[TowerTalk] Re: Slopers, half slopers, (using the tower)

[Guy Olinger, K2AV]

Towers are a problem for antennas that are not balanced with respect 
to the tower, e.g. have a balanced feedpoint at and perpendicular to 
the tower, or in the case of a yagi, have the boom attached to or 
directly over the tower.

If one END of a dipole is insulated but strung from a tower, there can 
be significant current induced in the tower. This also works to couple 
noise from coax shields and control cables coming up the tower to the 
dipole.

The amount of induced current varies significantly. In one model, 
without changing the dipole height or size, I increased the effective 
height of the supporting tower only 11 feet and changed the current in 
the tower from 1/10 of the current in the dipole to 2/3. The feedpoint 
Z of the dipole went from 78 j0 to 105 -j10.

This is a simple illustration of unwanted effects of a tower that 
isn't even connected. FEEDING the tower energy instead of inducing it 
only INCREASES the unpredictability.

Getting your "semi-L" arrangement to work involves all the same 
unpredictability as the sloper and the dipole example above.

Any antenna pulled unbalanced off a tower is highly unpredictable due 
to the tower, and needs some luck to hit a workable combination.

73, Guy.

----- Original Message ----- 
From: "doc" <kd4e at verizon.net>
To: <TowerTalk at contesting.com>
Sent: Wednesday, December 29, 2004 7:07 PM
Subject: Re: [TowerTalk] Re: Slopers, half slopers, (using the tower)


>>> That's why slopers worked against the tower should be called
>>> sloppers instead of slopers.
>>>
>>> They are sloppy systems that generally, if they work at all,
>>> work by good fortune rather than proper design.
>
> What about treating the antenna as semi-vertical "L"
> including the tower in the equation?
>
> In my case one leg can be a 55 foot Rohn 25 tower, at 55 feet
> I can directly/electrically attach a horizontal wire out another
> 80 feet to the feedpoint then another 135 feet to the far end
> insulated by a tie rope up in a tree.
>
> 100+ feet of the horizontal section will go across a shallow
> ravine thereby increasing the effective height at the
> feedpoint.
>
> Fed with open wire I am guessing this will be a decent
> NVIS doublet/dipole antenna on 160 and 80 and an interesting
> antenna for 40-10M.
>
>
> -- 
> Thanks! & 73, doc kd4e
> A blessed and joy-filled New Year to all!
>