In a message dated 4/30/01 8:42:04 AM Pacific Daylight Time,
Saw your response in regard to making ladder line, and had a question.
I am going to make some ladder line using some #14 (I'd use enameled or
insulated stranded) wire, I am planning on cutting some PVC spacers. My
question is what is the ideal width for the spacers ??
Steve: I thought you would never ask! Chuck Smith of Raibeam tipped me off
that the cheapest and best plastic spacer material is a Polly Propylene
tubing or rod from the hardware store and it's UV resistance--the main
factor. You drill a hole wire diameter or a few thousandths smaller 1/4"
from the end and then saw a slit on each end to the hole. Slip the wire in
the slot to the hole. Then secure the spacer to the wire with RTV rubber
cement. If a spacer is damaged somehow this is easy to repair. I had
recommended the plastic rod used in clothes hangers but I've been informed
it's not UV resistant.
There is the "K7GCO Magic 1/2 WL Length" I recommend based on the formula of
492/3.562 MHz X .98 VP= Ft (136.5') for open wire line with spacers every 3'
which is OK for straight runs. I recommend spacers every 1-2' where there
are bends. It lowers the VP just a percent or 2. Insulated wire lowers the
VP a few more percent. To grid dip the length to 3.562 MHz, short both ends
and suspend 5' off the ground. Keep at least 2' from a tower when running
parallel up a tower. This length is a 1/2 WL or multiple on all the major
bands or real close. 205' can be used for 1/2 WL multiples on 40-10M. This
length assures the "least reactance at the end of the feedline" created by
"QST suggested off lengths" of open wire line. The Johnson Match Box will
handle just about any load with this feedline length if the antenna is close
to resonance. There is a mfg that is interested in making a modern version
of the MB--with my mods. If "properly used" it is the best 2 knob tuner ever
made. Some just don't know how to use simple equipment and make it known
even on TT.
QST has contributed to poor use of open wire line more than anyone with the
"All Time Dumb Dumb Open Wire Line Advice" ever printed in their publications
with the continued statement "use open wire line of any length" when used
with Antenna Tuners. They have been told several times "it's a statement
made by those who have never properly used open wire line and antenna
tuners." In the 30's and 40's they used to give the proper open wire lengths
for multiband antennas and the proper tuner configuration of "Parallel or
Series" for Hi-Z or Lo-Z loads--at the end of the feedline. One other
deficiency even back then was they seldom ever showed a "series variable Xc"
in the link to ground. It provides a resistive load to the link. The Series
tuner was difficult to get to work without it and I doubt anyone at ARRL ever
got a Series tuner to work--without the Series Xc in the link despite showing
it that way repeatedly.
The main advantage of the MB is it will match 30-5000 resistive ohms and tune
out a fair amount of reactance without and connection changes and only 2
knobs to tune. This tuner doesn't need the Series Xc although it works great
with it but it's another knob to turn. One mod I make is to have the ability
to change the link tap from the 2 turn 50 ohm link tap to 3,4&5 turns for
certain loads. I tested the 275W and 1KW MB's for Johnson and on the 275 W
model they had the 50 ohm 2 turn tap connected to the 300 ohm 5 turn tap and
visa versa on one run. Check this. I have another mod that extends it's
range even more. I reverse the connections on the differential Xc--on
certain "Odd QST Open Wire Loads". W7RQT Patrick Buller in Issaquah, Wa
90027 425 747 7495 makes a KW MB version with that ability and a few other
bells and whistles.
Another mod is to add another coax SO-239 connector so 100 ohm balanced coax
can be used. I fed a 10M beam this way into a 100 ohm FD and the 275W AM
rating tuner will take the output of a 600W 30L1 linear--more on the lower
frequencies. It's a great feed system. Rain & UV are no problem--ever.
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