Alinco DX-70 to SGC Tuner Interface

Download the Schematics

I recently aquired an SGC-239 automatic antenna tuner that I couldn't wait to get installed in my Chevy Lumina where I already had an Alinco DX-70TH and a 40M Lakeview Hamstick. With all the enthusiasm in the world, I recently sat down to hook the Alinco DX-70 and the SGC auto-tuner together to make them play, but my spirits soon dropped. Fortunately, I was able to dig up a few staple electronics parts and put this project together in a few evenings.

The Problem:

The problem is quite simple - the DX-70 and the SGC-239 tuner were never designed to work together. Alinco did provide an Accessory port on the back of the radio as well as a tune button on the front panel, however some interaction with the tuner (or this interface box) is necessary for anything productive to happen. In a nutshell, this interface box's purpose in life is to wait for the tune button to be pressed, start the transmitter, and wait for the SGC tuner to finish it's job. It is a simple circuit for a simple task.

Theory of Operation:

As I've already stated, the circuit is very simple. If you study the schematic, you will see a few signals coming from the tuner and a few from the transmitter going into the microprocessor, a Microchip PIC16F84. All of the magic happens within the 'F84 and therefore a minimal amount of circuitry is involved.

You may also notice an extra five lines coming in from the DB9 plug that attaches to the Alinco. These aren't normally used - they're simply for programming the 'F84 while it is in-circuit therefore eliminating a potentially troublesome IC socket.

The way that the Alinco DX-70 is designed to work is that the Tune button on the front panel (actually Function-Tune) is pressed, which produces a signal on one of the pins at the accessory port. This signal lasts only a short time (about a second) which is picked up by the 'F84. Once the 'F84 sees that the operator wants to activate the tune sequence, it turns around an sends a PTT signal back to the Alinco through a 2N3904 transistor switch. At this point, the DX-70 begins transmitting a low power continuous-wave (CW) signal on the frequency of choice.

Once the SGC-239 tuner sees an incoming RF signal, it measures the frequency of the incoming signal and compares it to the last known good tune combination (i.e. it looks to see if you've changed frequency). If you have changed frequency, it will begin its tune sequence by comparing the forward and reflected powers - the SWR. It will try to tune the 50 ohm coax from the radio to whatever load is attached to the output port on the SGC. This is essentially a trial-and-error process, similar to how you would manually tune the antenna.

After a period of time, depending on how hard the SGC had to work to make the match, the SGC will hopefully send a signal back that it has tuned the antenna. It is possible for the SGC to give up because it is unable to make a match, however in practice, the Alinco will time-out long before the SGC is able to iterate through all possible tuning combinations. This would seem to be a serious problem, but in my experience, it normally works just fine. If the SGC absolutely won't tune a particular frequency within the given amount of time, often you can fake it out by shifting off frequency slightly and trying again (mine works the first time on almost all frequencies, through 40M which is where the antenna is roughly tuned for).

The 'F84 watches the SGC's signal to see if it has sucessfully matched the load, and if it has, it produces a Morse Code 'K' on a piezo element and stops the transmitter. If however the tuner has been unsucessful in making a match after a period of time (about 20-30 seconds), it stops the transmitter and sends three long tones to indicate a bad match.

Construction and Implimentation

The circuit is very simple and could easily be built on a piece of Radio Shack breadboard. I chose the PCB route to keep the board as small as possible and designed it around a small plastic enclosure purchased from Radio Shack.

As you can see, the board is fairly compact, but still very easy to work on. I used a single-sided PCB and only wound up with one jumper. The black round device towards the bottom of the board is the piezo element. The two DB-9 connectors are opposite gender and the signal lines are arranged in such a way that it would be possible to supply power to the tuner without this interface in circuit. By suppling steady RF to the tuner, it would tune the antenna without this interface - you just lose all status feedback and would have to manually key the transmitter using a CW key or a switch.

I used a Dremel routing bit in the Dremel drill press stand to cut out the plastic for the two DB-9's to fit through the side. The board was designed to fit within the box and uses the four lid screws to hold it in place. I also drilled a hole above the piezo element to aid the sound in escaping the enclosure.

