Jim Miller wrote:

OK, at cecil's suggestion I changed to a single 20 segment wire with a
split I source at 50%. I ran the SWR plot against a 300ohm alternate
impedence. Is that the way to see what effect a 300ohm twinlead will
have? Now I get a minimum 2:1 SWR at 5.6Mhz and a pretty smooth convex
curve out from there to 14.5Mhz with a max SWR along that leg of around
35 at 12.1Mhz.

That's more like it and you don't need a split-I source in the
middle of one wire. Split-I sources are used at the intersection
of two wires. 492/88' = 5.6 MHz. 984/88' = 11.2 MHz.

For 7 MHz and up, this antenna would be better behaved with 450
ohm ladder-line or open wire line. The best Z0 for the feedline
is Sqrt(Rmin*Rmax) ~ Sqrt(50*7000) ~ 600 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

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Hi Cecil

Thanks for your help. Looks like I've got a tiger by the tail. I'm
currently using #26awg fed by 300ohm ladderline to minimize visibility.
Both contribute to pushing the SWR higher across the spectrum. If I'm
not able to get the tuner to match this setup I'll try the 450 ladder
and and 18awg and risk the visibility. Actually I've gotten the antenna
higher than I thought I would be able to and the 18awg would probably
be ok. The ladder line is a bit more difficult to deal with at twice
the width unless I can disguise it or hide it behind a tree trunk. Then
there is still the issue of providing a guard against kids messing with
it.

I'm going to take a multimeter to some white and grey pvc conduit today
to see if I can detect any difference in conductivity as some have
speculated the grey was more conductive. Which ever I chose I'll have
to do some camoflage painting but if the resistance is high and
comparable for both I'll select the smallest diameter which allows the
ladderline to pass through without cramping.

The hardest part of all of this was getting the halyards up and that's
now done. Changing wires isn't a big deal as long as adequate
performance and visibility can be balanced.

tnx
jtm

On 13 Mar 2005 04:48:43 -0800, "Jim Miller" wrote:
If I'm
not able to get the tuner to match this setup I'll try the 450 ladder
and and 18awg and risk the visibility.

I have an 88 ft doublet of 26 ga. wire and 300 ohm twinlead approx 25
ft up. My MFJ-971 tuner lets me tune to SWR=1.0 at every ham
frequency from 80M to 10M. 160M not so good.

Some of the minima are very sharp and easily missed, especially on
60M. I know it is working because I am copied 60M NVIS phone 300
miles away using my 5-watt FT-817 with DYC-817 compressor.

Ken KC2JDY


Ken
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