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Gypsyjon 22-07-2019 11:53

Trimming VHF coax
I am a ham radio operator and i think I know the answer but would like sage advice.

I am putting a new VHF antenna on my boat and I only need about a 2 foot run of coax to the radio.

If I cut off several feet of coax on the antenna, am I going to mess up the SWR on said antenna?

I think not but welcome other thoughts.


tkeithlu 22-07-2019 12:03

Re: Trimming VHF coax
Nope, you're right. That's why it's coax.

MBWhite 30-07-2019 20:02

Re: Trimming VHF coax
I don't think you'll "mess up" the SWR, but there is a chance it will be affected and be aware the SWR could go in either direction.

a64pilot 30-07-2019 20:20

Re: Trimming VHF coax
Different lengths of Coax should not affect SWR

Doodlebug 30-07-2019 21:09

Re: Trimming VHF coax
Before anyone can answer your question correctly we need to know which antenna your asking about. Some VHF marine antenna use a tune length of coax, other do not and can be shorten. I canít remenber which antenna manfacture said not to use less than three feet of coax. Three feet or 5 feet you canít measure the loss with normal equipment.

Brian.D 03-08-2019 20:17

Re: Trimming VHF coax
Am I right in reading that you only need 2' of coax to connect your antenna to your radio? I would recommend a longer length. It will not hurt anything but if you decide to move either radio or antenna you are good to go.

tkeithlu 04-08-2019 05:23

Re: Trimming VHF coax
Marine VHFs are remarkably robust relative to SSB and ham radios when it comes to the antenna. A commercially made up coax for your antenna will be more than two feet, and probably cost less than buying the wire and the connectors. So, could I recommend that you use the shortest commercial coax, and hide the extra wire?

Bycrick 04-08-2019 07:05

Re: Trimming VHF coax
If I remember my antenna theory correctly from long ago, when coaxial cable is properly matched to the load, itís length is irrelevant except for increasing loss with greater length. There are also some antenna designs that depend on the coax shield as part of the antenna system. In this case, making the cable "too short" might have a bad effect. "Too short" in this context is something less than one wavelength at the operating frequency, I.e., about 2 meters (6í).

If the manufacturer of an antenna says the cable shouldnít be shorter than 1/4 wavelength, Iíd believe them.

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