100 Ohm Twin Lead
I have an antenna that needs 100 ohm twin lead to match the impedance
of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS |
100 Ohm Twin Lead
"Sonny Hood" wrote in message ... I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Hi Sonny Would you consider using two lengths of 50 ohm coax, side by side? Jerry |
100 Ohm Twin Lead
Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Sonny, the feedline cannot and does not have any influence on the resonance of an antenna. Resonance is determined by the physical dimensions of the antenna itself. Tell us about the antenna. How did you determine a 100 ohm input impedance? 72 ohm twin lead is available at selected suppliers. But, I am not aware of anyone making 100 ohm twin lead. that's why I asked about the 100 ohm value. |
100 Ohm Twin Lead
Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Use side-by-side runs of 50 ohm coax to achieve a balanced Z0 of 100 ohms. Tie the braids together at both ends. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
Dave wrote:
Sonny, the feedline cannot and does not have any influence on the resonance of an antenna. Resonance is determined by the physical dimensions of the antenna itself. Feedline length has every influence on the resonance of an antenna system if the SWR is not 1:1. Why is a G5RV resonant on 3.7 MHz when a 102 foot dipole is resonant on 4.6 MHz? Truth is that the feedline can be used to resonate the antenna system. I resonate my 130 ft. dipole on any HF band by varying the length of the feedline. Check it out at: http://www.qsl.net/w5dxp/notuner.htm -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? To answer your specific question, ZIP cord is close to 100 ohms Zo. However, it is relatively lossy (~3.5dB/100' at 10MHz). K8ZOA measured ZIP cord characteristics and reported them here a year of three back, Google for the info. There were commercial low impedance twin lines for RF use, but they are now obsolete (for good reason). Owen -- |
100 Ohm Twin Lead
Cecil Moore wrote:
Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Use side-by-side runs of 50 ohm coax to achieve a balanced Z0 of 100 ohms. Tie the braids together at both ends. -- 73, Cecil http://www.qsl.net/w5dxp Sounds like he may be trying to match a Quad or a Loop. If I am not mistaken, the usual way to do this is with a 75 ohm Q section. Side-by-side runs of 50 ohm coax with the braid tied together will yield 25 ohms because they are in parallel. Not 100 ohms. Or do I need to dive back into the antenna book? Gary N4AST |
100 Ohm Twin Lead
better get out the snorkle... if you tie the braid and the center conductor
together you get a 25 ohm line, i use that all the time for my stack matching boxes. wrote in message oups.com... Cecil Moore wrote: Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Use side-by-side runs of 50 ohm coax to achieve a balanced Z0 of 100 ohms. Tie the braids together at both ends. -- 73, Cecil http://www.qsl.net/w5dxp Sounds like he may be trying to match a Quad or a Loop. If I am not mistaken, the usual way to do this is with a 75 ohm Q section. Side-by-side runs of 50 ohm coax with the braid tied together will yield 25 ohms because they are in parallel. Not 100 ohms. Or do I need to dive back into the antenna book? Gary N4AST |
100 Ohm Twin Lead
Cecil Moore wrote:
Dave wrote: Sonny, the feedline cannot and does not have any influence on the resonance of an antenna. Resonance is determined by the physical dimensions of the antenna itself. Feedline length has every influence on the resonance of an antenna system if the SWR is not 1:1. C'mon Cecil, you know better! The antenna is resonant 100% by itself. 100 feet of wire is resonant within a few KHz of 4.68 MHz. Period. 120 feet of wire is resonant at ~ 3.9 MHz. Period. 65 feet of wire is resonant at ~ 7.2 MHz. Period. 33 feet of wire is resonant at ~ 14.2 MHz. Period. You are tuning a non resonant antenna to resonance with your stubs. You changed the scope of the discussion with the exact wording of your response. You used "antenna system". I used "antenna". Why do you persist in comparing alligators and kittens? |
100 Ohm Twin Lead
Dave wrote:
The antenna is resonant 100% by itself. Most knowledgeable people know that the resonant frequency of the antenna system can be changed by the length of the transmission line series matching section. Why don't you know that? Here is the SWR of my 130 foot dipole antenna on 40m achieved by varying the length of the 450 ohm transmission line. http://www.qsl.net/w5dxp/swr13040.gif Please look at that graph and then tell us that the feedline cannot tune the antenna system to resonance. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
