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

Re: CB radio - SWR problems
 

Sorry to butt in guys, I just want to say that this is a fantastic thread!

I've barely understood more than about 3 words but it's great fun to read nonetheless :D

Re: CB radio - SWR problems
 

That's a big 10-4 good buddy!

So it's 10-10 'til we do it again!

:D

Re: CB radio - SWR problems
 

Would now be an appropriate time for me to post this?

Re: CB radio - SWR problems
 

No

Re: CB radio - SWR problems
 

In 10 years, I don't think I ever said 10-4. :dozey:

Re: CB radio - SWR problems
 

Roger...

Its woirth testing the SWR meter against another one, as you would hate yourself if it was just a faulty metre its hapened to me a few times..

Cor I didnt even realise CB's were still going am tempted to get mine out of the loft now.

Penguin signing off

Re: CB radio - SWR problems
 

Jeez guys, this takes me back a bit. :D
Will have to see if my stuff still works, used to be a dab hand at tracking down wave-hogs with my trusty old DV27 on my car.
Does anybody still use AM by the way?

we will have to have a roll call of handles

mine were Sid Snot and Big Sid and used to help run the BTB CB club 20 years ago.

Re: CB radio - SWR problems
 

Being a huge "Convoy fan", I have always fancied getting into CB radios. Seems a bit more to it than I first thought :erm:

Re: CB radio - SWR problems
 

Eee, this brings back memories.... old ones but good ones.

I still got some of my gear packed in the loft, are people still using the hi/mid/lo bands, I thought they had all changed to UHF now?

What's the chatter like thse days? I left it when it got to just stupid kids playing music all day, swearing etc. Couldn't get a decent conversation in at all. Oh & I got another license...lol

Ther's nowt wrong with a Thunderpole, damn good aerial.

Re: CB radio - SWR problems
 

Yes but ugly....

Re: CB radio - SWR problems
 

dont forget that the SWR will only be great at the middle of the freq range that the ant is designed for.

just my tuppenny's worth, but it might be worth checking at a different freq?

Re: CB radio - SWR problems
 

Yeah we tried it on super-lo and super-hi (2:8 on both) and the 2:3 is channel 20 on the mid-band.

Re: CB radio - SWR problems
 

First thing is where have you measeured the SWR?
Swcond thing is NEVER tune the rings until you know what the SWR is.

The best way to check the SWR is to connect (f Possible) the swr to the antenna directly or as short s patch lead as possible.

The reading at the radio ens will be different...its what is at the antenna that counts.
Is it possible to ground the radio in the house?

If there is and SWR on the cable then it will read differently on a different lenght...its a wave after all.
If all else fails...stich a tuner / matcher in line to flatline the swr to the radio / finals see a good match :)

Re: CB radio - SWR problems
 

We measured the SWR at the antenna end of the coax. We only touched the rings after we couldn't think of anything else.

We've thought of using a matcher but as he's hoping to use the set-up for DXing, he doesn't really want to lose power.

Re: CB radio - SWR problems
 

Dare I suggest removing the twig from the roof & trying it elsewhere temporarily.

Re: CB radio - SWR problems
 

Not sure if that'll be much help, the place he wants to put it is ideal, it clears the roof by 6 feet and has nothing near it to cause a problem.

Re: CB radio - SWR problems
 

Is that the vibrating version...?! :naughty:

Re: CB radio - SWR problems
 

BTW its an Aerial. Ariel is for washing machines ;)

Re: CB radio - SWR problems
 

Dunno, found it on a website that does all sorts of Rubber Ducky related merchandise.

Re: CB radio - SWR problems
 

It would remove it from the scaffolding & allow him to check it's SWR without anything interfiering. (except the ground & you who is holding it :D )

Re: CB radio - SWR problems
 

We'll be trying some new and extra length coax today, we'll see how it goes :)

Re: CB radio - SWR problems
 

In the antron 99's there is a resistor that is there to protect against lightning strikes. On a number of these antenna's it has been noted that dropping the antenna can cause this resister to fail (i.e. faulty stock cough cough).

Also all the sections screw together, but what is inside them is a length of copper wire which is then soldered into the coupling that the next section screws into. Sometimes not many when manufactured the copper interconnecting wire breaks when the coupling is being fitted causing no connection after that section (check with a multimeter).

(I used to sell them)

But basically with a swr of 2:3:1 after checking that the coax cable is 50 ohms and placing a 50 ohm dummy load at the antenna end instead of connecting the antenna and checking the SWR to make sure that it does not read the same.
If it does then its either the plugs incorrectly soldered/faulty cable (check again with multimeter)or possible faulty SWR meter.

If all is ok at the cable end then it has to do with the antenna.

Also check that the rings on the antron are somewhere just over the half way point. If at the bottom or right at the top then its been adjusted in the past.

