Nags Head Report #5

[Andre' Kesteloot]

Estimating European Ham LF Transmission Signal to Noise Ratio
by Frank Gentges K0BRA

Prior to deploying an elaborate receiving setup, the potential signal
to noise ratio of European amateur LF transmissions may be estimated
based on measurements of the strength of European LF broadcast signals
and the noise on a quite channel.

If we measure the ratio of the LF broadcast station to the LF noise
alone, we can make estimates of how far below the LF broadcast the
amateur LF station would be radiating based on transmitter power
levels and antenna efficiencies.

Corrections can be made to the noise measurement to reflect any
decreased bandwidth of a more sophisticated narrow band receiver and
computer integration to be used in receiving the amateur LF signal.

Different propagation to the receive site can occur from the LF
broadcast site and the amateur LF transmitting site.  This can be
estimated as a delta function that can add to or subtract from the
nominal amateur LF signal strength.

These measurements can be made with relatively unsophisticated
selective voltmeters such as found in surplus such as Rycom or Hewlett
Packard.  Applying the calculated differences due to the different
ERPs and bandwidth/processing correction along with the estimated
propagation uncertainties will give the expected range of signal to
noise ratios of the received amateur LF station.  Based on this
information a decision can be made whether the station can be
received.

The relative ERPs can be calculated with a good degree of accuracy.
The noise  bandwidth / processing gain correction can be both
calculated and measured in the lab to a high degree of accuracy.

The propagation uncertainty is the least certain value to obtain.  It
reflects differences in frequencies, location and moment-to-moment
propagation variations.   While ground wave losses can be estimated
from simple mathematical models, sky wave losses are more complex.
Empirical measurements comparing with one another  the different LF
broadcast stations could be used to establish some values that should
reflect what would be expected with the amateur LF signals.

Once agreed-to values are available for each of these, a spreadsheet
can be used in conjunction with the selective voltmeter to quickly
assess the LF possibilities.

LF broadcast signal strength strip chart level recordings could be of
value in identifying when propagation is at a peak and a receiving
trip should be launched.  Since the LF broadcast stations are so
powerful the level monitoring setup does not have to be too special in
terms of siting and antennas.  More sophisticated computer monitoring
of these signals with a computer driven receiver could gather more
complete data.

Local noise may not reflect the noise that would be experienced at an
optimum receiving site.  The power line noise can be high enough to
hide any potential atmospheric noise and must be considered in
addition to raw meter level readings.

********************************************************