Hi Ed,
The affect of the T-vertical on a nearby HF Yagi is easily modeled in
EZNEC or your favorite antenna modelling program. You'll be surprised
by the results if the horizontal top is +/- 15% of resonance on 40 or 20
meters, for example. HF Yagi performance degradation occurs within ten
(yes, ten) wavelengths, but its especially severe within a few wavelengths
(wavelengths in terms of the HF Yagi, not 160 meter wavelengths).
There's no magic distance beyond which parasitic effects no longer occur.
If any conductor carries RF current, its magnitude and phase enters into
the determination of the radiation pattern of the antenna system, even if that
conductor is ten wavelengths away from the main radiator. Ask any
AM broadcast antenna engineer about this... (if you can still find one).
Maybe K3LR...
For many hams, installing a nearby second tower sets back their station
performance. Why? Because the antennas on the second tower degrade
the performance of antennas on the first tower and visa-versa. Exactly
the same situation occurs when a parasitic element -- in this case the
flat top of a T-vertical -- is installed within several wavelengths of HF Yagis
(wavelengths in terms of the HF Yagis, not 160 meter wavelengths).
We learn from our mistakes... I've made exactly the same mistakes with
T-verticals, multiple towers with HF Yagis for the same band, and HF Yagi
booms that are resonant within an HF band (e.g., when I installed a
72 foot boom 20 meter Yagi a few hundred feet from a 40 meter Yagi
on another tower).
For example, I have 20 meter Yagis on two towers spaced 300 feet apart.
The Yagis on the rear tower are badly degraded when pointing into the
the 20 meter Yagis 300 feet in front of it. They work perfectly well
when pointed at least +/- 30 degrees away from the 20M Yagis 300 feet
in front.
HF Yagi performance degradation is insignificant when an HF Yagi points
away from the horizontal top of a T-vertical. But when the Yagi is turned so
its the 3 dB b eamwidth intercepts the horizontal top of a T-vertical, all hell
breaks loose with:
- significantly degraded HF Yagi front-to-back ratio,
- additional sidelobes,
- splitting of the main beam of the HF Yagi into two or more lobes,
- reduced gain
The bottom line: If you care about the performance of your HF Yagis,
select the length of the horizontal section of a T-vertical so its not
within +/- 15% of resonance on any HF band you care about. 40 to
60 feet lengths do not interfere with any HF band other than 30 meters.
Or use an inverted-L rather than a T-vertical and you'll have no problem
at all, and -- from practical on the air performance -- an inverted-L is
imperceptibly worse than a T-vertical.
I no longer have any T-verticals...
73
Frank
W3LPL
----- Original Message -----
From: "Ed Sawyer" <sawyered@earthlink.net>
To: topband@contesting.com
Sent: Wednesday, February 19, 2020 6:13:02 PM
Subject: Re: Topband: T-loaded vertical
The answer to the top loading is that the top is essentially the same as if
it was an L on the antenna - just with the connection point moved to the
center. I would add maybe 5 ft on either side of the top to that equation
and see what it looks like when you install. And, like other verticals. If
it gives you a good SWR right off the bat, then your ground losses are too
high. It ideally wants to be around 20 Ohms. I can't get mine down past 30
Ohms because of the really poor ground in Vermont. I started with 24 and
kept adding 6 radials (1/4 wave) at a time until I saw no difference with
the feed point impedance adding the next 6. That had me end at 48 radials
per T on a 2 el phased array.
I was surprised by Frank's comments. I would get 50 - 75 feet away - but
"hundreds of feet away" would seem too far to have a parasitic element have
effect on a yagi. Especially a 10, 15, or 20M yagi. I guess it would be
easily modelled to see.
Ed N1UR
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