For the last month whilst testing my 472/500kHz transverter I've been using a highly compromised antenna. Essentially it is a very short Marconi tuned against my central heating system ground. The first 6-7m is a coax cable running next to pipes in the loft horizontally. Eventually the coax runs outside and up 5-6m to my 6m/10m halos. The coax inner and outer are strapped together when used on 500kHz. In the last week we have had a new central heating boiler fitted, this time in the loft, so there are even more copper pipes and wires running close to the coax. Reports appear to be a few dB worse on 500kHz as a result.
The time has now come to design a "no compromise" 500kHz Marconi! Looking at the geometry of my garden I should be able to erect a 5-6m vertical away from the house with some 10m of horizontal top loading. I can add some inductance near the top of the vertical section and tune it all against a reasonably decent ground (4 x 1m long earth spikes, a couple of radials and the central heating copper). Not sure how soon I can work on this but I would hope the ERP increase could be up to 6dB. At least with WSPR (and OPERA) one can do a series of tests over several days with each antenna and compare results. It will be easy to measure antenna current which gives a good indication of ERP when the antenna effective height is calculable.
26 Jul 2012
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4 comments:
Dear Roger,
I am experimenting with an L-shape wire antenna. Results are disappointing. I am considering a multiwound magnetic loop antenna for 600m. What's your opinion on magn loop antennas for this band ?
73 Ron
PA2RF
Ron, my single turn 80 sq m area loop using multistrand PVC covered wire worked well on 500kHz and even on 136kHz. It was tuned with a capacitance switch box in the house (use high voltage capacitors). Use as thick wire as you can manage as this affects the losses and hence the ERP. Ideal if you have lots of trees nearby which absorb energy from a vertical Marconi. Trees also make good supports.
Ron, should have added there is no advantage on TX of a multiturn loop. It is better to make a single loop with the wire (thicker) as the Q will be higher and losses lower.
Tnx for the advice Roger,
Will continue to experiment on this band. Local contacts in wspr and opera with a simple transverter works ok. My first dx on 600m is not made (yet). A one-wire loop as you describe might be someting... Good weekend, 73 Ron PA2RF
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