Hi Steve, with a vswr as low as 2:1 it would be tempting to match it right
in the 50 ohm feed line with a stub match.
For example. starting with 37R +j0, 40 degrees down the line the impedance
is 49.8R shunted by +165 ohms. The +165 can be canceled with 535 pFd
capacitor or a piece of 50 ohm coax, open ended and about 17 degrees long.
RG213/u has about 25 pFd per foot so about 21 ft of that would work.
The distance of 40 degrees will be influenced by the coax velocity factor
of course.
A series matching solution is also possible, using a combination of 50 and
75 ohm lines.
The conversion from 37R +j0 would be done with 123 degrees 50 ohm followed
by 21 degrees 75 ohm coax.
If the coax loss is low, a tuner in the shack would also be a good
solution.
73,
Gerald K5GW
In a message dated 11/11/2012 12:59:56 P.M. Central Standard Time,
nn4t@comcast.net writes:
Good afternoon. I have 118 feet of Rohn 45G which, at the moment, has no
antennas on it. It is guyed in 3 places using Philly and sits on 1.5 cubic
feet of concrete with a pier pin at the base. The tower is grounded (three
5/8" 8 foot rods per leg, spaced at 20, 40 and 60 feet from the tower in a
line and all connected with #2 copper) and I have layed out 33 130' radials
(#14 stranded copper, insulated) as the ground screen. If you disconnect
the ground rods (not the radials) the tower has a resistance of 37 ohms at
1.830 and an SWR of 2 (according to my MFJ 259B). I would prefer, however,
for lightning protection, to leave the ground connections in place and use a
gamma wire and capac itor to match the tower for use on 160. I have tried,
using a wire running from near the top of the tower, and with spacing from
the tower varying from about 28 down to 14 or so inches, to find the 50 ohm
tap point for the tower. This seems to occur, interestingly enough, at
about 3 feet off th
e ground. Since this seems a bit odd I consulted local antenna guru, N4KG,
and he too thinks the tap point should be significantly higher, at least
30-40 feet if not more. I don't have much experience with this to draw on so
I would like to ask the group for ideas or input. This also seems to vary
quite a bit from articles I have read on the subject in the ARRL antenna
book and in Low Band DXing, although virtually all of those articles concern
towers with some sort of top loading provided by yagis. Thank you in advance
for your advice, Steve, NN4T
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