G4AKW

Microwave ovens - The circuit shown is typical of bottom range microwave oven. Protection of the recifier diode and inrush current limiting is by a soft start circuit which puts a 10 watt 25 ohm resistor in the primary circuit for around a second. This slow start cct and the noise filter components are usually on a small pcb which is good without modification in home brew power supplies. The fan?, high voltage diode and transformer are useful for low voltage (eg 35v 30A - rewind the secondary) - picture shows a 700W transformer (output rating - transformer is >1kW rated) with the secondary removed and rewound with 2.5mm square mains wire to give 23v rms. After rectification with a 40A bridge rectifier and smoothing with 66,000uF @40V this gives a no load voltage of 31.6v measured. Note 23 x 1.414 = 32.5v minus two diode volt drops gives 31.3v. On full load with the 600W topband transmitter the volts drop to 20.9v (some mains volt drop with this measurement owing to shed being at...

This capacitor bank is currently wired with 22 2200uF capacitors in parallel. Maximum DC voltage is 350v and the total energy stored is approx. 3000 watt-secs (Joules). A discharge resistance low enough to achieve a 1nS pulse equates to 3TW peak pulse power. However, the ESR of the capacitors alone will limit the discharge time to around 100us, hence the peak pulse power will only be about 22MWatt. To confirm the marked value of the caps (2.200uF) a 17.5v DC supply was connected to one via a 100k resistor and the time taken to charge to 1v was measured. 15sec, gives a dv/dt=1/15 v/sec. The charging current (fairly constant) = 17/100k and the capacitance is therefore: C = i (const)/dv/dt = 17/100k / 1/15 = 17 * 15 / 100000 = 2550 uF

The specification and base pin out left is for an 813 Power Pentode in class C. Photos of my twin 813 amp are shown below. Its little brother amplifier shown is a 3 x 807 beam tetrode HF power amplifier which has evolved over the last couple of decades. In fact I'm pretty sure that one of the three 807 valves is out of my first transmitter, a No 19 set, purchased from G3WQN in 1970. Output power is about 150 watts on 160m, 80 and 40m. The 813 linear is currently configured as per G2DAF design with step up and rectified input for the screen supply. One pic shows the 813 beam power pentodes next to a 5u4 rectifier - for size comparison. Between 2kV and 2.5kV is needed on the anode and 400V for grid 2. The pentode construction gives very high gain and in class C (CW) 4 watts input should result in nearly 400W output (each). (ICAS conditions - not current bias cndx). So one in a box could be a nice add on to the FT 817ND. .

The first photo shows the two 1u shelves of the transverter sitting on top of the h/b 3u 80m qrp (3 w) cw tx/rx. The transverting (mixing) functions were turned off and the only modules used from the transverter were the Norton broadband receive amplifier and the linear amplifier. The linear produces 170 watts output on 80m (it gave 130 watts on 20m) and the next picture shows the linear amplifier splitter, high power combiner, 2 x dual Mosfet amplifiers and dual switch mode power supplies. The third photo shows a close-up of the 170 watt combiner and the input circuit of one the linear amplifiers. The 4 x IRF510 mosfets have proved very reliable (at a lower power level). The antenna (capable of handling up to 3 amps peak) and an 80m dipole was built using 300 ohm ribbon with the two conductors connected together. The 4th photo shows the dipole centre piece made from a chopping board. The DC rx in the QRP rig makes it difficult to copy cw in any but the best conditions for a DC rece...

Following on from the construction of the Norton amplifier, the real question to be answered is 'is an HF pre-amplifier necessary on 14MHz and if so what should its specification be'? The full system noise figure factor is given by f=fa + (Lc-1)(Tc/To) + Lc(Lt-1)(Tt/To) + LcLt(fr-1) where fa is the external noise factor given by: fa = pn/kTob pn = available noise power from a lossless antenna Lc = antenna circuit loss Tc = temp of antenna and ground surrounding (in range) Lt = transmission line loss (1dB in our case) Tt = transmission line temperature (288k as its quite mild today) To = reference temperature (go for 288k) fr = noise FACTOR of receiver Fr = noise FIGURE of rx = 10logfr (14 dB in our case) so the noise power in watts from all sources - pre detection is n = f k To b where f is as above the system noise factor which is a theoretical quantity taking account of all noise sources. For the purposes of finding out whether we really need a pre amp we will simplify the ma...

The 20m vertical antenna looks good, VSWR < 1.3 : 1 but RX might be a bit deaf. RX details: 1dB antenna cable loss: + 14MHz to 144MHz SBL-1 mixer (straight 6dB loss) :+ IC 202 144MHz receiver (8dB NF). Hence total receive noise figure is at least 15dB. Built a Norton HF preamp (2n5109) to try and improve situation. (Is this necessary given the noise level at 14MHz? see following. Photo below shows the circuit and the measured cbe voltages resulting from a 13.9v supply. First audible results were however not particularly impressive. The dominant noise is the external noise? and this is in excess of any receiver contribution - even at 15dB noise fugure? But I will look into this and quantify the position. OK, this is a simple circuit and the 50 ohm output load is transformed by the broadband auto transformer to the collector load. The actual turns ratio used was 3 to the tap and then 11 to the collector. The turns ratio is then 14/3 or 4.6 which is the voltage transformation. ...