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Re: Topband: T-loaded vertical

To: topband@contesting.com
Subject: Re: Topband: T-loaded vertical
From: w5zn@w5zn.org
Date: Wed, 19 Feb 2020 16:37:56 -0700
List-post: <mailto:topband@contesting.com>
So, what impact will the yagi have on the T element???

73 Joel W5ZN



On 2020-02-19 12:57, donovanf@starpower.net wrote:
Hi Ed,


A T-top at right angles to a Yagi will have little affect, but anything
close to broadside is very bad within 500 feet or less on 40 meters.
Fortunately a 60 foot top is just short enough to have minimal affect.
70 feet would be much worse.


Any top longer than 40 feet should have little affect on a 20 meter Yagi.


Happy modelling!


73
Frank
W3LPL

----- Original Message -----

From: "Ed Sawyer" <sawyered@earthlink.net>
To: donovanf@starpower.net, topband@contesting.com
Sent: Wednesday, February 19, 2020 7:50:13 PM
Subject: RE: Topband: T-loaded vertical



Frank, You will have me grabbing my EZNEC tonight and playing around.
In my case my tops are around 60 ft long and they are thankfully
perpendicular to my 20M yagi and about 120 ft away. However if I beam
west on my 40M yagi. I am 100 ft away and parallel to the wire. I need
to take a look at that.

It gets me thinking about the reverse. Could it “enhance” a direction
if properly designed?

My back T is 250ft even further so almost 400 ft from either tower.
Same T design.

I have personally found that for Ts that are only 70 ft vertical, like
mine, the ground losses of the long topped L are not as desirable as
the cancelled out horizontal lobes of the T. If the T were say 90 ft
of vertical, I don’t think the T adds as much value.

Ed N1UR



From: donovanf@starpower.net [mailto:donovanf@starpower.net]
Sent: Wednesday, February 19, 2020 2:13 PM
To: topband@contesting.com
Subject: Re: Topband: T-loaded vertical


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 beamwidth 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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