BF991 receiver RF preamplifier completed

While waiting for the boardhouse to make the printed circuit boards for my frequency synthesiser, ( see post on Mar 8, 2011 ), I reckoned I should have just enough time at long last to build the low noise 144MHz receiver RF preamplifier which I designed months ago, ( see post on June 16, 2010 for circuit schematic ).  For this piece of equipment I again made the pcb; the artwork for it is shown, ( actual size 4.2" x 2.2" ).
I built the circuit on single-sided copper laminated fibre-glass board, all components being mounted on the same side as the tracks and copper ground-plane; in effect surface mount technology even though only the BF991 mosfet, in a SOT143 package, is actually a surface mount device. A piece of double-sided board material was soldered vertically across the circuit board to act as a screen to prevent coupling between the input and output tuned circuits. The preamplifier is housed in a Hammond "Eddystone line" type 27134PSLA die-cast metal enclosure, 111x60x31mm.
Being somewhat larger than a matchbox, it's not the tinyest rf preamplifier that I have ever seen. Small size is not an issue for me, as it will not be installed internally in any other equipment but used outdoors close to the antenna. Besides that, the heavy-duty change-over relays that will be necessary to divert the transmitted signal around the preamplifier will add significantly to the overall size anyway.
Achieving a low noise figure for the preamplifier is strongly dependent on the C-C-L input bandpass filter and matching circuit which should use low loss, ( i.e., high 'Q' ), components. I easily obtained some suitable ceramic trimmer capacitors for C1 and C2, but sadly no silver plated wire from which to make the coil L1 !

Noise factors

Having achieved over the years considerable success with radio communication on the VHF bands using terrestrial modes of radio-signal propagation and reflecting my signals from the ionised trails formed by meteorites as they burn up in the earth's atmosphere, I am considering a new, formidable, challenge; reflecting my signals from the moon, to be able to contact amateur stations on the other side of the world on VHF. Signals received via the moon will be very weak, however, and at present noise may be a limiting factor. All radio signals are received in the presence of noise, from external atmospheric, man-made and galactic sources as well as that added by the receive equipment itself. Even an electronic component, such as a resistor, lying unconnected on the work-bench generates noise at the sub-atomic level, unless kept at a temperature of -273C ! Powered-up equipment at room-temperature is much noisier. I had been wondering if my equipment would be sensitive enough to receive signals reflected from the moon from other stations, as well as my own, or would they be buried under the noise and be undetectable.So I carried out an assessment of the noise performance of my VHF receive set-ups, for 50, 70 and 144MHz frequencies, and found it to be poor in each case. The noise is excessive for consistently successful 'moonbounce', though there is always the possibility of the occasional 'freak' contact. My assessment method, using my 144MHz set-up as the example, is shown in the upper picture.
I can't control external noise, but fortunately there is still a remedy; fit a low noise receive signal pre-amplifier as close to the antenna as possible. I set about designing one, first for 144MHz, based on a low noise n-channel dual gate mosfet transistor, type BF991, which is specified to 200MHz, ( circuit shown ). With a noise figure of 1.28dB and sufficient gain, this little amplifier should do nicely and allow a signal power of 1.306x10e-4 picowatts from the antenna to be detected with a signal / noise ratio of 10dB in a 2400Hz receiver bandwidth ! I will need to modify the circuit for the other two frequency bands mentioned, just by changing the attenuation of the 50 Ohm pad and the values of the frequency dependent components.
So three preamplifiers in all - my list of circuits to build during next winter just keeps growing !

Spring cleaning

Every year at around this time I like to clean the inside of my fan-cooled amplifiers; suck out the dust, remove dead insects and spiders, and also clean the fans themselves. Over the course of a year's use a lot of dirt gets drawn in and accumulates. Cleaning should ensure reliable operation over the next 12 months.
My 150 watt VHF amplifier for 144MHz has its top cover removed prior to undergoing this treatment. It uses the American, Eimac type 4CX250B, ceramic, external anode, tetrode tube; also shown with its ceramic chimney alongside.

Amplifier for 144MHz

Another project on the bench is a transmitter power amplifier for the 144MHz band based on the Russian ex-military GS35 triode tube, ( centre top ), and which should be capable of an output in the region of 700 watts. The bias board ( right ) is completed, as is the control and switching board, ( left ); but the project is on hold. Despite my great endeavours, I cannot locate a source for the needed 2000 volt, 2 KVA eht transformer !