[TowerTalk] HF Vertical on tower? RF Spill Over

[Tom Rauch]

>    In regard to radials being less effective than sleeves, it depends
>    which 
> way the sleeve points. 

The direction the sleeve points has no bearing at all on the
isolation impedance at the open end of the sleeve. It does have a 
bearing on the point where that isolation appears.  

>The open bazooka sleeve that points up and at the
> feedpoint is very effective.  However if the 1/4 wave sleeve open end
> points down as used in the 70 ohm Sleeve Vertical used way back in the
> 30's and frequently shown in the literature, it really swamps the mast
> with RF. 

Not at all. There can be no in-phase fields inside the sleeve, 
assuming it is more than several skin depths thick, no matter 
which way it points.

Actually pointing the sleeve down results in a more effective design 
in most systems, especially those with base-fed radiators above 
the sleeve.

When positioned open-end-down, the sleeve outside wall conducts 
in-phase currents with the antenna. This gives a collinear effect to 
the system, increasing gain along the horizon.

The choking impedance depends on the diameter of the internal 
conductor (the mast and feedline) and sleeve inside...and of course 
the sleeve's internal loss and electrical length.  A single sleeve can 
give quite high isolation, and that occurs no matter which way it 
points.    
 
> of high verticals don't work well.  Running 4 radials close to the mast
> does the same thing due to proxsimetry, capacity coupling and about 70
> ohms results at the feedpoint.  Horizontal radials give about 35 ohms. 
> Radials at about 30 degrees from the mast creates about 50 ohms and a fair
> amount of RF Spill Over. 

Actually RF excitation of the mast is maximum with horizontal 
radials (or radials that slope upwards). As the radials are sloped 
down at the outer ends, currents in the mast decrease. The ideal 
case is with an infinite number of radials sloping down parallel to 
the mast, which is....a sleeve!   

There was a CB antenna made that way.  I've
> duplicated all this with 2M models for talks at clubs and used neon lights
> as RF sniffers to show the levels of RF Spill Over.  I also used a small
> loop, a light bulb and a trimmer capacitor for a very sensitive sniffer. 
> Neon lamps have a threshold voltage before they ignite.

Measuring the induction field, especially with a device sensitive to 
the electric part of the induction field can be tricky. I certainly agree 
all less than perfect sleeves have noticeable mast excitation, and 
that includes three or four 1/4 wl radials at any angle.

The AEA isopole is an excellent example of how to avoid or greatly 
reduce this problem, and get gain in the process.  Someone knew 
what they were doing at AEA.
 
>    Now it occurred to me that the way to make 1/4 WL radials really
>    effective 
> and the Lo-Z they create at the connection point, is to attach them at a
> Hi-Z point for decoupling purposes. They will absorb a lot of RF there. 
> So 1/4 WL below the feedpoint where the RF Spill Over Z was at its highest
> voltage and impedance, I connected a radial.  PRESTO!  It cleaned the mast
> of virtually 100 % of the RF Spill Over.  The tip of this 1/4 WL radial
> was as hot as the radials above.  You could literally hear the RF sucking
> sound.  Would you believe the mast with the coax inside was stone cold
> with icicles forming on it in the summer.  The obvious conclusion is that
> the 2nd set of radials or just even with one of them 1/4 WL BELOW the Lo-Z
> feedpoint will effectively cool the mast.

That's another common way of solving the problem.
 
>    Now this is the principle behind the AEA Isopole.  It uses 2 1/4 WL
>    cone 
> sleeves below the feedpoint.  It could have been 2 sets of 3 or 4 radials.

But the radials wouldn't have been as effective.


73, Tom W8JI
w8ji@contesting.com

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