Dave wrote:
better get out the snorkle... if you tie the braid and the center conductor together you get a 25 ohm line, i use that all the time for my stack matching boxes. Nobody said anything about tying the braid and center conductor together. If the side by side run of 50 ohm coax is used as a parallel feedline, the braids are tied together and the Z0 is 100 ohms when the center conductors are connected across the load. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
"Sonny Hood" wrote in message
... I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS As I remember, common lamp "zip" cord (AWG 18 or 16) functions at about that Z (I'll have to check) gb |
100 Ohm Twin Lead
Cecil Moore wrote in news:x0N9g.86749$dW3.29542
@newssvr21.news.prodigy.com: Dave wrote: Sonny, the feedline cannot and does not have any influence on the resonance of an antenna. Resonance is determined by the physical dimensions of the antenna itself. Feedline length has every influence on the resonance of an antenna system if the SWR is not 1:1. Why is a G5RV resonant on 3.7 MHz when a 102 foot dipole is resonant on 4.6 MHz? Truth is that the feedline can be used to resonate the antenna system. I resonate my 130 ft. dipole on any HF band by varying the length of the feedline. Check it out at: http://www.qsl.net/w5dxp/notuner.htm All true, but there isn't much difference between a G5RV and a 102 foot dipole fed with 450 ohm line to a good balanced tuner. Back years ago I had a very effective vee dipole antenna for 80m that was composed of two identical wires from the top of my third story shack window insulated and tied off to trees and shrubbery. I fed it with home-made 600 ohm line, about six feet long and tapped it directly off the tank coil of my amplifier. It was LOUD all over the Pacific, South America, audible in Africa, but for some reason, nothing from Europe! -- Dave Oldridge+ ICQ 1800667 |
100 Ohm Twin Lead
On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Is this a single band or multiband antenna? If I recall correctly, you can use a different impedance feedline if it is a multiple of a half wave. At the half wave points, the impedance is the same as the beginning of the feed. Cecil or others here will correct the length error if I am incorrect. But I learned about a year ago that the impedance can be corrected by the multi-fraction length feedline of a different impedance. Maybe this is the principal of Cecil's feedline matching system. 73 for now -- 73 for now Buck N4PGW |
100 Ohm Twin Lead
"Sonny Hood" wrote in message ... I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS Does it have be twinlead? Certain LAN cable is made for 92 ohm Zo, which is might close. Only downside I can imagine is the small size of the center conductor: might limit XMIT power. |
100 Ohm Twin Lead
ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna
for 75 meters. The antenna wire is #12 multi-stranded tinned copper with 600 volt TEW insulation. The antenna is fed within one foot of the NW corner, via 25 ft. of 450 ohm twin lead to a 1:1 balun then via air dielectric RG/8 (21.1 ft.) then to a AT-1500CV. The formula 1005/f was used to figure the length and to cut the wire. However, due to the low height about ground (24-26 ft.); the ground capacitance changed the antenna resonance considerably. The target f (frequency) was 3.9 MHz with 257.7 ft. of wire. After installation in a rectangle configuration at 25+/- feet, the resonate frequency was 3.452 MHz. With a tuner matching the antenna to the "rf" generator (transceiver), the NVIS signal met the 100-200 mile range that was estimated. The antenna was tuned - cut (shortened) to increase the resonate frequency into the 75 meter phone band. After some estimating of desired length at the low elevation and cutting and pruning a resonate frequency of 3.790 MHz was obtained, that has a VSWR of 1.6 to 1. The resonance was found with an MFJ-269 without the tuner, 1.6:1 @ 3790 KHz. The tuner matches the load to 50 ohms for the PW-1. The area of the loop decreased and the perimeter length is 229.5 linear feet. Again with the use of a tuner throughout the phone band produced a NVIS signal range that was very good. K4WYS On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS |
100 Ohm Twin Lead
Dave Oldridge wrote:
... there isn't much difference between a G5RV and a 102 foot dipole fed with 450 ohm line to a good balanced tuner. Not much difference in performance. Lots of difference in the impedances seen by the tuner. For instance, built-in autotuners will usually match a G5RV somewhere on 80m, 40m, & 20m. There are usually certain troublesome lengths of 450 ohm line used with a 102 foot dipole that will prohibit a match by built-in autotuners. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
