[TowerTalk] short mobile antenna matching

[Jim Brown]

On 7/8/2013 4:52 AM, TexasRF at aol.com wrote:
> In that case, an impedance step down would be needed and that would require
>   a different matching scheme than what is normally used. It would be
> helpful if  the actual R ±j of the antenna feed point could be determined.

Yes, I agree. But the issue is more a matter of radiation efficiency 
than matching. The coax is not long enough for the loss to be an issue, 
even with significant mismatch. I've always found a mobile rig with a 
built-in antenna tuner to be helpful.

Other posters have wisely suggested a counterpoise.  Somewhere in your 
vehicle there is metal structure, and that can be an effective 
counterpoise. To make that structure useful, you may need to do some 
bonding to connect enough of it together.  I certainly did in my Volvo 
S80 with a hamstick on a license plate mount.  I had to bond from that 
mount to the trunk lid, and I had to bond around the trunk hinges.  When 
I started running mobile in a Toyota Sequoia (big SUV) I found almost 
nothing was bonded to anything, and as a result had massive RFI issues 
(the car's computers locked up).

  BUT -- the counterpoise, as well as the wire between the feedpoint and 
that metal will be part of the antenna. That's a good thing for the 
lower HF bands, but may not be as good on the higher bands. For the 
higher bands, you might want to add wire or metal strips to the roof and 
bond them to the coax shield. Pay attention to the millions of words 
that have been written about radial systems for 160M verticals, and 
scale that down to the bands where you want to operate.  Here's a link 
to a tutorial I presented to a couple of ham groups last year on that 
topic. http://audiosystemsgroup.com/160MPacificon.pdf

73, Jim K9YC