CB Antenna

Julian

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OK, first post in this section.
Which CB Antenna to buy and how do you use an SWR Meter?
Thanks for any input
 

Jon Wildsmith

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My favourites are the Sirio Hi Power's - http://www.thunderpole.co.uk/sirio_hi-power.htm

for the SWR meter, there's usually a switch to select SWR or PWR which you want in the SWR position and another switch to select FWD or REF which you want in the FWD position. Key the mike and turn the calibration knob on the SWR meter for full scale deflection of the needle, then switch to REF (Reflected) to see your SWR reading, un-key the mike. You test on channel 1 and then again on channel 40. If the SWR is higher on channel 40 the antenna needs to be shorter, if higher on channel 1 it needs to be longer. Adjust the antenna till it's the same on both channels if you want best SWR across all the channels or you can tune it to be better on some channels than others.
 

Crispin

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As for aerial, there are so many choices we could be here next week. It depends on what you want. Fixed, semi-fixed or mag-mount. The latter being the easiest option if you don't want it on all the time.

Fixed: No worrying about removing it. gets broken off by "lads" walking on the streets (Ask Paul)
Semi: You have a mount either mounted to the bull-bar / roof rack etc and clip in the whip part when you want to use it.
mag-mount: One very strong magnet and put it on when you want to use it.

The advantage of the mag-mount is you can move it. On a number of lanes with low branches, it sounds like an angry pianist on your roof. Gets comical with the sounds it creates. You can stick it on the bonnet which then saves you height.

Don't get a shorty aerial if you can. They have don't work as well as a 1/4 wave (which is 1.8m long). They do work, just at the extremities, the quarter wave whip will still work over most loaded ones.

If you can, mount the aerial in the middle of the "base". Base being, bonnet or roof. Mounting it to one side creates an uneven ground-plane (the roof) and your signal / SWR will suffer slightly. Again, when mine starts getting crackly, you'll be deaf. In a convoy, you won't notice a difference. When mounted off centre to the ground plane, your radiation lobe is distorted.
If you have a bad ground-plane, (no roof / bonnet etc) you'll struggle to get a good SWR, have a limited range as well as struggle to hear others from a distance.

SWR:
To test your SWR, mount the aerial in the desired place. Drive to an open space. By open, I mean, don't do it in your garage or parked next to your house. Move away from it.
* All (most) the aerials should have a grub screw in the base for adjusting the whip. Loosen it, find the mid point of the available play and tighten it slightly.
* Turn to channel 20 on the CB (mid point)
* Ensure the SWR meter is connected correctly (should be labeled)
* There should be a switch marked FWD / REF. Move to FWD
* Key your mike (transmit) and adjust the turning knob until the needle swings all the way over the the "set" mark on the dial. Should be on the right.
* Stop keying
* Change switch to REF
* Key mike again. needle should stay below 2. if it's in the red, stop keying! You'll break your CB.
* Adjust the aerial a mm or two up.
* Repeat above. if it gets better, keep going until it gets worse. if it gets worse, go the other way (shorten aerial)
Once done, tighten grub screw.

A bad SWR does 3 things: Poor reception, poor transmission and worst case, will pop your transmitter.

I'm sure someone else will come and add to it... :)
 

Steve Wright

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Julian said:
OK, first post in this section.
Which CB Antenna to buy and how do you use an SWR Meter?
Thanks for any input
Hi Julian

Very good description from Jon on the SWRing, but just a tip, make sure you have a good earth on the antenna

Steve
 
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Julian

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Thanks chaps, last one I got I just bought and put on, I think I might try a little harder with this one
 

TonyP

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The easiest thing for me was to find someone who new how to set up using a SWR meter.... ;) (thanks CP)

As for which aerial etc, well I used a local shop who sells CB radio's etc and although slightly more expensive than ebay, (about 10%) I think I ended up with a reasonable set-up for around £100
 

Jon Wildsmith

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Easy to explain in my head LC, not so easy out loud :? An over simplified explanation is that:

1) the ideal ground plane acts like a guide for radio waves. If you stick your antenna in the middle of a patch of dirt a good deal of the signal will be absorbed by the ground around the antenna because dirt is typicaly not a very good radio wave reflector / guide. A nice big sheet of steel on the other hand reflects / guides the radio wave energy that otherwise might have been absorbed so more of the signal gets into the clear.

