Here's one for the genius' of the forum

now... should i be getting a connectivity reading from the outer ring of my pl259 to the body on my car? As i seem to always have a poor signal on my stinger mag mount and have alot of issues trying to tune the antenna with the swr (the needle its just flickering all over the place when i key up)

So i tested the ground on the car to the pl259 and there was nothing i also tested the centre pin to the inner whip brass connector that was good connectivity but nothing regarding the ground.

Am i right in thinking that when a mag mount is used it uses the car body as a negative plane??

Ypdate: swr meter is ok now i have it a good clean and no the needle is steady i have tried to tune abit tonight but got abit late will have to have another go tomorow but i am still looking for answers to the above question about continuity from the car body to the outer ring on the pl259

A lot of mag mounts have got rubber or plastic bottoms or skirts on them, so you won't see a DC connection between the outer of the coax and the body of the car. The earthy side of the cable (and antenna) couples capacititavely to the car, and for some antennas it will work just fine.

Hi Matt,

As already mentioned a typical magmount works ground via capacitive coupling.

Continuity meters, multimeters, DMMs aren't going to be much help I'm afraid...even the clever ones with inductance and capacitance measurement can cause more confusion than anything.

I feel your frustration at trying to understand all this. We start with wanting to know "why", gain some snippets of information and then land up even more confused than anything!
It's that case of the old adage a little knowledge can be a dangerous thing .

If your'e up for a little bit of reading that doesn't doesn't come over with a ton of maths and condescension you couldn't do much worse than giving the Alan Applegate (K0BG) website a read. (He's an American Amateur Radio operator with a lot of practical experience and best of all, willing to share his knowledge).


Here's a link and specifically to his mobile antenna mounts page:-

K0BG mobile antenna mounts

Goes someway into understanding different antenna mounting systems. If you get the chance, have a browse through the rest of his site too.

Hope that helps

All the best,
Victor

I wish I could just go round the country helping sorting out peoples mobile installs and teaching them a few pointers. Unfortunately every time someone posts with a problem they're inevitably 100+ miles away.

Mau5matt you mention you get flickering SWR. Where you bring the coax into the car has the coax got pinched, flattened or crushed in the door/boot?

Sorry for the late reply.

Yeah swr meter is now sorted it was grubby inside and the knob was sticking but it is all good now i got swr down to 1.5 on ch 1 and 2.0 on ch40 i have since also learned that my local area is boombing on ch25 and not so much 19 as i thought that 19 was the go to channel but i am wrong i think... think i have this issue sorted now

And regards to the location where the coax comes into the car i have a ford rager pick up and there are loads of gromits in the rear i used one of them

Good to hear you've used a grommet, that'll save no end of grief.

If SWR is higher on Ch40 than Ch1 then then antenna is too long. If you've adjusted it to the shortest length you may find you need to cut it if it has a whip, the better the RF ground the shorter the antenna needs to be for a given frequency on a mobile install. Unfortunately manufacturers make the lengths assuming a poor RF ground which, to be honest, most mobile installations have. This means often there's not enough adjustment and it is out with the dremel. I use a fixed mount and did loads of bonding as per the guidance on the bonding page at www.k0bg.com. I typically have to cut 2-3 inches off CB antennas as at their shortest they're resonant just below 26MHz. That same vehicle before I did all of that and just used a magmount on the roof those antennas would just work fine out of the box.

Fortunately I own an antenna analyser so could see what was going on, without one I'd have thought there was a problem as very few CBs go down that far in frequency to test with just a SWR meter.