Would this be a good CB?

[Simon Baxter]

CB's run at 27mhz, or there about.
If you were to imagine a good old sine wave on a graph, then one complete cycle, or wave would be 11metres long (or there abouts)
Hence the term wavelength
That would a full wave.
When you see antennas and they are described as 1/2 wave, 5/8s wave, 1/4 wave etc, thats, 1/2, 5/8's or a 1/4 of the wavelength 11m.
Most mobile antenna are 1/4 wave meaning the amount of wire stuck out from the groundplane, where it becomes unshielded would be in the region of 2.75m long.
Hope that makes sense, it's a little more complex than that and I've rounded the figures a little.
If you m atch the size of the antenna to the wave length then it works better.
Obviously as frequency increases the wave length decreases, so the higher the frequency the shorter the antenna (thats why a car radio antenna is shorter than a CB antenna.
This is also where SWR comes in and why SWR changes when you change channel.
You can only truly set the SWR at one frequency, or channel as, as soon as you move away you have changed the wave length.
If seting SWR then always do it in the centre of the band, so channel 20.5 would be ideal if you have a 5kc dropper from channel 21 or a clarify knob as found on rigs with side band.
As for the length of the CoAx, I've never heard that one but I should imagine if you could be arsed to do the maths is proably something to do with the same.

[Simon Baxter]

Just re -read that and I missed a bit.
When you are setting the SWR all you are doing is fine tuning the length of the antenna to the frequency that you are using.
Yes, there are other factorys that affect SWR, reflections from objects close by for example that's why it's important to not mount antennas on bumpers and the like as it's not idea.

[Simon Baxter]

ideal!
grr.

[jed the spread]

Dork!!!!

Jed

[..lee..]

dork and spammer.

[lloydy]

Have just purchased this one
http://www.4x4cb.com/public/item.cfm?itemID=1492
Couldn't help myself and got a dayglo orange springer, just need to get my rims done to match

[silverbullet]

Then you can immediately modify the handset like this: viewtopic.php?f=39&t=78543

[PC52]

Pic on the small side but we haven't had any probs fitting the aerial on the corner



Have spoken with people 10 miles away, after swr on ch 25

[lloydy]

Pic on the small side but we haven't had any probs fitting the aerial on the corner



Have spoken with people 10 miles away, after swr on ch 25

Good to see someone else has tried it, have you run the cable underneath or through the interior?

[Simon Baxter]

For clive.
http://www.zen22142.zen.co.uk/Theory/antenna.htm
He loves sh!t like that he does...

I can just imagine him and jenny fashioning a home brew Dipole on the kitchen table.
Jamtenna, now theres an idea...

For the rest of us, the simple stuff...
http://www.transmissionzero.co.uk/radio ... o-antenna/

Like this guy said, I've always chopped my coax as short as possible so least amount of loss as possible.

I do have to admit to working out the frequency and wavelength of channel 20 on the CB then constructing a inverted V antenna in the attic when I was into side banding.. things you do when your a kid.

[syncropaddy]

Interesting, nerdy stuff there but a good read all the same. I subscribe to the theory that you buy a reputable product from the manufacturer and you plug it in and it works ..... cuts out a lot of faffing about for little real gain

[Simon Baxter]

Dork!!!!

Jed

No point spending a load of money and just wasting your investment on a poorly thought out antenna placement.

Slap bang in the middle of the roof is far and away the best place but not practical.

As high as possible and as far away as possible from parts of bodywork.

Always mounted to metal, although you could use a Westy roof, you would have to add some metal below the roof to act as a ground plane.

Centre of the vehicle will give good omnidirectionality (I just made that up), antennas fitted to a corner of a vehicle will make it more directional in the direction of the most groundplane.
So, antenna fitter to right rear will be directional toward the front left.
Not bad if it's just occasional use in convoy but all you people who try to keep in touch with other groups spread out over a large area will struggle.

There are inexpensive, illegal ways to cheap though...

[Simon Baxter]

I'm off for a dislexia test me, I'm such a mong.

Ways to cheat! not cheap.

[lloydy]

The midland cb has sold out so the guy recommended this one http://www.4x4cb.com/public/item.cfm?itemID=1720 has some good right ups on the web.
Some great posts on this thread, can't believe it can get so technical, i thought it would be much more plug and play

[PC52]

Good to see someone else has tried it, have you run the cable underneath or through the interior?

Cable is routed through the roof lining and down the a pillar.

Reception always been good and we ain't speaking to the man on mars so it does the job.