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Regenerative receiver for 27 MHZ WILL WORK?

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zeller

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Hi,

I am trying to modify the receiver below, to receive 27 MHZ. I have only changed the LC to detect 27 MHZ, I am not sure of how to attach the attena.The transmitter has a strong signal and will be less than I metre from the receiver.Can i just attach a small HORIZONTAL wire i.e 15cm LONG on the LC? will this suffice? Is the attena needed given the strong signal.Will this receiver work at 27 mhz by only modifying the LC?

Please help!!!

The receiver is here.

AM Receiver
 

E Kafeman

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It will work to retune LC circuit to 27 MHz but keep wires between coil and Q1 and RF ground short. Simplest is to make L1 as a air-core coil, which also can act as antenna or connect the antenna as in description via a impedance transforming coil.
What solution that is selected will affect tuning stability and bandwidth.
A short monopole can be connected a few turns from bottom of coil L1 as a test for better performance but it will also reduce center frequency so a retuning is necessary to find out if any improvements.
 

biff44

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Regenerative radios really stink. I would not waste a minute of my time on such a circuit, unless you are a history buff and want to figure out how circuits worked back in the 1930's. Build a superhet receiver with a common receiver chip.
 

kalyanasv

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It could ok as a hobbyist circuit or a learning principle. But that having said I built a superhet for my project 12 years ago..so.
The transmitters are typically the easy part in such stuff. Though your receiver is a 1m away: to ensure it works well.

I hope you understood the circuit well enough. i) There is a Darlington Pair in there so you want to be careful how you adjust
Instead of the 120K use a potentiometer (pot) if you reduce the resistance too much you might burn out the circuit with the current flow so you can use a minimum resistance in series with the pot to avoid burn out.
Personally i would not gone for the Darlington pair..but then..


The antenna is attached to the one end of the LC ckt.

At such frequencies you need a really long piece of wire for good reception at low power.... Its more of engineering approximation when building.
i.e. (0.95)*c/f=antenna wavelength.
Depending on the power requirement and quality you can choose from half wave, quarter wave...etc...


The circuit is for lower frequencies...I would look at FM receivers and down grade it for a 27Mhz but then that is me. ..Its part of the fun of engineering experiment!
 

zeller

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Regenerative radios really stink. I would not waste a minute of my time on such a circuit, unless you are a history buff and want to figure out how circuits worked back in the 1930's. Build a superhet receiver with a common receiver chip.

The problem with the AM receiver chips I have so far found do not go as far as 27 MHZ, my transmitter uses AM modulation what chip would you reccomend at 27 MHZ?
 

E Kafeman

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I have built a similar regenerative receiver with a small loudspeaker, that have been hooked to mf output in a spectrum analyzer. It have since many year been sitting there as a very helpful indicator tool when probing stuff without keeping an eye on the instrument and it is also useful as trigger source for further signal analyzing. Yes it is a simple receiver, but it does exactly what I need.
27 MHz wavelength is 11 meter => 1 meter TX-RX distance is very near field. In this situation have a well tuned inductor better efficiency as antenna then a quarter or full wave monopole antenna for which he anyway probably not have corresponding ground plane.
In near field does a loop have better defined directivity and static noise is better suppressed then in a wire antenna. If however a short monopole is preferred as antenna, offer the coil also the function to reduce any impedance mismatch for this very short antenna. A 10 cm long wire at 27 MHz result in low antenna efficiency but again, in this particular near field case, a 10 cm short antenna do probably result in better snr in a rather noisy and crowded 27 MHz frequency band then if a half wave dipole had been used.
 

biff44

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The trouble with regenerative receivers is that there are two parts:
1) the rf amplifier part
2) the self oscillating/quenching part

So if one of those is not working right, you can not tell easily which one needs work.

A superhet receiver operates in a straightforward way, and with simple tools (like an oscilloscope) you can trouble shoot it easily.

I would look at chips like the SA605
 
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FvM

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SA605 is mainly intended as a FM receiver, it misses an AGC option. Unfortuntately, I don't know if any of the various analog AM receiver chips, that has been previously available is still manufactured. E.g. TDA1046 has been well suited for a SW receiver up to 30 MHz.
 

biff44

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like with all these chips, you use the SA605 RSSI output as the AM detector.
 

zeller

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The trouble with regenerative receivers is that there are two parts:
1) the rf amplifier part
2) the self oscillating/quenching part

So if one of those is not working right, you can not tell easily which one needs work.

A superhet receiver operates in a straightforward way, and with simple tools (like an oscilloscope) you can trouble shoot it easily.

I would look at chips like the SA605

I am just using simple tools i.e resistors and transistors is the superhet easy to build? If not, do you know any chip with a superhet that works at 27 mhz AM? The sa605 datasheet does not seem to indicate suitability to am modulation.
 

E Kafeman

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Build a tuned coil and a simple diode detector. Done in minutes with simple components. That circuit can be tuned with a DC voltmeter. Works just as good as any RX circuit at that distance.
 

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