Rick,
Here's a problem quite close to what you asked. The outer (black) curve
is my 30M dipole running from a taller tree to a shorter tree, so it
slopes down. Note that the nulls are a bit different because of the
slope. The inner blue curve is the same dipole at 33 ft, where it used
to be. Both plots are for 10 degrees elevation. Note that while the
nulls of the lower antenna are less pronounced, the higher antenna beats
it by 3 dB in the top null and nearly 8 dB in the lower null. And in the
main lobes, the higher antenna beats the lower one by about 7 dB. These
curves are representative of what happens from 1 degree elevation to at
least 15 degrees elevation.
In Chicago, 10 years ago, I did exactly what you suggested -- I put a
"dog-leg" in my 80/40 dipole with the intent of filling in the nulls.
The antenna ran east-west at about 40 ft on one end and 30 ft on the
other, so I needed to fill those nulls to work west. :) I think it
might have worked a little bit.
But I also did exactly the other thing you described -- I fed that
antenna with 75 ohm twinlead, tied the two sides of the feedline
together, and fed it as a long wire against a counterpoise formed by
connecting to both ends of a big wrought iron fence that ran around the
front of my lot. That top-loaded vertical worked better on 80 than
loaded as a dipole, and it was my only 160M antenna. This was a great
example of not letting the perfect be the enemy of the good. It wasn't a
great antenna, but I had a lot of fun with it.
73, Jim K9YC
On Sun,11/23/2014 9:52 PM, Rick - DJ0IP / NJ0IP wrote:
GM Jim,
YES, strongly agree on the nulls.
I have had several QTHs here in DL and I have always oriented my 80 and 40m
dipole such that they theoretically should favor stateside. That enabled me
to work the most 3-pts stations in CQWW.
I have always had an ongoing 80m problem where in contests I would copy
Asiatic Russian stations (UA9/UA0) about 579, calling CQ, and I would call
and call but not be able to work them.
I have read and believed that a low dipole does not really have such deep
nulls and tends to radiate more omni-directional. As I said earlier, 40 ft.
was always my max height (33 ft. is the legal limit without jumping through
hoops for a building permit). You can push it a little and get away with
it. Eventually someone will call the authorities.
When I was using openwire fed dipoles, the solution was easy: tie both
sides of the openwire together, match it with a matchbox as a "T" antenna,
and use whatever was available (i.e. the beam' coax) as the counterpoise.
Sounds pathetic, but guess what? I could easily work the UA9. As soon as I
worked just one, I reconfigured it to normal.
With a coax fed dipole with balun at the feedpoint, this plan-B failed to
work. Just couldn't work the UA9, even when configured as a T. Of course
the loss in the coax was the problem here.
I even tried the "famous Carolina Windom", hung on an expensive fiberglass
pole, but to no avail. It was just as useless as my standard dipole for
working UA9. Their "trick with the radiating feedline" DOES NOT WORK.
PERIOD. I won't say it doesn't radiate; it just doesn't radiate enough to
make any real difference.
SURPRISE: at this current QTH in a valley on a mountain side, I had no
choice of direction when stringing the 80m dipole. It runs east/west. I
can work stateside and UA9 with it, no problem. Hmmmmm... 35 years of
following the book and getting it wrong.
Apparently there are deeper nulls on a low dipole than what the textbooks
would have us believe.
As you said, the higher the dipole, the less radiation off the ends.
>From modeling we have lots of data on dipole and on we have on-the-air
experience as well.
So how do we put it together and quantify this?
How much difference does height make on the nulls?
How about bending one of the legs so that the dipole is no longer in a
straight line?
How much do we lose in the main direction and how much do we pick up in
another?
Even if a dozen people model it and tell me, I won't fully believe it until
I have built and tried it.
Seems the only real solution is to move out into the country where we have
plenty of space and can erect two antennas.
Jim, you wanna sell me a couple of trees? ;-)
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
In my experience, what matters is where the null is, and it matters more for
high antennas than for low ones.
73, Jim K9YC
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