Transmitters and Link Tuners Used For Private Radio Broadcasting in the UK...PLUS.. a so called professional job from Dave!!!
large file..please be's fun

100W London Transmitter with built in Microwave tuner


50 watt transmitter from a Private Radio Station in the UK.

Exterior view showing enclosure. Durable construction augments uncluttered front panel layout.


Interior parts and layout found in the transmitter are ugly but fully functional copper clad 'spider wire' construction, with plenty of solder. This is the driver board showing the Oscillator to the right, pre-drive in the middle and the driver output to the left. Conventional input and output matching is used with trimmer capacitors used to maximize the power output. The coax output lead goes to the 50 watt amplifier output stage, coming up next.


This is the 50 watt amplifier section of the transmitter. Very basic but serviceable. It does not bother with a low pass output filter, damping resistors or stabilization of the output device against self oscillation, so costs are cut to the minimum. This amplifier will have good gain, but it may well be prone to the problems highlighted in our workshop article on amplifier stability.


A Band 1 Link Tuner From a UK Private Radio Station.

Compact 60 MHz Band 1 Link receive FM Tuner in small aluminum case. This receives a signal from a Link Transmitter, in the Radio Station Studio, some distance away. This type of radio tuner is called 'optimised'. This means that it is optimised to receive one frequency only, i.e. narrowband. This is the cheapest way to make a sensitive, yet selective tuner.

Inside View

The Band I Link Tuner interior showing copper clad construction and surface mounted components. This type of build is known as 'ugly bug' or 'spider wiring'. No printed circuit boards (PCB) are used in this model. Whilst the layout does not look as pretty as a PCB design , groundplane is abundant and I have seen PCB layout tuners whose performance would be shamed by the specs obtainable with this unit. The incoming studio signal at 60 MHz enters from the left hand aerial socket and is band pass filtered by the two tuned circuits (green variable inductors). At the top left is the local oscillator which runs at 49.3 MHz. The 60 MHz signal frequency and the 49.3 MHz are mixed in the dual gate fet (a quality low noise device) and the difference signal of 10.7 MHz is selected with the red rim toko inductor (centre of picture). This 10.7 MHz is the Intermediate Frequency (IF) and proceeds through a transistor amplifier and is sharply filtered with a ceramic filter of 280 KHz bandwidth. The 10.7 MHz signal, now filtered to avoid interference from other channels, goes into the IC, which is a CA3189 fm limiter/quadrature demodulator chip. This IC amplifies the 10.7 MHz IF signal by around 30 dB (1000 times) in it's internal limiting amplifiers, stripping off any unwanted amplitude interference and spikes. The amplified and limited high level 10.7 MHz IF (with the FM modulation) is then applied to the quadrature FM detection circuit within the IC (and also the 90 degree phase shift inductor on the right). From here, the original studio audio should emerge from the IC and is fed to the right hand phono socket. This feeds the main transmitter, shown further up this page.


Whilst everything above here has been constructed by enterprising young individuals who are not boastful RF engineers, the shambles below was constructed by a so called professional RF engineer popularly known as 'Arsehole Dave' who is particularly keen on quality(?). You may know him, he used to post messages on Alt.Radio.Pirate and his motto is:
"I have been an RF design engineer for longer than I can remember, so am probably just as thick now as when I started, so I am the weak link in the design of this unit ;-)"

Well, He said it! Please read on...

Not recommended, build quality very poor
Low Pass Filter with cut off at aprox 120 MHz

This is Dave's recommended layout for a finished product, a VHF low pass filter. As you can see, the connectors are airborne and the unit looks a complete mess. He is very choosy about his connectors though, saying that industry standard SO239 is 'Rubbish'! I have been told that the above workmanship is typical of his standards. Whilst this type of thing can obviously raise a laugh or two, do not under any circumstances operate a radio station with equipment build quality down to this low standard!

Now, the above filter was built to suppress the harmonic output of the RF Power Amplifier below.

found to be unstable and dangerous
MRF 238 RF Power Amplifier

Designed for an input of 3.5 watts to deliver 25 watts output on a 13.8v dc supply at 3 amps
This looks better! But unfortunately, Dave has got all the component values wrong in the output matching circuit, and the whole amplifier suffers from a lack of 'damping' so self oscillations are very likely to occur. Dave in his wisdom recommended this circuit and layout to a novice builder and encouraged him to broadcast night and day to test it. As expected, the circuit burst into oscillations. Which could be a major source of interference. It has to be pointed out that Dave did recommend this circuit as one to get maximum gain, but did not warn the user of the potential unstable nature of such an amplifier.

In conclusion, the privateers often can turn out a better job than some boastful "professionals".