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CB antenna radial myths and Legends!


Do radials on 5/8 wave antennas make a difference and can someone tell me why modern European made 5/8 wave antennas like the Sirio Tornado and the 827 etc all have 3 to 5 ft long radials in comparison to the USA models like the Interceptor 10K, Maco 5/8 wave and comtel's which have full size 9ft radials.

If 9ft radials are added to say a 5/8 wave silver rod or GP 5/8 wave will it perform better?

Also I wonder why Imax 2000's and Antron 99's use radials that are 45 degrees rather than horizontal.

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Re: CB antenna radial myths and Legends!


Hi Patrick.
I've always understood that to operate correctly, the radials on a 5/8 wave antenna should be 1/4 wave long. I suppose making them shorter saves metal and cost, but at some performance cost. Maybe many CB enthusiasts just see the 5/8 bit and buy an antenna on that alone without looking at the actual performance data, if published. (Which with the cheap stuff doesn't seem to happen - I wonder why.)

This could be why a basic 5/8 vertical without radials performs no better than its 1/2 wave relative? Addin radials to such an antenna may make it work better, but it may also cause a mismatch and resulting higher SWR, although I'm only guessing here.

As for drooping radials V horizontal ones, I've no idea, but the best antenna I ever had was a 5/8 with 3 45 degree 9 foot radials. Unfortunately destroyed by a direct lightning strike way back in the 80s - took the rig out as well as you may expect. emoticon

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Re: CB antenna radial myths and Legends!


A much better type of antenna that has more gain is the 1/2 wavelength vertical (Shown in figure 2). We know that the impedance of the 1/2 dipole is 70 Ohms when we attach the coax in the middle, but what if we were to attach our coax directly to the end? The impedance at this point is high, very high, so we must make a matching device to match the antennas impedance to the 50 Ohm coax. What would happen if we did not use this matching device? Well if you have been reading along, you would know that this would result in a very very high SWR.

There are several commercial 1/2 vertical antennas available, the two that I can think of most easily is the Solarcon A99 and the Shakespeare Big Stick. They provide slightly higher gain than the vertical dipole antenna.

The bandwidth of these antennas are good, they can easily span all the CB channels and more with a low SWR.

5/8 Wavelength Vertical

A higher gain antenna than the 1/2 vertical antenna is the 5/8 vertical antenna. As we can figure from the 5/8 wavelength rating the antenna is about 22 feet long (5/8 of 36 feet). This antenna is similar to the 1/2, it needs a matching device at the base to match it to the coax, it cannot be attached directly. This antenna has about 1.2 db gain over the dipole antenna and 1/2 vertical. Figure 2 shows both a 1/2 Wave vertical and a 5/8 Wave vertical antenna. It achieves this extra gain by concentrating its pattern out more at right angles from the antenna instead of wasting signal at high angles, see figure 3.

5/8 waves have better gain than a 1/2 wave and 45 degree 1/4 wave radials will bring the signal closer to the ground as opposed to sending it skyward (that is what is meant by DB gain). there is loads of info online. the above caption was taken from signal engineerings website.
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Re: CB antenna radial myths and Legends!


Tomorrow if the weather is okay, I will put the radials back on my Imax 2000, I have a bracket which allows me to mount them horizontal as apposed to 45 degrees, horizontal radials are easier to install at my QTH because of the shape of the roof.

As far as I gather the 45 degree angle helps to keep radiation of the coax on the imax but I have a choke balun anyway so that should do the same job. Any change of SWR or impedance is no issue with me as the antenna is fed through a tuner anyway.

I have seen some computer software evidence that would suggest that a 20 degree downward angle for the radials works best on the 0.64 wave imax and indeed there is a lower angle of radiation with the ground plain kit installed with little or no difference to local groundwave contacts, it also suggests with the GP kit installed the base of the antenna does not radiate as much which should reduce RF in the shack and reduce TVI.

I can't find much other info as regards to 45 degree versus 90 degree radials on a 0.64 wave antenna or indeed why the imax radials are 6ft rather than 8ft (maybe Solarcon tested it and the difference was too minimal?).

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Re: CB antenna radial myths and Legends!


A few years back i had a 5/8 on a 33 ft mast without radials. It performed really well.
I then bought some radials from a mate that were fitted to an original Sigma 2.
The difference was amazing. The received back ground noise was much lower and the transmitted signals were 1-2 S levels higher than before to repeat contacts around 25-30 miles away.

Last edited by 26FB1981, 23/May/2011, 6:17 pm
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Re: CB antenna radial myths and Legends!


I got the radials on and up in between the showers which were really heavy here on Sunday, I believe they do make a difference for the better, thing is because they are designed to go at 45 degrees and I have straitened them they are not entirely parallel like a cross, they are more like two Vs below the base, anyway not to worry, I gave everything a bit of a tighten up and a clean too while I was up and repainted my two K brackets as some rust had started. The no radial Sirio Gainmaster is starting to intrigue me now!

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Re: CB antenna radial myths and Legends!


Its all bumf Pat. get a quarter wave up.....lol
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Re: CB antenna radial myths and Legends!


Yes - a LOADED quarter wave, like the Thunderpole, remember them??

That loading coil really chucked out the radio signals!! (That's what some would have you believe.)

Incidentally, for the sake of something to type and totally off-topic, I have successfully fed my quarter wave vertical for 40m as a half wave on 20m.

They said it couldn't be done:
"You can't match to such a high impedance feedpoint."
"Too lossy."
"The capacitor will flash over."
"You'll cook the rabbits."

Well, it does, and with the correct tapping on the coil and a properly adjusted capacitor I get a true 1:1 SWR on the feedline. The lower part of the antenna now bites if I touch it whilst transmitting - most amusing!! emoticon

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Re: CB antenna radial myths and Legends!


 emoticon I had one of those quarter wave thunderpole type antennas, a Javelin Hy-Gain it was called and when I was on UK 27/81 it was ideal for the local sporadic summer skip to England. The Sirio starduster seems to be the only type about now that is similar.

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Re: CB antenna radial myths and Legends!


The reason why the radials on GP antennas slope downwards is because at the 45 degree angle its a better match to the normal 50 ohms coax, At the standard 90 degrees its a mismatch to normal 50 ohms coax.

Mark

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