> Jim describes nearly the exact problem I just developed. Have a Nye
> M...V (the Roman numerial 5 of the latest design) and am trying to get it
> to tune a full size half-wave 160 dipole fed with ladder line. Tuning is
> weird and unsatisfying on 160 (with balun inside), but fine on all other
> bands, including WARCs.
I'm not familiar with that particular model, but most all-band
commercial antenna tuners that I've seen work poorly at the extremes
of their frequency range, 160 and 10 meters. They're convenient for a
Field Day or d'xpedition, but you often pay for that convenience in
excessive loss in tuner and balun. A typical all-band tuner is a Tee
network with variable 200pF series caps forming the top of the Tee,
combined with a shunt inductor. These values give good adjustment
range with reasonable Q on 40m or 20m, but as you go down in frequency
the Q gets higher, and on 160 you end up with very high Q and high
losses.
A similar problem exists with the baluns that are usually built into
these tuners for feeding open-wire antennas. They don't have enough
inductance for 160m.
For non-portable 160m tuner applications, where you don't need the
flexibility of wide range bandswitching or matching a wide range of
unknown impedances, you are much better off building a tuner and
selecting components specifically for that band, and configuring it
for your specific antenna's feedpoint impedance. If you're feeding a
balanced antenna, you can also build a balun that is optimized for
160. It wont work on 10m, but will be more efficient on 160 than
anything you buy that is designed for "all bands".
Visit your local ham flea market and look for physically LARGE
variable capacitors and roller inductors. You want 2 capacitors that
have 1000 or 1500pF max capacitance. These are typically 12 to 15
inches long and 5 inches wide. Look for a large roller inductor with
at least 4 inch diameter and a heavy conductor. If it looks like #14
wire, its not heavy enough. These things are becomming hard to find
now. Another alternative is to buy them from a place like Nebraska
Surplus. You'll pay a lot more than flea market prices there, but its
still cheaper than buying a brand new commercial tuner that works
poorly on 160.
You need to experiment with the various L network configurations to
find one that allows you match your antenna. An L network usually
gives you lower Q than a PI or T for a given impedance transformation.
You want lower Q because you'll have lower losses and you can move
farther in frequency without having to re-adjust the tuner. I usually
do this with clip leads and low power until I've found the L
configuration that gives a match. There's about 8 different ways that
you can combine a coil and cap to make an L network, so you have to
experiment. You may have to use 2 capacitors in parallel to get enough
C. Having 2 capacitors also allows you to configure it as a Tee if
you need to. A Tee has greater flexibility in impedance matching range,
and the Q may be acceptable, though higher than with an L.
To make a 160m balun, stack four ft-240-61 ferrite cores together, and
wind as many bifilar turns as you can fit. If you use #14 teflon
insulated wire, you should be able to get at least 16 turns on it.
With a bifilar winding you can configure it as a 1:1 (basically a
coiled transmission line) or 4:1 balun. Experiment with both
configurations and use the one that gives the best balance, tuning,
and least heating. I usually do a crude balance test with a neon
bulb, looking for roughly equal brightness on both sides of the
feedline.
The acid test is key down, 1500W out for a minute or so, then check
for heating of the coil and balun. You know things are working well
if there is barely any perceptible change from room temperature. If
anything is hot to the touch, you can improve it. Check your balanced
feedline for temperature rise also. You may be surprised to find (as
I was) that you're wasting power in heating it also, particularly if
you're using ladder line with #18 steel conductors. I solved this
problem by making my own open wire feedline using #10 copper wire and
lexan spreaders. My antenna is electrically short on 160 however, and
you probably wont have this problem with one that is 1/2 wavelength
long.
73 + gl,
Ed Gilbert, WA2SRQ
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