1. Feeding the output of the RF modulator into an antenna. With no obstacles between the TV antenna and the RF modulator antenna, the best range I can get is 2-3 meters before reception is lost.
The output of the modulator is probably only a few milli-volts.
The modulator output is normally connected directly to the input of a tv receiver, so there is no need for a big signal, there is usually an attenuator on the output of the modulator.
Feeding the output of the RF modulator to the last stage (Q3 transistor, RF amplifier stage) of audioguru FM transmitter (Mod 4, 1 Mar 2005 version), and then use a 30cm antenna for the output. To my big disappointment, the TV can't receive anything at all even if the circuit and the TV are very close (worse then just feeding the output to an antenna directly).
The low output of the modulator is probably insufficient to drive the RF amp of the toy Tx.
In this state the amp will give even greater attenuation to the signal from the modulator.
Did you re tune the Tx output stage from Band2 (88 to 108Mhz) to Band 1 (50 to 70Mhz (ish))?
Feeding the RF modulator output to this simple single transistor amplifier circuit, **broken link removed**.
Good Grief! That circuit is a joke, how does "Instructables" get away with publishing such rubbish?
Why is it so bad?
It is untuned, at best has a gain of 1, the 10nF capacitor gives it a gain of about zero at any frequency above DC.
As I understand Channel 3/4 VHF frequency is just around FM frequency,
No it is not.
The old UK Band1 channel 3 frequencies were 56.75Mhz (vision) and 53.25Mhz (sound), channel 4 was 61.75 and 58.25Mhz. In the UK, television on these frequencies was abandoned about 30 years ago.
so why can't the approach work?
Apart from everything which I have said already, the vision carrier is ampltude modulated, a non linear amplifier will at best distort the signal and maybe remove the modulation altogether due to limiting action.
JimB