LF Antenna Setup at GB4RS

Andre' Kesteloot akestelo@bellatlantic.net
Sun, 18 Oct 1998 09:27:35 -0400

G0MRF@aol.com wrote:

> On 10/11 October, the special event station GB4RS was operational from the RSGB
> HF convention at Windsor.  The call was primarily used on two bands.
> 40m, where a large 4 square phased array was running "contest style" and also
> on 136 kHz LF.
> The LF demonstration station [...] was installed Friday afternoon by John
> G3WKL,  Peter DJ8WL, Graham G3XTZ and me.
> With any LF station.,  as much wire in the sky as possible, and even more in the
> earth, is a
> reasonable starting point.  The venue for the convention was a large country
> house with some very impressive Cedar trees and a  lawn that would do credit
> to any 18th green. - No possibility of digging that up for some buried
> radials!
> John came armed with fishing reel, catapult and lead weights and soon had a
> line over the top of a 70 foot high tree which was around 200 feet from the
> building.
> The antenna wire was from an old top band dipole.  The centre feed point was
> bridged with a short length of wire so it could be fed from the end and
> operated as a long (ish) wire.
> With the wire pulled up to within a few feet of the top of the tree, we were
> able to run it to the top floor of the building - abt 35 feet above ground,
> where it was tethered about 15 feet from the brickwork, and finally down into
> the room to the transmitter and matching components.  The total length ended
> up at 250 feet in an inverted L configuration.
> The earth was necessarily simple: A single length of copper pipe. Around 3
> feet of which went into the ground outside the operating room. A single
> insulated radial was also used. This was run around the base of the building (
> to avoid the ever present lawn mowers) and connected to a lightning conductor
> (!) at the far end. Total length 200 feet.
> At the transmitter, the 50 Ohm output was initially fed to a variable inductor
> and then  via an RF ammeter to the antenna.
> The station was switched on at 16.30 to see if our efforts would actually
> radiate.
> Ideally there should be a matching transformer between the TX and the loading
> coil, however this can be omitted if the system has a feed impedance thats
> reasonably close to 50 Ohms   With the TX keyed, we saw about 1 Amp of current
> in the antenna. A slight adjustment to the loading coil brought the system to
> resonance producing 1.5Amps of antenna current.  Total loading inductance  was
> 2.4mH.
> It was clear from the way the system was behaving that a transformer was
> needed, however when we went back to receive, we were immediately called by
> Derek G3HEJ and several others!   An LF pile up....Well almost.
> Before we could finish tuning the system Graham and Peter worked :
> The addition of a matching transformer showed the best power transfer occured
> at the 50 : 90 Ohm level.  Total current for the rest of the weekend 1.7 to
> 1.8 amps. Power output about 200 Watts.
> Saturday was expected to be quiet on the air as a lot of the UK operators were
> at the convention.  Even so GB4RS added
>       PA2NJN,  EI0CF G3XTZ and PA0SE to the log.
> An impressive display of photos, and items of equipment generated a lot of
> interest in the LF display as did a small fluorescent tube flashing in time
> with the CW.