Buck wrote:
Is this a single band or multiband antenna? If I recall correctly, you can use a different impedance feedline if it is a multiple of a half wave. At the half wave points, the impedance is the same as the beginning of the feed. i.e. at the antenna. You're right, neglecting losses, the antenna impedance is repeated every half wavelength. Neglecting losses, that 100 ohm antenna feedpoint impedance will be repeated at half- wavelength intervals for *any* feedline Z0, including 50 ohm coax. Cecil or others here will correct the length error if I am incorrect. But I learned about a year ago that the impedance can be corrected by the multi-fraction length feedline of a different impedance. Maybe this is the principal of Cecil's feedline matching system. The principal of the feedline matching system that I use is that every SWR circle crosses the purely low resistive point at a current maximum point. For dipoles that are 1/2WL on the lowest frequency of operation, the current maximum point is the ideal point at which to feed the antenna system. Adjust the length of the feedline until the current maximum point is at the transmitter and you will usually not need a tuner at all. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? Let's take a look at how the 450 ohm feedline "messed up the match". The SWR on the 450 ohm feedline will be 450/100 = 4.5:1. A feedline length equal to a multiple of 1/4WLs will transform the 100 ohms to ~2000 ohms or a 50 ohm SWR of 40:1 which is probably out of the matching range for built-in autotuners, for instance. You can change the impedance seen at the tuner back to 100 ohms by adding or subtracting 1/4WL of 450 ohm line which will make it a multiple of 1/2WL. As someone else said, one of the most common ways to match a 100 ohm antenna is to use 1/4WL of 75 ohm coax and then 50 ohm coax the rest of the way. Instead of "messing up the match", it fixes up the match. The SWR on 75 ohm coax would be 100/75 = 1.33:1. The impedance at the end of the 1/4WL matching section would be 75/1.33 = ~56 ohms, a good match to 50 ohm coax. If you are wanting to use the antenna for multi-band operation, change the length of the 450 ohm line until the match is not "messed up". -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
you are great cecil, you cut out the original post to make it look like i
had nothing to do with the original statement... if you will remember he was asking if he would get 25 ohms if he connected the braids together without the center conductors... i simply say that if you connect both the braid and center conductors you get a 25 ohm line... the implication being that if you don't connect the center conductors you don't get a 25 ohm line. thanks for keeping the fight thread going so well, now that you are riled up i guess you can branch out into some other threads and bog them down for the rest of the week. "Cecil Moore" wrote in message . com... Dave wrote: better get out the snorkle... if you tie the braid and the center conductor together you get a 25 ohm line, i use that all the time for my stack matching boxes. Nobody said anything about tying the braid and center conductor together. If the side by side run of 50 ohm coax is used as a parallel feedline, the braids are tied together and the Z0 is 100 ohms when the center conductors are connected across the load. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
great! glad the good old measure and trim method worked adequately... now
if you are really smart you will stop reading more replies to this thread and get on the air and make use of that nice new antenna! "Sonny Hood" wrote in message ... ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna for 75 meters. The antenna wire is #12 multi-stranded tinned copper with 600 volt TEW insulation. The antenna is fed within one foot of the NW corner, via 25 ft. of 450 ohm twin lead to a 1:1 balun then via air dielectric RG/8 (21.1 ft.) then to a AT-1500CV. The formula 1005/f was used to figure the length and to cut the wire. However, due to the low height about ground (24-26 ft.); the ground capacitance changed the antenna resonance considerably. The target f (frequency) was 3.9 MHz with 257.7 ft. of wire. After installation in a rectangle configuration at 25+/- feet, the resonate frequency was 3.452 MHz. With a tuner matching the antenna to the "rf" generator (transceiver), the