 GMT, Jim - NN7K wrote:
Owen Thanks Jim. I will insert comments inline... Keep in mind that I'am (primarily) VHF/UHF op. You might find this interesting (- the drawings)! This from the VHF'er, Nov, 1964-- Jim ACID TEST FOR A FLAT TRANSMISSION LINE Loren Parks K7AAD (BTW, Loren runs Parks Electronics, in Portland, Or. and has given up ham radio long ago!) The mark of a beginner is blind faith in meters. Most hams get over believing an "S" meter after a few years, but very few get to the point of doubting that SWR or power meter. You ought to try hooking half a dozen SWR bridges and power meters in series with your transmission line--all at the same time. If the line is not pro*perly terminated you'll probably get as many different readings as you have meters. The author is not stating what has happened, what he has observed, but an opinion on what he thinks may happen. Assuming that he is talking about the common reflectometer / directional wattmeter instruments that sample current and voltage in a very small region to determine SWR, that they are suited to the frequency and line, and they are of quality... The fact remains that on practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. Any difference between instruments for direct or indirect measurement of VSWR placed in a uniform practical line is accounted for by line loss and instrument error (eg its indication or disruption of the line). The line loss between adjacent meters in practical coaxial lines (which is where you are likely to be using an SWR meter) on practical loads is very low, and so quality instruments should read almost identically. To take an extreme example, using RG213 at 1296MHz, and two SWR meters spaced 0.3m (~1') apart, if the SWR on the line at the one nearest the source is 3:1, the SWR on the line 0.3m closer to the load is 3.1... you would probably not see the difference. If on actual test, two meters showed significant difference, there is instrument error in at least one of the instruments. .... lots of confused stuff clipped SWR bridges and power meters are very valuable instruments, properly used. End. Prophetic! Also, found a article by the same, useing a 400 foot run of RG-58 U for a dummy load, terminated with couple 2 Watt 100 ohm resistors (truth be known, at 6 meters, and up you dont even need the Yes, the input SWR of a 400' o/c stub of RG58 is about 1.1 at 50MHz, and gets lower with increasing frequency. resistors! The effect, tho is the same, on ANY freq, as long as the 1/4 (or the 1/2) wavelengths apply! Jim NN7K I don't really understand how the 1/4, 1/2 relate to the dummy load? Owen -- |
Are all RG8s created equal? (as promised)
On Tue, 05 Sep 2006 03:58:56 GMT, Owen Duffy wrote:
If on actual test, two meters showed significant difference, there is instrument error in at least one of the instruments. Hi Owen, The same thing will happen if you put a SWR (or Power) meter between the two mismatched ends of a transmission line. In this case it is not instrument error, it is systematic (application of the instrument) error. 73's Richard Clark, KB7QHC |
Are all RG8s created equal? (as promised)
Owen- see my comments below
The mark of a beginner is blind faith in meters. Most hams get over believing an "S" meter after a few years, but very few get to the point of doubting that SWR or power meter. You ought to try hooking half a dozen SWR bridges and power meters in series with your transmission line--all at the same time. If the line is not pro*perly terminated you'll probably get as many different readings as you have meters. The author is not stating what has happened, what he has observed, but an opinion on what he thinks may happen. What happens is IN a HIGH SWR Mis-match, is that the SWR (true) can only be measured at the load end. The reflected wave (Say infinity:1 is NOT a constant, over the entire length of your coax, but will take on the sinusoidial of the wavelength for voltage (or current). Obviously, the reflected wave repeats the voltage, every 1/2 wavelength (- coax loss's), going from a 1:1 match, to a infinity to 1 mis-match, every 1/4 wave length and, then back to a 1:1 match,the next 1/4 wave. If you were to graph this out for 1 cycle, you would see a start at 0 volts, raiseing to maximum volts, then dropping to 0 volts, and then to Maximum Negative volts.This repeats all the way from the source to the load! A perfectly matched coax, on the other hand will have the same voltage measured , no matter the length of the line (again ignoreing the line loss's). (straight line graph, not a sin wave). Assuming that he is talking about the common reflectometer / directional wattmeter instruments that sample current and voltage in a very small region to determine SWR, that they are suited to the frequency and line, and they are of quality... The fact remains that on practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. This is academic, as the True VSWR still exists at the load. If it is 3:1, at the load, but you measure 1.5/1 at your rig, the difference is because of line loss, NOT that the 3:1 swr has been corrected! Worse, (usually on short coax runs with a bum load, is the effect, that you have MORE POWER reflected, than was applied to the coax (an impossible situation)! Any difference between instruments for direct or indirect measurement of VSWR placed in a uniform practical line is accounted for by line loss and instrument error (eg its indication or disruption of the line). The line loss between adjacent meters in practical coaxial lines (which is where you are likely to be using an SWR meter) on practical loads is very low, and so quality instruments should read almost identically. To take an extreme example, using RG213 at 1296MHz, and two SWR meters spaced 0.3m (~1') apart, if the SWR on the line at the one nearest the source is 3:1, the SWR on the line 0.3m closer to the load is 3.1... you would probably not see the difference. If on actual test, two meters showed significant difference, there is instrument error in at least one of the instruments. Keep in mind, that WHEN a swr exists, the sin wave effect also exists irregardless of the freq you are on. and also, IF you measure at a null point where the forward power wave voltage cancells the reflected voltage (for 0 volts), the introduction of a 1/4 wave length (electrical), will force your meter to the voltage MAXIMUM ! 'corse, the best place to measure a mismatch is at the load,but try it sometime! Try about 13-1/2 inch long piece of coax, on 2 meters added to your mismatched load, and then remeasure the swr- you will see what Im talking about! SWR bridges and power meters are very valuable instruments, properly used. End. Prophetic! Also, found a article by the same, useing a 400 foot run of RG-58 U for a dummy load, terminated with couple 2 Watt 100 ohm resistors (truth be known, at 6 meters, and up you dont even need the Yes, the input SWR of a 400' o/c stub of RG58 is about 1.1 at 50MHz, and gets lower with increasing frequency. Only to say this why your swr APPEARS lower (coax loss) but that doesnt effect the REAL swr of the load! With the 400 foot of coax, the losses are so high, you wont even see any reflected at the source end- but at the load end, you will have infinity :1 SWR! Jim NN7K |
Are all RG8s created equal? More
Jim - NN7K wrote: Owen- Also check out : http://beradio.com/departments/radio...aves_antennas/ http://www.physics.montana.edu/demon...cherwires.html http://www.iop.org/EJ/abstract/0031-9120/8/1/002 Tho, both refer to the equivilant of ladder line, and one is on Lecher Lines, the effect on swr is the same. The lecher line was the way frequency was measured before tuned circuit/freq counters but is based on the phenomina that is described in measureing swr at various places in a feedline. It is an observable thing! Jim NN7K |
Are all RG8s created equal? (as promised)
On Tue, 05 Sep 2006 19:12:38 GMT, Jim - NN7K wrote:
Owen- see my comments below The mark of a beginner is blind faith in meters. Most hams get over believing an "S" meter after a few years, but very few get to the point of doubting that SWR or power meter. You ought to try hooking half a dozen SWR bridges and power meters in series with your transmission line--all at the same time. If the line is not pro*perly terminated you'll probably get as many different readings as you have meters. The author is not stating what has happened, what he has observed, but an opinion on what he thinks may happen. What happens is IN a HIGH SWR Mis-match, is that the SWR (true) can only be measured at the load The definition of VSWR is the ratio of a voltage maximum to the adjacent voltage minimum on the transmission line. That definition might imply that VSWR cannot be directly measured at a point, but it is possible to use a directional coupler to measure the forward and reflected wave components in an very small region (wrt wavelength) and to calculate the VSWR at that point (for all intents and purposes). Using a directional coupler / sampler, VSWR can be measured by this indirect method anywhere along a transmission line by insertion of the coupler / sampler. It is reasonable to talk of VSWR at a point. end. The reflected wave (Say infinity:1 is NOT a constant, over the entire length of your coax, but will take on the sinusoidial of the wavelength for voltage (or current). Obviously, the reflected No, you appear to have a misconception. It is true that the reflected wave component is not constant with displacement for a lossy line. The amplitude of the reflected wave component decreases exponentially from the load end towards the source end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the reflected wave component also changes with displacement (distance along the line). The amplitude of the forward wave component decreases exponentially from the source end towards the load end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the forward wave component also changes with displacement. The resultant of these two travelling waves is the standing wave pattern that can be observed with a voltage probe. You can find the derivation of SWR from those travelling waves in any reputable text book, I won't repeat it here. On practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. If you know the matched line loss and VSWR at one point, you can calculate the VSWR at any other point. .... clipped Owen -- |
Are all RG8s created equal? (as promised) try again!
