As K3VAT said, the length of a short antenna sets its radiation resistance and hence efficiency when the tuner is taken into account. Not gain, which varies very little from half-wave to infinitesimal dipoles (2.15dBi vs. 1.5dBi IIRC). People often get this confused, including magazine article authors!

Fractal antennas can be good when you have a lot of space and need a broadband antenna. Typically the opposite of amateur radio: we have limited space and (broadly) harmonically-related narrow bands.

For a 1/4 wave or 5/8 wave you have to ask what the antenna will use for a ground, as these are both 'ground plane' type antennas. A 1/2 wave does not have this problem. Note that a decent antenna may be more important for the transmitter than the receiver (e.g. model control). You may need to keep stray RF away from the rest of the circuit, so a well-behaved antenna may help.

If you have enough metal you can either use it as the antenna, or cut a slot in it and use the slot as an antenna. Otherwise a 1/2 wave fed via a simple balun will be best. If necessary you can fold it to reduce length, but this will change its feed impedance and reduce bandwidth.

The J-pole needs to be well away from nearby items as it is essentially an end-fed halfwave dipole with a quarter-wave matching section. It might work if the airframe is non-conducting, but a simple dipole would be much better - simpler and lighter too!

If someone's knowledge and experience with valves and RF is such that he considers using a 12AX7 for a VHF PA do you think he will be safe using a 4CX250B and the associated PSU?

The 6360 (QQV03/10) suggestion is good, but even there RF burns and widespread harmonics are a possibility.

If you want to play at low and fairly safe power level you could try the 12AU7/ECC82 - sort of a cousin to the 12AX7. It is actually two 6C4/EC90 power triodes in the same envelope, with a slightly reduced maximum anode dissipation.

For a sufficiently narrow band SLF~=SLC~=SLW, so just use two rectangular plates.

Isn't Tapatalk one of those annoying pieces of software which believes it is free to append advertising to forum contributions?

Sent by Firefox from my XP laptop  8-)

With zero stray capacitance (the simple 'ideal' case) you get SLC by sliding two rectangular plates.

SLW (wavelength) comes from a triangular plate, as overlap area varies as the square of the displacement.

SLF requires some stray capacitance; without this you get 'infinite frequency' with no overlap. Given the strays, some algebra then gives the required overlap area variation with displacement. Calculus (differentiation) then gives the required shape.

Someone running Linux could have a look and report back? I should have thought of that a few days ago, but I have just had to tidy the table and so clear away my Raspberry Pi for a while.

Good. My anti-virus reported it had foiled an attack from that site. Let's hope he can soon fix it and be back up again.

Yes, unless the front end was designed for lowest noise rather than reasonable selectivity and reasonable antenna match (which is more usual for HF) the dominant noise source will be the dynamic impedance of the grid tuned circuit. Grid noise can start to be a problem at the upper end of the HF range. It is only in really old (octal and earlier) valve receivers that valve shot and partition noise may dominate, and some of that may be from the mixer.

Simply putting in a higher gain valve (or cascode) is likely to cause AGC problems, as the RF valve will be nearly cutoff long before the IF valves (often 6BA6) have reduced gain by much. You may need to attach the RF grid bias to a tap on the AGC rail.

Another issue is tightness of antenna coupling. Most general coverage receivers have to use fairly loose coupling in order to get reasonable tracking and not allow the antenna to detune the grid circuit. Tighter coupling can reduce noise (by loading the tune circuit dynamic impedance down) but then the antenna has to be carefully tuned.

What is the value of capacitance which your antenna will have? Tell us that, and most of us can do the simple calculation to find the inductance which will have exactly the opposite reactance at your operating frequency.

If you can't tell us that, any answer we give will necessarily be wild guessing.

As has been said, in FM the amplitude of the carrier changes with amount of modulation.

In AM the amplitude of the carrier is unchanged by modulation.

Confusing, isn't it?

OK. I guess one is baseband bandwidth (e.g. 3kHz for us, 15kHz for mono broadcasters, 53 kHz for stereo broadcasters) and the other is maximum deviation (e.g. 75kHz for broadcasters).

This approximation avoids having to study the details of Bessel functions, which give the accurate figures.

Some context might help. I don't recognise the equation. Is Bw some sort of bandwidth?