Tom:
If you rearrange the diagram of your matching network you will see that a
series coil to ground [ shunt 3 uH in your email] with the feed line
attached at a point above ground is the same as the series coil +shunt coil
matching network I described previously. The network we are using has the
desired characteristic of allowing static charges built up on the antenna to
drain off to ground [or maybe allow the ground charges to go up the vertical
and cancel the charges being built up on the antenna]
You might find that fooling with two variables your match will improve. Make
the 3 uH say 5 uH and then tap the antenna to a point on the coil and the
feed line to a point between the antenna tap and ground. A lot more work
unless you have a good RF bridge to read X and R.
I happen to use two small roller inductors in a prototype matching circuit
to find the optimum value for each coil and then I fabricate good quality
coils with the same L values. With an AIM 4170[or similar instrument] you
can use the roller inductors to quickly see what the shape of the VSWR curve
will be for various selections of L. I select L values that give me the
greatest bandwidth at an acceptably low VSWR.
If I want better VSWR curves I can put a second low loss matching network in
place at the station. The name of the game is to push the power out the feed
line and at 1800 kHz line losses are quite low so 3:1 VSWR is no big deal as
long as your transmitter can load up at that value.
73, Tod,K0TO
> -----Original Message-----
> From: topband-bounces@contesting.com
> [mailto:topband-bounces@contesting.com] On Behalf Of Tom Williams
> Sent: Thursday, October 08, 2009 4:48 AM
> To: topband@contesting.com
> Subject: Topband: VERTICAL ANTENNA MATCHING
>
> I have a 1/4 wave inverted L. Prior to this season I fed it
> through a 2.25:1 UNUN. This summer I took out the UNUN and
> made a simple shunt coil of about 3 uH from bare copper wire,
> air wound. Put it in a plastic box at the base of the antenna
> and moved a wire tap around until I got the best match.
> Brought it in the house and soldered the tap in place. The
> match is MUCH better than what I got with the UNUN.
>
> 73,
> Tom N2CU <><
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