The AM Forum

This isn't an AM question (although it could be with REALLY large antennas). I would like to feed a pair of VHF/UHF antennas for polarization diversity. They'll be 90 degrees out and spaced far enough apart. The coax would go to a T-connector and there would be two lengths of coax feeding the antennas from the T.

Now, that gives you two 50 ohm antennas in parallel. What is a good formula for figuring out length of coax? And what type of coax should be used? Maybe 75 ohms?

Jon

connect each antenna after the 50 Ohms coax with an electrical  1/4 wave 75 Ohm just before the T. That transforms the 50 ohms into 100 Ohms. The T connects them in parallel and you get 50 Ohms again
You can assume the electrical length being 0.7 times the mechanical length. That will be good enough for PE and PTFE insulated coax. For air insulated or foam you should get the information from the manufacturer.

Very awesome, thanks!

Jon

You could mount them both on the same boom with one spaced a quarter wave ahead of the other and fed in phase. You'll get circular polarization that way. You could even mount the two antennas as an "X" if you wanted, with no difference. If you want polarization diversity, circular polarization would be the way to go. This would be a mono-band antenna unless you want to intersperse elements for another band on that same boom. Take a look at satellite antennas for ideas.


Don


Edit: Oh, and if you just want to work both vertical and horizontal stations, just mount one yagi on a 45 degree angle. It will be 3dB down from each polarization.

Quad hybrid another possibility.

Presentation matters as much as technical functionality, so I'm thinking about another satellite hunter/killer hip-mounted array similar to what I built the last time, but with 4 antennas rather than 2.

https://www.youtube.com/watch?v=dy2zB8bLSpk (https://www.youtube.com/watch?v=dy2zB8bLSpk)

Jon

Since it's VHF/UHf, a single quarter wave 75ohm coax most likely will not be long enough so be sure to use odd multiples i.e. 3/4, 5/4 wavelengths. For 2 meters using rg8 style coax 93" is usually good for 1 wavelength spacing setups.

The spacing is fairly tight, especially at 70cm. The power factor for the cable that I get is 80% and that gives me about a foot to deal with. So, even with angling both antennas at 45 degrees, there's still enough room to make it all work. The dual-band mobile radio would be mounted between those two antennas with the duplexer right beneath it.

Jon

It is not advisable to use more than 1/4 wave, than the error in velocity factor becomes more critical. Use normal 50 ohms cable and just before the T put a 1/4 wave 75 ohm coax

Most, if not all, modern dual band radios have a duplexer (technically a diplexer) built into the radio. This could be bypassed and each band sent directly to the appropriate antenna. With this, the loss incurred by both internal and external duplexers would be removed. Some older radios, such as the Kenwood TW-4000A, have no internal duplexer and are ready to connect to two seperate antenna arrays.

You can make your own phasing harnesses or buy them from places like:
https://directivesystems.com/
They make some nice stuff... with impressive "presentation" value. ;)

http://amfone.net/ECSound/K1JJ23.htm


Here is some good info I got from my friend, W6MYC, Mike who own M2 Antennas.  It applies to 6M Yagi phasing of two or four antennas, but can be scaled anywhere.

T


This homebrew 190' dual 8-bay 6M array was my result.... talk about a lot of climbing!

Either way works equally well. However, using odd multiple quarter wave feedlines to each antenna reduces the number of connectors in the system, eliminating a barrel connection thus reducing losses and potential weather problems.

Tom, it's not in Part 97 but there is a gentlemen's agreement to keep ERP under 1 million watts.

Here is my first attempt at what I'm talking about. Round two will have 4 antennas mounted at 45 degrees. 70cm in the center and 2m on the outside. Mono-band mobile radios (that are capable of breaking squelch) and a duplexer mounted in the center.

Compared to an Arrow antenna and a cross-band portable, this would be the satellite hunter/killer.

Jon