[TowerTalk] antenna FS measurements

[Jim Jarvis]

W/resp to antenna pattern measurement techniques, some very important things
have
been posted here, in fragmented form, from several knowledgeable guys:

*  You can measure at 0 degrees azimuth, using a friend a few miles away, on
groundwave.
*  Use the telephone (or 2 meters) as a backchannel for coordination &
control.
*  Vary the TX power, holding S meter reading constant.  Use a real
Wattmeter.
	Take data at fixed rotational intervals.
*  If possible, disable the AGC and digitize the audio in your soundcard,
instead of
	varying TX power. (The latter, of course, seems to lack a software
solution.)
*  Recognize that the pattern at 0 Az. isn't the same as at real arrival
angles.
	The data will be useful for comparing antennas against each other, or
against models,
	but NOT as an absolute.  It will be affected by terrain and structures. It
should
	be adequate to determine 'reasonably correct' function, decoupling of
feedlines or
	adjacent antennas, etc.  I have found that it validated specifications on a
Tennadyne
	T8, 3 el SteppIR, and multiple kt34A and XA's.  And, of course, identified
the 6dB f/b
	ratio on my vertical, which was coupled to the lpda via common-mode current
on the feedline.


As an aside, there's a program called "S-Meter", which takes meter readings
via the radio
interface, for those rigs which report S meter data.  While this leaves you
open to
measurement error due to the AGC response curve, it should produce a
visualization of the
pattern.  My sense is, most S meters tighten up above S9, and may expand
below S3 or so,
and it will tend to understate gain and f/b, and overstate the depth of
nulls.

The LPL technique, of holding S meter constant and varying TX power, will
eliminate that.
So would turning off the AGC and digitizing the audio.

If anyone finds a program to support the latter, I'd like to know about it.

n2ea
jimjarvis at ieee.org