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From: "Laurence KL1X" <hellozerohellozero@hotmail.com>
To: rsgb_lf_group@blacksheep.org
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Date: Sun, 10 Jun 2007 18:28:14 -0800
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Subject: Re: LF: 136kHz tests - M0BMU ERP
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Nice experimentation - too noisy to receive anything from you but appreciate 
the signal  whist in XE


Now in San Antonio TX -
Laurence KL1X/5


>From: "James Moritz" <james.moritz@btopenworld.com>
>Reply-To: rsgb_lf_group@blacksheep.org
>To: <rsgb_lf_group@blacksheep.org>
>Subject: Re: LF: 136kHz tests - M0BMU ERP
>Date: Sat, 9 Jun 2007 12:50:50 +0100
>
>Dear LF Group,
>
>Thanks to all for the reports on the beacon signal. The DFCW30 beacon ran
>through until about 0400utc this morning.
>
>The purpose of the tests was to do some field strength measurements with a
>new mobile measuring antenna set-up I have been working on recently. This 
>is
>meant for determining transmitted ERP and antenna efficiency on 136k and
>500k. I am still in the process of calibrating the system, but it should be
>quite accurate now. I initially used my antenna in inv-L configuration
>(about 40m long, 10m high top section) with about 1kW TX power and made
>field strength measurements at 38 locations between about 1km and 6km 
>around
>my QTH, and then calculated the ERP for each location. Most of these were
>within +/- 2dB of the average; I rejected 3 that were outside this limit as
>probably being affected by the receiving location. The average of the
>remaining 35 came to 0.18W ERP. I then repeated the measurements at 6 of 
>the
>former locations, with the centre of the antenna span raised to 14m with a
>fibreglass pole, making an inverted-V configuration. This increased the ERP
>to 0.59W average (most of the time, the DFCW30 signal was being transmitted
>at this level).
>
>The antenna current was close to 4A for both configurations, but the loss
>resistance for the increased height antenna was reduced from about 63ohms 
>to
>51ohms, so lower TX power was required. The overall efficiency works out to
>0.010% for the inv-L antenna configuration, and 0.040% for the inv-V
>configuration. So the fairly modest increase in average height of the top
>wire of the antenna increases the overall efficiency by a factor of 4. The
>effective height calculated from the antenna dimensions is about 9m for the
>inv-L and 12m for  the inv-V. But calculating Heff from the field-strength
>measurements gives 4.5m (inv-L) and 7.8m (inv-V). This could be interpreted
>as the effective height of the antenna being reduced by the screening 
>effect
>of the surrounding trees, buildings etc. From these figures, about 4m is
>knocked off the effective height of the antennas, giving substantially less
>ERP than what would be calculated from the antenna dimensions and current.
>
>I did similar measurements back in 2003 with similar antennas, and the
>antenna efficiency was better by a factor of 3 or more then. The loss
>resistance has also increased substantially, from 37ohms for the inv-L and
>29ohms for the inv-V. This is not so surprising, because several trees
>around the antenna were just seedlings then, but now they have grown to a
>similar height to the antenna wire.
>
>These are just preliminary results, but they do show how important it is to
>get the maximum height possible for the antenna. Unfortunately, this is
>usually the most difficult dimension to increase!
>
>I hope to repeat these tests on 500kHz in the next few days.
>
>Cheers, Jim Moritz
>73 de M0BMU
>
>
>
>

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