[BC] RE: Slant wire feeds

[DANA PUOPOLO]

Yep.

Monopoles also need lightning protection at the feed point, as a strike can
induce a lot of voltage into the outrigger wires as it runs down the tower.

-D



------ Original Message ------
Received: Fri, 20 Jan 2006 06:36:16 PM PST
From: "Phil Alexander" <dynotherm at earthlink.net>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Subject: Re: [BC] RE: Slant wire feeds

On 20 Jan 2006 at 10:52, Milton R. Holladay Jr. wrote:

> Wouldn't that vary with the electrical height of the tower ?
> M
> ----- Original Message ----- 
> From: "Lewis Munn" <looey323 at yahoo.com>
> To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
> Sent: Friday, January 20, 2006 10:49 AM
> Subject: [BC] RE: Slant wire feeds
> 
> 
> >   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.

Installation requires a bit more attention, but correctly done with heavy
wide copper strap it lasts as long as a series fed tower.

As for current, consider the loop from Tx/feedpoint to tower to ground 
to Tx as the primary of an autotransformer, thus critical from a loss 
standpoint. Unless that loop is very low loss, efficiency (in the FCC 
sense) is lost. What is critical is the strap between base and feed 
point/Tx. The ground radials can be the normal 120 at least a quarter 
wave long. There is no more current in them than in a series fed tower, 
but that strap to the tower base cannot be over-emphasized. That carries 
the heavy loop current. There is a strong induction field near the base
above the ground strap. Under the new standards it may require fencing.
Beware!

The same is true of the slant wire. A LARGE conductor is needed. I like
to see at least #4 bare, stranded copper. The attach point on the tower
is also important. The lug on the end of the top of the slant line needs
to be bolted with a heavy galvanized steel bolt and drawn very tight.
If the tower has round legs, one of those heavy clamps that looks like
water pipe ground clamp on steroids, IOW properly sized for the tower 
leg, would be a good idea.

The slant wire need not be the traditional 45 deg. I've been forced into 
using angles as high as 70 deg with good results. Setting the attach
point to the tower in these cases can be a little tricky. Plan at least
half a day of down time with a climber on the steel.

The best match seems to be a little below 50 ohms, perhaps 35 or 40,
however 50 is a good compromise, and a good installation will have a 
very flat "curve" compared with the same tower series fed - AND the 
"ATU" is a variable vac. cap. with a torroid for current if an old
installation is reworked today.

BTW, forget the myth about lightning protection. A grounded shunt fed
tower MUST HAVE a spark gap at the slant wire feed point. Lightning 
will come straight down the slant wire. In a good installation you may 
see more zaps across the spark gap than on a similar series fed tower. 
A shunt/slant wire is sympathetic to cloud to ground strikes within 
several miles. Ignore that at your peril. However, isocouplers and 
lighting chokes are so cheap and easy - none there. <g>

If you find one of these things, chances are it will have the thickest
ground strap you have ever seen, and it is a 50/50 chance the tower
will be an old Windcharger. I have never seen one that was put in 
after the mid-50's.


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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