[TowerTalk] HF Vertical on tower? RF Spill Over

[Tom Rauch]

Verticals on higher bands always work better up in the clear than 
they do on the ground in a cluttered environment. That isn't 
necessarily true on 160 and 80, where surrounding structures are 
short in terms of wavelength and ground losses are less.

I've had very good success with verticals above yagis, and intend to 
install a nice trap vertical on top of my rotating tower as soon as I 
get the other antennas mounted.

>    The main difference obtained by differences in height will depend on
>    how 
> much RF spill over on to the coax shield, mast & tower.  It increases the
> angle of radiation.  I've mounted 1/4 WL verticals on top of yagis using
> the DE as it's ground plane, 1/2 WL verticals with no RF spill over on
> towers and also over yagis.  They work great high off the ground when they
> don't have any RF spill over.  I'd add the donuts (lots of them) at the
> bottom of the and give it a try.  Buy the Palomar RF current meter and
> measure it before and after.  

Unless you insulate the base of the vertical (where the shield 
connects) from the tower, adding sleeves on the feedline won't do 
much. All that happens is the current shifts to the tower.

Some groundplanes use insulated mounting brackets with a choke 
balun, and the radials all float for RF. This prevents the feedline and 
supporting structure from having common mode currents. 

Another scheme surrounds the mounting structure and feedline 
with an open skirt. The end of the skirt has to have a reasonable 
amount larger than the structure inside or it generally won't choke 
well. 

> The higher they are the more the tower predominates and the less affect
> the antenna has.  One set of radials doesn't stop it all.  That's why the
> AEA Isopole has the least RF Spill Over.  The 2nd 1/4 WL Sleeve kills it. 
> The 2m Ringo was bad, they tried to clean it up and put the extra
> decoupling stubs in the wrong place. 

Stubs (radials) are always less effective than a 1/4 wl decoupling 
sleeve. I'm not so sure Cushcraft put the radials in the wrong spot, I 
am sure four radials are not enough to decouple a mast.


73, Tom W8JI
w8ji@contesting.com

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