Dean,
With respect, TLW doesn't agree with my results!
Taking the " 450-Ohm Window Ladder Line" option, 60ft long, with a 50
Ohm load, at 29.5MHz TLW predicts a loss of 0.356dB - that's almost
exactly half the 0.71dB loss I measured in Test 7.
It's no coincidence that there is a 2:1 discrepancy - the TLW data and
the Antenna Book table entries for 450 Ohm window are optimistic by
almost exactly that factor.
Let's do some simple maths:
1) Suppose we inject 1A into the Ladder Line under matched conditions.
Zo is quoted as 405 Ohms so the line input power must be 405W.
2) At 10MHz the RF resistance of 100ft of #18 solid copper wire is 8.6
Ohms; so, flowing 1A the copper loss is 8.6W for each wire, making 16.2W
total for the two wires.
3) So line output power = 405W - 16.2W = 388.8W
4) Therefore line loss is 10*Log(Pout/Pin) = 10*Log(388.8/405) = 0.18dB
That's just over twice the Antenna Book figure (0.08dB) and the TLW
figure (0.082dB).
I know this has been pointed out many times, but it never seems to get
corrected.
If you can see anything wrong with my maths, please let us know.
73,
Steve G3TXQ
On 09/08/2013 17:33, RDStraw wrote:
Thanks for the heads-up, Jim. And nice work, Steve!
I went back and looked carefully at the Nov 2009 QST article by W1ZR and
WB1CGM. Nice to see that my old TLW program gave the right numbers. It's
been about 10 years since I did any work on it. It's also nice to know that
the matched-line loss figures in The ARRL Antenna Book for open-wire and
window-line transmission lines have withstood the test of time. So far as I
know, those loss figures have been in the book for at least 50 years.
I'm starting serious preparation for Pacificom 2013. Jim, I'll be asking
permission to use some photos from your white papers on your web site. I do
briefly want to touch on the topic of current-choke ("balun") loss due to
high SWR with multiband antennas, such as the famous 132-foot long dipole.
I'll get back to you with a request for reproduction of specific photos or
figures.
73, Dean, N6BV
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