[BC] BC] Shunt fed antenna options /was/ Slant wire feeds vs series fed

[Phil Alexander]

On 30 Jan 2006 at 23:56, Scott Bailey wrote:

> Phil,
>     With all the fighting these days between local zoning officials 
> and AM broadcasters over towers, it comes to mind that what one 
> should consider if in a situation where the station is a non-d, say 1 
> KW, is to approach the owners of an existing cell tower, (of a 
> correct height for the AM frequency) and do the slant wire deal with 
> them on a lease.
>     I read where Clearance Beverage, did this for a station in 
> 1995.  The station had a slant wire, feed into a 300' Communications 
> tower. From what I understand, it worked well.
>     I have a 150', Pirod, Self Supporting, Grounded tower, on 1560. 
> I've been approached by different groups wanting to lease tower 
> space, but I haven't moved on it yet, due to I was looking into a way 
> to maintain the coverage I have now.

Generally speaking, the problem with cell towers is the lack space for
a ground radial system. No AM band radiation system works well without
a reasonably extensive ground system.

Recent work suggests that far less that the normal 120 radials can be
used, but there is a significant penalty in field intensity. 

In your case, you have a ground system so that would not be a problem.
If you feed the tower with either a slant wire or a skirt, that will
permit running tenant lines up the tower without the typical problem
of crossing a base insulator.

I'd feel better if your tower was about 25 ft taller IOW 175 ft. because 
100 deg seems to make a very nice shunt fed height. However, the 85.6 deg 
of the tower you have should be OK and will be improved because it is self 
supporting which usually increases the tower's inherent impedance somewhat.

If you go the skirt route, you will pay significantly more, but the
kit will be pre-engineered for your application. Slant wires are
cheap in terms of material, but you will need to find someone who
can correctly set one up without losing coverage. 

IMHO that is a trade-off that is as much preference as science. I happen
to prefer the slant wire approach because it can give a very flat load
and the ATU can be a vacuum variable in a box at the bottom end of the
slant wire, but you will find others who prefer the skirt approach
because the impedance is more easily manipulated. There are differing
schools of thought about that, because power that you can't measure
can be wasted and coverage lost as a result. However, if a slant wire
is not VERY well grounded, there will be excessive ground losses there
too. Almost anyone can put up a series fed tower and get good results
from it. This is not true of a skirt or slant wire fed shunt excited
tower. You really, really need someone who understands them to set it
up. 

Tower electrical continuity is always important, but for a slant wire,
continuity in the part of the tower below the feed point is doubly
important because it carries a high current loop. This is also true
of the ground lead between feed point (bottom end of slant wire) and
tower base for the same reason. Thus, I like galvanized towers and
4 inch ground straps. The slant wire itself needs to be heavy. #6
stranded wire is a minimum in my book. The larger the wire, the lower
the inductance. Conventionally, the feed point should be located about
the same distance from the base as the attach point is up the tower,
IOW a 45 deg slant. However, in unusual circumstances it is possible
to feed as close as 10 to 12 ft from the tower base with very good
results by making other compensations.

Both Nott Ltd. and LBA Group Inc. have good records of supplying skirt 
equipment for feeding and detuning towers and unipoles. A skirt has the 
advantage of having the feed point at the tower base, rather than
in a separate enclosure some distance away. 

In the event of design question about either a skirt or slant wire
feed system the answer can be found in careful modeling of the tower
and feeder. This is especially the case with the skirt which is a 
little more complex and critical in terms of spacing of wires.
For this reason, with a skirt, you need a good tower crew who will 
follow installation instructions exactly.

One of the differences between these two methods is the critical work 
on a skirt is mostly at the top, while with a slant wire it is at the 
bottom. 

Either way, you'll be converting an expense into an income generating
property which can't be all bad. :)


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





-- 
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.375 / Virus Database: 267.14.23/243 - Release Date: 1/27/06