This is primarily for Fred, K1VR, but all are welcome.
The original shunt feed article by John True, W4OQ and later N4BA,
from Ham Radio, May 1975, is still very up to date. The omega match
will work fine, sometimes in reverse direction. Original shows a
series capacitor and a shunt capacitor across the coax feed.
Sometimes the shunt capacitor needs to be a smaller value on the other
side of the series cap. That is between the bottom of the gamma rod
and ground.
Gary Breed, K9AY, wrote an update on using two wires in parallel to
broaden the match bandwidth in an issue of the Low Band Monitor by
Lance Johnson Digital Graphics, K0CS is the editor.
A tower that is equal to or longer than an electrical quarter wave,
including top loading can also be gamma fed with only a series
capacitor if you are willing to experiment to find a spot to connect
the top end of the rod to the tower that will make the impedance at
the bottom of the gamma rod is about equal to 50 + XXXj
where XXXj is any inductive reactance that can be tuned out by a
series capacitor.
It is best measured by a General Radio bridge, but I am told Autek has
an instrument that will do the job.
Fred described a 91 foot tower with a couple of beams on top and some
side mounted beams as well. He was talking about a tall gamma feed
and either shunt or gamma matching. If a rod at nearly 90 feet is too
tall for 50 ohms, experiment with a jumper between the tower and gamma
rod to find the right point and then cut off the rod or make the
jumper permanent.
It means climbing to experiment, but may make things easier. The
spacing on the series capacitor should be sufficient for about 5KV.
It can be calculated by multiplying the antenna current - about 5.5
amps at 1500 watts into a 50 ohm load - times the capacitive reactance
times 2 for a 2:1 SWR times the value of sign wave peak to average
voltage. I can remember if this is 1.732 or 1.56 or something else.
Best to use a 7.5 or 10 KV vacuum variable unless you have a large
value of capacitor.
Don't forget the gamma rod has an inductance of almost one microhenry
per meter for average wire size and maybe 0.8 microhenry per meter for
hard line and typical conduit sizes.
Hope this helps.
73 George K8GG
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & KM9P
|