Mobile antenna counter poise ??

[Chainsawgang]

It seems I remember a post a while back from DTB (I think) re: using a short 9' or so counter poise . If I remember I think it was to use in a truck cab. Can anyone direct me here ??
I slip-seat so , I was wondering if a counter poise would work with my slipseat radio box as the numerous groung conditions I run into can really be a nightmare !! Any tips here guys to get a consistant SWR while switching vehicles?

 

[W5LZ]

Chainsawgang,
Interesting idea! Depends on what you want to use that 'counterpoise' for. If it's to be part of the antenna system, it isn't gonna work very well 'cuz it's in the wrong 'place'. If it's going to be used as a ground for the radio, make it a 1/2 wave length long instead of a 1/4 wave long. But don't touch it, you'll get a shock near the end of it.
You're dealing with two different characteristics of that 'counterpoise'. If it's close to the right length for the band you operate on, and if it's at the feed point of the antenna, it can certainly act as the "ground" portion of an antenna. If it's not in the 'right' place, it'll do no good at all.
The other characteristic is that the 'right' length of conductor for a particular frequency acts sort of like a very good RF ground, a 'black-hole' RF can fall into and not stay on the equipment to 'buzz' you when you touch it (among other things). That 'black-hole' length is an even multiple of a 1/4 wave length at the frequency of use. The 'other' side of that characteristic is that odd multiples of a 1/4 wave length will act as an open circuit, or a lid on that 'black-hole', and there won't be any RF going there. Get ready for some shocking experiences!
Ain't all this electronics stuff just great $#*T?
- 'Doc

 

[Chainsawgang]

Interesting DOC !!!
Now let me make sure I'm doing this right .....
1/4 wave about 9 feet ?
1/2 wave then about 18feet?
And do you have any suggestions as where or how to mount the counter poise ?
I was considering bolting it to one of the mirror mount mounting bolts>

 

[W5LZ]

Chainsawgang,
Oooo boy! Lots of variables in that, so there's no absolute, for sure answer.
try doing it that way and see what happens. Not the 'best' in the world? Change it, or just don't use it, see what works.

Yes, those are the common lengths for 11 meters. Exact? No, but then it depends on several things. It'll get you close though.

For that 'counterpoise' connected to the radio, put a large alligator clamp on the end of it and clamp to metal. Works just as good and not near as 'picky' about lengths...
Good luck.
- 'Doc

 

[freecell]

a counterpoise (the word itself refers to a balancing action) is a conductor but more generally a system of conductors used as a substitute for earth ground in an elevated antenna system, more commonly referred to as ground plane radials.

any effective rf ground return requires a low inductance path which is always much less (as short as possible) than a quarter wavelength at the frequency of operation.

since the purpose of a counterpoise is to mirror the currents existing in the antenna (balance) the effects on swr are negligible and provides no solution to maintaining a consistent swr when the antenna is moved from one vehicle to another. one exception to this rule is when the vehicle cab is constructed predominantly of fiberglas or some other non-conductive material.

also since most antennas in this application are end fed a counterpoise already exists. if not then the swr would be 3:1 or higher no matter where the antenna was moved to. if you never see an swr approaching that figure then you're wasting your time and your money. if you're worried about small changes well below 3:1 then you need to count the cost.

 

[Chainsawgang]

Thanks for the responses guys ! I'm still waiting to see if anyone remembers the situation that DTB was referring to ....if I remember correctly; I think DTB suggested trying a counterpoise for an RF or related issue.
Anyway , I'm just experimenting so, I'll give it a try there may be NO positive gain from this. But, it's not expensive or troublesome so hey , I'll give it a shot .