From: "James Moritz" <[email protected]>
Reply-To: [email protected]
To: <[email protected]>
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
