Why not leave the power constant and use a switchable attenuator to keep the
s-meter constant? That would seem to give more accurate and more repeatable
[mailto:firstname.lastname@example.org]On Behalf Of Pete Smith
Sent: Monday, June 28, 2004 7:02 AM
To: Larry Phipps; email@example.com
Subject: Re: [TowerTalk] Actual LP Performance vs Tribanders
At 02:08 AM 6/28/2004, Larry Phipps wrote:
>Jim, I'm not that familiar with the beacons, but since your post I did a
>little research. There are a couple of major problems.
>First, the transmissions are very short... there wouldn't be time for more
>than one sample per beam heading.. and it would take almost 2 hours just
>to gather the samples for one rotation (36 samples). The signals are going
>to be all over the place during that time frame... and that doesn't take
>interference into account. Timing would also be critical... your computer
>clock would have to be dead nuts on. There's also really no accurate way
>to correlate the signal strength to anything else minute-by-minute, so the
>levels would be more or less meaningless.
>Even with a 20 minute continuous carrier at 100W, I doubt the received
>strength of the beacons would be enough to be useful for plotting the
>pattern of a beam with 30dB F/B ratio. You would need a stable signal
>about 50dB above the noise floor... probably something around S9... and
>you'd have to listen to make sure there is no interference while the
>samples are being taken.
Frankly, I think these are show-stoppers, as Larry says. Another issue is
that S-meters can be quite non-linear -- for example, my Mark 5 is (very
roughly) 2 dB per S Unit below S-9, and then fairly abruptly transitions to
~6 dB per S-Unit.
That's why I use the trick, told to me by W3LPL, of varying the transmitter
power to maintain a reference S-meter level, rather than counting S units
at constant power. In the test of my 40m beam that I ran last week, power
required to maintain the reference level varied from 20 watts to 1400 as I
rotated the antenna. Of course, the accuracy of the power meter is still
an unknown, unless you know how it is calibrated, but a 5-10 percent error
in power is still pretty small when converted to dB.
I used the phone to coordinate with a neighboring ham, but even if you
interspersed phone transmissions and steady carriers to do it over sky-wave
distances, it wouldn't take very long to plot an antenna pattern this way.
73, Pete N4ZR
The World HF Contest Station Database
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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