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Message-ID: <5.1.0.14.0.20020724171432.00a84530@gemini.herts.ac.uk>
Date: Wed, 24 Jul 2002 18:57:37 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: DL2NDO/P
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Dear Mal, LF Group

At 16:01 24/07/2002 +0100, you wrote:
>For those that like figures and data to speculate, invent or guess erp 
>emitted by others on LF, I measured the signal today from DL2NDO/P and 
>other associated callsigns used at various times.
>on 136.5 khz.
>
>Level meter  SPM-12 (calibrated)                       Time 1020 utc
>
>Signal level -50 db
>Noise floor -90db
>Signal above noise -40 db
>
OK then - the problem here is that these are only relative measurements, so 
it is not possible to derrive field strength or ERP from them. But we can 
estimate a value for the field strength of DL2NDO/P at your QTH using the 
previously determined ERP of around 160W. At a range of 845km, and using 
G4FGQ's software with "type 4" ground again, this comes to 31dBuV/m. This 
gives us the basis of a rough calibration for your antenna/receiver. The 
bearing from your QTH to Donnebach is about 125degrees, while your ant has 
maximum pick-up on 0/180 degrees, so the effective signal level is reduced 
by a factor of 20 log[cos(180-125)], which is -4.8 dB. Therefore, if your 
antenna were aligned on Donnebach, you would get a signal level of -45.2dB. 
This makes the "antenna factor" (31+45.2) = 76.2dB. Then, to measure the 
field strength of other signals, you will have to calculate a further 
factor of -20log(cos [theta]), where the angle theta is the difference in 
bearing between the source of the signal,and the maximum of the antenna, to 
compensate for the signal and antenna not being aligned (one reason why it 
is handy to be able to turn a loop!). Add this factor plus the antenna 
factor to the signal level reading to get field strength in dBuV/m - eg:

-50dB (sig level)  +4.8dB (bearing correction) + 76.2dB (antenna factor) = 
31dBuV/m

-Back to were we started.

This is only a very rough calibration of course, since it is very indirect 
with many sources of error, but it gives you a reasonable idea, so better 
than speculating, guessing and inventing. You can do a similar calculation 
with DCF39 to check. Of course, changing the tuning, input impedance, 
antenna etc will need a  re-calibration. Using these figures, your noise 
floor works out to -13.8dBuV/m in 25Hz, or about -3dBuV/m in 300Hz, which 
is quite similar to what I get here under good conditions.

You could also do a similar calculation using SpecLab to calibrate the 
signal level - here you have to take care not to adjust the RX gain (and 
keep the AGC turned off) between calibrating and measuring. To interpret 
the noise level you would have to take the bandwidth/resolution settings 
into account.

.....No attempt here to interpret or calculate voltage at rx input....

This depends on how you have set up the SPM12 - if you have set it to read 
dBu, then the reading is in decibels relative to 0.775V, so is a direct 
measurement of voltage anyway. If set to dBm, then it is measuring input 
power, and you would have to take into account the input impedance to get 
the input voltage.


Cheers, Jim Moritz
73 de M0BMU