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Date: Thu, 14 Jun 2007 14:58:19 +0200
To: rsgb_lf_group@blacksheep.org
From: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
In-Reply-To: <001f01c7ae26$3474ab40$c2137ad5@w4o8m9>
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Message-Id: <20070614125512.BD7D431E702@smtps01.kuleuven.be>
Subject: Re: LF: M0BMU ERP on 500k
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Hello Jim,

based on radiation resistance it should be 13.5 times better on 504 
kHz [ (504/137)^2 ].
I guess that the remaining factor of 4.4 [ 59/13.5 ] is due to lower 
losses on 504 kHz.

BTW:
Simulating your 10 x 40 m inv-L antenna on gives a radiation 
resistance of 0.36 Ohm on 504 kHz (versus 0.026 Ohm on 137 kHz).
With 0.4 A antenna current the radiated power is (should be) 58 mW.
Taking into account the theoretical gain of a short vertical (4.8 dBi 
/ 2.6 dBd) the EIRP should be (have been) 173 mW EIRP or 105 mW ERP.
It seems that even on 504 kHz still a lot of power is "absorbed" near 
the antenna.

73, Rik ON7YD

At 03:49 14/06/2007, you wrote:
>Dear LF Group,
>
>To complement the 136kHz field strength measurements done a few days ago, I
>have now made some measurements on 503.8kHz. The average of 57 measurements
>with an antenna current of 400mA came to 43mW (-13.7dBW), and the antenna
>efficiency 0.59%. This is for the 10m high, 40m long inverted L, which was
>one of the antenna configurations used for the 136k measurements. The
>antenna efficiency on 500k is 59 times greater than on 136k - illustrating
>how much easier it is to "get out" on 500kHz, in spite of the poor
>efficiency on both bands.
>
>Qualitatively, the field strength results were rather similar to previous
>experience at LF - a few dB variation in ERP occured between measuring
>sites, with most of the results being within 2dB of the mean value. I
>rejected 4 measurements that were made along the road where the overhead
>phone line to my QTH runs - as was the case on 136k, some of these showed
>several dB variation from the average, and it seems the phone line acts as
>an extension of my antenna over quite a wide frequency range (unfortunately,
>not a practically useful one, though - it clearly does not do much for the
>overall ERP, I suspect it is also responsible for coupling some of the local
>QRM into the TX antenna). I made measurements at distances between about
>230m and 6km according to the GPS receiver, and a wide range of different
>directions - plotting ERP against distance did not reveal any particular
>varying trend, indicating a F.S. proportional to 1/distance relationship
>holds over this range. Plotting ERP against bearing indicated no detectable
>directional effects. This is what you would expect for an electrically small
>500kHz vertical antenna.
>
>I also re-measured the dimensions of the antenna, since various changes have
>been made over the years. I calculated a new value for the effective height
>of 8.3m (previously, I had used 9m). Working backwards from the ERP
>measurements, the effective height at 503.8kHz is 5.8m, and at 136kHz it is
>4.5m. Whatever is reducing the radiation from the antenna is frequency
>dependent, by about -3dB at 500k and about -5dB at 136k.
>
>Cheers, Jim Moritz
>73 de M0BMU


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