----- Original Message -----
From: "Jay E Ostrem" <wc7m@vcn.com>
To: "Gary Breed" <gabnjb@earthlink.net>
Sent: Tuesday, October 09, 2001 12:52 AM
Subject: Re: Topband: shunt feed help needed
>
> >
> > Use the measured data to design a matching network. An L-network is
often
> > quite adequate; a T-network can compensate for the reactance slope and
> give
> > you both an efficient impedance transformation and a wide SWR bandwidth.
A
> > little algebra is all that's needed. I can provide more details if
> desired.
> > (Unfortunately, neither the ARRL Antenna Book nor ON4UN's Low Band DXing
> > covers T-network design adequately.)
> >
> > You climb the tower once and do all the rest on the ground!
> >
I agree with Gary, I only want to climb it once! I've used the "cheating
method" almost exclusively. There are two caveats though.
1. I believe the Q of the L network is related to the ratio's of the Input
and Output impedances, if I remember my theory correctly. Try to get the
drop wire "close" to 50 ohms, otherwise the bandwidth gets narrow and the L
Network gets warm (throws sparks if its really bad). I found this out by
not putting the drop wire high enough. The 120ft tower was heavily top
loaded with aluminum, and the near 50 ohm location was much higher than the
30ft I put the drop wire at.
2. When using a T network, beware of the commercially made units, they
usually lack enough capacitance on the output cap. This limits their
range,
and raises the Q. Again arcing and heating occur. I use 1000pf on a
homebrew T network, and it works well.
73 Jay W7CW in Wyoming
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