2) objects close to the antenna affect its feed point impedance. You don't need to understand impedance in detail just know that the wrong impedance also affects the antenna's efficiency. A mobile antenna will have been designed to work sat in the middle of a decent ground plane, like a sheet of steel and have the right impedance when installed like that. If you don't put the antenna in a position with a good ground plane the impedance won't be what the designers intended and the antenna won't be as efficient as it should be.

A poor impedance match because of poor antenna positioning often shows up as the best SWR you can get still being quite high.

I tried :D
 

Chris

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Hi Jon. Great explanation, but I wasn't asking for me. It was sort of rhetorical? It was an adjunct to SWR reading being OK but CB still not performing. People sometimes mistake ground plane for being an 'electrical' earth - something to do with continuity, like a circuit.

Chris
 

Steve Wright

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Jon Wildsmith said:
Easy to explain in my head LC, not so easy out loud :? An over simplified explanation is that:

1) the ideal ground plane acts like a guide for radio waves. If you stick your antenna in the middle of a patch of dirt a good deal of the signal will be absorbed by the ground around the antenna because dirt is typicaly not a very good radio wave reflector / guide. A nice big sheet of steel on the other hand reflects / guides the radio wave energy that otherwise might have been absorbed so more of the signal gets into the clear.

2) objects close to the antenna affect its feed point impedance. You don't need to understand impedance in detail just know that the wrong impedance also affects the antenna's efficiency. A mobile antenna will have been designed to work sat in the middle of a decent ground plane, like a sheet of steel and have the right impedance when installed like that. If you don't put the antenna in a position with a good ground plane the impedance won't be what the designers intended and the antenna won't be as efficient as it should be.

A poor impedance match because of poor antenna positioning often shows up as the best SWR you can get still being quite high.

I tried :D
Wow you are getting good at this Jon, I could not described it any better.

But just to add its always better if you can to place the antenna right smack in the center of the roof of the car, not always possible but as high as possible.

Thats why the guys in big trucks do very well with TX and RX although it used to be always better up and down the motorway, but not too much to the sides of the truck.

Steve
 

Chris

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Now what I don't know is what is the effect of adding an amp to a 'not so good' set up. So if ground plane is poor will an amp help? I know that if SWR is poor ie reflected waves in the red, then an amp is not the answer. There is a lot going on in any installation. I know that some of the fiberglass cabbed lorries use a special aerial with an artificial GP built in.

C
 

Jon Wildsmith

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Obviously adding an amp to a CB is illegal ;) but provided the mismatch is not so great as to destroy the amp you will put out a bigger signal but not as big as it should be. If your SWR is really poor then most of the amps output will be as a cab heater :lol: It won't make you receive any better either unless it has a pre-amp (which is another can of worms!) and antenna efficiency affects both TX and RX. With a well set up / sited antenna you shouldn't need an amp anyway.

EDIT - forgot the artificial ground planes - AFAIK they change the antenna impedance to compensate for the lack of ideal ground plane so overcoming point 2 in my last post but some of the signal radiated is still absorbed by the close objects i.e. fibreglass roof that a real ground plane would have guided / reflected into the clear as in point 1 in my previous post.
 

Chris

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Illegal, oh yes, obviously. I would never do that. Especially if, say, I were in remote parts of err, let's say, Scotland in dense woodland. Hypothetically speaking.

Ahem.

Well, technically adding an amp isn't illegal as long as it doesn't boost output to over 4 watts -ok ok another ridiculous argument. Just that someone may have a 1 watt CB.


Mine has been working great for two years, then suddenly, when I key the mike, it sets off the alarm sounder. Huh? The rev counter drops to zero and my indicator lights for auxiliary equipment light up. What the ....

Re wired this weekend and all is well again.


