On 1/27/2012 5:10 AM, Steve Hunt wrote:
> The ladderline loss figures quoted in the Antenna Book - which I believe
> are also repeated in TLW - are out by a factor 2. It's very easy to
> demonstrate that with a simple I^2R loss calculation. I'm happy to show
> the sums if you need convincing.
Based on verbal correspondence with two ARRL technical editors I trust
and know well, there's considerable content in ARRL publications that
has been there for a long time, and doesn't get as much (if any) review
as is needed. AFAIK, N6BV is responsible for TLW (and HFTA). Several
years ago, I was studying some of the discussion in the ARRL Antenna
Book, which he edited, on the topic of the fields associated with
vertical antennas and the radial system. It was quite good, but fairly
dense, and in using figures and drawings to follow it, noted some typos
that made it impossible to follow. I also spotted some other typos.
Those errors had been for at least one edition back, as were a few
others I pointed out to him.
When I was working with N0AX a few years ago on updates to the Handbook,
although he liked what I had written and most of it was incorporated,,
we were stonewalled by "the old guard" in their defense of existing
Handbook content on EMC that is wrong and/or seriously outdated, and the
old stuff remains. At my insistence, the disputed content was
subsequently submitted for peer review, and last I heard, my material
was supported by that panel.
The lack of competent technical review for ARRL publications, along with
the dumbing down of content, have gotten to the point that I cringe at
least several times at the gross errors in every month's QST. There
USED TO BE very good stuff in QST that you could sink your teeth into.
Now, N0AX's regular contributions are almost the only thing you can
trust in the magazine.
One of the major deficiencies in published data for transmission lines
is good numbers for loss and shielding effectiveness in the MF and HF
spectrum. Loss is most often quoted in dB/100 ft and rounded to one
decimal place. This yields values of 0.1 dB/100 ft and 0.2 dB/100 ft for
most coax at 1 or 2 MHz, which is lousy resolution (0.1 dB is the result
of rounding between 0.05 and 0.149; 0.2 the result of rounding from 0.15
to 0.249). Now, loss at 1 or 2 MHz is quite difficult to measure, first
because it is small, but also due to termination effects (that is, Zo is
not a constant 50 ohms, but rather varies with frequency and is
increasingly reactive below HF, but generators and loads are a pure 50
ohms resistive). I've attempted it, and you've got to measure at least
500-1,000 ft to get reasonable precision. Why does this matter? Several
reasons, Some hams need to run 500-1,000 ft to their antennas, so the
difference can matter. Second, many hams are spending big bucks for
premium coax when they may not need to.
And when have you seen data for shield transfer impedance for ANY coax?
Or a definition for it? Yet it is a fundamental property that
quantifies shielding effectiveness and allows computation of actual
numbers for how much RF gets through a shield!
73, Jim Brown K9YC
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