## A tutorial on initial design of operating conditions for a valve amplifier

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.

Assumptions:

• output network efficiency 90%;

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

## Thinking about SOTA, EFHW and EMR safety

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’ (0.005,13).

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

## SPICE model of Zcm of a Guanella 9:1 current balun

At Why the preference for Guanella 1:1 current baluns for HF wire antennas I compared a 1:1 Guanella balun with a 4:1 using the same component baluns.

Broadly, the findings were that the common mode impedance of the 4:1 balun was around a quarter of that of the component baluns, and mediocre at that.

This article extends the reference, documenting the SPICE model extended to a 9:1 balun.

Above, the same component baluns were interconnected to make a 9:1 balun. Note that Zo of the TL section has been increased to 150Ω to suit the notional nominal 50 to 450Ω broadband transformer. Continue reading SPICE model of Zcm of a Guanella 9:1 current balun

## APRS duplicate removal – trial #3

The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.

In an attempt to limit the propagation of posits and hence the probability of corruption / delay etc, I have experimented with a path of WIDE1-1 on a recent trip to Canberra (about 400km for the round trip).

Whilst this should prevent packets getting to the Wagga, Newcastle and Tamworth regions which have been the main cause of corrupted posits and mapping defects, it does so at the risk of some loss of posits as some digi infrastructure was never updated to the “New N paradigm” of more than a decade ago and they ignore WIDE1.

Above is a zoomed in view of the Canberra end of the trip, and I am pleased to say that the zig zag double backs that have been evident in recent trips did not occur. The principal reason is that with a path of WIDE1-1, the packets did not pass through VK2RWG-1/VK2KAW. Continue reading APRS duplicate removal – trial #3

## Chinese 18650 Li-ion cells – Ultrafire capacity test

I purchased a torch (flashlight) on eBay recently. It was described as using CREE T6 LED array, and supplied with two 4200mAh 18650 Li-ion rechargeable batteries with charger for A\$25 inc post.

Above, the cells are clearly marked 3000mAh, way short of the advertised 4200mAh… but what is their actual capacity.

Above are the results of discharge tests, the first digit is the cell number and the second is the test. The first test is charged with the supplied charger, the second test is with my charger. Continue reading Chinese 18650 Li-ion cells – Ultrafire capacity test

## End fed matching – analysis of VK3IL’s measurements

David, VK3IL posted EFHW matching unit in which he describes a ferrite cored transformer matching unit that is of a common / popular style.

Above is David’s pic of his implementation. It is a FT140-43 toroid with 3 and 24t windings and note the 150pF capacitor in shunt with the coax connector.

The popular belief is that these are a broadband impedance transformer with impedance ratio equal to the square of the turns ratio, 64 in this case and therefore a broad band match from 3200Ω to 50Ω.

To his credit, David took some measurements of several different variations and reported them in his article.

Above are David’s measurements of the subject transformer.

Lets explore the matching detail for the case of a 3.3kΩ load at 22MHz, and using the 150pF shunt cap.

Superficially, you might convince yourself that this is explained by the turns squared story, but the 150pF doesn’t reconcile with that story, nor does the variation with frequency, eg the rapid change above 22MHz. Continue reading End fed matching – analysis of VK3IL’s measurements

## ATU efficiency

Much is written about ATU efficiency, about the need for them or not, and often in subjective terms like “lossy ATU”, and most of it lacking quantitative detail.

The little quantitative detail is almost entirely for purely resistive loads… as if that is typical of real life conditions.

The most common configuration used today is the ‘high pass T match’, but a range of other configurations are seen as being superior… though usually without quantitative evidence.

MFJ claims

More Hams use MFJ-949s than any other antenna tuner in the world! Why? Because the worlds leading antenna tuner has earned a worldwide reputation for being able to match just about anything.

… so let’s make some measurements with a reactive load on a MFJ-949E. Capacitive loads tend to be very common for antenna systems at lower HF, so let’s choose a load of 50Ω with a 100pF silver mica cap in series at 3.6MHz. The reactance of the cap is -442Ω, so the load is 50-j442Ω, and the 50Ω part is a RF power meter (RFPM1).

The test setup then is:

• a standard signal generator (SSG) on 3.6MHz with 20dB precision attenuator so that we are confident that Zs=50Ω (important to the adjustment of the ATU for maximum power as indication of 50Ω match);
• MFJ-949E;
• 100pF silver mica capacitor (low loss);
• RFPM1

The SSG was adjusted for -10dBm out directly into the RFPM1, then the ATU+cap inserted and ATU adjusted for maximum power indication. Power indicated was 1.4dB lower, so InsertionLoss and TransmissionLoss are both 1.4dB.

Above is a simulation of the T network in RFSim99, component values are adjusted for a match and inductor Q is calibrated to the measured loss of 1.4dB. Continue reading ATU efficiency

## RG-6/U for lower HF

RG-6 has become a popular 75Ω transmission line for ham stations, and I have used it to good effect in many applications.

(Duffy 2007) extolled the virtues and gave implementation information, but cautioned:

Some types of RG−6/U use a CCS centre conductor and will have higher loss at low frequencies that shown in Fig 1, depending on the thickness of the copper cladding which may vary from cable to cable.

I have used RG-6/U with solid copper centre conductor widely on HF, and measured performance has always been consistent with expectation.

However, RG-6/U with solid copper centre conductor has become very hard to obtain, and products that remain available such as Belden 1694A are quite expensive.

This article documents measurements at low HF on a 100m roll of Quad shield RG-6/U purchased for UHF TV cabling.

The method used was to measure input impedance of the open circuit terminated 100m line section at a range of resonant and antiresonant frequencies, and from those to calculate Matched Line Loss (MLL) in dB/m.

Above is an example measurement around 3.74MHz. Zin is 213.4Ω at 3.74MHz. In this case I have used an AIMuhf one port analyser, but any instrument that can measure impedance in the range 10-500Ω would suit this particular scenario. Measurement of short low loss cables will yield more extreme impedances and may not be within range of some instruments. Continue reading RG-6/U for lower HF