Re: [TowerTalk] Actual LP Performance vs Tribanders

[Pete Smith <n4zr@contesting.com>]

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
was updated on June 5, 2004
2728 contest stations at
www.pvrc.org/WCSD/WCSDsearch.htm  

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