[Lf] [Fwd: LF: Puckeridge Experiments]

[Andre' Kesteloot]

James Moritz wrote:

> 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