TopBand: 160m 4-square

[Peter Nesbit VK3APN]

At 08:17 AM 6/02/97 GMT, Joe AA6WG wrote:
>   Retirement is getting near and a QTH with a 4 square for 160m is not so far 
>away. A few questions have come up regarding construction of the array and 
>keeping it up. I plan to use 200 feet of Rohn 25G, run 4 nylon ropes off the 
>top and suspend 4 1/4 vertical wires from the ropes.
(snip)
>   In light of recent comments regarding Fresnel Zones I am also wondering if 
>I should extend all the radials to the limits of the lot. It is a 40 acre lot 
>and the array will be in the center so the shortest radial could be approx 500 
>feet long and others may reach out to approx 700 feet. Would I be improving 
>the reflection properties in the Fresnel Zone with these extended radials? 
>   
>Joe AA6WG  
>

The answers are evident from theory.

Buried radials behave partly as resonant structures, and partly as
travelling wave structures. The less energy is reflected from the ends of
the radials back to the centre, the more they are operating in the
travelling wave mode. For long radials in good ground, the travelling wave
mode predominates, and resonance effects all but vanish. In such cases, the
length relative to a wavelength will have no special significance.

A quarter wave length is popular because if the ground conductivy falls, say
after a long dry spell, the resonant mode of the radials starts to dominate,
and they will continue to provide a good low impedance return for the
antenna current. An added benefit is that because the ends of the radials
are at a high impedance, they will couple fairly efficiently to the outlying
earth, increasing their effective length. In any event, buried radials are
not strongly resonant, because they do not have a clearly defined length
when the length and dielectric constant of the included ground path is taken
into account.

Consequently, I would definitely make the radials as long as possible, for
maximum low-angle radiation. Where they cross over, I would bond them
together and cut off the excess length, because to do otherwise would waste
wire to no benefit. This also avoids any problems caused by wires touching
each other at the crossover point.

Enjoy your new antenna. I've already commenced building an attenuator to
prevent receiver burn-out caused by your signals.

73s,
Peter VK3APN


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