I don't think this is all that odd. If you think of an elevated ground
plane (elevated high enough to minimize ground coupling effects) as a
vertical dipole with the bottom portion splayed out (which is pretty
much what it is functionally), then changing the ratio of vertical
length to radial length is essentially the same as feeding a dipole off
center to raise the feedpoint impedance. For the first three decades of
my ham radio career I was limited to vertical antennas, and I used that
effect all the time to get a resonant 50 ohm feed without additional
networks.
Adding a second set of radials to get the two band capability you see is
really nothing more than the equivalent of a fan dipole, and the only
thing I would caution there is that in my experience there is enough
interaction between elements of a fan dipole that it can be difficult to
simply model it and have it come out dead on. I always had to do some
significant tweaking of lengths and the direction of those tweaks
(lengthen or shorten) never seemed completely intuitive to me.
You can also get some interesting combinations by using more than one
vertical element with the verticals spread far enough apart to reduce
their coupling ... again, similar to a fan dipole.
73,
Dave AB7E
On 9/10/2014 10:53 AM, Bill Turner wrote:
Some more results modeling HF ground planes with EZNEC.
I took my existing 40 meter ground plane with a 33 foot vertical
element and elevated radials at six feet high, which had a resonant
impedance of about 30 ohms, and reduced the radials down to a mere
five feet. Now the antenna is resonant in the 60 meter band with an
impedance of about 97 ohms.
Presto - a two band antenna with SWR of less than 2:1 and no traps or
switching and just one simple vertical element. 2:1 is well within the
capability of my rig and at such low frequencies, the loss from SWR is
negligible. Azimuth and elevation patterns are essentially the same as
with full length radials.
In other words, just add four five foot radials and get a second band.
This is too easy! :-)
Looks OK on the screen but will reality be as good? There might be a
gotcha in there somewhere. We'll see.
73, Bill W6WRT
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