Topband: data follow up to W4ZV and K2AV comments

[ea5rs at ono.com]

I find SNR reports from RBN very useful and RBN a great tool, but you have to understand the numbers and what they really mean. Let me explain that with a real life example.

Almost all skywave HF signals are affected by amplitude fading which follows a Rayleigh distribution when plotted against time.

I recently made a simple test by transmitting an SSB two-tone signal (with the aid of the two tone audio generator built into the K3) from my EA5 QTH and monitored the received signal at a SDR (WebSDR) receiver in YO-land on 14 MHz (2,500 km path). The transmitted spectrum of this signal is two continuos, equal and constant amplitude carries spaced 1 KHz.

You would expect the amplitude of both signals at the remote end equal at all times as they are two carriers separated only by 1 KHz transmitted with equal powers from the same transmitter, same QTH and with the same antenna .... Well, you were wrong

Fading is frequency selective and I found a difference between the amplitude of both signals ranging from -5 to +25 dB (with a mean of roughly 0 dB) over a 140 second time period. The fact the maximum difference is not symetric around 0 dB is just random and due to the limited duration of the experiment.

So if I had used single time samples of the amplitude levels from these signals (the data provided in terms of SNR from RBN), I could have estimated a -5 to a +25 dB difference between both signals when the average difference is 0 dB.   Remember this is two signals with equal power transmitted from the same TX and antenna, let alone if signals are originated in different transmitters at different locations (and so with a different ionospheres between them) with different antennas and not sampled at exactly the same time instant.

So be careful how you interpret the RBN supplied SNR numbers.

73 de Juan EA5RS

 

PS: If someone is interested I can supply the recorded audio of this experiment as well as analysis data

 




from [Tim Duffy]
[Permanent Link][Original]







To:
"'Bill Tippett'" <btippett at alum.mit.edu>, <topband at contesting.com>

Subject:
Re: Topband: data follow up to W4ZV and K2AV comments

From:
"Tim Duffy" <k3lr at k3lr.com>

Reply-to:
k3lr at k3lr.com

Date:
Wed, 28 Aug 2013 22:02:07 -0400

List-post:
<topband at contesting.com">mailto:topband at contesting.com>









For the record:

40 meter antennas at W3LPL:
Two full size 3 ele Yagi's at 190 feet over 100 feet - 48 foot booms.

40 meter antennas at K3LR
Three full size 4 ele OWA Yagi's at 260 feet over 190 feet over 118 feet.
48 foot booms. 7 take off angle combinations are available.

QSOs for 40 meters in this contest:

K3LR 1767 Europe and 144 Asia
W3LPL 1686 Europe and 122 Asia

73,
Tim K3LR

-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Bill
Tippett
Sent: Wednesday, August 28, 2013 6:17 AM
To: topband at contesting.com
Subject: Re: Topband: Effect of trees- tree appreciation

         Long delayed response to

http://lists.contesting.com/pipermail/topband/2013-August/041954.html

K2AV wrote:

"By common
expectation LPL and LR should have a propagation advantage over NY4A.
LPL and LR both have excellent stacked 3 or 4 element 40m yagi's. But
note how as the fourth mode is engaged, both LR and LPL fall off
because they are not cleanly engaging the mode, most likely because
the increasing elevation angle is starting into a notch in the yagi
vertical pattern.  Also note that NY4A carries the best signal for
most of the 24 hours. This is an evaluation of the NY4A 40 EU quad vs.
known excellent installations that is hard to argue with."

 From the ARRL results database, sorted by maximum 40m QSOs (after 
log-checking):

http://bit.ly/19YAF1f  don't worry Frank...it's OK  :-)

Rank            Call            40m QSOs

1.              K3LR            2000
2.              W3LPL           1911
3.              WE3C            1862
4.              KM1W            1818
5.              NY4A            1809

K3LR could be expected to have a higher total because of better prop 
to JA but the other 4 should have had similar propagation to EU, 
which would dominate their results.  There's more to this story than 
RBN spots from a single location shows.

73,  Bill  W4ZV


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