| Author |
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Delerius
| | Posted on Thursday, July 18, 2002 - 10:18 am: | 
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Can anyone tell me the true proper way to set the swr's with this configuration? I have a Dosy meter that I want to keep connected inline all the time so my travel path would be, Radio -> Amp -> Dosy -> Antenna. Now I have heard that you should remove the amp and place the swr meter in its place to set the swr's. Since I am leaving the meter inline, should I leave the amp inline and have it turned off when setting the swr's or should I remove the amp and connect the two coax's together where the amp would be? Right now I have a 3 foot coax running from the radio to the amp and another 3 foot running from the amp to the meter and 18 feet running from the meter to the antenna. I am assuming that if I set the swr's by going from the radio to the meter then to the antenna my swr's will be off when I put the amp and 3 foot coax back inline. Also in regards to coax length, am I correct to believe that as long as that I am in the 50 ohm range, I will be ok? I was thinking about moving my amp further away from my meter and if I do, I need to use a 6 foot piece of coax instead of 3 foot. Any help would surely be appreciated. |
Dx431
| | Posted on Friday, July 19, 2002 - 2:29 am: |
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To get the lowest swr, you want to remove the amp and jumper....connect the jumper from the radio into the meter....get the lowest swr possible then replace the amp. You should be good to go. The swr will fluctuate a bit with the amp on, that is normal. Moving your amp isn't going to make a differance. Hope this helps. |
Delerius
| | Posted on Friday, July 19, 2002 - 10:24 am: |
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After doing some investigating, it seems that coax length has no real factor in one way but yet in another it does. What I mean is, for 100% power transfer, I would want a 1/2 wave length from the radio to the antenna. If I would use RG-58, which has a velocity factor of .66 and if my main comm. channel would be 27.185 mhz, then the length of coax should be 12 feet. By using 12 feet also, I should attain very true swr's. Am I correct to say that when going by 1/2 wave length measurement, I would also be limiting Common mode currents? If this would be the true method of determining coax length, for my installation situation, I could have a 3 foot coax going from the radio to the amp, another 3 foot coax going from the amp to the meter and have a 6 foot coax going from the meter to the antenna. Now for what I also read was that coax length has no factor and by placing a swr meter any where on the line, I will see the same results with minor fluctuations. It was said that I should use the shortest piece of coax to go from point a to point b. So in the end, what is the right method? Am I correct to believe the reason why cb shops tell me to use 18 feet is because it is half of a wave length without the velocity factor calculated in (a wave length being 36 feet)? If anyone knows of a web site or personally has more info with different test results regarding this, I would love to see it. |
Tech671
| | Posted on Sunday, July 21, 2002 - 9:34 am: |
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What radio, amp, meter, coax are you running? What antenna and where/how is it mounted? |
Marconi
| | Posted on Sunday, July 21, 2002 - 12:18 pm: |
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Delerius, you are correct about the resonant 1/2 wave line reflecting your best shot at a true SWR reading. So, if you can get your antenna tuned (set your SWR's as you note) to 50 ohms with very little or no reactance, then it should not matter how long the lines are. Really it should not matter what you have in line either, if the devices input/outputs are non-reactive and near 50 ohms. IMHO, this has to do with what the manufactures are indicating when they describe their coax as having a characteristic 50 ohm impedance. They are telling you that everything in the system has to be 50 ohms and their line should reflect a true match when the load is purely resistive.
If you used the 1/2 wave line you note above with a good non-reactive 50 ohm dummy load at one end, and an antenna analyzer at the other, you should see approximately R=50 ohms and X=0 reactance depending on the load and the type of connection. I would first check the load with the analyzer hooked up directly to the load. This way you can see what it reads at the frequency you want to check just to get an idea of the values you might expect to see on the analyzer.
If you do this with several random and tuned lengths of coax you may note some strange and maybe unexpected results. Only the resonant 1/4 wave and the 1/2 wave will typically reflect very little or no reactance (X). The impedance may not show exactly 50 ohms as you would expect, but it should be close and the reactance should be no more than X= (+ -) 1.
Considering these observations, I prefer to use jumper lines that show very little value for X =<(+ -)1 or reactance. This way if you have some reactance in your antenna or another device in line you will not additionally complicate the impedance match as much. For example, some amps show a heavy reactance to a system and they often are guilty of acting up by increasing this reactance to the system. This is why a change in jumper lenghts can sometimes help solve this type of problem.
Personally, I tune antennas with a tuned 1/2 wave line cut for the frequency I want to use as center without anything in line but the meter and a non-reactive jumper. If I can get the antenna correctly tune, then I will use anything random and it seems not to matter.
Marconi |
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