Dear Geoff,
I think Rik is right about shunt-feeding the tower being extremely
inefficient. The further up the tower the shunt feed is connected, the
better it will be, but even if the connection is right at the top (i.e. a
folded monopole configuration), it will still be very inefficient. At 136k,
the shunt feed would nearly behave as a short circuit - a folded monopole
60m high could be modeled as a vertical monopole with perhaps of the order
of 1 kohm capacitive reactance, shunted by an inductive reactance of some
10s of ohms, due to the shorted stub formed by the shunt feed. This would
make the circulating current in the matching network of the order of several
tens of times larger than the "useful" antenna current, increasing the
losses in the matching network by something of the order of 1000 compared to
an insulated mast with matching components of the same Q. In your case, with
the feed point far below the top, things will be even worse.
I suspect that a wire insulated from the mast would be a better bet, even
if it can only be attached part of the way up - many people are getting
reasonable results with wire antennas 10 - 20m high, although 60m would be
nicer, of course! If the problem is simply one of accessibility, one idea
might be to treat the mast as a big tree, and get a line over or through it
using bow and arrows, catapults etc ... then again, if it is someone's
telecoms mast covered in microwave antennas, this might not go down too
well!
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Geoff
Sent: 16 July 2004 13:02
To: [email protected]
Subject: LF: Feeding grounded tower on 136KHz
Hello,
New to 136KHz.
Callsigns: ZC4CW or ZC4LOW (for experiments).
I need advice on using a grounded tower as a vertical radiator on 136KHz.
|
