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
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