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Sunday 26 March 2017

thoughts turn again to the magnetic loop antenna

With the Blackpool rally now 2 weeks away, my attention has again turned to the magnetic loop antenna I plan to build for 10-12 metres (though 12-metres may not be immediately possible due to lack of equipment for that band), sizes, capacitor values, even the feedline from the antenna to the radio is now being worked out.

The capacitor can probably be sourced from the Blackpool rally, the copper tube to make the main loop is available from Wickes as it is the cheapest I have seen it, Homebase don't sell it for some unknown reason and B&Q sell it for more than Wickes, I hope to get the tubing this Saturday for both the loop itself (copper) and the support (plastic), the coupling loop can be made out of RG58 or some other coax with the braid and centre shorted together, or the same copper tube as the main loop.

The design I had seen by Larry WD0AKX also incorporated a meter, though I plan my final build based on this design it will have an enclosure over the capacitor which will contain a motor on a gearbox to tune the loop with the capacitor and I will keep an SWR meter on the feeder, for initial tests though tuning will be done with the capacitor and a plastic tool to avoid stray capacitance from my hands, my final build will also be suited to 50 watts (maximum on the Intermediate) if I find a capacitor that can handle the resulting voltage but until I get my intermediate I would only transmit at up to 10 watts, for CB use I'd always be at 4 watts or 12 watts PEP for SSB on the midblock.

Naturally I also want both my CB and my 10-metre rig to both be connected so I don't have to go behind them to swap the coax over, this requires a switch, preferably one capable of dropping the output of the "disconnected" radio into a dummy load and the "connected" radio onto the antenna, such a switch does exist, my good dummy load is, however, rated only to 50 watts, though this would protect the finals of both radios, though the SS9900 does have a HI-SWR function that prevents TX if SWR exceeds a preset limit however I prefer the reassurance of a load always being there, as repair of the finals is something I have no time to do, this idea makes sense as the President Grant II does not have a high SWR safety cut out, much like many off the shelf CB radios.

Hopefully I can start gathering what I need to build the magnetic loop antenna this week save for the capacitor which I hope to get from Blackpool or Ripon if it comes to it.

73 de M6RSQ

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