Results of antenna tests

[Andre Kesteloot]

James Moritz wrote:

> 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