Jerry,
Radials on the surface do not need to be 1/4 wavelength long since they
don't serve to resonate the structure like elevated radials do. They are
there to improve ground conductivity only. Radials between 1/8 and 1/4
wavelength are considered optimum. So on 40m, even your 10 foot radials are
not far from that range and would be useful. Your 60 foot ones may even be
longer than they need to be. After 1/4 wavelength, you are beyond the point
of diminishing returns. However, there is no harm in any of the radial
lengths you propose.
There may be some directivity but I bet it would be very hard for you to
detect it.
As for the number of radials, there is a rule of thumb that says you
reach a point of diminishing returns when the distance between the tips of
two adjacent radials is less than .02 wavelengths or so (assuming they or
equal length). Any closer than that does little additional good. So that
implies that you will actually have fewer 10 foot radials per degree than
you have 60 foot radials per degree. It seems odd, but it's all about the
density of wire over the surface area. At only 10 feet from the base of the
antenna, you don't need many wires to make up the required density. So for
40 meters, plan for 0.02 x 40m = 0.8m or approx 2 1/2 feet between the tips
of the radials regardless of length. (For grossly unequal length radials,
this means 2 1/2 feet between the tip of the shorter radial and the closest
point on the adjacent radial.)
The other important point is that the current density in the ground has
the same distribution as the vertical, so your 10 foot radials are at least
in the right place to reduce some losses.
If you have no ability to lengthen the shorter radials, they will be less
useful on 80m and 160m, of course. But like everything else in ham radio,
we do what we can do within the limitations and make the best of it. I, for
one, would consider building an 80m or a 160m vertical for the radial field
you propose.
I am inclined to think that at 40m you are on the high frequency end of
the range where a vertical can outperform a horizontal antenna. Verticals
are good to use when you can't get a horizontal antenna high enough. But on
40m, you might be able to get an inverted vee high enough to outperform a
40m vertical. Its just a thought.
Regards,
Dudley - WA1X
Date: Mon, 29 Nov 2004 17:50:03 -0500
From: "Jerry Keller" <k3bz@arrl.net>
Subject: [TowerTalk] Uneven Radials
To: "(Reflector) TowerTalk" <towertalk@contesting.com>
Message-ID: <012e01c4d665$c6cc5f60$6400a8c0@homebrew1>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
Gentlemen:
For a 40M 1/4 wave vertical at one end of my back yard, I have room for
radials on the surface to run out 60' to the west, 30' to the east, 25' to
the north, and only10' to the south. I'm in eastern PA, and the earth here
has pretty good conductivity.
(1) Should I expect any directional consequences? I would think so, but in
what directions?
(2) I had planned on 60 radials evenly spaced. Would it help to have extra
radials in the directions with the shortest runs?
(3) How would the answers be different for 80M? 160M?
73, Jerry K3BZ
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