@hk34119 Airband transmissions are brief and pretty narrow, you may be simply missing them. If you know the frequency of the local control tower and have a frequency counter you can generate a signal to tune the receiver more precisely.
Alan - a fun project. Highly recommend your circuit as a starter. Built just the detector. On its own it will just drive a crystal earpiece on FM stations but highly recommend adding a 1 transistor amp. I found that increasing the 2.2k to 5.6k and the drain resistor to 3.9k produced slightly better audio. @hk34119 the RF choke is not critical. I tried 2.2uH and 100uH and both worked.
I think that the drain of the second FET should be connected to a tap on the second tuned coil should make it work better, should you be tuning 118-136MHZ, and a duel gate MOSFET would make a better RF preamp.
Gave this a try a few days back. Built dead-bug style on a piece of copper clad with short leads getting very nice FM broadcast but noting on the airband or other paging sounds (we get lots of POCSAG/Flex here as well; one of them sound weird in AM). For the RFC I used a ferrite bead '22uH' my cheap Inductance meter says. Any thoughts? Thanks.
I know this was posted over a year ago. I was wondering what your thoughts would be on a project I want to build. I really like your circuit design. What i'd like to build is a NBFM receiver for the American NOAA weather radio at 162.4 - 162.55 mhz. Thank you for any help and or advice you might have.
Dear VK2ZAY, I have been a regular on your channel and website. I came across this video after watching VKY3E's 2 transistor vhf fm radio. I built 2 versions to test different components and how they acted. I had a problem with antenna as it was too sensitive to the touch and my own body radiation I guess, as it will tune if my hand was at close distance and i could hear great music and voice equally. what should i do for a better antenna that doesn't need me to touch or move. thnx Khalid 73
why is the 2-35p varicap to ground rather than parallel to the coil like a traditional tank circuit? wouldn't a cap to ground attenuate the RF? I've been trying to build a receiver for a very long time and the more I research the more I find I don't know..
The capacitor is still effectively parallel to the inductor. The path for the circulating current is completed through the bypass capacitor. The main reason to do this is so you can ground the capacitor rotor which helps avoid tool/body capacitance effects pulling the tuning.
Is it possible that I can use it for UHF and VHF? and how far can it receive the transmitter? I have ATV (Amateur Television) projects back in the 90's.
+Dr Pepper 23 sure, but at UHF the stability won't be great, it would need to be robustly built. Range wise, well that depends upon the antennas at each end, the path in between and power level of the transmitter. If you shield the receiver completely you could measure its input sensitivity, it is probably a few 10s of uV for 12 dB quieting or something.
hi, I want to build my own fm receiver, I see your in your circuit you use CB to match antenna impedance and that is also the way I'm trying to design my circuit. but A LOT of the examples online have a CE configuration with large input impedance... my question is, what are the advantages of high input impedance vs impedance matching... or when would it be desireable?
Depends upon the antenna impedance really, the CB is better for coax/dipoles, etc. It also has much better reverse isolation which is important for superregenerative receivers to reduce radiation from their oscillating detector. CE has some voltage gain and matches end-fed and short antennas better, but it has less isolation.
