FT-817 Automatic Loop
April 30th 2012 Update: Version 1.0 Released on Google Code
First things first:
"I live in the US" Legal Disclaimer:
Magnetic Loops are extemely dangerous. The extremely high voltage that develops can cause serious injury or even death. Use Magnetic Loop at your own risk. Actually, you should never use a Magnetic Loop. Please don't. If you do I will not be responsible in any way.
Also CAT interfaces are very dangerous. The undocumented 0xBD command could damage your FT817 in an irreparable way. You should never use the CAT interface of your FT817 in the way presented in this article. Please don't. If you do I will not be responsible in any way.
I really like my portable magnetic loop antenna and I believe it is the best compromise in terms of performance and portability. My Loop for HF is so small and light than I normally transport it on my bike. Some others use their Magnetic Loops as pedestrian mobile.
There is, however, something unpleasant about the Magnetic Loop experience that the user cannot avoid noticing: tuning. If you ever tried one you know what I'm talking about: each time you change the operating frequency - even just by a few KHz - you will have to tune the antenna again. What a nuisance! Wouldn't it be nice to have a loop that magically tunes itself?
Well, here it is! This project is a proof of concept for an Automatic Magnetic Loop Antenna: you press a button and the antenna is ready to be used. Such a loop will be portable, cheap and finally also easy to operate.
The project is based upon arduino Arduino, more specifically an Arduino UNO but I think any other variant will be just the same.
The initial idea was to use a SWR directional coupler and use the feedback to drive the logic of an Arduino board. While investigating the various options for the coupler I realized that my FT817 already measured the SWR so started asking myself if it was possible to somehow "read" such information from the outside. Apparently it was possible thanks to an undocumented CAT command (0xBD) that was created by Yaesu just to do that. Moreover, an Arduino CAT library for the FT817 had already been written by VE9QRP and published under GPL.
The necessary Servo to drive the variable capacitor is a Hitec HS-322HD and Arduino has a Library for it that works right off the box. The Servo will position itself within a range of 180 degrees in just a few second. Just add a LED and a pushbutton and you are ready to go.
The VE9QRP library doesn't compile on latest one (Arduino 1.0) so I had to port it.
Then I added the missing 0xBD command that is necessary to read the SWR from the radio. After some debugging (done with a USB / TTL interface and a terminal emulator) I was ready for the actual tuning script.
The first attempt sampled the entire 180 degrees - one degree at a time - and performed an SWR sampling for each step: the lowest SWR sample indicated the position I was looking for.
It worked almost immediately. Then, in order to obtain faster tuning, I modified the procedure: first the capacitor moved in "big steps" of 5 degrees each, then in "small steps" of 1 degree. At the end I opted for yet another solution, which sampled continuously the SWR to determine the arean in which the tuning was possible.
Thanks for reading, best 73's de Alain WW3WW