NVIS signal met the 100-200 mile range that was estimated. The antenna was tuned - cut (shortened) to increase the resonate frequency into the 75 meter phone band. After some estimating of desired length at the low elevation and cutting and pruning a resonate frequency of 3.790 MHz was obtained, that has a VSWR of 1.6 to 1. The resonance was found with an MFJ-269 without the tuner, 1.6:1 @ 3790 KHz. The tuner matches the load to 50 ohms for the PW-1. The area of the loop decreased and the perimeter length is 229.5 linear feet. Again with the use of a tuner throughout the phone band produced a NVIS signal range that was very good. K4WYS On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS |
100 Ohm Twin Lead
"g. beat " @spam protected wrote in message ... "Sonny Hood" wrote in message ... I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS As I remember, common lamp "zip" cord (AWG 18 or 16) functions at about that Z (I'll have to check) gb I once measured some cheap zip cord at 7 MHz. got something like 85 Ohms. Tam/WB2TT |
100 Ohm Twin Lead
"Sonny Hood" wrote in message ... ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna for 75 meters. The antenna wire is #12 multi-stranded tinned copper with .............. Sort of like what I have. Full size 75m loop fed at one corner. I connected a 1:1 current balun directly at the antenna , feeding a 1/4 wave section of RG11 Foam coax. All 50 Ohms downstream from there. SWR at resonance is about 1.2:1. No tuners. Tam/WB2TT |
100 Ohm Twin Lead
On Mon, 15 May 2006 08:26:39 -0400, Sonny Hood wrote:
ANTENNA DESCRIPTION: I recently installed a horizontal loop antenna for 75 meters. The antenna wire is #12 multi-stranded tinned copper with 600 volt TEW insulation. The antenna is fed within one foot of the NW corner, via 25 ft. of 450 ohm twin lead to a 1:1 balun then via air dielectric RG/8 (21.1 ft.) then to a AT-1500CV. The formula 1005/f was used to figure the length and to cut the wire. However, due to the low height about ground (24-26 ft.); the ground capacitance changed the antenna resonance considerably. The target f (frequency) was 3.9 MHz with 257.7 ft. of wire. After installation in a rectangle configuration at 25+/- feet, the resonate frequency was 3.452 MHz. You would expect the feedpoint impedance of a full wave loop (cut using your formula) to be around 90 ohms at resonance. You have then connected 25' of 450 ohm line. Lets assume this is Wireman 551 line, the input impedance to this line section at (assuming 3.7MHz) should be around 141+j282 (and line loss would be ~0.1dB). You then have an unspecified length of "air dielectric RG/8" (new to me) which will transform the impedance further. You seem to intend to then use an ATU to transform this impedance to a satisfactory load for the transmitter. Where did you make the measurements with the MFJ269? If it was not at the antenna feedpoint, did you really expect resonance at the measurement point to coincide with loop resonance, and is resonance at either point important to efficient operation of the entire system? Owen With a tuner matching the antenna to the "rf" generator (transceiver), the NVIS signal met the 100-200 mile range that was estimated. The antenna was tuned - cut (shortened) to increase the resonate frequency into the 75 meter phone band. After some estimating of desired length at the low elevation and cutting and pruning a resonate frequency of 3.790 MHz was obtained, that has a VSWR of 1.6 to 1. The resonance was found with an MFJ-269 without the tuner, 1.6:1 @ 3790 KHz. The tuner matches the load to 50 ohms for the PW-1. The area of the loop decreased and the perimeter length is 229.5 linear feet. Again with the use of a tuner throughout the phone band produced a NVIS signal range that was very good. K4WYS On Sun, 14 May 2006 16:34:44 -0400, Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS -- |
100 Ohm Twin Lead
Cecil Moore wrote in news:dY%9g.76844$H71.68934
@newssvr13.news.prodigy.com: Dave Oldridge wrote: ... there isn't much difference between a G5RV and a 102 foot dipole fed with 450 ohm line to a good balanced tuner. Not much difference in performance. Lots of difference in the impedances seen by the tuner. For instance, built-in autotuners will usually match a G5RV somewhere on 80m, 40m, & 20m. There are usually certain troublesome lengths of 450 ohm line used with a 102 foot dipole that will prohibit a match by built-in autotuners. True, I'm somewhat spoiled since I've always used fairly wide-range tuners. Even my Intellituner will match a 64-1 load. -- Dave Oldridge+ ICQ 1800667 |
100 Ohm Twin Lead
