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Re: [TowerTalk] The Vertical and the Balun

Subject: Re: [TowerTalk] The Vertical and the Balun
From: "Dan Zimmerman N3OX" <>
Date: Thu, 15 May 2008 13:23:50 -0400
List-post: <">>
>Your antenna should
>work better than the Zero Five on 160, but will require a tuning network at
>the base.  40 will be quite difficult due to the high impedance at the base.
>It will probably perform well on 30 with the proper matching network.  Above
>30 meters, as others have pointed out, you won't like the radiation pattern.

Matching networks for my 60 foot vertical wire should work on a 65
foot vertical with some adjustment:

That might give you a starting point for 50 ohm matching of a 65 foot
vertical on 30/40/80/160.  I haven't tested my networks at high power,
so I haven't thought much about component ratings (and honestly, I
don't know what voltage rating some of my capacitors have) but #10
wire inductors should be sufficient for full legal limit generally.
You might want to check ones you have to wind on a PVC form to make
sure the form isn't getting hot.

I've since switched to a separate shunt coil for 160m, I think it's in
the 3uH neighborhood, but I'm not sure.  The tapped coil thing with a
coil that diameter was too touchy.

I built a fancy motorized wafer switch to switch networks.  If you
were clever with relays you could do things much more easily (and
probably have a single tapped shunt coil of modest size)

I can see why some people would just throw up their hands and suffer
with the 4:1 balun, but I think in this particular case, where the
vertical is intended for 80/160:

1) you don't need any matching network for 80m.  The base impedance of
the antenna will be a good match to coax and you can use a tuner in
the shack no problem to bring down the SWR at the band edges without
worrying too much about line loss.

2) The 160m matching network is just some wire.  Plain old #10 bare
solid copper is what I used.  It helps to have a good grooved coil
form ( but if
you were to wind the big coil on plain old PVC conduit with nylon
string trimmer line seperating the turns as you go, you  could end up
with a coil that was nearly as nice.  Please notice that I was only
using a little less than half of the turns on that coil at the time...
I built it to try to match a 40 foot vertical before (which was a
pretty terrible antenna base loaded)

But aside from the difficulty of constructing a 5" diameter big coil,
the matching network can be built with supplies from Home Depot, so it
doesn't make sense to fiddle with an expensive 4:1 balun that only
makes a little bit of difference to the mismatch when you can toss
some wire on a form and efficiently match.

Just to be clear, I'd recommend a matching network that's not a single
tapped coil for 160m, but a network very similar to "80m low" with an
independent shunt coil somewhere in that ballpark (might be 12-13
turns instead of 9, I adjusted it experimentally for good match) and
the series coil is as described under the 160m network, something
between 25 and 30 turns of #10 wire on a 4.5" diameter form 4.5" long.


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