Tech-Talk
Part 7
Welcome to Part 7 of our discussion.
I was going to start talking about coax this time, but I received two questions from readers that I think bear answering here.
Rich from Connecticut asked "If my coax is 50 Ohm, and I'm
connecting it to 450 Ohm ladder line, doesn't that mean I should use a
9:1 Balun? Back a long time ago when I was in school, 50 x 9 = 450, and
I don't think that's changed". His math is correct, but...
Way back in Part 1 of this series, I mentioned that if the
SWR is anything but exactly 1:1 -- that is, the antenna's impedance
exactly matches that of the feedline -- then the SWR will vary at every
point along the line. By extension, if, and only if, the antenna does
match the feedline, then the SWR will be a constant 1:1 all along the
line. Another interesting point is that, ignoring losses, the SWR will
repeat every electrical 1/2 wave along the line. So if we've built a 50
ohm antenna, no matter what feedline we use, we'll see 50 Ohms
impedance every 1/2 wave regardless of the feedline. But back to Rich's
question. Since it's unlikely that we have built an antenna that is
450 Ohms resistive impedance, it's highly unlikely that at the input end
of the ladder line we're looking at a 450 ohm impedance. Furthermore,
9:1 baluns tend to be more narrow-banded, and more subject to saturation
and overheating at moderate mismatches. They are also generally harder
to find and more expensive. My advice? Stick to a high-quality 1:1 or
4:1 model and use a good quality tuner if necessary.
Jim from California notes "I adjust my antenna tuner at low
power and get a good match. But when I go to full power, my SWR
changes. Why does that happen?"
The short answer is -- it doesn't happen, at least in most
cases. Remember that SWR is, by definition, derived from the ratio of
the power going forward toward the antenna to the power reflected back
from the antenna. That ratio does not change based on power level.
Therefore, the SWR does not change.
But there are several possible explanations for what you're
seeing. One is that you have RF on the outside of your coax. This can
get into the meter circuit and affect the reading -- and higher power
will affect it more. You may also notice other "RF in the shack"
symptoms. Take a look at some of the previous installments below
concerning baluns. Another possibility is that you did not use enough
power to tune properly. SWR meters typically use diodes to rectify the
RF and drive the meter. If there is insufficient power, the diodes will
not fully conduct, and the meter reading will be off. Try tuning at
about 15 watts.
There is a special case where the SWR actually will change
with power increase. A corroded or dirty connection, or a fault in the
coax, may allow some arcing over at higher power that will not happen at
lower levels. Possible, but not probable... look to the first two
causes as the most likely suspects.
Finally, it's a good idea to check your meter for accuracy.
Most of us can't afford the high quality meters that professionals
use. But if you ask friends and fellow club members, you may find that
someone owns, or has access to, a known good meter to compare yours to.
That's it for this month. We'll continue our discussion next time. Until then,
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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