CB Woes...

Hi guys, I'm looking for some advice on fixing high SWR on my CB set up. I know there are some CB gurus on here.

CB is a Midland 77-095; it's been sat in the shed for 20 years but I don't see why it shouldn't work. As I had the head-lining out of the disco I ran some new coax to a new dome-mount in the centre of the roof, to see if I could get it working. Aerial is a new 3' springer.

I've put it all together and I can receive but I get massively high (~3) SWR readings if I press the transmit button.

I've used my multi-meter and I have good continuity all the way from the metal of the roof through the braiding layer in the cable to the metal of the plugs on the back of the radio and SWR meter. I also have good continuity down the centre of the cable, from the plug to the aerial mount to the plug at the radio. There is no short circuit between the core and braid layer of the cable.

I've tried SWR readings on Ch 1, 20 and 40 and they all come back with a reflected signal strength of very close to 3. Small difference in readings across the range suggests the aerial needs shortening, but the fact that they are so high to start with suggests there is a more major problem.

The aerial itself is a metal whip with the coil at the bottom and there is good continuity between the tip and the scewthread at the bottom.

Any ideas please folks?

M

sounds like the rig is fooked. If everything else cable earthing etc is ok. Damp does terrible things to communications devices as does dust and spiderwebs. have you opened the CB and cleaned the circuit boards and contacts off.

just a quick thought are you close to any buildings and have you tried taking an additional earth from roof panel back to battery

Don't know why you are bothering! I've got a CB in mine but no bugger ever talks to me!

Well clear of any buildings whilst testing, have even tried driving to the top of the local hill... I've not tried running an additional earth; it would be easy to do just to test (will try that the next time it stops raining).

Redhand - I also wondered about that as shed has been damp, but it receives ok. Will try the extra earth first as that is easiest and then perhaps open the box up.

If there's anyone who has a known good CB in the Cardiff area I'd happily exchange biscuits for the chance to try my rig on your aerial and your rig on my aerial. That would sort out where the problem is.

McBad said:

Well clear of any buildings whilst testing, have even tried driving to the top of the local hill... I've not tried running an additional earth; it would be easy to do just to test (will try that the next time it stops raining).

Redhand - I also wondered about that as shed has been damp, but it receives ok. Will try the extra earth first as that is easiest and then perhaps open the box up.

If there's anyone who has a known good CB in the Cardiff area I'd happily exchange biscuits for the chance to try my rig on your aerial and your rig on my aerial. That would sort out where the problem is.

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Does sound like a blown output. but do try a known good cb and a 2nd swr meter just to rule out the obvious.

Mustbemental said:

Don't know why you are bothering! I've got a CB in mine but no bugger ever talks to me!

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Have been told they go up 2 channels and talk about ya.

Mustbemental said:

Don't know why you are bothering! I've got a CB in mine but no bugger ever talks to me!

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Yes, things have changed since the 80s when I last played with CB. Then every kid had one in his bedroom. This afternoon all I could find were old men chatting about trucks on one channel and some guy going on about his son's three girlfriends on another. Long term I hope to do some gentle lanes and stuff so that's why I was looking to put the CB in.

Mustbemental said:

Don't know why you are bothering! I've got a CB in mine but no bugger ever talks to me!

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That's 'cus no one wants to go up the back channel like you.

Bastids the lot of ya!

Have you tried lengthening or shortening the twig.. Can't remember which way, but it should bring it down..

My understanding is that twig adjustments only come into play at an SWR of less than about 2.5; anything above that is fault that cannot be cured by changing the aerial length.

McBad said:

My understanding is that twig adjustments only come into play at an SWR of less than about 2.5; anything above that is fault that cannot be cured by changing the aerial length.

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I had this problem on tractors quite often, a mix of antenna length and ground plane area, it also used to happen if signal was bouncing off the cab,

I will try and find some more info on it as I have a friend who is a radio engineer, he has sorted these problems for me in the past

neiltce said:

I had this problem on tractors quite often, a mix of antenna length and ground plane area, it also used to happen if signal was bouncing off the cab,

I will try and find some more info on it as I have a friend who is a radio engineer, he has sorted these problems for me in the past

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Errmm I'm a licensed amateur radio operator. the problem is in the CB or the swr meter, not the antenna setup. If he's described the setup/problem correctly. then his SWR should be way below 1:2..

redhand said:

Errmm I'm a licensed amateur radio operator. the problem is in the CB or the swr meter, not the antenna setup. If he's described the setup/problem correctly. then his SWR should be way below 1:2..

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Lol so is he , and radio engineer , but he has sorted mine out when the needle was knocking on the wall ..