So: therefore, Lecher Lines can't work?? They
DON'T show the TRUE wavelength of a signal?? So, then, why were these devices used? to get light to see to read the manual?? Obviously V/F vs. V/R is EQUAL everywhere on a given line! which makes the lamp glow at the SAME intensity everywhere on the line?? Don't think so! Jim Owen Duffy wrote: On Tue, 05 Sep 2006 19:12:38 GMT, Jim - NN7K wrote: Owen- see my comments below The definition of VSWR is the ratio of a voltage maximum to the adjacent voltage minimum on the transmission line. NO! the defination is V(MAX) forward/ V(MAX) reverse (at a given point)! That why they use 2 directional couplers (or one that rotates to change directions , like BIRD)! Like 2 signals, going in opposite directions, at the same freq, at the same time! That definition might imply that VSWR cannot be directly measured at a point, but it is possible to use a directional coupler to measure the forward and reflected wave components in an very small region (wrt wavelength) and to calculate the VSWR at that point (for all intents and purposes). This is HOW the Isotron "Antenna" is made to work- literally loading your coax to radiate! No, you appear to have a misconception. It is true that the reflected wave component is not constant with displacement for a lossy line. Point being : it ain't even, in a LOSSLESS line! The amplitude of the reflected wave component decreases exponentially from the load end towards the source end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the reflected wave component also changes with displacement (distance along the line). AS does the forward wave, combined with it! The amplitude of the forward wave component decreases exponentially from the source end towards the load end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the forward wave component also changes with displacement. The resultant of these two travelling waves is the standing wave pattern that can be observed with a voltage probe. Which is why a lecher wire works But, you will find points that will measure a great swr, and yet it will be higher than a kite at the load end! Jim. Jim |
Are all RG8s created equal? (as promised) try again!
On Tue, 05 Sep 2006 23:48:41 GMT, Jim - NN7K wrote:
The definition of VSWR is the ratio of a voltage maximum to the adjacent voltage minimum on the transmission line. NO! the defination is V(MAX) forward/ V(MAX) reverse (at a given point)! Hi Jim, Are you so quick to impeach your Lecher line testimony from only one paragraph earlier? Owen's definition is perfectly correct, classic in fact. It may not be the only one, but historically (as in Lecher lines) it certainly preceeds yours by decades. However, it does suffer from accuracy (but that hardly vindicates the common Bruene circuit). There are at least half a dozen ways to measure SWR, each one being more appropriate for certain regions of the magnitude. Some SWRs that are very low or very high would be impossible to measure using either method described above. Only academics are concerned with those methods. I've put my hand to most of them. I wasn't an academic, but NBS standards did demand their use. Some techniques are infinitely better than either described above (this presumes Owen's description consisting of a raw detector which introduces up to 23% error from square law problems). 73's Richard Clark, KB7QHC |
Are all RG8s created equal? (as promised) try again!
On Tue, 05 Sep 2006 23:48:41 GMT, Jim - NN7K wrote:
.... The definition of VSWR is the ratio of a voltage maximum to the adjacent voltage minimum on the transmission line. NO! the defination is V(MAX) forward/ V(MAX) reverse (at a given point)! That why they use 2 directional couplers (or one that rotates to change directions , like BIRD)! Like 2 signals, going in opposite directions, at the same freq, at the same time! No, you are quite confused / wrong. VSWR is *NOT* V(MAX) forward/ V(MAX) reverse (at a given point). VSWR=|Vmax|/|Vmin| VSWR=(|Vf|+|Vr|)/(|Vf|-|Vr|) You seem to be mixing up the forward and reflected wave components, and the resultant SWR pattern. You need to go back to the text books and work it through. .... Owen -- |
Are all RG8s created equal? (as promised)
Owen Duffy wrote:
Using a directional coupler / sampler, VSWR can be measured by this indirect method anywhere along a transmission line by insertion of the coupler / sampler. In fact, if one measures the forward power, Pfor, and the reflected power, Pref, at a point on a transmission line, SWR = [SQRT(Pfor) + SQRT(Pref)] / [SQRT(Pfor) - SQRT(Pref)] -- 73, Cecil http://www.w5dxp.com |
Are all RG8s created equal? (as promised) try again!
