Topband: T-loaded vertical

[donovanf at starpower.net]

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 at earthlink.net> 
To: topband at 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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