Is it even possible to use stock inductors for those inductors?
I've had success with 1uH RF chokes for the 27 - 40 MHz region in oscillator collector circuits. These would resonate with a trimmer in the few tens of pF. For a secondary wrap 2 or 3 turns of enamelled copper wire around it. Altronics and Jaycar sell them, with the former much cheaper.
The inductance values for the pi network on the output will need to be much lower than 1uH. As a rule of thumb, 1uH and about 440pF is good for 7 MHz, so for 27 - 30 MHz then 0.22-0.25 uH would be OK. Something on a T37-6 toriod would be OK but to cut costs it should be possible to make something from enamelled wire wound on a 5 - 10mm former. Here's where a low value inductance meter will be handy. Low value slug tuned coils sometimes come up in the Rockby mailer and may be OK.
In relation to RF transistors, a 2N2222A (especially the metal can version), is OK up to 50 MHz and puts out a bit more than the BC548. I think Talking Electronics favour the 2N3563 for small signal VHF stuff over the BC548 - this would also be fine for low power. You see overseas circuits try to push a lot out of a 2N2222A but be safe and run them at maybe 20 - 30mW or so only.
I've found that a garden variety LM386 audio amp works well as an AM modulator for transmitters up to about a half watt. Use the standard circuit but without the coupling capacitor between Pin 5 and the speaker. Instead put in (say) a 10 ohm resistor (this protects the LM386 as a near short will blow it up).
You'll need a crystal oscillator and one or two RF amp stages to get the RF up to a few hundred milliwatts (if that's your aim). Instead of connecting the last stage to the positive rail (probably via an RF choke), connect it to the free end of the 10 ohm resistor from the LM386. Then maybe a 0.1uF from there to shunt RF to ground. You'll then have an AM transmitter.
As to its legality, rules vary between countries. Here in Australia we have a 'Low Interference Potential Device Class Licence' (look it up on acma.gov.au ). It basically says that you can use low power on a generous selection of frequencies without needing to take out a licence. Conditions are that you mustn't interfere with anyone and must accept interference from other users.
Permitted power outputs vary with frequencies. They're very often 100 mW (plenty of power to have lots of fun with a 1 - 5km range) but there's some (including around 27 MHz) where you can run 1 watt. I like around 36.864 MHz as you can double 18.432 MHz crystals and there's less interference than 27 MHz. Other countries may have 'short range device' provisions or similar.
The cheapest way to start experimenting is to buy a set of toy walkie talkies for $6-7. These are often AM with super-regenerative receivers. Maybe get 2 sets, so you have one good receiver, one good transmitter (to use as a reference) and then there's two left over to pull apart or modify.
Peter
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