CB - Getting a decent SWR reading?

[muddyplugger]

I found an existing CB mount and lead in the Defender (previous owner fitted I presume), so I assumed all would be well when I connect the rig to one end and my new aerial to the other! (first mistake I hear you say).

The trouble is I cannot get anywhere near a decent SWR reading, best is well over into the red +5 or so.

The aerial is mounted on the wing, which being aluminium is a poor ground plane, so I ran a wire from the base to a decent earth but no change!

My first thoughts are just to buy a new lead/mount and replace the lot, but has anyone experience of best postion on the Defender to mount it? Does it matter?

[simonr]

I don't know where people get this idea that Al makes a poor ground plane!

Al is a better conductor than steel - and will thus be a better ground plane!

Issues with it being poor are generally down to having a poor connection to it.

First try measuring the SWR with the antenna disconnected from the SWR meter. I suspect this will give a similar reading to what you have now.

If it does, then you either have a short or a break in the circuit.

Test with a multi-meter. There should be no connection between the centre core of the plug and the shroud. There should be a good connection between the centre core of the plug which connects to the rig and the antenna (where it screws on to the base).

Even the most cra@@y antenna should give you better than 3 and I would expect to end up with an SWR of between 1.2 and 2.0. In practice this will be plenty good enough. Aim for about 1.5

If the above all checks out OK (I suspect it wont) then, it must be the length of the antenna.

You can use the following procedure to tune the length of the antenna.

Measure the SWR on Chan 1 and Chan 40. If it is better on Chan 1 than 40 your antenna is too Long

If it is better on 40 than 1, the antenna is too Short.

Change the length in increments of 2mm or so and stop, ideally when it is the same on 1 and 40, or when the readings reverse.

Finally measure the SWR on Chan 20.

Si

[western]

Alloy works fine for my Ham radio's as a ground plane, make sure the grippng part has access to clean metal for a start, the conductor centre is not touching the outer braid anywhere otherwise it'll duff your rig's amp.

SWR info

[steve_a]

I'd suggest checking the lead running from rig to antenna first. This was the problem with mine, it worked one day and died the next, new lead and no problems.

multimeter test should give you the information, however in my case I couldn't see anything amiss, I assume I had some water ingress, which in your case if the lead hasn't been in use for a while would seem pretty likely.

ally is fine as a ground plane, a nice flat area is best, my IFOR canopy on the back has too many ridges, so my gp is better with just the open back than with it, but it still gives around 1.5 - 2

[minivin]

if aluminium was a poor ground plane, then all the aircraft in the world would be having a major issue with their IFF's, Dopplers, UHF, VHF, HF, GPS et cetera. However it's all in the bonding.

I got some woven braiding that is usually used to sheath cables to provide EMC screening. Puller it down to it's smallest shrink point and then put it down some heatshrink to protect it. I then put a ring crimp on one end and mounted that at the aerial mounting disc and run it down to the cappings on the tub. Was getting 3.5 at channel 40, but now get 1.8 on channel 1 and 2.2 at channel 40. The reason for this being that my aerial was mounted on a 6" aluminium disc on top of a FFR antenna arm, not enough material for a sufficient ground plane

[muddyplugger]

That is all good info thanks

I dont know where I suspected that Aluminium was a poor ground plane, old wives tale?

I believe then that it is the lead/connection, as judging by the rest of the vehicle wiring its bound to be bodged! (but I'll get the meter out and check).

Cheers, Anth.

[simonr]

One last suggestion! Replace the coaxial cable with something decent. The stuff they use is usually dreadful!

Good quality RG58 50 ohm is reasonable with soldered on connector at the CB end. I like Minivin's idea of crimping a ring terminal to the braid and bolting this to the body skin adjacent to the antenna base. Good belt & braces approach.

The other option is a type of antenna which does not require a ground plane. Not the silly little spiky things that are supposed to replace it, but something like a folded J which doesn't need one at all (which is what I used for 2m / 70cm). Don't know if such a thing is available for CB, but a mobile amateur 10m antenna should work OK.

Si

[cols110]

Removed your wip and chuck in a straightened steel coat hanger that is a few inches to long, then remove an inch at a time and check your SWR, I ended up removing 6 or more inches from mine to get the correct reading, then once you have an approx lenght do the same to your wip, but with a bit more finess with the fine tuning.

I had a similar problem, and tried everything and could`nt sort it, and it ended up just being I was`nt taking enough lenght off the wip.