Dear Peter, LF Group,
I tried some rough field strength measurements on the Loran noise in the
136kHz band, in an attempt to confirm your calculations.
A a field strength reference, I injected a known signal EMF into a
single-turn loop RX antenna, using a clamp-on current transformer. The
transformer primary has 25 turns, so the induced voltage in the loop is 0.04
times the sig gen output voltage. The equivalent field strength is
calculated using the formula V = 2.1e-8*fNAE, which at 137kHz means the
induced loop EMF for 1uV/m and a 4m^2 loop is equivalent is 11.5nV,
requiring a sig gen output of 0.29uV. I used an RA1792 RX and SpecLab to
compare the amplitude of this test signal with the Loran lines. The idea of
this set-up is that preamp and RX gain does not affect the comparison, and
the only thing that has to be assumed is the relation between loop EMF and
field strength.
Using this system, I made a quick survey of most of the 136kHz band. The
Loran lines are not equal in amplitude; a variation of about 10dB can be
present between adjacent lines in a repeating pattern (see the attachment,
which is actually the "wanted" Loran signal, where 0Hz on the scale is
100.000kHz - the pattern of amplitude variation is also similar throughout
the 137kHz band, but obviously the overall amplitude is much smaller). I
logged the minimum and maximum Loran line amplitudes in 200Hz segments,
given in dBuV/m:
136.0 - 136.2: -6.1, -15.3
136.2 - 136.4: -6.8, -16.6
136.4 - 136.6: -6.5, -17.0
136.6 - 136.8: -8.6, -14.6
136.8 - 137.0: -5.3, -14.1
137.0 - 137.2: -5.9, -16.3
137.2 - 137.4: -6.9, -14.6
137.4 - 137.6: -6.1, -16.4
137.6 - 137.8: -7.0, -16.2
So, give or take a few dB, the overall level of Loran noise is fairly
constant over the band, allthough individual lines can differ by 10dB or
more. Averaging together all the min and max figures in dB gives a "typical"
level of -11dBuV/m. Rugby is roughly 100km distant from here, so if ERP =
(Ed)^2 /50, and typical field strength is 0.28uV/m, the "typical" Loran line
has an ERP of 16uW.
I also tried measuring the ERP of the signal at 100kHz. The problem here was
that the max. RX bandwidth is 6kHz, and the pulse width is noticeably
stretched compared to the pictures of the theoretical Loran pulse, so the
peak amplitude is probably reduced by the filter. I measured the peak
amplitude of the Loran pulses at the RX IF output on a 'scope, and compared
this with a CW reference signal in a similar way to the previous
measurements. The peak field strength worked out to 19mV/m, making the peak
envelope ERP 72kW. I also used a true RMS mV meter (Racal 9301A) to compare
peak and RMS amplitudes at the IF output. These were 57mV and 8.5mV
respectively (not sure what crest factor the 9301A can cope with; for the
Loran signal it must be quite big, but I used the 30mV range instead of the
10mV range to hopefully reduce error from this source), which makes
peak/mean power ratio 45:1. Thus the mean ERP would be 1.6kW. Due to the
reduced bandwidth, these figures for power are probably a bit low. When I
get time, I will get the covers off the RX and see if I can't repeat the
measurement in a wider bandwidth.
Comparing to Peter's figures, it would seem that my measured values are
lower than his calculated values by something of the order of 10dB; 0.128 mW
ERP at 100km should result in a field strength of about 0.8uV/m,
or -2dBuV/m for each Loran line, wheras I get values between -5
and -17dBuV/m. My ERP figures for the "wanted" 100kHz signal are lower than
what Peter assumed, but that could be expected because of the restricted
bandwidth of the RX I used, which is only about 1/3 of the nominal Loran
band. But I would note that most Loran TX antennas seem to be of the 1000'
or more umbrella variety, while at Rugby, I think they must be using one of
the old "T" antennas which was presumably the one used for the 73.25kHz
signal that used to be such a pain. When we visited the site in 2003, I
guessed the T antennas, which were slung from the main 820' masts, but at a
level well below the VLF top load, were at somewhere around 600' high, so it
might be that the Rugby Loran has significantly less ERP compared to typical
Loran stations.
Hope this is of interest,
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: Peter Martinez <[email protected]>
To: <[email protected]>
Sent: Friday, March 03, 2006 9:54 AM
Subject: Re: LF: LORAN spurious emission levels
> From G3PLX:
>
> I calculated the radiated power of a LORAN line now, but I would like
other
> people to check if I did it right, because the result is surprisingly
small.
.....
>
> If we estimate (from the info given by Marco) that each LORAN transmitter
> radiates 7.5kW mean power, then the radiated power in each LORAN line is
> 1.71e-8 * 7500 = 0.128 mW.
>
> I don't know if I should believe this! Can anybody find a mistake or at
> least check my calculations? All the figures needed for the calculations
> are in this email. Does anyone have any evidence that any of these figures
> are wrong?
>
> 73
> Peter G3PLX
>
>
>
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