[BC] RE: Slant wire feeds

[Phil Alexander]

On 20 Jan 2006 at 7:49, Lewis Munn wrote:

> All,
>    
>   The AM station in Glasgow, MT has a slant wire feed also.  
>   I have measured it, and it is not any flatter than a similar 
>   series fed tower, or a skirt fed unipole I saw in Hillsdale, 
>   Michigan years and years ago.

Then I suspect there is a problem somewhere. The ones I've seen
have had remarkably "flat" drive point "curves."

>   The major advantages I have seen are in lightning protection, 
>   and in getting transmission lines up the tower.

I've found this to be a myth. If anything, slant wire fed shunts 
are worse than series fed towers when it comes to attracting "zaps" 
at the drive point. They need a spark gap for safety. I've seen 
series caps blown to bits on these "grounded" (not to RF and 
lightning is RF) towers.

As for getting lines up and down the towers, yes, I do love the
simplicity. <g>

>   A disadvantage is that he actual RF currents at the real base 
>   of the tower end up to be quite high, so there needs to be 
>   extraordinarily good grounding of the tower base to a good 
>   ground system.

The ground radial system is not remarkable in any way. In fact
many of them are 120 90 deg radials of #10 solid copper wire.
This works fine as the ground radial currents are the same as
for any other radiator, series fed or skirted.

However, the best way of looking at a slant wire fed shunt is
seeing the wire, tower below attach point and ground strap
between tower base and drive point as an autotransformer primary
circuit. There is a large induction field inside that loop. The
strap is really important and it should be wide and well bonded.
That is one of the secrets to making these things work. Same
can be said for the slant wire itself. A large (=> #4) stranded
conductor works best.

>   And it is a good idea to check it physically a bit more 
>   often...not just put it in and forget it for 40 years.

Well, if you use 3" x 3/16" copper like in the old WE installs,
and copper weld everything (as was common back in the day) you
CAN forget it for 40 years and it will still be going strong.
However, if it was thrown in slip-shod, like some were, then
a roll of 3" or 4" strap will work wonders.
>    
>   I recently redid the base area grounding for a slant fed 
>   system, and appeared to recoup about a 5% field gain by 
>   putting in extra-heavy strapping over the somewhat corroded 
>   #6 wire that had been used to ground the tower base originally.

Yes, replacing and inductor with a low loss ground DOES help. <g>
>    
>   As an aside, a bunch of copper transmission lines, if properly 
>   bonded to the tower, makes the tower loss itself smaller and 
>   increases the effective field.  Did a DA tuneup in Fargo, ND, 
>   and both towers had 3-1/2" FM lines up them bonded every 20 feet.

You can accomplish something similar by welding one leg of a steel
tower at every joint from top to bottom. Flanged sections are prime
candidates for this. A couple of short (1" to 1-1/2" long beads) at 
the flange joints without significant heating of the steel (that is
structurally important) works wonders.
>    
>   The tuneup showed that rather than the 1 ohm loop loss of a 
>   galvanized tower permitted in the Rules, we had about 1/3 ohm 
>   loop loss, and the pattern was expanded to the very limits in 
>   every direction.

Back a long time ago they would assume a 2 ohm ground loss, and I
put a system in with a very good ground system (< 0.5 loop loss on 
a really difficult close spaced array with high mutuals and low base 
R's). They granted program test authority subject to inserting a 7.5% 
resistive dissipation in the CP. No good deed goes unpunished.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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