weird ground plane mobile K-40 magnet mount question?

[niceguy71]

hello everyone
not been into CB's since the 90's but kept all my stuff ( never going to sell it) and been meaning to put it all back up.... I had a big 4x4 Blazer with a K-40 magnet mount on the roof and a President Grant FM AM SSB radio they worked great!!!! I just retired and want my set up back.
now I have a 2 door 4X4 F150 pick up, ( small roof area) no place to mount that huge radio.... but I'll try to floor mount it.... anyway my question
I pull the truck into a garage every day so I can't put the magnet mount K-40 on the roof, it would hit the bottom of the garage door. and it's too much work to get in the bed and take it off every time I go in or out of the garage. .... so I was thinking maybe I can take a piece of angle iron and a flat piece of thick metal, maybe 7" X 7" and screw the angle iron to the side of the bed and weld the metal so the whole rig sits on top of the bed rail???
that way for everyday little trips I can listen and who knows maybe talk on it if the SWR is not too high.... then if I want to use it I can jump in the pick-up bed and pull the magnet mount K-40 off the bed rail and set it on the roof..... but I know a CB antenna needs a ground plain ..the signal needs to reflect off the cars metal..... not much metal on a pick up bed rail and my bed has a rubber ( not plastic) bed mat so I don't know if that will ground plain..... any thoughts??? will it work ok???
Question part two
if it does work I can just bolt a piece of angle iron to the side of the bed rail and permanently mount the K-40 trunk lip mount to the angle iron..... but where would it work best at the end by the tailgate or in the middle??? or where it would look nice right behind the cab???? the K-40 is 57 inches tall and the cab is about 25 inches above the bed rail so it would still be above the truck cab....
any help would be greatly appreciated.

 

[merlin]

If not already, ground strap the bed and cab to the frame.
This effectively makes the whole truck the ground plane.
Your greatest gain is to the longest distance of the truck.
IE: antenna at the right rear, your best gain is off the left front.
Mounting just behind the cab center would be mostly omnidirectional,
but keep the coil a good foot away from any metal above the mounting plane.
Adjust the antenna for best SWR and that should work for you.

 

[niceguy71]

thank you Merlin, I'll do a conductivity test on the bed and cab and frame but I would imagine everything is connected by being bolted together. I guess I'll give it a shot and see what happens

 

[merlin]

All too often, cabs and beds are shock mounted to the frame making for a not so good ground.
Why I suggested ground straps.
In cases where you have mirror mount antennas, even the door to cab needs a ground strap. so you get the idea here.

 

[mastr]

thank you Merlin, I'll do a conductivity test on the bed and cab and frame but I would imagine everything is connected by being bolted together. I guess I'll give it a shot and see what happens

Be aware that "connected" for DC (your multimeter shows a low reading) and "connected" for RF (there is a good RF path between all parts) can be very different things.

 

[niceguy71]

Be aware that "connected" for DC (your multimeter shows a low reading) and "connected" for RF (there is a good RF path between all parts) can be very different things.

I didn't know that?? Thank you Mastr how can I tell without fancy equipment?

 

[mastr]

...how can I tell without fancy equipment?

Easiest way "without fancy equipment"- use your handy model 1 pair of eyes. If whatever item on your vehicle (1) moves on hinges, (2) is mounted with rubber/plastic, (3) mounted in some way you cannot see - the RF connection is questionable.

How to tell if your changes such as adding bonding straps make a difference "without fancy equipment"-- A CB field strength meter is very cheap. Typically near field RF strength will increase as bonding improves. The first 2 or 3 straps usually make the most difference.

I'm not going to discuss antenna mounting except to say that optimum aesthetics, convenience and performance will not be found in the same location.

(DISCLAIMER - this is my experience, I will consider opposing views, but am not likely to be convinced)

 

[merlin]

A lot of misunderstanding about magnet mounts.
Nearly all cases, there is enough capacity to couple the ground in an RF sense.
DC will show open circuit, but this is 27 Mhz RF.
Say your bed is rubber mounted and antenna at a corner. 8 foot bed, there is enough ground plane to be effective. Just tune the antenna to the bed.
There will still be some parasitic ground plane toward the cab.
master mentioned a field strength meter. With the radio transmitting, you can walk around the truck and see the variation in radiation. you can find the strong and weak directions with that.
I had a mag mount 1000M Wilson on the cab of my truck. A near perfect match even through the protector film and paint. My field was very much omnidirectional.
This, in spite of no ground strapping.
From Portland Or. I could talk to a friend in LasVegas Nv. (good skip back then.)
The big difference you will see mounting other than the cab roof is a change in directional patterns, generally stronger to the longer distance. (toward the ground plane.)