[TowerTalk] Actual LP Performance vs Tribanders

[Michael Tope]

One of my colleagues at work fly's model Helicopters. Seems like a model
helicopter with a small beacon transmitter might be the way to make a
poor man's HF antenna range. Alternatively, a tethered helium balloon with
a small beacon might be another way to build a cheap HF antenna
range.

Mike, W4EF........................

----- Original Message -----
From: "Jim Lux" <jimlux at earthlink.net>
To: "Larry Phipps" <larry at telepostinc.com>; <towertalk at contesting.com>
Sent: Monday, June 28, 2004 11:37 AM
Subject: Re: [TowerTalk] Actual LP Performance vs Tribanders


> At 02:08 AM 6/28/2004 -0400, 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.
>
> Yes, I agree that you'd have a problem doing the measurement in one pass
> through.  You'd have to do a statistical approach over many days and
hours.
>
>
> Timing you can get from the signals themselves, and the PC clock is good
> enough to sequence it. As long as you're within a few seconds of the
"real"
> time, it's fairly straightforward to track (programs like BeaconSee do it,
> for instance).
>
> You could correlate the signal strength on the antenna under test to a
very
> short monopole antenna  or small non-resonant loop (which will be fairly
> consistent in azimuth and elevation response).
>
>
>
>
> >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.
>
> Kind of depends on the accuracy you require.  If you want tenth dB
accuracy
> on something that is 30 dB down, it would be a challenge (I doubt there
are
> many antenna ranges or network analyzers that can do that well).  If you
> are willing to tolerate 1dB errors (out of 30), and, say, 0.5 dB out of
> 10dB, I think you could do it with, maybe, 10-15 dB SNR. Depending on the
> integration time.  If the beacon is sufficiently narrow band (which the
> NCDXF beacons are, during the test tones), your measurement bandwidth
> (which sets the SNR) can be quite narrow (probably limited by ionospheric
> doppler spread). Maybe 5-10 Hz?
>
> Using  the noise level in VOACAP for residential areas (-145 dBW/Hz),
> you're only going to be seeing something like -105 dBm noise
> floors.  Running a quick VOACAP from a monopole in Tangier to a swwhip Los
> Angeles, (june SSN=100) it looks like you'll have SNR (in 1 Hz) of above
15
> dB for about 4 hours.  That's going to be a bit marginal.
>
> However, as you point out, big broadcast stations would be a better bet.
>
>
>
>
> >My tests used commercial stations with BIG signals that transmit
> >continuously, like WWV. I was testing a very broadband beam, so actual
> >frequencies weren't that important. Testing a LPDA or SteppIR should also
> >work OK with my software. You could probably make it work with a ham with
> >a strong signal though, who would be willing to make several  5 minute
> >continuous transmissions while the data was gathered.
> >
> >If you happened to have a neighbor within a few blocks, you could get
> >meaningful results, especially if your antennas are high. You could use
> >the TRX-Meter utility that comes with TRX-Manager since only one pass,
and
> >one sample per 10 degree heading would be needed. I got the idea for my
> >program from trying TRX-Meter, but added the ability to integrate
multiple
> >samples over time to smooth out the effects of fading on skywave signals.
> >Further away than about 10 wavelengths would give you a nice pattern, but
> >I doubt that it would correlate into similar skywave performance.
> >
> >Larry N8LP
>
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>
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