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low frequency AM bug receiver

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scubadog

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Hi

I am looking to build a receiver capable of picking up my dads ferret transmitter collar.

I know that the collar transmits on 457kHz frequency, the same as avalnche transmitters.

I have seen am receiver circuits but i cannot find any that i can tune in to anything less than 500kHz.

For example this circuit can be tuned down to 550kHz, but what do i need to change to be able to reach 457kHz?

Thanks
 
i was going to say you could use a standard AM radio for this, but you start getting to the point where the received frequency is close to the IF frequency, and you will have problems caused by the original signal bleeding through the mixer, causing interference and reduced sensitivity. you're better off getting a "general coverage" or "communications" receiver that has continuous coverage from 100khz to 30Mhz. these are usually dual conversion type radios. the first IF in these is usually something like 50Mhz, and the second IF being 455khz, but since it's dual conversion, the original signal won't be bleeding through the mixer as it would with a single conversion receiver
 
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If you're happy receiving audio a cheap radio should do.

The AM broadcast band is ~ 531 - 1611 kHz, so you just have to reduce it slightly. In cheap radio receivers the frequency is adjusted using a variable capacitor which interacts with a ferrite inductor. If you increase the capacitance, the frequency will drop.

I'm not sure, but I think the standard variable caps are 60-160pF for 531-1611kHz, so to decrease the bottom freq to 450kHz, you could try soldering a 100pF cap across the variable cap which will reduce the lower frequency to 420kHz (if the var cap is 60-160pF).
 
the reason i said a standard AM radio would have problems is that the IF is 455khz, and if you re-tune the oscillator to shift the receiver's range down to 455khz, you will have your original signal bleeding through the mixer into the IF, and you will have two signals in the passband 2khz apart. there are reasons standard AM radios don't go below 500khz, and LW/AM/SW radios (single conversion) leave out the range between 400-500khz.

plus, just soldering a cap in isn't going to shift the reception range. you have to change both the RF stage tuning, AND the local oscillator frequency. when the radio is tuned to 457khz, the oscillator frequency will be 912khz, and the difference between the received signal and the oscillator will be 455khz (the IF frequency. but since those mixers are so cheaply made, the received signal of 457khz will be passed through also, giving a 2khz whine in the receiver.
 
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By 'cheap', I was referring to TRF AM radios which is basically the tuned circuit followed by an amplifier (I realise that heterodyne radios are cheap too [and probably easier to come by], so I should have been specific).

If you can't find one in a shop, you can make one yourself using tuned circuit followed by an amplifier. ZN414/5/6 might be available to help also.
 
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that's actually not a "direct conversion" radio, it's a tuned detector, the receiver is called TRF (Tuned RF). direct conversion uses a local oscillator the same frequency as the received frequency, and a doubly-balanced mixer, the output instead of being at a fixed IF frequency, is audio. a direct conversion receiver can also receive CW and SSB as well as AM with no additional devices like a BFO
 
scubadog said:
...I am looking to build a receiver capable of picking up my dads ferret transmitter collar.

I know that the collar transmits on 457kHz frequency,...
Click to expand...

I'd start with an older portable superhet transistor AM radio. Discard the mixer/local oscillator, retaining only the tuned RF stage/loopstick, 455kHz IF stages, detector, and audio stages. Tune the IF transformers just slightly higher in frequency. Fix-tune the RF stage/ferrite loopstick to 457kHz.

This will make a sensitive TRF receiver, where you can use the directional properties of the loopstick to locate the bug...
 
Not sure MK484 availability, TA7642 is much more common now. For a LOng Wave TRF receiver using TA7642, you may increase the loop turns or make use of the radio coil calculator for exact LC figures.
 
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