q's on 1TR regen RX

Dear Harry,
I want to construct your Regen receiver, as it is small and very portable. But it will be nice if you can clarify me some of my doubts.
1) can I use bc547/548/549 for the transistor
2) I want to listen to local hams (within 200Km radius) transmiting 10 watts SSB in 40m band, Is it that sensitive?
3) can you suggest a 741/lm324 audio amp to go with this one such that i can make the whole receiver in to a cigar box
4) can you tell me how big and antenna requirement for this circuit?

Regards
Ashok( ashok[dot]s[dot]das[at]gmail[dot]com)

1) The coil required for tuning is WL/4 but the antenna coupling should be determined by the impedance. If the tuned coil has a top impedance of, say, 1000 Ohms at resonance then for 50 Ohms the antenna coil ratio should be about root(1000/50) = 5:1 (40T + 8T for the antenna). This can be an 8T coil or an 8T tapping.

In reality I would go for a much lower tapping, in the interests of loading/stability.

2) 6% or 10% are the same order of magnitude - in the same region.

If you were to use a coil for, say, 465KHz then you would have a lot of turns and the "rule of thumb" I quoted would be quite accurate, but as the number of turns reduces with higher frequencies then you can find a larger deviation form this.

When you build such a circuit you will normally need to adjust your coil turns a little from the calculated values in order to take into account the construction method, component tolerances, transistor gains, and any other factor that can affect the coupling or feedback. Once you have achieved a working prototype then you can go ahead and publish your design, as-built.

Finally, if you were to build a receiver covering, say, 1MHz to 2MHz then the tuning capacitor across the tuned circuit would need to have a 4:1 capacitance ratio to get a 2:1 frequency ratio.

A 4:1 capacitance ratio will also give you a 4:1 impedance ratio, depending on the frequency you are receiving. You could have an impedance ration that varies from 500 Ohms to 2000 Ohms. Just to cover 1MHz to 1.5MHz would give an impedance ratio of 2:1.

An antenna tapping should be a "reasonable match", that is to say, with a 2:1 frequency ratio you can expect the 50 Ohms antenna input impedance to vary from 25 Ohms to 100 Ohms. External changes to the antenna itself, when coupled directly to the oscillator, can have a large effect on frequency, so an input impedance of 5 Ohms or less can be chosen in order to reduce the unwanted frequency "pulling" effects.

Calculations are not always an exact science, but are often a compromise. The final figures I have in the article were a result of practice.

Very best regards from Harry

hi Harry,

great site that you have! i found your regenerative revceiver there and have some questions, hope you don't mind.

1)
you said the antenna and the main tuning coils should be

number of turns = wavelength/4
T=160/4
  = 40

but the schematic shows the antenna coil to be a lot smaller than the tuning coil, and usually that is the case in practice. so should the antenna coil really be the same as the tuning coil, 40T?

2)
you said the base and reaction coils are both 6% of the tuning coil, which is 4T, but,

tuning coil = 40T
(6%)(40)=2.4T
and 4T is actually 10%.

so so you mean 10%, but not 6%?

thank you for helping!