Chris
 

Jon Wildsmith

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Battered and Blue said:
Well, technically adding an amp isn't illegal as long as it doesn't boost output to over 4 watts -ok ok another ridiculous argument. Just that someone may have a 1 watt CB.
Technically the equipment has to have been certified for use on the CB frequencies to be legal and you won't find a certified amp ;) What you could legaly do is get an amateur radio licence and be able to transmit up to 400 watts legaly on other frequencies :mrgreen:

Hypothetically speaking, judging by how useless OFCOM are at dealing with genuine nuisances, I doubt they'll have much time for processing complaints from some dense woodland in a remote part of Scotland :shock:

Your problem with the alarm and rev counter were probably caused by a bad connection on the antenna or its feed wire and all that reflected (poor SWR) energy has to go somewhere and some will escape into the cars wiring. Stray RF (radio frequency) can do funny things :D

Are we making a rhetorical FAQ here?
 

Chris

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Jon, I think you have nailed it there. I reconnected to a different power source and put a new antenna mount on the vehicle. I found that the actual stem was loose in the clamp bit on top of the mount. I couldn't loosen the grub screws at all, so I drilled them out and migged the aerial through the hole! OK caveman repair, but all seems well again. Like many techno bits, I don't pretend to understand CBs, I just know when they are working and what to do when they don't. But in terms of maximizing potential, that's a different matter.

We have the usual 40 UK channels and up there we never hear anyone on any of them. We thought about vhf/uhf or whatever, but actually sticking a 50watt amp on has been all that we needed. We can now cover the whole estate between us.

Chris
 

uHu

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Lot's of good info here. This turns out to be a very good thread. And:
That, Crispin, is the very best recipe I've seen, in any forum. You must be a radio-tech.

Just want to emphasize that the roof is always best.
Anywhere on the roof is almost always better than anywhere else.
Wing is second best, but far from "roof-quality"
Bumper, bull-bar, spare-wheel holder, roof rack, etc, is always more complicated. Good contact with the body of the car is required. If mounted on any other part than the body, you need wide copper braids from the antenna base to the nearest part of the body for good ground. (With a large roof rack you might get away without, without to much compromise)

Re GP & other types of antenna etc:
A radio signal has to have a reference, much like you can't connect a bulb to only one pole of a battery. For a GP antenna, the signal is in reference to the Ground Plane, which is not as easy as an electric ground, but a radio-signal-ground. I.e. the GP antenna has to have a large body of mass with good conductivity as a reference. Other antennas, like the bipole, has no need for a GP; half of the antenna (or bipole-element) works against/towards the other half. E.g you old TV antenna.
 

Julian

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A simple question would seem to have created a perfect FAQ about CB radios
Thanks for all the input, I think most things have been covered :shock:
 

BIG clean GREEN

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OK so under advice I bought a sirio hp 3000, just fitted it and I'm not happy with the SWR readings, following the advice from 4x4 CB where I bought it from, I cut 2" off the top of the antenna, (due to it not being used in central europe)
when I'm on Ch 20 the Fwd is on set in the red and the Ref = 1.7
when I'm on Ch 01 the Fwd is on set in the red and the Ref = 1.4
when I'm on Ch 40 the Fwd is on set in the red and the Ref = 2.2

The aerial is as low as it will go in the base, so do I either start from an higher aerial position or cut more length off the aerial ?? as it is at the moment I'm no better with the Sirio than I was with the springer !

Advice please chaps

Nick.
 

Jon Wildsmith

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Cut some more off Nick, probably another inch at least.

P.S. Comparing the SWR of the two antenna doesn't tell you what their radiation efficiency is like, you can have a 1.1:1 SWR and barely be radiating a signal ;) Your Sirio more than likely out performs the stinger by a large margin already.
 

Chas

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BIG clean GREEN said:
OK so under advice I bought a sirio hp 3000, just fitted it and I'm not happy with the SWR readings, following the advice from 4x4 CB where I bought it from, I cut 2" off the top of the antenna, (due to it not being used in central europe) Advice please chaps Nick.
I've bought myself a Sirio HP 3000 too, and was told to start by cutting 2" off, if that is necessary why do they make them that long :?:, why would Central Europe make any difference :?: , I haven't used it yet as, like Manuel from Faulty Towers :lol:, I know notheeng about CB's and my CB set is buried in the dash and awkward to get at to connect an SWR meter.
Chas
 
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