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Re: [TowerTalk] Actual LP Performance vs Tribanders

To: Larry Phipps <>, <>
Subject: Re: [TowerTalk] Actual LP Performance vs Tribanders
From: Pete Smith <>
Date: Mon, 28 Jun 2004 07:02:02 -0400
List-post: <>
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
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