On top of that, based upon the modeling I've done over the years I've
never found that a vertical dipole has any appreciable advantage over a
1/4 wave ground plane. Certainly not enough to justify the
significantly greater hassle.
I also don't think that a "ground plane" necessarily needs a lot of real
estate for the radials. N6BT got great results just coiling up a single
radial in a circle around the base of the vertical some distance off the
ground (he called it a VOR), and when I modeled that approach it looked
virtually the same as more elevated radials.
In fact, I've long thought that maybe we could just consider whatever is
below the feedpoint of a quarter wave vertical to be a counterpoise and
nothing more ... essentially just a place for balancing currents to have
somewhere to go. To that end, I recently modeled (using EZNEC Pro2+ v7
and NEC5) a 160m Inverted-L with nothing but a 4 foot wire under the
feedpoint connected to the center of a 6ft by 6ft ground screen
positioned about a foot off the ground. I put an inductive load in
series with the 4 foot wire and adjusted it for resonance. The net was
about 18 ohms real with no reactance, and the pattern and gain was about
what you'd expect from an Inverted-L. It's not a radial system, it's a
tuned counterpoise with a series inductance and a capacity hat.
It's a pretty twitchy setup, though, with the bandwidth being a bit
narrow (if it was being fed by 18 ohm coax the 2:1 SWR bandwidth would
be about 40 KHz) and resonance being quite dependent upon the actual
distance above RF ground ... which is almost never where the surface of
the earth is. It certainly would need an L-Network to match the
feedline. But my point is that simply thinking in terms of a
counterpoise might open up some opportunities for alternatives to a lot
of wires. It's not like we're going to change the far field vertical
pattern no matter what we do at the base of the antenna.
In fact, I just checked Tom's website and his latest antenna, the
Park-5, looks suspiciously like it uses a similar technique ... although
I can't be certain of that. It appears to have hairpin coils for
matching against the three tripod legs that likely perform the same
capacity hat function as the ground screen I modeled. His manual even
mentions that proximity effects can be significant.
73,
Dave AB7E
On 1/12/2026 10:16 PM, Tom Hellem wrote:
My hat is off to all of you fine gentlemen who contributed to this
discussion.
You have helped me see the light.
I think the reasonable conclusion is that a center fed vertical dipole is
a very difficult thing to make work, especially if the feedline cannot be
brought away from the antenna horizontally for an appreciable distance.
I’m going to try base feeding it with an LC network and see if I can get
better results.
Many thanks to everyone who provided input.
Tom
K0SN
On Mon, Jan 12, 2026 at 6:57 PM Jim Brown <jim@audiosystemsgroup.com> wrote:
On 1/12/2026 5:23 PM, Jeff Blaine wrote:
Tom, I'm emphasizing what Jim mentioned earlier. If the antenna
resonant point moved by adding the choke, then that means the feedline
was participating to some extent as "part of the antenna."
Yes, but in Tom's configuration, even with the world's greatest choke,
the feedline can still be a parasitic element.
My belief is that without a feedline choke, you simply cannot guarantee
that the feedline will not participate. So every antenna here (19 of
them) has a feedline choke of some form.
Yes. And feedline chokes also reduce interstation interference, whether
SO2R or multi.
73, Jim K9YC
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