Re: LF: Re: M0BMU 500kHz /P

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Thanks for the information Jim. The 15.3m fibre glass pole looks useful. I presume this was one bought from an internet supplier and that it collapses down for transportation.  I can just make out some guy wires in the photo.

Next time I'll get better organised for a 2-way QSO.

73s
Roger G3XBM

On 29 March 2011 00:47, James Moritz <james.moritz@btopenworld.com> wrote:

Dear Dennis, Roger, LF Group,

Thanks for the additional reports for M0BMU/P. Here are some details of the station - see the attachment for pictures:

The TX antenna was a 15.3m fibreglass mast with 4 x 10m long top loading wires sloping down to just over 9m. The ground system was 4 x 1m ground rods spaced on a 2m circle round the base of the mast. The antenna was tuned with approx 300uH fixed inductance near the base of the mast, and connected to the 18 - 70 uH fine tuning variometer in the shack about 15m away by a transmission line wire about 1.5m above the ground. The antenna ground was also connected to the tuner via a return wire about 300mm off the ground. The tuner contained the variometer, ferrite cored matching transformer, and a common mode choke to isolate the antenna ground return from the shack ground. This arrangement puts the tuning and matching controls in the shack, and I found it was easy to adjust, with no problems due to unwanted resonances or ground loops. The total antenna resistance was only about 11ohms, including 2 or 3 ohms due to the loading coil and variometer, so antenna current was approx. 3A with 100W PA output. I estimate the antenna effective height was 11m, giving Rrad of 0.53ohm, and ERP of 8.6W.

The receive antenna was a single K9AY terminated loop, oriented to reject wideband noise coming from the Brookmans Park BC site to the west, and give good reception of Eu stations from the east. I found a terminating resistor of about 200ohms gave a good null - this is rather lower than the values quoted for the HF bands. The RX antenna was positioned about 50m from the TX antenna, to minimise interaction, and give the RX preamplifier an easier life!

I used my 200W EER transverter rig, although just for CW on this occasion, driven by an IC-718 as the IF. The minimalist shack ("hutch" might be a better name for it :-)) was supplied with 130m of mains cable going to an outbuilding and into a slightly dodgy mains socket, but much less hassle than a generator or batteries. It was also quite handy to be able to follow the cable and find the gate in the dark!

Cheers, Jim Moritz
73 de M0BMU

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