[ARDF] US ARDF Championships: 2m AM receivers...

[Jay Hennigan]

Matthew Robbins wrote:

>     4.  I got the impression that the Ackerly attenuator display (and
> the experience of several 2m AM events) kept me out of trouble a few
> times, especially on the way to 5, where I stopped twice to check what
> turned out to be reflections, and decided to continue east because the
> signal strength seemed too low.  Another example is the start (on 2m),
> where the corridor pointed (more-or-less) east toward 3, but I decided
> to go north to the gravel road.  When 3 came on, it wasn't strong
> enough to make me go after it.  I'm still not sure if 3 is the right
> first transmitter.  I felt like going to 1 first gave me a lot of
> information about the course that I wouldn't have had if I'd started
> into the low area between the start and 3.  I meant to ask the people
> with the Ukrainian and other 2m gear how it worked for them.

I agree that the numeric range indicator on the Ackerly box is very
useful.  I too was fooled at the start that 3 was farther away and it
cost me.

>     I think the biggest lesson from the 2m course (for the people who
> didn't already know), was that 2m AM is a lot harder than 2m MCW, and
> you really need to practice with 2m AM.

AM vs. FM isn't as significant to me as keyed vs. continuous carrier.
Most ARDF receivers have an audio S-meter (whoopie) function.  With
continuous carrier the tone is audible for the entire cycle.  Keyed
carrier causes the tone to follow the code elements.  This takes some
adjustment for people used to continuous feedback.

One thing that can make a big difference when hunting AM transmitters
is the modulation depth.  If using a conventional AM receiver with
manual gain control and no AGC, the relative loudness of the recovered
audio should directly correspond to signal strength.  Weaker signals
sound weaker (quieter).  If, however, a strong carrier has low audio,
it will sound weak to a conventional AM foxhunt receiver.  Too much
modulation, in addition to generating spurs, will cause the illusion
that a transmitter is stronger than it actually is.  Overmodulation
will also affect peak-reading S-meters and whoopie-mode receivers as
the carrier peaks are much higher than the average power, especially
if the degree of overmodulation is different on different transmitters.

The IARU region 1 rules indicate that 2M transmitters should have a
modulation depth of 75% plus or minus 5%.  The rules don't specify that
the modulating tone should be a sinewave, but I would recommend it.  At
least not a squarewave.  The rules also say that the antennas should
be two to three meters above ground level.  I know that the practice
antennas were less than two meters above ground, not sure of the ones
in the event (too busy looking at the map and for the flags).  I know
from other events that antennas close to the ground can cause problems
with perceived signal strength as the radiation pattern will be
distorted.  But, any variation from the ideal will tend to affect
all of the competitors similarly, so it really boils down to the skill
and experience of the contestants.  And most of us from the USA have had
little experience with AM 2M signals, and less with keyed-carrier AM
signals.


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Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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