Jim - NN7K wrote:
Owen Duffy wrote: The definition of VSWR is the ratio of a voltage maximum to the adjacent voltage minimum on the transmission line. NO! the defination is V(MAX) forward/ V(MAX) reverse (at a given point)! If V(MAX) forward = 100V and V(MAX) reverse = 0V, then the SWR is infinite? If V(MAX) forward = 100V and V(MAX) reverse = 100V, then the SWR is 1:1? Doesn't that sound exactly backwards to you? -- 73, Cecil http://www.w5dxp.com |
Are all RG8s created equal? (as promised)
Quite true- the point I'm tryint to make is, that
when makeing swr measurements, that the APPARENT measured swr, need have little/no relation to what the true swr , depending on Where the swr is measured in a line. at least with cheap meters. and it really gets disconserting, when you calibrate a SWR meter, (forward power , to the set line, and then measure a reflected swr that is Beyond the set line! Admittedly, much of that is from too big a sampleing line, but it is beyond the ability to have MORE power returned to the source, than the source provided (perpetual motion machine)! And, that you can prune a feed line and lower your swr (measured), but that doesn't change the load's mis-match, does it? And that lines (1/4 wave, shorted, or 1/2 wave open) form a very high impedence. Translation: whatever is on that end of that line will repeat (ZL) every 1/2 wavelength, will it not ? And , the opposite would also be true (1/4 wave back, and in odd multiples thereof, would this not show a dead short (electrically), in a connected line? Where am I wrong ? just curious -Jim Cecil Moore wrote: Owen Duffy wrote: Using a directional coupler / sampler, VSWR can be measured by this indirect method anywhere along a transmission line by insertion of the coupler / sampler. In fact, if one measures the forward power, Pfor, and the reflected power, Pref, at a point on a transmission line, SWR = [SQRT(Pfor) + SQRT(Pref)] / [SQRT(Pfor) - SQRT(Pref)] |
Are all RG8s created equal? (as promised)
Jim - NN7K wrote:
... but it is beyond the ability to have MORE power returned to the source, than the source provided Did you know a reflection coefficient can be greater than 1.0? that you can prune a feed line and lower your swr (measured), but that doesn't change the load's mis-match, does it? If the Z0 of the feedline is different from the Z0 of the SWR meter, then the meter is not measuring the SWR on the feedline. And that lines (1/4 wave, shorted, or 1/2 wave open) form a very high impedence. The (virtual) impedance is (Vfor+Vref)/(Ifor-Iref) The closer that Iref gets to Ifor, the lower the stub losses and the higher the (virtual) impedance. -- 73, Cecil http://www.w5dxp.com |
Are all RG8s created equal? (as promised)
On Tue, 05 Sep 2006 22:52:47 GMT, Owen Duffy wrote:
.... It is true that the reflected wave component is not constant with displacement for a lossy line. The amplitude of the reflected wave component decreases exponentially from the load end towards the source end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the reflected wave component also changes with displacement (distance along the line). The amplitude of the forward wave component decreases exponentially from the source end towards the load end, and the rate at which it decreases is given by the line's "matched line loss". The phase of the forward wave component also changes with displacement. .... On practical lines at HF and above with practical loads, the SWR decreases smoothly from the load to the source because of line loss, the lower the loss, the lower the rate at which SWR falls as you approach the source. .... I have plotted theoretical forward and reflected power for a detailed model of RG58C/U at 100MHz with a severely mismatched load to illustrate the underlying thing that is being measured by a reflectometer. The plots are at http://www.vk1od.net/lost/NN7K.htm . Comments? Owen -- |
Are all RG8s created equal? (as promised)
On Wed, 06 Sep 2006 07:36:13 GMT, Owen Duffy wrote:
I have plotted theoretical forward and reflected power for a detailed model of RG58C/U at 100MHz with a severely mismatched load to illustrate the underlying thing that is being measured by a reflectometer. The plots are at http://www.vk1od.net/lost/NN7K.htm . Comments? Hi Owen, Interpretative comment - You state here "theoretical" plots of a model. At the reference you state "indicated forward power, reflected power." To me this implies data taken, not generated from a model's formula - but my interpretation may be a stretch. Data source comment - However, you go to rather elaborate lengths to specify a load of 2+0.5j (sic) and a line of 50.36571-j0.17872. What was the point of injecting reactances here if you simply wish to illustrate the degradation of SWR with feedline loss? They add to the implication of data taken, not generated - except for 5 place resolution at 100 MHz which is (excuse me) absurd. Elaboration of the implications of interpretation and data source comment - Further implying this was data taken at the bench is the declaration "THE REFLECTOMETER IS NOT CALIBRATED FOR THE LINE Zo." Again, my presumption circuits peg on that one because the reactances are so easily removed from emphatic concern if you are in control of the "virtual" workspace. Informational comment - What is supposed to be the total length of line? Was it matched at its input, or simply fed by a transmitter? Picky comment - Graph font sizes are a squint too small. Last, really picky comment - Your copyright notice at the bottom is off by a year. 73's Richard Clark, KB7QHC |
