RF  Power  Options

(Based on a talk given by VK2KU in April 2002 to the NSW VHF Dx Group)

Factors Affecting Station Effectiveness

• Location, low horizon in important directions

• Antenna Gain

• Feeder Loss

• Transmitter Power

• Receiver Noise Figure

Transmitter power is by no means the most important factor affecting overall station performance. Furthermore, increasing the transmitted power significantly is one of the more expensive upgrades to a station. However, sooner or later many operators decide to upgrade the output power of their station. What are the options on 144, 432, and 1296MHz?

Output Power of Common Transceivers

* transceiver option

Most of these transceivers are no longer available new, but are included here because some of them at least appear regularly on the second-hand market.

On 144MHz all commercial transceivers seem to have an output power of about 50W +/-3dB, a bit less on 432MHz (80W +/- 3dB), whereas on 1296MHz an output power of 10W is almost universal.

A realistic target is therefore an output power of 200W +/- 3dB on 144MHz and 432MHz, and 100W +/- 3dB on 1296MHz. How can we achieve these power levels?

Transistors or Valves (Tubes)

While the older generation of radio amateurs grew up with valve amplifiers and feel comfortable with them, it is a fact that many amateurs today have never used valves, and do not feel comfortable with valve circuits. Whether you go for a transistor amplifier or a valve amplifier, you can choose either to build it yourself or to buy off the shelf (or second-hand). A third option, especially in the case of valves, is to modify ex-commercial equipment, such as the well-known AM17 amplifier on 144MHz, built by AWA.

Here are some things to consider before choosing between transistors and valves:

• High power transistors are expensive,
• Valves are sometimes available quite cheaply, especially Russian types,
• High power transistors are easily destroyed in a moment's carelessness,
• Valves are relatively robust against damage, and will have a long life if run sensibly with adequate cooling,
• Commercial solid-state "bricks" are expensive new (e.g. $800 for 160W on 144MHz), and all models seem to require 13.8V at currents in the range 25A (for 160W) to 50A (for 350W). 24V bricks are not readily available although they would be more efficient, and easier to supply with DC.
• Commercial valve linears are also expensive, but available.
• Valve linears are comparatively easy to build, but require more elaborate power supplies, especially tetrodes. High voltage supplies of 2000V at 0.5A or more need to be built with great care, and with safety always a major consideration, but they are not to be feared, only treated with great respect.

Commercial Solid-State Linears on 144 and 432MHz

Not all 432MHz bricks contain a receive preamplifier.

Solid-State at 1296MHz

It is hard to find commercial bricks at 1296MHz, but it is possible to build one using several Mitsubishi modules M57762, each of which will give 18W output - cost around $150 each.

Commercial Valve Linears

There is no market in valve linear amplifiers with only 400W output, since most countries allow considerably higher powers than this. 

Henry market an 800W amplifier, which would be very comfortable at 400W p-p.

Lunar-Link (K1FO) make a 1500W amplifier.

Building Valve Linear Amplifiers

The basic choice is:

• Triodes in a grounded-grid configuration - gain about 13dB, or x20.
• Tetrodes in a grounded-cathode configuration - gain about 23dB or better, x200.

You will want to design for about 500W output, so that the amplifier is not working flat-out, so triodes require a drive power of 25W, tetrodes only 2.5W.

Valve efficiency in converting DC to RF will be up to about 62%, so you need to choose valves with a total anode dissipation of 400W to 500W minimum.

A good centrifugal air blower is needed, and this can be one of the harder things to source. Muffin fans are useless, being unable to generate enough pressure to move a sufficient volume of air along the constricted air path through the valve anodes.

Which Valve?

Pairs of valves may be used either in push-pull (W1SL design), or in parallel (W2GN, K2RIW designs).

Russian valves fall into two groups:

• Valves sold on the export market through Svetlana, such as the 4CX400a, slightly cheaper than equivalent Eimac types,
• Valves for the domestic market, with type numbers beginning with "G", but available directly from various sources, much cheaper than Svetlana types.

The 2C39a has been the workhorse for decades on 1296MHz, but it is hard to generate serious power levels.

The GI7b is a Russian valve, cheap to buy. A pair of these is a good choice on 144MHz or 432MHz if you have the 25W to drive them. There have been serious problems with tuning drift on 1296MHz, due to distortion of the internal grid structure when heated, and these problems are yet to be satisfactorily solved.

One or several GS15b tubes is a possibility for 1296MHz, but designs are yet to be proved.

The 3CX400a is very satisfactory on 432MHz, but this tube is only available new at rather high cost.

A pair of 4CX250b or 4CX350a tubes has long been the norm on 144MHz, but the 4CX350a will not work at 432MHz. The higher power output of the 4CX350a is only available by running them at higher anode voltage, since the maximum cathode current is the same for both types. A pair of 4CX250b's is close to maximum ratings at 400W output, and some people will prefer to run larger tubes more gently.

The 4CX400a is a Svetlana tube, only available at new prices on the Russian export market. Larger Russian tubes like the GU74b are available much more cheaply.

The 3CX800a7 is an excellent modern choice for 144MHz and 432MHz, but is expensive.

A Russian GU74b is a good choice for 144MHz, and readily available at a reasonable price.

For 432MHz the GS23b is widely used by EME operators running 1500W. Unfortunately there is no similar 800W dissipation tube suitable for 432MHz, but there is nothing wrong with using the GS23b at 400W - one should last a lifetime! For those who prefer triodes, there is the GS35b.