Coax length question?

[ak7an]

I am in the process of installing a UHF only antenna on my Can Am Maverick Trail 1000 SXS. from where the NMO mount will be to the SO239 on the back of the radio is at the most 18 inches. I don't need the 15ft of coax that comes with most antenna mounts. I was thinking along the lines of 3ft, enough to have some slack to be able to connect the antenna analyzer for testing. I was wondering what length should be avoided? Operating will be mostly between 438 and 450 MHz, and the antenna is a 5/8 wave 3db gain with a spring for those pesky low hanging branches.
Thanks
Ed
AK7AN

 

[mmckenna]

The correct length of cable is whatever you need to get from your antenna to your radio with some slack.

There really isn't any length to avoid. A properly installed and tuned antenna will negate the need for that.

 

[Ubbe]

I was wondering what length should be avoided?

If everything are exactly 50 ohm then it shouldn't matter, but usually it isn't and a 5/8 antenna might be of the kind that cannot be tuned. If you measure SWR then add a metal crocodile clip to the top of the antenna to see if the SWR goes down and the antenna are too short. If the SWR goes up then you can probably cut the antenna 1/2 inch shorter and see if that improved SWR. But it could just as well be the 1/4 wave bottom part of the 5/8 that needs to be trimmed. Some antennas are fastened with an insex screw at the bottom and that usually allows it to be extended a bit and check if the SWR changed.

Make some test coax cables you can use to extend the coax. Make one 11cm (1/4 wave) and another 5cm (1/8 wave) Add an additional 22cm (1/2 wave) length to your antenna coax before cutting it to the length that are more appropriate and use a SWR meter and transmit. Insert the 1/8 wave extension and check if the SWR improved. Then switch to the 1/4 wave extension and check SWR. Then add both extensions and check SWR.

If any extension length improved SWR then add that length to your coax but reduce by 1/2 wavelength (22cm). You can use those extensions for other tests for your antennas at home, even for receive only scanner use.

/Ubbe

 

[JustinWHT]

Guy called me looking for some coax cable for an antenna.

Me: "How long do you need it?"
Him: "I'm installing an outdoor antenna, so I guess I'll need it a long time."

I coax-ed him out the door.

 

[spongella]

15 feet isn't that long and won't result in any significant signal loss. Depends if you want to go through the trouble of cutting the coax and resoldering a new connector. Think of any future installations of that antenna that might suffer from such a short (3') length.