Are the radials on the ground or elevated? If they are on the ground, how
do they get their from the 4 foot high base? Did you try directly feeding
it with the coax center going the vertical and the shield going to the
radials? After hooking it up directly (no match) connect an antenna
analyzer and see where it is resonate and what the radiation resistance is
and then make your adjustments to the height. You can raise the resistance
using a hair pin coil - this will also lower the resonant frequency a
little.
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] 160m 1/4 wave vertical help needed
From: AB5MM
Date: Sun, 16 Sep 2007 17:24:05 -0500
List-post: <mailto:towertalk@contesting.com>
Fellow Top Banders,
I have a matching problem with a Rohn 25G, 160m ¼ wave vertical and am
hoping some of you can help me out.
<> The tower is 124'-9" in over all height. The tower bottom insulator
is located at 4'-3" above ground level thus making the vertical radiator
a total of 120'-6". All guy wires are insulated from tower as well as
being broken up throughout their length. <>
For now, the ground radial system is made of 22 each, 125' long, 14ga.
copper wire. I plan on installing another 14 radials next weekend.
Total number of radials will be 36 at 10 degree spacing. The soil here
in the Sabine River Valley bottomland has a "good rating" of 15mS/m. <>
My friend (K5ZO) and I have tried every thing we can think of to get
this antenna to resonate anywhere in the 1.8 to 2.0 MHz range. We've
tried both low pass and high pass "L" networks, a high pass "T" network,
low pass "Pi" network, and a parallel inductor with capacitor, in series
with the center conductor. <>
The best match we were able to achieve was with a 500pF cap in series
with center conductor. This gave us a 1.7/1 swr at 2.113 MHz. (R=64 &
X=31) All matching efforts have been made right at the tower feedpoint.
1. Is the tower too short?
2. Do we need to place a capacity hat at the top?
3. Would a short stinger out the top (for added height) help?
4. What about all the various matching schemes we've tried? Are we
missing something here?
<>Any input or suggestions would be greatly appreciated. Please feel
free to post to the group as I feel this information would be helpful to
others as well. <>
Thanks & 73, <>
Steve AB5MM
ab5mm@9plus.net
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|