[TowerTalk] antenna FS measurements

[Tom Rauch]

> I'd suspect the short term stability of the MJF
attenuators
> is reasonably good.  Most things ARE, for short intervals.
>
> The big source of error is likely switches in the
attenuator.

I don't know how the current switches are, but the pads I
have can sit for several months in my dusty shop and never
give a moments trouble. The only problem I've had was after
transmitting through one.

I never see variations as I wiggle connectors or engage or
disengage switches.

My real reason for pointing this out is people get ideas
from reflectors and Internet chatter. The methods used by
most people to evaluate antennas and other things could be
greatly improved with a wide range attenuator pad. Also,
people should understand what NOT to do. Measuring an HF
antenna at a distance of a mile or more is not only
unnecessary, it actually INCREASES the error.

I think the 1 mile stuff comes from the same place the silly
"120-radial for 100% efficiency" thing comes from,
misunderstanding what the FCC requires. The FCC does not
REQUIRE measurements at a mile. As a matter of fact, they
would kick out any data submitted using only that distance.

You look at the slope of signal loss with distance to obtain
a MEAN value of ground conductivity, you then use that slope
as a correction to ESTIMATE soil conductivity. You then
compare measured to expected FS based on that average soil
conductivity over a long path, and estimate efficiency. This
is where the mile comes in. They had to set SOME distance to
talk about, so they normalize to one mile (or kilometer)
rather than 1.1572 miles or 999 feet.

The entire process is good within 3dB if you know HOW to use
the data and how to make measurements, which most modern BC
engineers who's papers I have read obviously do not. They
even include flyers (obviously wrong readings) in results
without resolving them by taking the value of FS from
several very close points to the flyer. In an effort to
prove a specific point (like four elevated radials=100%
efficiency) I've seen them extract only the data that moves
FS upwards, and toss out all the other data.

Making an accurate measurement does not require $10,000 and
10,000 hours, it only requires having simple basic stuff and
knowing how to use it. People misuse simple power meters,
and more complex equipment is misused even more often than
that. I was even at a spectrum analyzer demonstration where
the HP rep used the analyzer incorrectly!

73 Tom