Topband: Shunt Fed tower bandwidth replies summary.

[Herb Schoenbohm]

Ken,

I found over the years that an increase in the wire size is very beneficial in
the BW and also the overall efficiency department.Feeding a grounded tower can
present some problems if not engineered carefully.  The ratio between a Rohn 25
and a shunt feed of #12 wire can make it difficult to get a precise match..  My
solution was to construct the shunt out of discarded CATV cable (found in huge
piles behind the local Cable company after a storm waiting to be taken to the
land fill). Type 500 or 750, is 1/2 inch or 3/4 inch OD aluminum and  makes a
very excellent shunt feed for 160.  Another answer in increasing the bandwidth
and lowering the Q is to construct a skeleton cage with drop wires around the
tower.  It runs the distance of the tower and connects at the top to the
steel.  A tapped shunt wire selects the appropriate point to match the feed
line and a series cap removes the remaining reactance.  Cortana sells a skirt
kit for BC towers and recommends tapping up from the bottom rather than down
from the top, I guess because there is less climbing to do.

If you don't like to climb up and down to find the exact spot a T match will
also make the match easier.  Another solution is a the slantwire feed,
sometimes resulting from an insulated guy point near the ham shack.  Connecting
a single wire type tuner in the ham shack to the slant wire is a very
convenient way, perhaps the easiest way to get a grounded tower to radiate on
160. (This is quick way to get a self supporting or cell site tower to radiate
for a weekend contest at a location in the country at a communications
facility.)

Good Luck and thanks for the information.

Herb (KV4FZ)



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