"k1drw" wrote in news:1170509048.239078.207990
@a34g2000cwb.googlegroups.com:

Hello:
I am looking words of advice on conducting an informal exercise to
evaluate performance of 75m mobile systems. I have done much reading
of rec.radio.amateur.antenna threads and have learned much from Cecil,
Reg, Keith, Roy, et al. The msg threads addressing how measurements
were taken during past shootouts have been very helpful but equipment
used in those events seem to be beyond my capabilities.

I am aware that the traditional method of comparing mobile whips is by
some form of field strength measurement near to the transmitter antenna.
The measurement needs to be performed sufficiently far from the antena to
be in the "radiation far field" zone for meaningful results... but is
still potentially affected by the nearby environment (ground, structures
etc), and is a measurement of performance at near zero elevation.

I did some thinking on a more meaningful end-to-end test, and described a
technique at http://www.vk1od.net/lost/mobiletest/index.htm , which may
be of interest.


I am part of an ARES/RACES HF Mobile team. We are in the early stages
of assembling our team and have conducted a couple of field
exercises. I believe I adequately understand many of the issues
related to operating 75m mobile and have concluded (from assessing our
mobile configurations) that our systems are VERY compromised. As we
conduct our EmComm field exercises we take the opportunity to
experiment and make system adjustments that clearly improve
performance. Primarily we are using 100w mobile hf rigs with 75m
Hamsticks or Hustler resonators. We understand that the vehicle, the
antenna, the ground, etc comprise the "System". One important note
is that we are not focused on achieving DX communications. Our field
of operations is primarily Groundwave within a tri-county area in
Eastern Pa (many 600-700 ft ridges and valleys, heavily forrested),
perhaps 100 miles max. We do use and experiment with NVIS (using
restricted space configurations: tilted whip, Hamstick dipoles, etc)
however, this query is relative to Groundwave.

I would like to add an occassional field exercise expressly for the
purpose of measuring the performance of our systems (for improvement
and detecting problems). We currently have no equipment for this
purpose. One device I see advertised and would like some feedback on,
is the MFJ-802 Field Strength Meter.

As far as I can understand from its meagre online manual, it is an e-
field probe (at low frequencies), and uncalibrated.

I have peformed some field strength measurements using an untuned small
square loop, with balun and (50 ohm) crystal detector feeding a digital
multimeter. Knowing the Antenna Factor of the loop from NEC models (
http://www.vk1od.net/SmallUntunedSquareLoop/ssulNEC.htm ) and having a
measured the crystal detector DC output vs RF input, I was able to make
measurments of absolute field strength which reconciled with other
(albeit non-traceable) measurements and modelled expectations. I am
working on a development of this kit that uses a LCD panel meter / diode
detector / termination for a self contained portable field strength
meter. Another variation being considered is using a log detector IC (DC
- ~500MHz) for the termination / detector.

Anyway, it may be of interest.

As for calculating efficiency, won't you need to integrate the power flux
density over the hemisphere... a much bigger task than making FS
measurements on the ground at a hundred paces.

Owen