Now, however did I know this thread would go on 'n on, 'n on 'n on...... Cecil Moore wrote: Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? Let's take a look at how the 450 ohm feedline "messed up the match". The SWR on the 450 ohm feedline will be 450/100 = 4.5:1. A feedline length equal to a multiple of 1/4WLs will transform the 100 ohms to ~2000 ohms or a 50 ohm SWR of 40:1 which is probably out of the matching range for built-in autotuners, for instance. You can change the impedance seen at the tuner back to 100 ohms by adding or subtracting 1/4WL of 450 ohm line which will make it a multiple of 1/2WL. As someone else said, one of the most common ways to match a 100 ohm antenna is to use 1/4WL of 75 ohm coax and then 50 ohm coax the rest of the way. Instead of "messing up the match", it fixes up the match. The SWR on 75 ohm coax would be 100/75 = 1.33:1. The impedance at the end of the 1/4WL matching section would be 75/1.33 = ~56 ohms, a good match to 50 ohm coax. If you are wanting to use the antenna for multi-band operation, change the length of the 450 ohm line until the match is not "messed up". |
100 Ohm Twin Lead
On Mon, 15 May 2006 14:26:11 GMT, Cecil Moore
wrote: Buck wrote: Is this a single band or multiband antenna? If I recall correctly, you can use a different impedance feedline if it is a multiple of a half wave. At the half wave points, the impedance is the same as the beginning of the feed. i.e. at the antenna. You're right, neglecting losses, the antenna impedance is repeated every half wavelength. Neglecting losses, that 100 ohm antenna feedpoint impedance will be repeated at half- wavelength intervals for *any* feedline Z0, including 50 ohm coax. So far, so good. If the antenna is fed with 50 ohm coax, there will be great losses, however, if I am correct, there will be negligible losses with 450, or even less with 600 ohm twin-lead. A 2:1 balun at the end of this 600 ohm wire should net a low-loss, but good match for his antenna? Cecil or others here will correct the length error if I am incorrect. But I learned about a year ago that the impedance can be corrected by the multi-fraction length feedline of a different impedance. Maybe this is the principal of Cecil's feedline matching system. The principal of the feedline matching system that I use is that every SWR circle crosses the purely low resistive point at a current maximum point. For dipoles that are 1/2WL on the lowest frequency of operation, the current maximum point is the ideal point at which to feed the antenna system. Adjust the length of the feedline until the current maximum point is at the transmitter and you will usually not need a tuner at all. OK, I think that is a more technical explanations of what I said, so we are in agreement. But for the sake of clarification, let me ask this... I have a yagi with about 25 ohms at the antenna. I connect 600 ohm twin-lead and run it 1 1/2 wave lengths to my radio. This, I assume, gives me the highest current at the end of the twin-lead. So I connect it directly to the radio with a PL-259 and get an acceptable load for the transmitter with low-loss in the feedline? -- 73 for now Buck N4PGW |
100 Ohm Twin Lead
Dave wrote:
if you will remember he was asking if he would get 25 ohms if he connected the braids together without the center conductors... Here's the original posting: ************************************************** ************************ I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? K4WYS ************************************************** ************************ It bears no resemblence to your assertion above. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
Buck wrote:
W5DXP wrote: You're right, neglecting losses, the antenna impedance is repeated every half wavelength. Neglecting losses, that 100 ohm antenna feedpoint impedance will be repeated at half- wavelength intervals for *any* feedline Z0, including 50 ohm coax. So far, so good. If the antenna is fed with 50 ohm coax, there will be great losses, ... A 100 ohm antenna causes an SWR of 2:1 on 50 ohm coax. Lots of hams don't even bother with a tuner when the SWR is 2:1. The additional losses in 50 ohm coax due to an SWR of 2:1 are pretty much negligible on 80m for reasonable lengths of feedline. I have a yagi with about 25 ohms at the antenna. I connect 600 ohm twin-lead and run it 1 1/2 wave lengths to my radio. This, I assume, gives me the highest current at the end of the twin-lead. So I connect it directly to the radio with a PL-259 and get an acceptable load for the transmitter with low-loss in the feedline? The SWR on the 600 ohm line will be 600/25 = 24:1 The SWR on 50 ohm coax will be 50/25 = 2:1 For a 25 ohm single band antenna, I would be inclined to go with the coax. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
