CB radio - SWR problems
 

I spent most of yesterday evening trying to set up a neighbour's CB radio equipment and more importanly, his antenna.

He's got a Ham International Jumbo, fully expanded with an EpRom chip (super-lo, lo, mid, hi, super-hi plus UK40) and the arial is a Solarcon-99 with groundplane kit. He's using exactly 21 feet of coax.

In theory that should be an ideal set-up but for the life of me I cannot get his SWR below 2.3 - I've tried adjusting the tuning rings at the base of the antenna to no avail and I've plugged the SWR meter in to the arial end of the coax to ensure the problem isn't there. He's using high quality coax, so I'm mystified as to where the problem is.

This probably doesn't make much sense to most people but I'm sure I've seen a few threads here in the past about CB radio so.....'elp!

Re: CB radio - SWR problems
 

Check the connections on the coax - if they are not making good contact the SWR can go though the roof.
If you have any spare, try some more coax as well (low loss is better if you have it).

Re: CB radio - SWR problems
 

The PL plugs are all firmly soldered and the coax is low-loss.

The thing which is confusing me is if it was due to a bad connection then the chances are the SWR would be through the roof, but when it's as close as 2:3, it's got to be something daft.

Re: CB radio - SWR problems
 

Try another ariel if you can - that will at least eliminate the coax, connections, CB etc.

Try moving the ariel - position can make a difference. Is the mounting well grounded. Can the length be altered as well as the tuning rings.

At the end of the day it will work at 2.3 (not brilliantly I admit) and shouldn't damage anything - but it would be better to get below 2.0. Other than this I cannot say - it's a good 10+ years since my CB radios saw the light of day.

Re: CB radio - SWR problems
 

He's only got the one ariel and seeing as he paid £70+ for it plus the groundplane kit I don't think he fancies getting another!

The ariel is on a scaffolding pole which clears the top of his roof by what looks like 6 feet, there's nothing in the immediate area which could cause any direct interferance.

Re: CB radio - SWR problems
 

A couple of suggestions:

1) make sure the aerial isn't "grounding" on the scaffolding pole, ie the connections are properly insulated.

2) If you can, try the aerial on another rig to see what the SWR is on that. It could be that the aerial itself is duff, if so, take it back.

3) You could always ask the shop where he got it to check the aerial out too.

Re: CB radio - SWR problems
 

Had it been a grounding problem or an issue with the arial then I'm sure the SWR wouldn't be as low as 2:3.

Re: CB radio - SWR problems
 

If you haven't got a dummy load and he doesnt obviously want to spend money on another aerial, perhaps you could try having a length of coax with PL259 plug on one end and pigtail the other end of the coax and join an 8 foot 6 inch piece of wire to each connection. This will make a half wave dipole and will at least let you know if the CB itself is OK, it is a slight possibility that the output transistor, filter/matching circuit or even changover relay is at fault. I had a look at one CB for a local guy who couldn't get the SWR down and it was because the power supply was faulty and had lots of ripple on the supply.

Also beware of cheap SWR meters, the diodes in all SWR meters can go leaky with age, but it's suprising how many of the cheap ones go faulty and will not even read a good match into a 50 Ohm load.

I would personally knock up a dipole as above, and make it a bit longer and trim each leg until the SWR comes down. If this works OK at least you know the set is OK, and the length of coax and the plug at the CB end are OK as well.

In the days before I could afford a dummy load for my radio gear I used to use a standard 60Watt bulb with a coax soldered diredctly to it, the resistance is around 50 Ohms when cold but quickly alters and causes a bad SWR as it warms up! :D

Re: CB radio - SWR problems
 

I'll try the dipole suggestion, although I though each half of coax would need to be 7 feet long, not 8 and a half as you've said?

Re: CB radio - SWR problems
 

No 7 is too short!

I knwo a dipole for 28MHz is about 16 foot 7 inches, it can be worked out from the formula 300/f

f in your case is 27MHz, you must then take away the correction factor for the difference between the velocity factor of the wire and free space. It usually works out about 95%

To work it out in good old feet and inches use 495/f , my 28.5 Mhz example is 17.403 feet, multiply by 0.95 for the velocity factor and you get 16.533 feet, divide by 2 to get 8.266 feet per side. Your dipole being 27MHz will be slightly longer, and also remember the length is affected by the wavelength it's located above ground, the figures are for free space calculations above ground.

PS: The 300 in the equation comes from the speed of light 3x10-8 meters per second in free space, the 95% is cos its travelling in a wire.

Re: CB radio - SWR problems
 

Aha, perhaps it's the length of his coax then, I was always told it should be multiples of 7 feet - we'll try 25 and a half feet instead then.

Re: CB radio - SWR problems
 

I'm not really sure where that myth started about the length of coax. If the aearial and feed system is matched to the transmitter it shouldn't make any difference, however if there is a mismatch you will get standing waves on the coax with high voltages/current at odd wavelength multiples on the coax. I think it really only comes into play when a system is not correctly matched, also in the ideal world you wouldn't connect a dipole direct to coax because the coax is 50 Ohm and the dipole has a nominal impedence of around 72-75 Ohm. (covered it cos I can't remember) also the dipole is a balanced device and the coax is unbalanced, so in the ideal world we would fit a unbal-balance transformer at the aerial feed point, failure to fit this also affects the radiation pattern of the aerial.

I think just connecting your 8 and a half foot per side or whatever to a piece of coax should prove the point, just ignore me being bl**dy minded again. :D

Re: CB radio - SWR problems
 

As far as I know, the coax length makes no difference - I certainly never measured it in all my installations.

Re: CB radio - SWR problems
 

With an angled groundplane, the coax should stay within the groundplane, or it will pick up RF and disturb the characteristics.

It can also help, to add a coil of a few turns at the the minimum acceptable bend radius of the coax, as this chokes any RF on the outer that can cause SWR meters to mis-read.

One thing though, if the antenna would be a perfect match with a good ground or a flat groundplane, an angled groundplane will raise the impedance.

Re: CB radio - SWR problems
 

I've suggested all this to him today and he's considering going back to a traditional Thunderpole :eek: :D