Ground plane question, antenna length question

[Lynn]

Hi all,

Quick question 1: Does the ground plane have to be one contiguous piece of metal?
Quick question 2: Is there a minimum length for coax?

Longer explanation: I'm thinking of building a 'module' that would drop in an opened sunroof (and be clamped in place, of course). Antennas mounted on top, radios mounted on bottom.

For the main piece sealing the sunroof off, I was thinking of making it of aluminum versus plastic or wood. That way it would extend the ground plane created by the roof of the vehicle, but I'm not sure if it has to be grounded to the rest of the roof. The easiest method for construction would include a rubber weather seal that would electrically isolate the sunroof section from the rest of the roof. Would that be an issue?

Also, my plan could require less than a meter of coax to connect the CB to the antenna. Seems like I read somewhere that one should idealy have a minumum amount of coax. Something like 4 meters? Seems to me as long as you can tune the antenna for a low SWR, the length of coax shouldn't matter, but I know there are lots on this board who know more than I.

 

[Finlay]

Answers :

1. No. Inexpensive ground planes are built out of radial wires all the time. It should be electrically connected, but doesn't - strictly speaking - need to be. A car/truck isn't a great groundplane anyway, it's just better than nothing. I wouldn't overthink it - a mobile antenna is a bag of compromises to start with; a few extra square inches of ground plane won't make or break it.

2. Yes, the minimum length is the length needed to get from the radio to the antenna. Longer than that, and you invite problems - but those aren't unsolvable, either and only an issue if it affects your installation.

 

[gary in ohio]

For VHF or UHF your sunroof sized metal piece would be enough. for HF you need a bigger ground plane and a much better mount point.