Building a backpack with a mobile radio.
Using SO-239 antenna mounting points. The pack will be used in horizontal and vertical orientation. Is it okay to split the antenna coax feed to two SO-239 mounts, leaving one open? I can cap either unused connect. But will this cause problems? The range is centered at 155 MHz. Do I need a better termination at the unused mount point, than just a cap?
Sorry. One more detail that may matter.
The coax split point can be within inches of the antenna mount, or further back. Less than 3 feet total from antenna to radio. Split at any point.
You can connect one antenna and leave the other open.
Is this a transceiver, a radio that transmits and receives? If so, be sure to observe RF exposure guidelines. Manpack radios put the antenna close o your head.
You are adding an open stub to the feedline. Which will contribute considerable reactance. If you knew the complex feedpoint impedance of your antenna, then you could make the stub provide a conjugate match…for ONE of the antenna connections, but it would be totally wrong for the other, because you’d be swapping the stub length with the location, and vice versa.
One way to solve the issue would be to make the two branches of the Y exactly 1/4 wavelength long (do mind the velocity factor of the coax you use) and then place a shorted plug in the unused SO-239. At 155 MHZ, 1/4 lamda is fairly manageable. You’d want to make the coax for each arm a bit shorter to account for the electrical length of the connectors out to the actual short. This also keeps rain and dust out of the unused connector.
OR you could make the two arms of the Y much shorter (but still equal) and make a shorted-stub for the unused connector such that one arm of the Y and the stub + connectors add up to 1/4 lambda.
Yes, a shorted 1/4 wave stub really appears as an open. This is the basis of many highly selective filters, such as the duplexers used on repeater installations. So as a bonus you’d get some protection against spurious emissions and receiver overload due to very strong out-of-band interference.
I decided I should put some numbers on my warnings. I used This Excel spreadsheet to calculate that if the arms of your Y are 4" long and solid dielectric RG-58 (VF=0.66) then it will present -j91.6 Ohms. (91.6 ohms capacitive reactance)
If the antenna is matched (50-52 Ohms resistive, SWR=1.0) then the branch going to the antenna will be 50 ohms resistive.
50+j0 in paralell with 0-j91.6 Ohms = =38.5- j21.0 ohms or 43.85 < 28.61 deg. Ohms
Per https://chemandy.com/calculators/return-loss-and-mismatch-calculator.htm
This results in an SWWR of 1.72:1 which your transmitter might be OK with, but it will increase feedline loss, and note that this assumes antenna is a perfect match. A typical antenna won’t be, and the stub could make it better if you are lucky, or worse if you are typical.
Missed edit window:
38.5- j21.0 ohms is of course 43.85 < **-**28.61 deg. Ohms not 43.85 < **+**28.61 deg. Ohms
Thanks very much for the very detailed and informative replies.
I suspected the issue would not be a simple one. We already push our distances with the packs. And the users are not careful with them. I will instead try to arrange a rotating mount that will hold up to the abuse.
Moderators can close this off. I got great replies.