At HC-500 I showed some VNA plots of the HC-500 matching a 50+j0Ω load at 3.5MHz.
The following commentary is on a single load scenario, a 50+j0Ω load at 3.5MHz, and while the results are not simply extensible to other loads and frequencies, it does provide some interesting insight into the devices.
THP HC-500 (Ultimate Transmatch (McCoy 1970))
Above is the behaviour of the unmodified HC-500 (an Ultimate Transmatch).
Losses at match are 12% of input power. At its rated 500W maximum power, that is 60W (which might seem high but heat tolerant insulation materials are used). On modification to a T match, losses at match were reduced to 8% or 40W at rated maximum power.
Continue reading A tale of three tuners
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%. Continue reading HC-500
At Improved cooling for the MFJ-949E I described a modification to the ATU to improve its cooling using a fan and run on timer.
The run on timer described was based on a Chinese STC15F104E DIP8 8051 like microcontroller.
Because the programming tools for the STC chips work so poorly, and the lack of documentation of their protocol, there is no simple way to update only the calibration data in EEPROM. I have ported the algorithm to an ATTINY25 which doesn’t cost a lot more but had a much better development environment and a range of tools to allow EEPROM update without overwriting the FLASH image, and as well it will run my bootloader, ATB.
This article describes a generic run on timer based on an Atmel AVR chip, a ATTINY25 though the code will also run in ATTINY45 and ATTINY85.
The circuit is very simple, the DC output from the forward power detector is connected to the input pin which turns the BC548C transistor on at input voltage greater than about 0.7V. The high value of base resistor ensures very light loading of the forward power detector.
Continue reading A generic run on timer using an ATTINY25
Sixth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
This article documents measurement of impedance.
The antenna system is a G5RV with tuned feeders (9m of home made 450Ω open wire). The tuned feeders terminate on the balun described in this series, and it is located on the outside of the antenna feed entrance panel shown above. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6
Fifth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
Installation / testing
The balun packaged in a non-conductive housing was designed to have minimal stray capacitance to ground to minimise common mode current with asymmetric loads.
Above, the balun is attached to the exterior side of the antenna feed entrance panel using a male to male N adapter, done up very tight. The feed line connections are liberally coated with marine grease to prevent ingress of water and oxygen, a measure to reduce corrosion. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5
At A look at internal losses in a typical ATU I demonstrated that it is quite easy to raise the temperature of the coil in the MFJ-949E to an unsafe level, even with quite modest power.
The most heat sensitive component in this ATU is the coil, specifically the coil supports which are probably polystyrene, and the glass transition temperature of polystyrene is around 100°.
This article documents modification of my MFJ-949E to reduce the risk of damage under some operating conditions. Continue reading Improved cooling for the MFJ-949E
A correspondent asked for a walk through of use of a couple of my online design tools for a 6m 350W single ended valve PA using three QQE06-40 valves. The request was perhaps inspired by a design he had seen, but I sound a caution about a large number of parallel valves (6 sections in this case).
Key design parameters:
- HV power supply fully loaded: 1200V;
- Power output: 350W;
- Class: AB1;
- Vak min: 180V (from datasheet anode curves);
- Pi output network, Q at least 12 (for reduction of harmonics on the FM broadcast band), select 15.
The datasheet gives max supply voltage at 450 for plate and screen AM, which implies max ‘instantaneous’ DC supply voltage in AB1 SSB telephony of 900V… so 1200V goes beyond the guaranteed ratings. Of more concern, it is probably close to 1400V lightly loaded, 56% greater than the maximum instantaneous supply implied by the AM specifications.
- output network efficiency 90%;
- valves load share equally.
An advantage of a high Q design is that it requires higher input C which makes accommodating the self capacitance of the 6 valve sections somewhat easier. A disadvantage is lower efficiency.
Above is a calculation using Calculate initial load line of valve RF amplifier. Note the anode dissipation, a total of over 190W is quite high for 120W total valve rating (though these are pretty robust valves). Total DC anode current will be almost 0.5A, so the HV power supply must deliver 1200V @ 0.5A. The 0.5A figure is within the absolute maximum of 720mA (for 6 sections).
Continue reading A tutorial on initial design of operating conditions for a valve amplifier
There seems to have been a revival in use of the so-called End Fed Half Wave antenna.
The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.
Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.
A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (σ=0.005, εr=13).
Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima. Continue reading Thinking about SOTA, EFHW and EMR safety
Fourth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a weep hole would be advisable in a permanent outdoor installation).
Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.
The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.
Differently to the example shown in the earlier articles, this prototype uses twisted PTFE insulated wires which have voltage breakdown higher than the XLPE shown earlier.
The self resonant frequency of the built balun was measured as 7.4MHz and the predictive model above calibrated. The balun has high choking impedance on the lower half of HF.
Next installment: Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5.
Calculate initial load line of valve RF amplifier was written as a companion to my RF power amplifier tube performance computer tool to provide a starting point for building a model, but as it turns out, the initial load line (and related values) is a very good estimate and further modelling may not be needed.
Although written for an application to valves, it is quite applicable to any active device, keeping in mind that it assumes a linear transfer characteristic.
The update provides for both single ended and push-pull configurations.
For example, the requirement is for a single ended Class C bipolar amplifier to deliver 25W from a 13.8V DC supply. What is the ratio for a broadband output transformer to 50Ω.
Above is the solution. The required Rl is 3.3Ω, and the required turns ratio is (50/3.3)^0.5=3.9. a 1:4 (turns) transformer would be selected for a prototype. Bear in mind that output power would fall to around 20W at 12V DC supply.
Another example is the common 100W 13.8V Class B push-pull design.
With a requirement for around 3Ω collector to collector (or drain to drain), a transformer with 1:4 turns ratio would be selected.