Dear LF Group,
Glad to see my intended experiments are generating some
interest. At the moment, my main priority is to get everything up to
Puckeridge in working order, which is quite an effort, but here are
some further details of what I hope to do, and the point of doing it:
The inverted L antenna will be located 130m from the main mast,
off the main ground system and with it's own ground spikes. The
coax feeder will run to the base of the mast, but will be decoupled
using a common-mode choke. The main antenna will be grounded
while the inverted L is transmitting. This should ensure the two
antennas have the least coupling possible whilst still being on the
same site. It will be possible to monitor RF current flowing in the
main mast while transmitting from the small antenna, so that the
degree of coupling can be assesed. It should be possible also to
try the main antenna 'floating', or resonated.
ERP will be calculated on the basis of estimated radiation
resistance and measured antenna current for each antenna. For
the inv. L this should be about 20 milliohms and 2A, for the main
mast about 0.7 ohms and 0.34A, both giving about 80mW radiated
power in theory. Final adjustments will be done based on the actual
current that can be achieved on the day into the small antenna.
By definition, two transmitters/antennas giving the same ERP will
yield the same signal strengths at equal distances. But a number of
things may modify the actual ERP obtained, in comparison with the
calculated figure:
i) The effects of the environment around the antenna - eg. houses,
trees, could absorb some of the radiated signal - is not taken into
account in simple calculations of ERP.
ii) Different sizes of antennas may have different radiation patterns
- some have suggested that small antennas have predominantly
high angle radiation, which does not propagate usefully for DX
communication. Theory says that this should not be a significant
factor unless the antenna is a large percentage of a wavelength
high, which even 100m isn't, at 136kHz.
iii)There are also other factors that may play a part, such as
penetration of the EM field into the ground, and radiation of
horizontaly polarised signals, which may affect field strength at a
distance.
Having a large commercial antenna and a small amateur antenna
at the same location, in the same environment, gives a simple way
of testing whether the theory as applied to large, commercial
antennas is also valid for the 'wet string' antennas that amateurs
have to use. This is important both from the regulatory viewpoint
(are you exceeding the 1W limit?) and the communications
viewpoint (just how much signal are you putting out?).
Measurements by myself and others suggest that most of the time,
signal strengths are down on what would be expected from theory.
Equipment details:
TX: 0-350W, VFO + Mosfet PA
RX: Homebrew superhet
Creaking 386 laptop for beacon and QRSS keying - no receive
software.
2 Tuners - 1 with motorised tuning for remote control of small
antenna, one with power attenuator for main antenna.
Sundry test gear for setting up and checking purposes - frequency
counter, selective voltmeter, RF ammeters etc.
1.5kW Generator
Since it is a one-man effort, I expect to have my hands full, and am
not sure what it will be possible to accomplish. However, the main
thing will be to get plenty of comparative reports. Field strength
measurements would also be very useful. I hope to also put out
some beacon signals, probably during Saturday evening, with 3
configurations, ie. inv. L antenna, 80mW ERP, main antenna,
80mW ERP, main antenna 1W ERP, perhaps around 137.2 -
137.3 kHz. I can't leave it on all night, mainly because the
generator will have to be re-fuelled, but I'll do what I can - let me
know if you have better ideas!
Cheers, Jim Moritz
73 de M0BMU
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