One last point is that when SWRing in an antenna if the SWR is high on the highest band then on the lowest band (or frequency/channel) then the antenna is classed has electricially to long and needs to be shortened.

The opposite can be said if the SWR is higher on the lowest band/frequency/channel then on the highest band/frequency/channel then the antenna is to short and needs to lengthened.

Be carefull when adjusting the rings make a tape mark to pop them back it needed.

Re: CB radio - SWR problems
 

Wow, that's a post for the most helpful thread if ever I saw one.. lol

Re: CB radio - SWR problems
 

Ok peeps, thanks for all the help....we bought new coax and made it a little longer, SWR is now down to 1:3 in the middle and about 1:6 top and bottom ends, an nice smooth signal - excellent! :D

Re: CB radio - SWR problems
 

The Problem Could Be The Amount Of Coax I Was Always Led To Believe You Should Do It In 9 Foot Lengths Ie 9,18,27 Etc

Re: CB radio - SWR problems
 

Thanks - but I sorted this out almost 18 months ago ;)

Re: CB radio - SWR problems
 

Interesting first post, regardless I'm fairly sure that this issue is now a bit dead considering the thread is more than 2 years old? :)

Re: CB radio - SWR problems
 

haha, lmao. I thought 'do people still use these?' then saw the thread had been revived from the basement. lol. How is the SWR anyway?

Re: CB radio - SWR problems
 

Right Then How Do I Make A Dipole Out Of Coax

Re: CB radio - SWR problems
 

I know this thread has been dug up from the past, but can I just say that as long as a length of feedline(coax etc) is correctly terminated wiith the correct impedence at both ends, then the length of the feedling doesn't matter at all, never has and never will.

As to how to make a dipole out of co-ax, work out the length of dipole you need/require, half the mesurement and then strip that length of outer insulation from the coax, make a hole in the shielding and pull the inner conductor and it's insulating shield through outer shielding, this gives you the two dipole "legs".

Hang you dipole in your chosen spot with the feeder coming down at right angles at least a quarter wavelength.

Re: CB radio - SWR problems
 

hehehehe the ham international jumbo is a pig to set up for swr purposes. its difficult to se up with a low swr on all channels. i ahve had a number of rigs in the past includeing a certain cobra that had the full job done to it ie recieved 24to 30megs with all the in betweens as well . all modes. and was one of the famous rigs known as the gold. it was built by a famous biulder who lived in plaistow. east london anyway swr has to be a happy medium anything under 2.1 will do. as long as yu are not pushing too much power. on my cobra it pushed to 40 watts pep that is really the limit. jumbos in my view are too big and not as good as they pretend to be. also look out for the jumbo copys ie the uk models. (they are crap ) my old antenna was a silver rod 5/8 wave... and a BIG MAC I not been on air for many years. i currently hold a class 2 amateur radio licence. now own a yaesu xxxxxx hf/vhf /uhf. very old but very good... with a beam half the size of our house( was)

Re: CB radio - SWR problems
 

If you mean the old class B license, it doesn't exist anymore, it was merged with the class A when the morse test was dropped and is now known as an advanced class license.

It goes: Foundation(M3), Intermediate(2E0) and Advanced(currently M0)

Jon, M1CBH

Edit: I realise that FlashGorden will know the above already, just posted for the benefit of others

Re: CB radio - SWR problems
 

I AGREE WITH RUSS B by the way my call sign is 2E1GAD i have not used the call sign since 2000 shame really ! the last two years live in a block of flats with my wife. my ex has got most of my gear.

Re: CB radio - SWR problems
 

I've lived in a ground floor flat with no garden, no place for aerials etc, not good at all.

Re: CB radio - SWR problems
 

You must have been licensed just before me: M1DTJ

How do you do

All my gear is stuffed into a box under a load of shoes :)

Re: CB radio - SWR problems
 

July 1997

I had a couple of years of inactivity when I lived in a ground floor flat. As soon we moved in to this house I had a 2m/70cm co-linear put up on the side of the house.

I quite often get asked what i've got clamped to the clothes line post as well(HF vertical).

Do either of you(Flashgordon, Uncle Peter) plan on becoming active again?

Re: CB radio - SWR problems
 

I have not been active for yonks. I am G8PZI and I used to live right on the top of the Downs in Surrey but I moved to the bottom of the Downs and could only work locals on 2m. and 70 cms. so I gave up. GB3NS was the local repeater but as far as I am aware it has been relocated.
Ah, happy days having fun with the squeakys

Re: CB radio - SWR problems
 

I probably will actually once I've managed to sort out my licence reissue. Doubt I'll do anything other than mobile or portable though. I'm out on the road a fair bit and often contemplated putting a set in the car once I've managed to find a way through to the battery that is :)