I lost my ATAS 100 a few months ago and decided that I wanted to try something a bit different, and a bit more effient for mobile operation. So I decided on the Little Tarheels II (200 watts) antenna, which covers from 6 to 80 meters, but eventually settled on the Little Tarheels HP (High Power, 500 watts), 6 to 40 meters. The the antennas are not that much larger than the ATAS antennas, still fits on a car, but is more efficient.
But, most importantly, I didn't want to loose the automatic tuning from the FT100 to the ATAS100 (or an ATAS120). So I built a circuit using an Arduino, a voltage regulator and a relay card (2 relays). The parts cost me under $15 plus a few junk box parts. Incidentally, this same controller will work for any larger screwdriver antenna with minor mods (mentioned below). Oh, and this should also work for the FT857 as well, which is compatible with the ATAS antennas.
There are a couple other things required, such as a small inductor (about 100 to 150 microhenry) and a few .01 uf caps for filtering and a 1 to 2K pot. This circuit is used to detect the voltage from the FT100 (8 volts for Down and 11 volts for Up) and one of the analog inputs on the Arduino is used read the voltage (divided down from 11 volts to under 3 volts so as not to damage the Arduino input which has a limit of 5 volts). As I mentioned, I ended up with the Little Tarheels HP antenna, which has a coil to ground for matching. This would ground the voltage from the FT100, so I had to add a .1 uf cap (needs to be 600v or larger) in the coax line. This circuit is essentially a "bias tee", something similar to:
I used another Arduino analog input to read the antenna motor current. I have a 5 ohm resistor in series with the motor to ground. When ever this voltage jumps, which indicates the motor stall current, I know that the motor has hit one end. Keep in mind that the ATAS100 has a mechanical clutch that allows it to spin for a long period of time without damage. Basically, if the antenna is moving down and hits the stall current, we just turn off the relay, back it up just a bit to take the strain off the antenna and motor, and then just wait for the voltage to change direction. If we hit the stall current going up, the antenna is lost and so we just run it down to the bottom.
So, from that point on, just monitoring the voltage and moving the antenna up or down just follows what the FT100 would send to the ATAS100(120). Switch bands, press "Tune" and it moves the antenna until a match is found!
I used the my new antenna and circuit on my trip from southern MN to western SD and it worked great. I really like the fact that it works exactly like the ATAS100 for tuning, but I had a wonderful time on 40 meters in a conversation for about 45 minutes with a guy in Washington state. I was also on 20 and 17, wrapped up in a couple long conversations. I can't give you a side by side comparison of antenna signal strengths, but I do firmly beleive this is a MUCH better antenna. Time will tell.
This was my first attempt at the circuit and the only disappointment is that I found a number of birdies in the receiver (mostly on 75M, which I don't use), but I'm certain that I can easily eliminate these with some bypass caps and putting the the circuit in a metal box. The current case has a metal base plate and a plastic covering, it's what I happened to have laying around to build this first circuit.
If there is interest in this project for other's to duplicate, I could publish the circuit and the code required to duplicate this project for others.