It can vary significantly even more quickly than that. My friend Hank,
K7HP, once described to me some experiences he had many years ago
working for a company that was doing some propagation studies for the
military. They had a large array of HF antennas fed to a bank of
equipment, configured such that they could observe rapid changes in the
arrival angle of signals. Hank told me that they could see changes in
as little as 100 msec.
My rig is an Elecraft K3 with two identical receivers that can be phase
locked for diversity mode, and after Hank's comments I fed my 40m yagi
into one receiver and my tribander into the other receiver. Both
antennas are about 12 feet apart on the same tower, so I was able to
feed the two audio channels from the K3 into the sound card of my
computer and display the relative time difference between the two
signals on the screen using an audio oscilloscope application triggered
by one of the signals. Signals arriving from a higher angle would, in
theory, have more path length difference between the two antennas.
I wasn't able to see absolute arrival angle since the transmission delay
of the two coax lines was not equal, but I could see real time
variations in terms of "phase" jitter in the display of the
non-triggered signal. I used the carrier signal of strong 40m SWBC
stations for the test, waiting until there was no speaking for a stable
signal. A rough calculation based upon the separation between the two
antennas translated the jitter to about 10m degree changes faster than
my eye could follow, and occasional jumps of 20 degrees or more in much
less than a second. That could have a major impact on an antenna with a
notch in the pattern.
Some day I'll put up a couple of identical vertically spaced horizontal
dipoles with feedlines having the same transmission delay to play around
with this some more. I think it might be interesting.
73,
Dave AB7E
On 5/15/2011 5:45 AM, Rob Atkinson wrote:
> Skywave propagation varies too much, even during a time of only one or
> two seconds. You have to do the testing over a ground wave or space
> wave (line of light) path.
>
> 73
>
> Rob
> K5UJ
>
> On Fri, May 13, 2011 at 5:49 PM, pehaire<pehaire@comcast.net> wrote:
>> The true test would be have the other end of the contact on the land line
>> and test with him over maybe a 10,000 mile path or the path one is
>> interested in
>>
>> Since a contact is 95 percent propogation a good op at the other end can
>> possibly copy a signal down about 10 db below other signals.
>>
>> 50 years ago when I was at the 12th USASA field station I could copy the
>> weakest signal with 2 stronger stations on top of him.
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