The PIC16F84 does need to be programmed with the firmware provided at the bottom of this page. Anyone who has dabbled with Microchip PIC processors undoubtedly will have a programmer for the 'F84. With the propper adapter cable, you can actually program the chip after it has been soldered into the PCB. If you just want to build the interface and not worry about re-writing the firmware, I would recommend programming the chip before construction and skip the building of an in-circuit programming cable.

After putting the lid on, i slid my box under the front seat near the transmitter so that it can be heard from the driver's seat.

Tuner/Antenna Installation

I also want to touch briefly on how I connected and mounted the SGC tuner as well as my antenna configuration. As I stated above, this unit is installed inside of a 1993 Chevy Lumina sedan. The DX-70 is mounted (okay, swedged :-) ) between the two front seats under the middle of a bench seat. I purchased a separation kit for the remote head and mounted it to the front of the console. The interface box is left sitting on the floor-board under the passenger seat.

Tuners are most efficient when placed as close to the antenna feedpoint as possible. The whole reasoning is beyond the scope of this article, but it has to do with radiating feedlines and excessive coax loss due to high-SWR's. I was able to mount my SGC within about four feet of the base of the antenna which includes a little bit of slop that allows me to still open the trunk lid. I crawled in the trunk of the car (literally... I wonder what the neighbors thought?) and found a metal plate that stands vertically that supports to top of the back seat. Being close to the antenna and out of harms way, it was perfect.

I pulled the cover off the SGC which has four holes in the bottom of it, apparently for mounting. I marked these holes on the metal plate and very carefully, as not to go through the back seat, drilled pilot holes for four sheet metal screws. Before actually attaching the SGC cover, I used some steel wool to remove some paint so the SGC would have a good electrical connection to chassis ground. You can see in the previous picture the fruit of my efforts.

SGC has a funny way (in my humble opinion) of connecting antennas and feedlines to their tuners - they use screw terminals which are sometimes several inches apart. The feedline coming from the DX-70 can be seen in the picture above. It was just standard 50 ohm coax with terminals soldered onto the center conductor and braid. The antenna side of the connection was a little different - see the picture below.

Since the output terminals were separated by several inches, I was somewhat weary of using standard coax and leaving the center conductor exposed. As it turns out, I happened to find a short length of RG-55 double shielded coax (it actually has two separate braids for the shield) in the junk box with a BNC connector on the end. I was able to fish the center conductor and inner braid through the outer braid, thereby keeping the coax shielded most of the trip. I put a short piece of heat shrink over the lose braid to prevent it from shorting out against the center conductor. See the closeup below.

This piece of coax is about two feet long and terminates into a BNC connector that is left hanging near the top of the trunk lid.

My antenna configuration is a little varied depending on the situation. Under normal circumstances when I just need some basic HF communications for a few days, I have a 5 inch magnet that I use to hold a Lakeview 40M Hamstick. With a couple of 50lb test fishing line guy wires into the clothes hangers inside the back doors, this antenna setup has shown to be reasonably stable (except when you're backing up quickly into low-hanging trees, right Jon?). This setup has the advantage that it is quick and easy to setup and take down, and is relativly low-profile. The disadvantage was that before the auto-tuner, I had to carry multiple antennas in order to operate on multiple bands.

I also have a Lakeview Carolina Bugcatcher that I put out occasionally. This obviously draws a lot more attention which in itself makes it worth the effort. :-) Because of the increased size, I always use a triple-magnet pad with the same guy wires described above. I have yet to have any problems with this configuration (structurally) even after months on the vehicle and at least two trips to Colorado (in the woods). The advantage to this system is that it is semi-multi banded by changing a tap on the coil. The disadvantage to this was that it takes a lot of effort to place and remove those three big magnets, and consequently I have three rings on the trunk-lid where the magnetism has altered my paint.

Conclusion

Although I have only recently installed the tuner, I think that it will open up a new world of flexibility in when and how I operate HF in the mobile. Hopefully the increased flexibility will help foster more of a willingness to actually turn it on and operate, rather than just talking about it.

PCB Patern

View the source code

Download the firmware (machine code for 16F84)

Created: 10 January 2005  02:52   Last Modified: 22 January 2005  22:46