Return-Path: Received: (qmail 17766 invoked from network); 26 Sep 2001 13:16:11 -0000 Received: from unknown (HELO warrior-inbound.services.quay.plus.net) (212.159.14.227) by excalibur.plus.net with SMTP; 26 Sep 2001 13:16:11 -0000 Received: (qmail 26451 invoked from network); 26 Sep 2001 13:14:01 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by warrior with SMTP; 26 Sep 2001 13:14:01 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 15mENp-0001hV-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 26 Sep 2001 14:05:49 +0100 Received: from cmailenv2.svr.pol.co.uk ([213.218.77.54]) by post.thorcom.com with esmtp (Exim 3.16 #2) id 15mENk-0001hJ-00 for rsgb_lf_group@blacksheep.org; Wed, 26 Sep 2001 14:05:44 +0100 Received: from [62.21.157.140] (helo=w8k3f0) by cmailENV2.svr.pol.co.uk with smtp (Exim 3.22 #1) id 15mEN2-0007wQ-00 for rsgb_lf_group@blacksheep.org; Wed, 26 Sep 2001 13:05:01 +0000 Message-ID: <004a01c1468d$43af9cc0$8c9d153e@w8k3f0> From: "Dick Rollema" To: rsgb_lf_group@blacksheep.org References: Subject: LF: Re: Field strength measurements by M0BMU Date: Wed, 26 Sep 2001 15:10:46 +0200 MIME-Version: 1.0 Content-Type: text/html; charset=windows-1252 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2314.1300 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Content-transfer-encoding: 8bit

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 -----

Van: James Moritz

Aan: rsgb_lf_group@blacksheep.org

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