My 6-Meter Junkbox Tuner

Shopping in the local emporium found me face to face with a Tokyo Hy-Power HL602V 6-Meter amp, which is their 200 watts beast. I had been considering one of these for a while to complement my FT-950, which I use on 6-Meters. Knowing how long they take to arrive when ordered, the quality, and also that the price was only going to increase I decide to buy it.

Having got it home (and smuggled it past the XYL) I installed it and tested it into a load. It achieved the stated 200 watts output but the first thing it needed was a fan. Running 200 watts SSB got it warm but at just 100 watts carrier for RTTY it was generating serious amounts of heat!

Fortunately THP provides a DIN socket on the rear of the linear with various output. This included switched +VCC. I modified the mobile mounting bracket to carry a small PC fan. Although it's a bit noisy the finished result looked okay and provided the necessary cooling for those long RTTY overs. A little more careful work saw the SO-239s in the junk box and replaced with a nice pair of N-Type females.

Having spent so much on the amp I was concerned about running it into a bad VSWR. My HB9CV would easily take the power but was presenting the amp with about 1.5:1 which is quite a few watts of reflected power when running 200 watts. What I needed was a tuner to protect my investment.

I had seen a couple of designs on the net so I decided to have a go and build my own. The design used two variable capacitors and an inductor in a pi network. But the designs I saw tended to use plastic enclosures to avoid interaction with the coil. As I was going to run 200 watts I decided a metal enclosure would be safer and more stable. To avoid interaction the sides of the box would need to be 3.5 times the diameter of the coil from the coil.

So I needed a fairly large enclosure for this project. -- The most suitable product I found is provided by Maplin Electronics. Their enclosure AB15 is 202mm in length, 152mm deep and 76mm tall.

http://www.maplin.co.uk/Module.aspx?ModuleNo=1728&criteria=enclosure&doy=29m7#Faq

The last two figures were the most important. The coil would only be 3x the diameter of the coil from the top and bottom of the box but I felt this would be okay.

Aluminum has to be the easiest metal to work with and 20 minutes had all the holes drilled and punched ready for all the components. The tuner didn't have to match a very broad range so the coil is 3 turns of 2.5mm copper wire at 3/4" diameter I had a couple of variable capacitors in my junk box which were about 50pf. I made sure the capacitors were fixed well and all the joints soldered. I suspended the coil from the wires between the coaxial sockets and the capacitors so it was in the middle of the box (away from the sides). This also makes it easy to remove and adjust or replace the coils.

Initial testing on low power (50 Watts) went well; the tuner easily matched the antenna. I screwed the box closed, re-tuned and everything worked fine running 100 watts. I increased the power while carefully watching the reflected power. The unit needed a small re-tune but everything still seemed fine.

On the air I went back to an EA calling on SSB from southern Spain. The band was closing and he was just about readable but gave me a 59 in return so the amp and tuner were doing their thing.

This tuner handles plenty of power but has a limited range. Increasing the range with a larger inductor and capacitors would work for tuning HF antennas for 6M but if you do this tuning may become more fiddly and I would suggest decreasing the power.

The ATU needs some cosmetic finishing + knobs but it will do for now... I can beautify it when the Sporadic E season ends!

Member Comments: