Topband: T-loaded vertical

[w5zn at w5zn.org]

So, what impact will the yagi have on the T element???

73 Joel W5ZN



On 2020-02-19 12:57, donovanf at 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 at earthlink.net>
> To: donovanf at starpower.net, topband at 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 at starpower.net [mailto:donovanf at starpower.net]
> Sent: Wednesday, February 19, 2020 2:13 PM
> To: topband at 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 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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