N-type JFETs don't cut off until their gate voltage is well below their source voltage. This is called pinch-off. The pinch-off voltage Vp is the Vgs where the drain current Id drops to some tiny value (basically zero, usually some small number of uA specified in the datasheet along with the corresponding Vp). You can use this fact to bias JFETs to a specific quiescent current by arranging the drain current to drop voltage across a resistor Rs in the source (ie set a specific Vgs for a desired Id). This is generally called "self-biasing" and can be calculated from Idss and Vp of the device. Idss the drain saturation current and Vp the pinch off voltage are easily measured. The Idss is the Id with the Vgs == 0 (i.e. gate/source shorted). The Vp is the Vgs with a tiny Id current flowing, I use a 1M resistor in the source to measure it, putting the JFET well into pinch off.. To arrive at the required value of Rs you use the JFET active region drain current equation Id = Idss * (1 - Vgs/Vp)^2 and solve with -Vgs = Id*Rs. There is a calculator on my website that does just this for you, but it helps if you know fundamentally that Id is a square-law relation to Vgs with Vp and Idss being the important coefficients: www.vk2zay.net/calculators/jfetBiasing.php Note that Vp is *negative* for an N-type JFET, the gate-channel diode has to remain reverse-biased for normal active-region JFET operation so all Vgs we care about for biasing are also -ve - hence the Vgs/Vp in the equation above is always +ve, even for pJFETs. Vsg = -Vgs = Id*Rs. Many people ignore the signs of Vp and just work with the magnitude. My calculator doesn't care about Vp's sign either. Technically it should handle -ve Ids for pJFETs, but it does not, magnitudes work fine either way.
Yes, it likely has a much larger value, so you'd need to series it with a smaller capacitor to reduce its effective capacitance. Also, depending upon its construction it may be quite lossy or not even capacitive at VHF, but in my experience the cheap polyvaricons designed for AM BCB service will work OK at VHF, at least the dual-gang 160 pF/60 pF ones with wide metal straps for terminals.
Yes ....making projects at work.....stick it to the man!! :)
Thanks for another great, easy to build, project. Really like the idea of building on a tin top
in a dead bug style.
Such neat and tidy soldering
@hk34119 Airband transmissions are brief and pretty narrow, you may be simply missing them. If you know the frequency of the local control tower and have a frequency counter you can generate a signal to tune the receiver more precisely.
I work at and live near an airport, so this project is in my list of things to try.
Alan - a fun project. Highly recommend your circuit as a starter. Built just the detector. On its own it will just drive a crystal earpiece on FM stations but highly recommend adding a 1 transistor amp. I found that increasing the 2.2k to 5.6k and the drain resistor to 3.9k produced slightly better audio. @hk34119 the RF choke is not critical. I tried 2.2uH and 100uH and both worked.
you should have a lot more subscribers, thanks for making these videos
Great video! Please make a video about biasing BJT's and FET's like you mentioned.
Nice work mate !
Can't you make a video step by step showing how you built it? That would be just awesome! Thanks. I'm sure you would get more subs then ;-)
pretty cool with antenna buffer. I'm building one like this.
I think that the drain of the second FET should be connected to a tap on the second tuned coil should make it work better, should you be tuning 118-136MHZ, and a duel gate MOSFET would make a better RF preamp.
Gave this a try a few days back. Built dead-bug style on a piece of copper clad with short leads getting very nice FM broadcast but noting on the airband or other paging sounds (we get lots of POCSAG/Flex here as well; one of them sound weird in AM). For the RFC I used a ferrite bead '22uH' my cheap Inductance meter says. Any thoughts?
Thanks.
Yes, BJTs and FETs!
Good work. How about a NBFM design or option add-on for this one in part 2?
Thanks.
I know this was posted over a year ago. I was wondering what your thoughts would be on a project I want to build. I really like your circuit design. What i'd like to build is a NBFM receiver for the American NOAA weather radio at 162.4 - 162.55 mhz. Thank you for any help and or advice you might have.
Dear VK2ZAY, I have been a regular on your channel and website. I came across this video after watching VKY3E's 2 transistor vhf fm radio. I built 2 versions to test different components and how they acted. I had a problem with antenna as it was too sensitive to the touch and my own body radiation I guess, as it will tune if my hand was at close distance and i could hear great music and voice equally. what should i do for a better antenna that doesn't need me to touch or move.
thnx
Khalid
73
why is the 2-35p varicap to ground rather than parallel to the coil like a traditional tank circuit? wouldn't a cap to ground attenuate the RF? I've been trying to build a receiver for a very long time and the more I research the more I find I don't know..
