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What kind of SWR mismatch to expect	Reply
by N0ZYZ on June 11, 2008	Mail this to a friend!
If I were to use 75 ohm coax on 2 Meters. I have alot of High quality Quad shield 75 ohm that I used when I installed satalite systems and I am just wondering if I can get away with using it on 2 Meters, I know on HF I can expect 1.5:1 but I am unsure if this changes for 2 meters? Charley

RE: What kind of SWR mismatch to expect	Reply
by W5FYI on June 11, 2008	Mail this to a friend!
Assuming the antenna remains at 50 ohms nonreactive, SWR on a 75-ohm line would be 1.5:1 for a maximum power loss of 0.177 dB (4 watts lost out of every 100). However, many 2-meter antennas are either slightly more than 50-ohms or slightly less. A ¼-wave vertical over a ground plane might have a Z around 30 ohms, which would put SWR on a 75-ohm cable at 2.5:1, for a maximum 0.883 dB loss (18.47 watts lost out of every 100). Further, a 50-ohm Z SWR meter is of limited use on 75-ohm coax. Therefore, I would say that if you knew your antenna Z was close to 75 ohms, or you can make it 75 ohms, or match it to 75 ohms with a ¼-wave matching transformer, go ahead and use the TV coax. On the other hand, if you're not too sure, and need every dB you can squeeze out of the system, stick with 50-ohm transmission line. Just my 2¢ worth....

RE: What kind of SWR mismatch to expect	Reply
by WB6BYU on June 11, 2008	Mail this to a friend!
It doesn't make any difference what frequency you are using it on. Well mostly... In practice there would be a slight change due to the difference in line losses on 80m vs. 2m, but otherwise the same principles of SWR apply. But that doesn't mean you would see an SWR of 1.5 : 1 at the rig unless the ANTENNA had a 75 ohm input impedance. If the antenna is 50 ohms, then the 75 ohm feedline would see an SWR of 1.5 : 1, which will increase the losses slightly. But an SWR of 1.5 : 1 on 75 ohm cable means that the impedance at the rig end could be 50 ohms, 112 ohms, or somewhere in between with some reactance. So the rig may see an SWR of 1.1 or 2.25 : 1 or some intermediate value, depending on the feedline length. There are several different ways to use 75 ohm feedline in a 50 ohm system: 1) use it and ignore the SWR 2) trim the length of the coax to get close to 50 ohms at the rig 3) if the antenna is adjustable, adjust it for best 50 ohm match to the rig with the coax you are going to use. 4) use 1/12 wave matching sections of 50 and 75 ohm coax at each end to transform up to 75 ohms and back to 50 ohms 5) use wideband 50 : 75 transformers at each end 6) use shorted stubs or other methods to transform between 50 and 75 ohms.

RE: What kind of SWR mismatch to expect	Reply
by KD8HZC on August 2, 2008	Mail this to a friend!
I use 75 ohm cable on a 2 meter GP in my attic. I got the SWR down to just under 2, but at a 75' run there is line loss. It works, may not be the best but it works. I primarily use it for a back up antenna anyway.

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