HC-500

In the early 1970s I purchased a Tokyo High Power Labs HC-500 ATU based on recommendation of other hams and the seller’s representations (Dick Smith Electronics) that it was a T match with 200pF capacitors.

The circuit configuration is of the so-called Ultimate Transmatch, an invention of (McCoy 1970) that claimed a bunch of advantages over the ordinary T match.

The HC-2500 would appear to use the same circuit.

It wasn’t long before several authors waded into the Ultimate Transmatch over its poorer efficiency. With an ambitious name like Ultimate Transmatch, it had a lot to live up to… but it failed.

Within months, an reconfigured topology appeared entitle the SPC Transmatch, but it also had issues.

The reality is that none of these designs is ultimate, they all have advantages and disadvantages and are mostly used in ignorance of those.

So, I have had this HC-500 which worked well enough I suppose, but was quite difficult to tune on some loads that ordinary T matches handled with ease. It has always been my intention to reconfigure it to a T match be rewiring the grounded stator of the input cap to parallel it with the other stator… a minimal modification to get rid of the shunt capacitor and use it to help to keep coil voltage down on some loads.

Before performing the modification, I measured transmission loss when matched to a 50+j0Ω load at 3.5MHz using a two port VNA.

Screenshot - 11_04_16 , 20_26_07

Above, transmission loss is 0.54dB, efficiency is 88.3%.

HC500mod

Above is a pic of the modification. The ground wire to the top end of the coil originally passed through the left hand stator solder tag, grounding the stator. The wire was removed from the tag and routed directly to the top end of the coil, and the bare copper wire added to connected the left hand stator in parallel with the right hand one so it is now a T match with input cap to 440pF.

As a side not, this is a beautifully built ATU, all insulating materials (switch wafer, capacitor cheeks, coil former) are ceramic, wire is heavy gauge, case is strong and generous space allowed around the coil, insulating shaft to the capacitor knobs. I have not seen a modern commercial ATU with all these features.

This modification increases minimum C of the input cap which might be an issue for some extreme loads on the highest bands.

Screenshot - 11_04_16 , 21_14_11

Above is the sweep of the modified ATU, transmission loss is 0.35dB, efficiency is 92.2%. Whilst not a large improvement, it validates analyses that showed the inherently lossy topology and supports the proposition that an ordinary T match is likely to be more efficient on most loads.

Screenshot - 11_04_16 , 21_16_12

The plot above was using the T match configuration but using the 3.8MHz coil tap. It can be seen that it degrades efficiency significantly… don’t use the 3.8MHz tap unless you cannot match using the 3.5MHz position.

References

  • McCoy, L. July 1970. The Ultimate Transmatch In QST July 1970 p24-27, 58.