To have an idea of what kind of spread of SNRs one can expect from skimmer
reports, I performed a test this morning
I recorded two overlapping CQ messages with two different callsigns, one had
the sidetone on 500 Hz (EA5RS) and the other at 900 Hz (EA5BY), and applied
it to the MIC input of my SSB/USB transceiver (just like you would do in
AFSK). I first equalized amplitudes to make sure both tones generated equal
output. I then waited for the skimmer reports (and answered to those who
called!).
So it is the same power, QTH, antenna, ..... only selective fading can
introduce a difference between data reported for both "stations"
Some skimmers reported only one of the calls (some BY, some RS) and some
reported both.
I cannot guarantee the skimmers decoded both calls exactly at the same time
(maybe there are some seconds difference)
Here are the reports from the skimmers that decoded both calls at the same
minute:
21046.6 EA5RS 19-Aug-2014 0714Z CW 12 dB 34 WPM CQ
<DJ9IE-#>
21047.0 EA5BY 19-Aug-2014 0714Z CW 12 dB 34 WPM CQ
<DJ9IE-#>
21041.6 EA5RS 19-Aug-2014 0711Z CW 27 dB 33 WPM CQ
<SK3GW-#>
21042.0 EA5BY 19-Aug-2014 0711Z CW 25 dB 33 WPM CQ
<SK3GW-#>
21046.6 EA5RS 19-Aug-2014 0708Z CW 22 dB 33 WPM CQ
<SK3W-#>
21047.0 EA5BY 19-Aug-2014 0708Z CW 26 dB 34 WPM CQ
<SK3W-#>
21044.6 EA5RS 19-Aug-2014 0704Z CW 29 dB 34 WPM CQ
<SK3W-#>
21045.0 EA5BY 19-Aug-2014 0704Z CW 30 dB 33 WPM CQ
<SK3W-#>
14047.6 EA5RS 19-Aug-2014 0655Z CW 7 dB 33 WPM CQ
<DK0TE-#>
14048.0 EA5BY 19-Aug-2014 0655Z CW 7 dB 34 WPM CQ
<DK0TE-#>
21044.6 EA5RS 19-Aug-2014 0652Z CW 23 dB 35 WPM CQ
<SK3GW-#>
21045.0 EA5BY 19-Aug-2014 0652Z CW 24 dB 35 WPM CQ
<SK3GW-#>
I think this shows averaging is necessary if you want precision
measurements.
73
Juan EA5RS
-----Mensaje original-----
De: Topband [mailto:topband-bounces@contesting.com] En nombre de Tom W8JI
Enviado el: lunes, 18 de agosto de 2014 20:26
Para: topband@contesting.com
Asunto: Re: Topband: Skimmer calibration
Since no one likely knows the gain of a reference antenna within a dB or so,
splitting hairs doesn't matter.
Ten dB would jump right out, while 3 dB might get lost in the QSB.
When I was comparing high dipoles to verticals on 160, I collected reports
for about a year. It was thousands of reports.
I probably could have done it in a week or two with skimmer, but then I
would have had to repeat it for seasonal changes. I'm sure a good test
protocol using skimmer could be worked out.
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <topband@contesting.com>
Sent: Monday, August 18, 2014 12:47 PM
Subject: Re: Topband: Skimmer calibration
> On Mon,8/18/2014 4:53 AM, Tom W8JI wrote:
>> A live comparison of S/N ratio or relative level over time is with very
>> few exceptions an excellent comparative test. It is much more accurate
>> than S meters or absolute levels without a comparison reference. As such,
>> the RBN is a great tool for evaluating systems.
>
> Yes. BUT -- my experience has been that I must average hundreds of data
> points to get meaningful data. The reasons are simple -- we must contend
> with QSB, and as Tom noted in another post, nulls in the patterns of
> antennas at both ends. A few years ago, I tried to compare two 160M
> antennas using JT65 and W6CQZ's JT65 RBN. On a good night, I would see
> reports from 3-4 stations east of the Mississippi. I alternated between
> the two antennas for hours, putting the reports in a spreadsheet, and
> studying the data. Modelling predicted differences of a few dB, and I
> never found that the data was good enough to confirm the models. The
> antennas are passive arrays of fairly tall verticals of a quarter wave or
> less, so there are no vertical nulls in their pattern. I can clearly hear
> their directivity on RX, but their gain is what I was trying to confirm.
>
> 73, Jim K9YC
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2014.0.4745 / Virus Database: 4007/8057 - Release Date: 08/18/14
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|