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Author Topic: elevated vertical-radials  (Read 1195 times)
KC8RPD
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Posts: 121




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« on: August 15, 2012, 01:08:20 PM »

I'm preparing to do an elevated installation for my Butternut vertical.  My question is simply this: how closely can the individual band radials be spaced before interference becomes a problem (I'm planning on 4 equally spaced radials per band.)
 
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KF6A
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« Reply #1 on: August 18, 2012, 01:18:04 AM »

I'm preparing to do an elevated installation for my Butternut vertical.  My question is simply this: how closely can the individual band radials be spaced before interference becomes a problem (I'm planning on 4 equally spaced radials per band.)
 
I don't think anyone can answer that with any reasonable amount of accuracy since each locations environment is unique. All you can really do is as you said, symmetrically space them, but you would probably do better by ground mounting it and putting down as much copper in the ground or on the ground that you can.
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WB6BYU
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« Reply #2 on: August 18, 2012, 08:01:36 AM »

I think interaction may be a better word than interference, as it
has nothing to do with QRM or QRN.

You'll see installations and commercial products that cut the radials from a
single length of multi-conductor cable.  That's pretty close spacing!  The
closer together the radials are, the more the resonant frequency may shift,
especially with the higher bands.  To some extent, you can make up for
this when you tune the antenna.  Or you can connect a pair of radial sets
together as a dipole and check the resonant frequency:  generally the
lengths need to be adjusted somewhat then they are close, but this
depends on a number of factors.

So you can get by with very close spacing, though the radial lengths might
need adjustment to maintain resonance.  If you can keep the end of the shorter
ones spaced 6" or more from the longer ones, there will be less tuning interaction.
Wider is better, closer may still work.
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N8BOA
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« Reply #3 on: August 21, 2012, 07:50:50 AM »

I used traps 2 groups of three for a total of six each group Serbs three bands via the traps
Look up coax traps
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KC8RPD
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Posts: 121




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« Reply #4 on: August 22, 2012, 12:18:18 AM »

>>I think interaction may be a better word than interference, as it
    has nothing to do with QRM or QRN.

Point taken, it's the term I should have used.

Your comment re multiconductor cable jarred a memory.  I think you're right there, too.

Lastly, some advice for those with a ground mounted vertical:  be careful if you use a string trimmer around your antenna.  I found it'll remove the coax from the vertical with remarkable speed.  If you're not paying attention, you won't discover your carelessness until some time later.

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W5DXP
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« Reply #5 on: August 22, 2012, 04:49:25 AM »

>>I think interaction may be a better word than interference, as it has nothing to do with QRM or QRN.<<
Point taken, it's the term I should have used.

[Side Bar] In physics the term "interference" has a much broader meaning than QRM and QRN. Constructive interference is what allows a Yagi to have gain in one direction and destructive interference is what happens off the back of the Yagi. Interference is what happens to allow an antenna tuner to achieve a 50-ohm Z0-match. Interference is why a 1/2WL dipole has a 50-75 ohm feedpoint impedance. Interference is what allows one to see yellow from an RGB TV. Often, interference is deliberately designed into a system.

In a transmission line with reflections, the reflected wave interferes with the forward wave to create the standing wave pattern - but the two waves interact only at an impedance discontinuity.

Destructive interference is what keeps opposing elevated radials from radiating much RF (their fields tend to cancel). Your closely spaced radials will certainly interfere with each other. But interference can be good, bad, or not worth worrying about. Destructive interference between opposing resonant elevated radials is a good thing because that destructive interference energy tends to get radiated by the vertical element.

The book, Optics, by Eugene Hecht has a very good chapter on interference (and superposition).
« Last Edit: August 22, 2012, 04:56:21 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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