Several months ago, I was contacted by several people on this board who were
interested in learning more about the E.F. Johnson Antenna tuners. Please be
advised that the March, 2001 (latest issue) of Electric Radio Magazine has a
very interesting article describing both the 250 watt and kilowatt models. This
is the only source I have ever seen that has actual side by side photographs of
both units showing their rather massive internal components along with
schematic diagrams of each. The article also explains how to modify them to
operate on 160 meters. The author is using them to center-feed a 180 foot long
hertz (dipole) antenna for use on his favorite bands of 160, 80 and 40 meters.
I have not verified his math, but the author states that on 40 meters, a 180
foot long dipole will act a double extended zepp and has 3 db gain and can be
used as a dx antenna. This seems reasonable. (Note: the terminal impedance will
not be anywhere near 50 ohms; probably closer to 1000 ohms and will have a
large unknown reactive component so a tuner or matching stub is needed.) The
author is using common 450 ohm low loss ladder line to connect from the tuner
to antenna. The author's comments about using the Johnson tuners exactly
duplicate my experience; except I used home made, unknown impedance - 5"
spaced, open wire line with excellent results. The tuners really work great
for what they were designed to do, provide tremendous bandwidth ( 80 -10 meters
operation) with very high efficiency using balanced feed to a dipole, hertz or
any balanced type antenna. They were not really intended to be used with
coaxial cable, but can be modified to work with coax. (I really discourage
modifying them or the use of coax in this manner.) I think the author said he
is operating with about 300 to 500 watts of 100% plate modulated a.m. (his
favorite mode) through them with no evidence of arcing or heating of the
components. I do not remember his name or call but this info. is in the
article. For anyone wanting to understand exactl
excellent book entitled Reflections, Transmission Lines and Antennas by Mr.
Walter Maxwell or his series of six articles entitled "Another Look at
Reflections" published in QST in the mid 1970's. - 73's- Corn - k4own.
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