The capacitor is still effectively parallel to the inductor. The path for the circulating current is completed through the bypass capacitor. The main reason to do this is so you can ground the capacitor rotor which helps avoid tool/body capacitance effects pulling the tuning.
@@vk2zay thank you so much for the reply.
Is it possible that I can use it for UHF and VHF? and how far can it receive the transmitter? I have ATV (Amateur Television) projects back in the 90's.
+Dr Pepper 23 sure, but at UHF the stability won't be great, it would need to be robustly built. Range wise, well that depends upon the antennas at each end, the path in between and power level of the transmitter. If you shield the receiver completely you could measure its input sensitivity, it is probably a few 10s of uV for 12 dB quieting or something.
i have a question about where you found a plastic flathead screwdriver...
xenochris it is actually ceramic and crazy expensive, specially designed for adjusting RF circuits. I rarely use it.
Is there any advantage to building on a tin top as opposed to a piece of strip veroboard?
neil groves no, not really. Copper is actually better.
hi, I want to build my own fm receiver, I see your in your circuit you use CB to match antenna impedance and that is also the way I'm trying to design my circuit. but A LOT of the examples online have a CE configuration with large input impedance... my question is, what are the advantages of high input impedance vs impedance matching... or when would it be desireable?
Depends upon the antenna impedance really, the CB is better for coax/dipoles, etc. It also has much better reverse isolation which is important for superregenerative receivers to reduce radiation from their oscillating detector. CE has some voltage gain and matches end-fed and short antennas better, but it has less isolation.
vk2zay thanks for that it makes sense now :)
I'm a JFET newbie. How can this work with the gate grounded?
N-type JFETs don't cut off until their gate voltage is well below their source voltage. This is called pinch-off. The pinch-off voltage Vp is the Vgs where the drain current Id drops to some tiny value (basically zero, usually some small number of uA specified in the datasheet along with the corresponding Vp).
You can use this fact to bias JFETs to a specific quiescent current by arranging the drain current to drop voltage across a resistor Rs in the source (ie set a specific Vgs for a desired Id). This is generally called "self-biasing" and can be calculated from Idss and Vp of the device. Idss the drain saturation current and Vp the pinch off voltage are easily measured. The Idss is the Id with the Vgs == 0 (i.e. gate/source shorted). The Vp is the Vgs with a tiny Id current flowing, I use a 1M resistor in the source to measure it, putting the JFET well into pinch off.. To arrive at the required value of Rs you use the JFET active region drain current equation Id = Idss * (1 - Vgs/Vp)^2 and solve with -Vgs = Id*Rs. There is a calculator on my website that does just this for you, but it helps if you know fundamentally that Id is a square-law relation to Vgs with Vp and Idss being the important coefficients:
www.vk2zay.net/calculators/jfetBiasing.php
Note that Vp is *negative* for an N-type JFET, the gate-channel diode has to remain reverse-biased for normal active-region JFET operation so all Vgs we care about for biasing are also -ve - hence the Vgs/Vp in the equation above is always +ve, even for pJFETs. Vsg = -Vgs = Id*Rs. Many people ignore the signs of Vp and just work with the magnitude. My calculator doesn't care about Vp's sign either. Technically it should handle -ve Ids for pJFETs, but it does not, magnitudes work fine either way.
is there a possibility tha i could change the trimmer tuning capacitor with an old AM plastictuner?how should i do it?thanks
Yes, it likely has a much larger value, so you'd need to series it with a smaller capacitor to reduce its effective capacitance. Also, depending upon its construction it may be quite lossy or not even capacitive at VHF, but in my experience the cheap polyvaricons designed for AM BCB service will work OK at VHF, at least the dual-gang 160 pF/60 pF ones with wide metal straps for terminals.
thank you very much sir
@David Wanklyn Yeah the AM/FM quad-gang ones normally have pretty good VHF performance on the smaller caps intended for the FM part of the radio.
@David Wanklyn Nice! Glad it worked.