Magnetic Loop FT817





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Published on May 7, 2013

Homebrew QRP Magnetic Loop Antenna with auto-tuning control unit linked to FT817 CAT serial port. http://www.gm4wzg.co.uk/antennas.html

Update - April 2017

Thanks for the encouraging comments over the last few years. Really it was just a proof of concept which turned out to be very successful. There are a few concerns about operating procedure and the possibility of causing interference to others. I had indeed given that some consideration. The unit needs to be calibrated once on setting up in a new location. It carries out this calibration itself automatically, transmitting at the bottom and top end of each band and tuning to get the best SWR. Superficially, it might seem anti-social to not check if the frequency is in use first. On closer examination, it takes longer to send QRL? twice than it does for the tuner to tune up. Also, for at least 80% (just a guess guys!) of the second or two of tune up time the antenna is hopelessly out of tune and radiated power is negligible. Remember that the rig switches to the lowest power mode possible to do this (100mW if I remember rightly ?). At no time is the unit unattended while this is taking place.

Nevertheless I have modified the software to not actually transmit if there is evidence of activity on the frequency, using the S-meter output to gauge the occupancy. Since it will now only transmit on what appears to be a quiet frequency it now truly is less intrusive than the case of an operator listening for 20 seconds then sending QRL? twice before commencing to call.

It's not easy to take a one-off project and put it into production. I toyed with the idea but the selling price would have been prohibitively high. Experts in production techniques might be able to do it cheaper but I'm retired and I do this as a hobby so please accept my apologies for not taking this any further.

Likewise it is surprisingly difficult to fully document and support a project like this. Something I find a challenge anyway. There is no circuit diagram and the software is not in a very readable form. I occasionally answer specific questions about how I did one or two things and the folk who have designed their own versions seem to have been very succesful.

A final thing I would say is that I enjoy using loop antennas, and its nice to have remote control of them. But in truth, the autotuning nature of the thing always does seem to impress folk, but in reality its 99% about having a loop antenna and getting on air with it rather than worrying about making it autotune. If you fancy making a loop for the first time, then don't hesitate - there are thousands of sources of info and completed projects on the web and you'll find it very satisfying - Arduino or not !

Thanks for your support and understanding




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