[Antennas] CB Intenna ?

[Terry Conboy]

On 2011-05-21 10:00 PM, Hue Miller wrote:
> I recently rediscovered a CB curiosity I obtained long ago. I bought this at some kind of store closeout.
> It is "the Original CB Intenna". "Stop CB Rip Offs with the Intenna. It's Completely Inside - The Car Skin Is The Antenna!"
>
> Here's what it consists of: a small one inch metal cube box with 2 holes in it topside for screwdriver
> adjustment of what's inside. One side has a coax female. The other side has a single insulated wire about
> 20 inches long coming out. Unfortunately I don't know where the instruction sheet is right now, but I
> think it's set up like this. You mount the metal coupling box on your dashboard, plug the CB jumper coax
> into the coupler and the other end to the CB. The single wire you run up to the rear view mirror and
> screw fasten it there so that it actually makes contact with the car chassis. I think it must work by
> having the RF path from the rear view mirror spot, around the two halves of the windshield, and back
> to the CB radio ground.
It sounds like it is intended to make the opening around the windshield 
into a slot antenna.  The signal would be primarily vertically polarized.

I built a very rough EZNEC model of the top of a large car, and the feed 
Z came out about 3 ohms in series with 450 ohms of inductive reactance.  
This can be matched with two capacitors - one in series with the feed 
wire to tune out all but about 12 ohms of inductance (about 13 pF) - 
then a second cap across the feed to ground (about 470 pF) which along 
with the remaining inductance forms a virtual L-network giving a 50 ohm 
feed.  Due to the high Q (450/3=150) of the antenna, the 2:1 bandwidth 
at 27 MHz is relatively narrow, a little under 200 kHz, depending on losses.

The tuning will vary a lot depending on the size of the vehicle.

Since the feed Z before matching is pretty low, the resistance of the 
feed wire, connections to the roof and dash, and the caps (ESR) needs to 
be minimized to get reasonable efficiency.

73, Terry N6RY