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To All from PA0SE
Jim, M0BMU, deserves great praise for his fine job of measuring
field strength at a large number of locations and I
certainly would like to receive the graphs he made. Thanks Jim in
advance.
I could not resist the temptation to model Jim's aerial using
K6STI's computer program AO.
This yielded a radiation resistance of 0.017 ohms. At an aerial
current of 2A radiated power becomes 0.017 * 2 * 2 = 68mW. This is
EMRP, Effective Monopole Radiated Power. A short vertical monopole has a gain of
1.829 (2.62dB) over a half wave dipole in free space. As ERP is related to such
a dipole calculated ERP at M0BMU is 1.829 * 68 = 124mW. Jim mentions 150mW
ERP but he also says the aerial is slightly over 8m average height. I used
8m for the simulation and this may explain the difference between 124W and
150mW.
AO also permits to compute what is called the "near field" but
in reality is the total field (near field plus far field) up to a distance of
2km. I did this for Jim's aerial in steps of 200m. Both
the electric component E of the field in V/m and the magnetic component A
in A/m are computed.
In the far field E/H = 120 * pi = 377
ohms.
In the following table E/H is shown for Jim's aerial and for
my own one.
At
M0BMU
At
PA0SE
Distance (m) E/H
(ohms) Distance (m)
E/H (ohms)
200 493
200
536
400
256 400
262
600
286
600
288
800
317 800
318
1000
336
1000
336
1200
347 1200
348
1400
354
1400 356
1600
359 1600
361
1800
364 1800
364
2000
366
2000
365
Very near the aerial the magnitude of
E/H depends also on the configuration of the aerial, as
shown in the table for the aerials of M0BMU and PA0SE. But the difference is
already negligible at 400m distance. (My aerial is a T with the top of the
vertical leg at 18m height).
The tables show that even at 2km E/H is different
from 377 ohms, which means the influence of the near field has not yet
disappeared.
In the far field both E and H decrease in a linear way with
distance (at least in the region where field strength measurements are
being made, say up to 10 km from the transmitter).
So going from 1km to 2km both E and H should decrease by a
factor of 2.
The computer shows that at M0BMU that factor is 1.92 for E
and 2.09 for H. This also shows some contribution of the near field to
the total field. But for FS measurements the discrepancy
is probably small enough to be neglected; also taking into account
that other factors can cause variations in FS of several decibels.
In dry weather aerial current at PA0SE is 1.95A. Radiation
resistance is calculated at 0.043 ohms. So radiated power is 1.95 * 1.95 *
0.043 = 164mW EMRP = 300mW ERP.
At 5.83km distance, well into the far field zone, field
strength showed ERP to be 95mW.
A difference of 5dB.
But the measurement was made at one location only, though
selected carefully for the absence of disturbing metal objects. One never
knows what is below the surface of course but the FS did not fluctuate when
moving around a bit so it seemed a good location.
But Jim has shown us that for serious work a single measurement is
certainly not good enough.
73, Dick, PA0SE
---- Oorspronkelijk bericht -----
Verzonden: dinsdag 25 september 2001
17:18
Onderwerp: LF: <TECH> Results of
antenna tests
Dear LF group,
I have now made a total of 155 field
strength measurements on 135.923kHz, all using the same antenna with 2A
antenna current. I suspect this is the most detailed FS measurements yet done
on an amateur 136k antenna, and some of the results are quite interesting.
The antenna is an inverted L, with slightly over 8m average height,
and a single top wire 40m long running east (the feed end) to west. The
antenna current was measured at the bottom of the downlead, where it joins the
loading coil. I made measurements at distances between 148m and 8.1km, with a
wide range of different directions between TX and RX. The equipment used was
my homebrew, car- mounted, ferrite rod antenna and preamp, with an SPM-3
selective level meter. This combination was calibrated using a Helmholz coil
arrangement. Distance and bearing between TX and RX was measured using a GPS
receiver.
I worked out the ERP in dBW for each measurement, and
plotted these results against the distances and bearings at which the
measurements were taken, using a spreadsheet. The graph of ERP vs. distance
shows that at ranges below about 600m, the calculated ERP increases rapidly,
suggesting this distance is the minimum that could be considered "far field".
There are also several points where the ERP was 10- 15dB higher than average;
plotting ERP against bearing revealed that these occur in a sharp peak around
a bearing of 270 degrees, up to a distance of 1km or so. It turned out that
all these points were measured on Bulls Lane, a road going due west from my
QTH. Closer inspection showed that there are overhead phone lines on poles
along this road, one of which connects to my QTH. The overhead lines run
parallel to my antenna within 10m in places, so I expect the unusually high
FSs were due to signals being coupled into, and propagating along, the phone
wires. The wires go into buried cables after about 1.2km, and the field
strength returns to normal.
The area in which I live is a mixture of
urban and countryside; in urban areas there are many buildings, while in the
rural areas there are overhead phone or power lines by the sides of most
roads. This makes "ideal" locations for field strength measurements hard to
find, so most measurements were made at less than perfect locations. In spite
of this, the ERP figures for most points are within 2 or 3dB of the average,
showing quite consistent measurements can be made under these conditions. I
tried returning to the same location on subsequent days; the measurements were
within a 1dB range. Also G3NYK supplied some plots of the test signal received
at his QTH; these remained stable within 1dB or so as well. So one can be
reasonably confident that the observed variations are due to the location, and
not drift in TX output or RX sensitivity.
I looked at potential causes
of changing ERP results - apart from the phone lines and short distances, I
found that in at least one case, measuring on top of a motorway bridge
enhanced the signal level by a few dB. Once I had excluded these deviant
measurement points, the average ERP was -11.2dBW, equivalent to a (geometric)
mean of 75mW, compared to a calculated value of 150mW. There was little
evidence of directional behaviour of the antenna, although there is perhaps a
slight "bump" of 1 or 2 dB around 120degrees bearing. Interestingly. this is
roughly the direction the aerial masts at Brookmans Park transmitting station
are in, so they might be having a slight parasitic effect. However, there are
only a few points in this direction, and they were all taken on one road, the
Ridgeway, which as one would expect is elevated above the surrounding ground.
The conclusions so far - it is possible to make reasonably consistent
field strength measurements on amateur LF antennas, provided sufficiently
detailed measurements are available to identify disturbing factors, which can
cause errors of 10dB or more. In the mixed urban and rural environment in
which these measurements were done, one can expect variation of about +/- 3dB
between measurements - repeating a measurement at the same location gives
results within 1dB. The "far field", where field strength is inversely
proportional to distance, starts at about 600m distance in the case of this
antenna. At shorter distances, the field strength decreases more rapidly with
distance. The measured ERP is 3dB down on the calculated value in this case,
roughly in line with previous measurements. As would be expected, there is
little or no directional behaviour in a 40m long, 8m high inverted L at
136kHz.
If anyone would like to play around with the figures I have so
far, I have all the data in an Excel spreadsheet, which I could e-mail to
them. I originally tried to post a couple of the graphs to the reflector, but
it was having none of it, so if anyone would like to see the results let me
know, and I'll send them individually.
I plan to do similar tests using
different antennas, and also on 73kHz, in the near future.
Cheers, Jim
Moritz
73 de M0BMU
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