When you KNOW you have a low impedance antenna at low frequency you can
improve life for the tuner by adding a transformer. Not a balun, a
conventional ferrite cored transformer. There have been such designs in
QST and ARRL Handbooks for feeding short verticals on 80 and 160. At
least 20 years ago, maybe longer. Though the conventional 1:4 balun
operated with the high impedance side to the tuner and the low impedance
side to the antenna can be a benefit operated as a unun which may
require unhooking the ground from the center tap of the high impedance side.
Or if its the usual case causing a low impedance, a short vertical with
no loading coil, you can raise the feed impedance by adding a loading
coil, not making the tuner be the loading coil too.
Without digging out a design, I'd suggest a start for a ferrite
transformer would involve a F200 core, with 12 to 15 turns on the
primary and a 5 turn secondary tapped every turn. Pick the connections
that give the lowest SWR without the tuner.
Or a loading coil that resonates the short antenna which still make end
up with a low resistance but cuts the reactance problem which is part of
the problem that fries a tuner, and then add the ferrite cored
transformer which was the emphasis on those short verticals in QST.
73, Jerry, K0CQ
On 11/24/2010 7:05 AM, kf6e@mail.com wrote:
I would strongly recommend the MFJ-998, unless your antenna is very close to
1:1 SWR anyway.
They rate tuners by the range of impedances they can match, and by the power they can handle. But
it's "or," not "and." If a tuner is rated at 6 to 1600 ohms matching
capability and 300 watts, it will handle 300 watts only around 50 ohms load. You must decrease
power when the impedance varies greatly from the 50-ohm nominal load, especially at the low
impedance end. I blew up an LDG AT-600 Pro (rated at 600 watts) with 175 watts on CW, running into
a low impedance antenna. LDG repaired it for free, but I can't use that power on that frequency on
that antenna with that tuner.
I've been very pleased with my MFJ-998. I've run up to 1000 watts through it
over a wide range of antenna impedances with no problems.
73,
Frank
KF6E
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