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AN OLD ANTENNA COUPLER (....As Used By An Old Radio Operator!) Recently, I've had several questions about the antenna coupler you've seen in some of the VK3BVW radio shack pictures. It's a fabulous old unit and I use it every day. So this post and the YouTube video below take a closer look at the Tokyo Hy-Power Labs HC-500A. So, what's an antenna coupler? It is a impedance matching device, used between the antenna and the transceiver or receiver. It is often referred to as an antenna tuning unit (ATU) or simply an antenna tuner. But actually that title is really a bit of a misnomer! In fact, it has no influence on, or tuning of, the antenna itself. The insertion of an antenna coupler in the circuit allows for the maximum transfer of RF energy from a transmitter to the antenna, and for the optimum impedance matching (and hence energy transfer) between the antenna and a receiver. The Tokyo Hy-Power Laboratory was founded in 1975 by Nobuki Wakabayashi, JA1DJW with its factory near Tokyo in Saitama Prefecture. The company's first products were the HC-500 and HC-2500 antenna couplers for the HF bands. Several years later, new products appeared that included high quality linear amplifiers. Later, they added wide-band mobile linears, automatic antenna tuners and solid-state high powered RF amplifiers. Regrettably, the company finally closed its business on December 24, 2013. You can learn more about the company by heading to the Tokyo Hy-Power Users Club . Manuals and circuit diagrams for most of the company's products are still available on the Users Club website . I purchased my HC-500A during 1978, making it 42 years old. It's been a long-time partner in my radio hobby activities, and I love it! I expect it will be still going strong for another 42 years.....although I personally don't expect to be still going that long!! https://medxr.blogspot.com/2016/10/an-old-antenna-coupler.html 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, a reconfigured topology appeared entitles 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. Above, transmission loss is 0.54dB, efficiency is 88.3%. 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 note, 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. 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. 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 on this load. Inductor The inductor values are 0.09,0.17,0.9,2.5,6.7,8.9 µH.