Are all RG8s created equal? (as promised)
On Wed, 06 Sep 2006 04:45:57 GMT, Cecil Moore
wrote: Jim - NN7K wrote: ... but it is beyond the ability to have MORE power returned to the source, than the source provided Did you know a reflection coefficient can be greater than 1.0? It is true that reflection coefficient can be greater than 1.0. The reflection coefficient *CANNOT* be greater than 1.0 where Zo is purely resistive. So, where a sampler is calibrated (nulled) on a purely resistive load (eg 50+j0) as is most commonly done, it should never show a reflection coefficient greater than 1. A reflectometer calibrated to a resistive load and that shows a "reflected" reading greater than the "set" reading is inaccurate / defective / a poor design. Owen -- |
Are all RG8s created equal? (as promised)
Owen Duffy wrote:
The reflection coefficient *CANNOT* be greater than 1.0 where Zo is purely resistive. Yes, a reflection coefficient greater than one is caused by the interaction of the Z0 reactance and the load reactance in an effect resembling resonance. -- 73, Cecil http://www.w5dxp.com |
Are all RG8s created equal? (as promised)
On Wed, 06 Sep 2006 10:36:52 -0700, Richard Clark
wrote: On Wed, 06 Sep 2006 07:36:13 GMT, Owen Duffy wrote: I have plotted theoretical forward and reflected power for a detailed model of RG58C/U at 100MHz with a severely mismatched load to illustrate the underlying thing that is being measured by a reflectometer. The plots are at http://www.vk1od.net/lost/NN7K.htm . Comments? Hi Owen, Hi Richard, Interpretative comment - You state here "theoretical" plots of a model. At the reference you state "indicated forward power, reflected power." To me this implies data taken, not generated from a model's formula - but my interpretation may be a stretch. For avoidance of any doubt, it is entirely theoretical. Line characteristics are based on an RLGC model of the line derived from Belden's published specs for 8262 (RG58C/U). Firstly, I should not have said "indicated forward power, reflected power" as the "indicated" value is a voltage or current, ie the square root of power, it is the deflection on the indicator meter if you like, the meter current on the reflectometer. I have changed the text. The meaning "indicated" is that it is the voltage or current driving a reflectometer meter, calculated from the sampler and line conditions. Data source comment - However, you go to rather elaborate lengths to specify a load of 2+0.5j (sic) and a line of 50.36571-j0.17872. What was the point of injecting reactances here if you simply wish to illustrate the degradation of SWR with feedline loss? They add to the implication of data taken, not generated - except for 5 place resolution at 100 MHz which is (excuse me) absurd. The stated line Zo precision is a result of lazy cutting and pasting, and it should not be seen to imply that level of accuracy. I have changed it. The load (2+0.5j) is relatively unimportant, but it was a load to create a high VSWR, higher than would be practical for such a feedline at that frequency, extreme to satisfy the people who assert strange things can only be observed at extreme VSWR. The reactance is just a non zero value for no other reason than to test the models behaviour on a complex load. What is significant, is the load VSWR is very high at ~25, I would suggest higher than practical, and therefore extreme. Elaboration of the implications of interpretation and data source comment - Further implying this was data taken at the bench is the declaration "THE REFLECTOMETER IS NOT CALIBRATED FOR THE LINE Zo." Again, my presumption circuits peg on that one because the reactances are so easily removed from emphatic concern if you are in control of the "virtual" workspace. A reality of lossy lines is that Zo is not purely resistive. A source of instrument error is a sampler that is calibrated for a nominal Zo (being Ro). Fig 1 shows the modelled VSWR (which is what accounts for increased loss, impedance transformation etc), and Fig 2 shows the calculated indication for a reflectometer calibrated or nulled at 50+j0. There is a small difference, and the difference becomes minute at lower VSWR. Informational comment - What is supposed to be the total length of line? Was it matched at its input, or simply fed by a transmitter? The x scale is in wavelengths (as noted on the page), Frequency is 100MHz, line length covered by the graph is therefore about 9m. Picky comment - Graph font sizes are a squint too small. Ok, it is a quick cut and paste from Mathcad. For a permanent article, I would try to do better, for I too have aging eyesight! Last, really picky comment - Your copyright notice at the bottom is off by a year. Thanks, I have fixed the footer (which is common for most of the site). The graphs are a quick and dirty presentation, but it is a very