The SWR on the 600 ohm line will be 600/25 = 24:1 The SWR on 50 ohm coax will be 50/25 = 2:1 For a 25 ohm single band antenna, I would be inclined to go with the coax. Ok, I learned something, but I would be inclined to go with the 1/2 wave of 75 ohm and then the 50 ohm ;) -- 73 for now Buck N4PGW |
100 Ohm Twin Lead
Buck wrote:
The SWR on the 600 ohm line will be 600/25 = 24:1 The SWR on 50 ohm coax will be 50/25 = 2:1 For a 25 ohm single band antenna, I would be inclined to go with the coax. Ok, I learned something, but I would be inclined to go with the 1/2 wave of 75 ohm and then the 50 ohm ;) "1/2 wave of 75 ohm" would make matters worse for a 25 ohm antenna since the SWR would be higher than for 50 ohms. What you need is 1/4WL of 35 ohm coax like RG83. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
On Tue, 16 May 2006 13:24:37 GMT, Cecil Moore
wrote: Buck wrote: The SWR on the 600 ohm line will be 600/25 = 24:1 The SWR on 50 ohm coax will be 50/25 = 2:1 For a 25 ohm single band antenna, I would be inclined to go with the coax. Ok, I learned something, but I would be inclined to go with the 1/2 wave of 75 ohm and then the 50 ohm ;) "1/2 wave of 75 ohm" would make matters worse for a 25 ohm antenna since the SWR would be higher than for 50 ohms. What you need is 1/4WL of 35 ohm coax like RG83. OOPS, I went back to the original problem with 100 ohm -- 73 for now Buck N4PGW |
100 Ohm Twin Lead
Buck wrote:
Cecil Moore wrote: Buck wrote: Ok, I learned something, but I would be inclined to go with the 1/2 wave of 75 ohm and then the 50 ohm ;) "1/2 wave of 75 ohm" would make matters worse for a 25 ohm antenna since the SWR would be higher than for 50 ohms. What you need is 1/4WL of 35 ohm coax like RG83. OOPS, I went back to the original problem with 100 ohm Then you should use 1/4 wave of 75 ohm, not 1/2 wave. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
On Tue, 16 May 2006 14:30:52 GMT, Cecil Moore
wrote: Buck wrote: Cecil Moore wrote: Buck wrote: Ok, I learned something, but I would be inclined to go with the 1/2 wave of 75 ohm and then the 50 ohm ;) "1/2 wave of 75 ohm" would make matters worse for a 25 ohm antenna since the SWR would be higher than for 50 ohms. What you need is 1/4WL of 35 ohm coax like RG83. OOPS, I went back to the original problem with 100 ohm Then you should use 1/4 wave of 75 ohm, not 1/2 wave. ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm coax, it is 1.33:1 ?? If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax, it will be a close match? Why did the impedance change to 52 ohms? -- 73 for now Buck N4PGW |
100 Ohm Twin Lead
Buck wrote:
ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm coax, it is 1.33:1 ?? If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax, it will be a close match? Why did the impedance change to 52 ohms? Because of the energy bouncing back and forth between the 100 ohm load (that mismatches the 75 ohm, so causes a reflection) and the 50 ohm feed (that mismatches the 75 ohm section, so causes a reflection). If the 1/4 wave section has an impedance that is the geometric mean of the impedance on each end, it creates a perfect matching resonator. So the perfect 1/4 wave matching section between 50 and 100 ohms would have an impedance of the square root of (50 times 100) or 70.7 ohms. 75 is pretty close. |
100 Ohm Twin Lead
Buck wrote:
ok, a 100 ohm load fed with 50 ohm coax is a 2:1 swr. Fed with 75 ohm coax, it is 1.33:1 ?? Yes, and 75/1.33 = 56.25 ohms. For a perfect match to 50 ohms, one would need Z0 = SQRT(100*50) = 70.7 ohms. If the 75 ohm is fed with 50 ohm coax 1/4 wave down the 75 ohm coax, it will be a close match? The SWR on the 50 ohm coax will be 56.25/50 = 1.125:1 Why did the impedance change to 52 ohms? It didn't. It changed to 56.25 ohms. The load is 100 ohms and the Z0 is 75 ohms so the SWR is 1.33:1 at the current minimum/voltage maximum point. To find the impedance 1/4WL away, divide Z0 by SWR, 75/1.33 = 56.25 ohms. -- 73, Cecil http://www.qsl.net/w5dxp |
100 Ohm Twin Lead
On Wed, 17 May 2006 12:13:57 GMT, Cecil Moore
wrote: It didn't. It changed to 56.25 ohms. The load is 100 ohms and the Z0 is 75 ohms so the SWR is 1.33:1 at the current minimum/voltage maximum point. To find the impedance 1/4WL away, divide Z0 by SWR, 75/1.33 = 56.25 ohms. Thanks, I understand how that works now. -- 73 for now Buck N4PGW |
All times are GMT +1. The time now is 02:25 PM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com