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PIC Based Antenna Tuner

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LAN_403

Dave Marshall

Wed Aug 01 2007, 04:34AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I'm about to undertake the construction of a mobile HF antenna known commonly as a 'screwdriver antenna'. Basically its a heavily inductively loaded vertical antenna that uses a battery powered screwdriver motor to adjust the inductance of the loading coil, and thus tune the antenna.

Its a pretty elegant and clever design, and with a good controller, they are pretty effective and very easy to use. The only problem is, the higher quality controllers are criminally expensive. $120 is on the low end.

Since I'm building the antenna itself, I figure I'll build the controller too. I've already got a manual tuning control figured out, using a magnet on the shaft of the motor and a stationary sensor to measure a position on the coil relative to the starting position. Along with a couple binary decade counters and a Binary-to-Seven segment LED driver to count each rotation, this should let me tune the antenna roughly once I get aquainted with the antenna's characteristics. Fine tuning would then be done with the SWR meter of the radio.

I'm interested in a way to add memories to the controller. This is a feature only the most advanced controllers offer, but is extremely handy for use when driving. My basic idea is to manually set the antenna to the proper location, have a PIC poll the binary counters, and store the value into memory. When I push button #1, it pulls the value from the counter back up, and proceeds to move through the coil until the counter output matches the stored output. Just how complicated would something like this be to implement? I'm fairly inexperienced with PICs, but I do know a bit.

Dave

Carbon_Rod

Wed Aug 01 2007, 06:15AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

You could use a simple USB-to-LPT and a 2803 Darlington PNP driver for two 0.7' steppers.

Personally I have not experimented with LF/HF adaptable systems since I was a kid and came across one of these... (I recall its intended use was as a variable inductor.)

Cheers,

Dave Marshall

Wed Aug 01 2007, 09:42AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Thats a very common sort of setup for what is often called a 'rotary inductor', used in higher quality antenna tuners and the like.

Mine won't be quite so complex. Because its going to be mounted on a car, moving parts would almost certainly end badly. Debris, precipitation, low flying birds, etc.

The typical screwdriver antenna setup has the coil telescoping into the mast of the antenna, with a long piece of threaded stock being the only thing that rotates. As the stock rotates, a nut at the bottom of the coil pushes it out of the mast, or pulls it in depending on the direction of travel. Copper finger stock at the top of the antenna mast makes contact with the coil, so what ever portion of the coil is outside the mast is your load.

The mechanics of it are pretty much set in stone. I kinda got chased off the computer in the middle of that post, so I had to rush. My only real question is if it would be feasible to give the tuning control box memories to store coil settings.

As an example, say I want to transmit on 14.300Mhz. The antenna has a tuning range of 7-54Mhz. 7Mhz would be with 100% of the coil extended outside the mast, 54Mhz would be with the coil completely retracted. Say for this example, the antenna has a 1:1 SWR with the coil extended 4". With 24 threads/in stock, that would equate to approximately 96 turns. I'd like to be able to set the antenna to 96 manually using the up/down tuning buttons on the control box, then push a button and have the PIC poll the binary counters that are keeping track of how many times the thing goes around. It takes that value and stores it in memory, and assigns it to a certain button on the control box.

I'm going to build the controller so that the antenna automatically retracts when power is removed from the controller (handy, because the counters aren't going to remember the last setting before power was removed). When power was restored, I'd push button 1, and it would start extending the antenna until it detected that the counters were again displaying the number 96.

I've screwed around with PICs quite a bit, but only doing simple repetative tasks like turning stuff off and on, blinking LEDs, etc. I've got very little experience working with more dynamic tasks, or working with memory. Would a middle weight 8 bit PIC like the 16F870 be capable of something like this?

Dave

Wolfram

Wed Aug 01 2007, 11:08AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Why not just implement the counters in the PIC instead of using external counters? Should be much simpler.

If you're going to buy a PIC, get an 18F, something like 18F252 should work fine. Many of the quirks that make assembly programming on 16Fs annoying at times are nonexistant in the 18Fs, and C compilers are easier to find, if that's your cup of tea. If you already have a 16F, it should be more than capable of the task.

Bjørn

Wed Aug 01 2007, 12:31PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Most PICs have at least 64 bytes or more of built in EEPROM so they are fine.
Any PIC with enough pins will do nicely.
Remember to have some sort of end stop switch so it does not get more and more confused about the position. (and remember to account for failure of any detector so the program does not keep the motor running until something burns up)
Do as much as possible in software, like the counters and 7 segment conversions.

It is not a complicated project but it will take some time if it is the first PIC project on that scale. It could be done in a day if you can ask someone that can point you in the right direction each time you get stuck.

Dave Marshall

Wed Aug 01 2007, 03:15PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Like a certain admin type person who will be visiting me in September, perhaps. :)

I was pondering the whole situation earlier, and sketched a perfectly effective way to do it with only analog logic. It would require about half a dozen 14 pin chips per memory entry though, and wouldn't be easily reprogrammable (would take a minute or three per entry to reprogram). It might do for a temporary controller until the PIC based system is up and running though.

I'm actually stuck on the end stop switch. The only easy way to do it will be to have the switch on the exterior of the antenna, which just exposes it to the elements.

I was made a very tempting offer for a commercial antenna today, so I may not build the antenna itself. My design was pretty much a blatant copy of the model I may purchase though, and it will need a controller. I'll just have to wait until I get the thing so I can figure out how an end stop switch could be rigged.

Thanks for the advice so far guys.

Dave

Bjørn

Wed Aug 01 2007, 04:36PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If the design is solid enough you can run it in one direction until you are 100% sure it would have reached the end and the motor is stalled, or add a simple current detector so you can stop it instantly when the motor is about to stall.

Dave Marshall

Thu Aug 02 2007, 05:29AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I had considered that. I don't really know how it would be done, but it seems like it would be fairly straight forward. I went ahead and bought the commercial antenna. One of these:

I should have it a couple days after I get home next week. I'm gonna start off with analog tuner for starters with 2 or 3 memories, just so I can have something usable for my upcoming trip home to Pennsylvania.

Dave

Bjørn

Thu Aug 02 2007, 02:45PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I hope they do posting and handling better than product photography or they will send it to Reykjavik in a pigs bladder.

There are many ways to detect the end point. The are A/D converters in the PIC to monitor the current very accurately. There is the rotation sensor. You could add a fast resetting polymer fuse to stop the motor.

Dave Marshall

Thu Aug 02 2007, 03:12PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Haha! I think all those photos are taken by customers, rather than the company itself. I've tried it before. Taking a reasonably decent photo of something thats 3" wide and 8' tall is not easy!

I'm actually astonished at how well this logic based tuner is coming along. I'm in limbo here waiting for airlift home in 3 days, so I've got little better to do.

I sketched out the entire thing, including all the detals, and I'm quite surprised how easy it is. Granted, it would take some 15 ICs, but could have all the features the PIC will offer. I'm pretty much just going to wing it with a basic logic based counter for my road trip later this month, no memories, no auto retract etc.

After that, I'll have Chris R visiting for about two weeks, so I can corner him and improve my PIC-fu.

I'll look into those options for stopping the motor. Not familiar with the sort of fuse you referenced, but I'll definitely give it a look.

Dave

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