detailed transmission line model underneath. I acknowledge there are all sorts of error sources in real life, but keeping sight of the underlying phenomena helps to flag whether measurements look invalid. Back to the original proposition that asserted that changing feedline length can vary measured VSWR dramatically, and that shortening a feedline might improve VSWR, there are reasons why that might happen, but it is not explained solely by what is happening on the inside of the feedline. A possible explanation for example is that the feedline is not decoupled and carries common mode current, and that changing the feedline length changes the load impedance seen by the load end of the transmission line. So, enough from me, can someone produce details of a scenario with a very high load VSWR where there are multiple sweet spots with 1:1 VSWR on a practical line? ("VSWR" here means a property of the line at that point, not what might be read by an unsuitable or defective instrument, and an extremely long line is not "practical".) Thanks... Owen -- |
Are all RG8s created equal? appology
Thanks, Owen, and all and sorry to cause so much
grief. I fall on my sword! Have believed 99.44% of what was in the old VHF'er Magazine, as was started by a consortium of many engineers , originally by Doug Demaw, W8HHS, as editor, and finally Loren Parks,K7AAD, an ex engineer from Tektronix, and covered the Gamut of VHF, UHF, and Microwave-- to early satelites and Moonbounce. Was instrumental to me getting on air, many moons ago. Tho much I've learned (and much I've forgotten!) over the years. I have built tuned coaxial finals, ect., and from former job, haveing to use tuned lines (mainly,to couple cavities together , and then tune them to use on same antenna. IF those lines changed, the things detunewhen you remove your test equipment! Also, I get a little bull headded after all theses years! Stuck on Stupid, if you might! Owen- your charts are quite illuminating! and, finely, The meter referred to , if memory serves correct, was made by SWAN, just before they went out of business (made for the "CB" trade, more than amateur radio. and the lines used in it were on a Printed Circuit board! and this probably before they knew how to get the correct impedence , on a p.c. board! I shall return to my lair-- Jim NN7K Owen Duffy wrote: On Wed, 06 Sep 2006 04:45:57 GMT, Cecil Moore wrote: Jim - NN7K wrote: ... but it is beyond the ability to have MORE power returned to the source, than the source provided Did you know a reflection coefficient can be greater than 1.0? It is true that reflection coefficient can be greater than 1.0. The reflection coefficient *CANNOT* be greater than 1.0 where Zo is purely resistive. So, where a sampler is calibrated (nulled) on a purely resistive load (eg 50+j0) as is most commonly done, it should never show a reflection coefficient greater than 1. A reflectometer calibrated to a resistive load and that shows a "reflected" reading greater than the "set" reading is inaccurate / defective / a poor design. Owen -- |
Are all RG8s created equal? appology
On Wed, 06 Sep 2006 23:24:45 GMT, Jim - NN7K wrote:
Thanks, Owen, and all and sorry to cause so much grief. I fall on my sword! Jim, I have been following this thread for some time and wish to complement you on your response. Normally, on the NG egos seem bigger than brains and seldom does anyone admit making an error. You are to be complemented for you forthrightness. It speaks highly of your character - a rare quality in this day and age. Very 73, Danny, K6MHE |
Are all RG8s created equal?
On Wed, 06 Sep 2006 23:24:45 GMT, Jim - NN7K wrote:
Thanks, Owen, and all and sorry to cause so much grief. I fall on my sword! Have believed 99.44% No need for that Jim, the discussion has challenged your thinking (and mine), and that is part of the learning process. of what was in the old VHF'er Magazine, as was IMHO, it is not a very good article. Forward Voltage, Forward Current, Reflected Voltage, Reflected Current, Current, Voltage, Zo, VSWR, impedance and propagation constant are all relevant, related but different and the article doesn't adequately draw the distinction. Indicators might sample one or more of voltage, or current, or directional power. They are all different, a voltage sensor or a current sensor alone at a single point will not allow you to determine VSWR, a pair of directional power sensors will. Your Lecher lines example needs a voltage or current sensor, not a directional power sensor (which won't work), whereas a VSWR meter needs a pair of directional power sensors, not a voltage or current sensor (which won't work). My view is that the article is imprecise, confused and contains bad advice. It does play to one of the archetypal myths of ham radio! I am considering whether I should create some more graphs of current, voltage, and impedance etc that illustrate how those behave, and incorporate it in a more complete permanent article on my web site. (IIRC there were some graphs of voltage, current, phase in Fred Terman's book, but I am thinking of going a little beyond that.) Thanks for the response Jim. 73 Owen -- |
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