Tech-Talk
Part 3
Welcome to Part 3 of our discussion.
As promised, this month I have some rather eye-popping
information for you. We're going to see why your SWR meter can lie to
you. Or as another fellow says, "SWR meters make you stupid."
The material in this section was prepared using 2 programs
from the ARRL Antenna Book CD. Roy Lewallen, W7EL's EZ-NEC antenna
modeling program was used to get the basic antenna data. That info was
fed into Dean Straw N6BV's Transmission Line for Windows (TLW) analysis
program. If you like to play with antennas, both programs are great
tools.
Using EZ-NEC, I "built" a 40 meter dipole at about 33 feet
high. As expected, the SWR was about 1.6:1 at the center of the band
and below 2:1 at the band edges. Then I opened TLW, specified 100' of
RG-8X coax as the feedline and added the feedpoint impedance (77 -j13)
calculated by EZ-NEC. TLW calculated the total loss in the coax as just
under 1 dB. Most of that was the inherent ("matched line") loss in the
coax. The slight SWR mismatch added just a 10th of a dB to the total.
None of this was surprising; it's basic antenna theory. But read on...
Let's take that 40M dipole and -- through a tuner -- use it
on 20M. An SWR meter in the shack will read about 6:1. Not great, but
well within the range of most manual or external automatic tuners. So
the tuner does its thing, our transmitter sees 50 ohms, and happily
pumps 100 Watts out into the coax. But how much of that power actually
radiates, and how much is lost in the coax? Would you believe that a
mere 6 Watts actually goes out as RF? The remaining 94 Watts are lost
-- converted into heat -- in the coax!
Hmmm.... I must need better feedline, eh? So (using the TLW
program) I replaced it with some nice, low loss LMR-400. Much better
now. I'm radiating 16 Watts. "Only" 84 Watts lost in the coax.
Yikes! Well, I decided to go whole hog and swapped the LMR-400 for some
nice 1-1/4" hardline. Which, in the real world, would have set me back
about $500 or more when you include connectors. Only to find out that
I'm still losing 1/2 of my power in the feedline!
Maybe the old-timers were right about using ladder line.
And sure enough, when I feed that antenna with 100 feet of 450-Ohm
ladder line, my loss plummets to almost nothing -- just about 1/2 of a
dB. Now almost all of my power is heading out to that rare DX. And of
course, now I can actually hear that DX station. Remember that antennas
are reciprocal. That 94% loss on transmit is equally a 94% loss on
receive.
But how can the losses be so staggering when my SWR meter shows only 6:1?
Here's the first clue... each time I upgraded the coax, my
SWR increased. With the hardline, my meter showed nearly 50:1. No,
that's not a typo. Fifty to one. But that's not the worst of it.
Remember that SWR is, by definition, the degree of mismatch between the
impedance of the feedline and that of the antenna. But I didn't change
the antenna at all, and each of my feedlines was 50 Ohm. Then how can
the SWR increase? And the answer is -- it can't.
But what my meter reads can (and did) change. Why? Because
the high loss in the coax absorbed most of the reflected power. The
meter was seeing the full 100W forward power, but only a fraction of
what was coming back down the line. TLW tells me that the actual SWR at
the antenna feedpoint is about 100:1. That's just what we would expect
from a full-wave dipole -- an impedance of about 5000 ohms.
And as if all of that wasn't bad enough, keep in mind that I
used a lossless tuner, and good quality coax in new condition. Tuners
do have loss. And coax used outside does get more lossy with age.
That's it for this month. We touched briefly on ladder line today. Next time we'll take a closer look.
73 for now,
John Bee, N1GNV
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