how to convert AM/FM receiver to shortwave?

Thread Starter

Vignelli

Joined Nov 13, 2023
20
I have an old tuner I want to convert to shortwave. plenty of room to work in.

convert_am-sw_01.jpg
my plans were to add L in parallel with the coil on the bar antenna to reduce it, according to

Lt = (L1*L2)/(L1+L2)

then, given the equation f^2= 25,330/(L*C) and the fact that the AM band on this radio covers 520-1620 kHz, whatever L was, the same range of C but a different L would receive shortwave. specifically, if L was a typical 250 uH, adding 2.0095 uH in parallel changes the frequency range to 5.8-18.0 MHz.

later I can update it to switch between 2 fixed L's in parallel with the bar antenna to switch between 2 shortwave ranges.

I could do the same exact thing to C instead (in series) but that would make tuning all wacky because of the properties of

Ct = (C1*C2)/(C1+C2)

L is fixed and wouldn't have that problem.

so now my problem is actually doing it. I tried using an LCR meter on the bar antenna to get a reading, to determine which wires to add the L in parallel with. but no combination of wires gave any reading.

convert_am-sw_02.jpg

convert_am-sw_03.jpg
maybe I couldn't get a reading because it was in a circuit. so I used test leads to connect L in parallel with various wires on the bar antenna, and none of them changed the frequency of reception at all.

? it should!

why wouldn't this work? is there some other method?


next question:

I came across a video where some guy did a variation of that by wrapping a loose wire around the coils on the bar, about 1/4" spacing between turns, then simply connecting the ends together. attach to that a random wire antenna. it works. I'm not sure exactly how.

the closed circuit of the wrapped wire must be forming an inductor, inductively coupled to the AM bar antenna, and the random wire injects shortwave signal. but I don't see how it can do that since it's a closed circuit without a ground. the current doesn't circulate. maybe what's going on is it acts as a step-up transformer that changes the frequency? I don't know.

I tried it with the loose wrapping and it worked, but was pretty lacking. many AM stations bled through. absolutely plagued with ghost images. high noise floor and low sensitivity. I would rate the reception about as good as the worst shortwave portable radio.

then contrary to all the instructions and this guy's video, I wrapped it tightly to increase inductive coupling even if it would shift the frequencies too far down. fastened the wires to the case with clothespins to prevent them from unravelling.

convert_am-sw_04.jpg
the results were good. only the strongest AM stations bled through, sensitivity improved noticeably and noise floor decreased slightly. fewer ghost images. on the dial it runs 8.2 to 15 MHz which is acceptable. still doesn't compare to a decent cheap radio but it was fast and easy. and I got to convert a stereo to a shortwave radio.

so, the question is, why does this work at all? it shouldn't do anything.
 

LesJones

Joined Jan 8, 2017
4,172
You do not seem to have done anyting to alter the tuning range of the receiver. Even if you change the tuning range of the first RF amplifier stage and the local oscillator you will probably have poor image frequency rejection when tuned to higher frequencies due the the low IF frequency. (Probably about 455 Khz.) Do you understand how a superhet receiver works ? If not you need to do some research.

Les.
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
You do not seem to have done anyting to alter the tuning range of the receiver. Even if you change the tuning range of the first RF amplifier stage and the local oscillator you will probably have poor image frequency rejection when tuned to higher frequencies due the the low IF frequency. (Probably about 455 Khz.) Do you understand how a superhet receiver works ? If not you need to do some research.

Les.
I was unable to change the tuning range of the receiver by changing L of the bar antenna. that was what the question was, why doesn't it change or how else to do it. I would not change the tuning range of the first RF amplifier stage, unless you mean to say the AM bar antenna is part of that. neither for the AM LO which would nevertheless have insufficient range to change to shortwave. not worried about image rejection, I can't do anything about it.

yes, I understand how the superheterodyne architecture works and I assume it remains functional when changing the oscillation frequency of the antenna to get a different frequency range.
 

MrChips

Joined Oct 2, 2009
30,467
While it may be possible to achieve what you are attempting to do, it would require a lot of knowledge and effort.
A better option would be to down convert the SW band to 1600kHz and use the AM radio to receive this signal. Then you would have created a dual conversion receiver.
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
Is this the equivalent circuit?
View attachment 307467
no, the wrapped wire is only connected to itself in a closed loop along with a long random wire, not to the tuning capacitor. and what I was trying to accomplish at first was entirely different, simply adding L to the bar antenna with an inductor on the side that's not inductively coupled to the bar.
 

Ya’akov

Joined Jan 27, 2019
8,973
In a superhet receiver with a loopstick antenna, the loopstick is part of the first stage determining the receive frequency. The resonant frequency of the antenna (as selected by the coil's inductance, the ferrite core, and the value of the tuning capacitor determines the frequency tuned.

Notice the split capacitor—it is two capacitors mechanically coupled so they tune together. This is because one of them is tuning the antenna/filter and the other is tuning the LO (Local Oscillator) at the same time. This is necessary because the job of the local oscillator creates the IF (Intermediate Frequency) fed to the next stage.

This IF is a fixed frequency. That is, it is always the same no matter what the tuned frequency of the first stage happens to be. The LO does its job with a mixer. A mixer is a circuit that combines the LO and the output from the antenna resulting in signals at the sum and difference of the two frequencies due to destructive and constructive interference. This is where "het", short for heterodyne, comes from.

The most common IF chosen for broadcast receivers is 455KHz, so the LO's tuning is contrived such that there will be a signal at 455KHz no matter where in the tuning range the dial is adjusted. This has the great advantage of being able to make the following stages highly optimized for that one frequency. There can be more than one IF frequency to improve various performance parameters.

Along with the desired IF signal there can be one or more image signals. An image is a signal caused by a frequency other than the one of interest that also falls out at the IF frequency because the difference between it and the LO equals the IF. Image rejection is an important performance parameter for a receiver. For the various IR schemes to operate, the stages must be properly tuned.

The bottom line is that changing the resonant frequency of a loopstick in a receiver designed to use one will change the receive frequency—but it will also run all the careful design that depends on the original resonant frequency range of the loopstick. It will never be a very good receiver. You could rewind the loopstick and replace the running capacitors to make it works on a different band, but you will also need to ensure the LO is operating as expected and the various filters are functional in that range.
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
While it may be possible to achieve what you are attempting to do, it would require a lot of knowledge and effort.
A better option would be to down convert the SW band to 1600kHz and use the AM radio to receive this signal. Then you would have created a dual conversion receiver.
is there a simple way to downconvert the SW signal? I would be interested.
 

nsaspook

Joined Aug 27, 2009
12,778
no, the wrapped wire is only connected to itself in a closed loop along with a long random wire, not to the tuning capacitor. and what I was trying to accomplish at first was entirely different, simply adding L to the bar antenna with an inductor on the side that's not inductively coupled to the bar.
Well, you are coupling (by overlapping the coils) that small coils value of inductance in parallel into the total tuning circuit. So even if you don't have a galvanic connection, you have an equivalent electromagnetic one.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,467
Yes. Firstly, you create an LC tunable circuit designed for the desired SW band.
Then you create local oscillator that is tunable to the RF plus or minus the IF which is 1600kHz.
You mix the RF and LO signals to produce 1600kHz.

1699910959784.png
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
In a superhet receiver with a loopstick antenna, the loopstick is part of the first stage determining the receive frequency. The resonant frequency of the antenna (as selected by the coil's inductance, the ferrite core, and the value of the tuning capacitor determines the frequency tuned.

Notice the split capacitor—it is two capacitors mechanically coupled so they tune together. This is because one of them is tuning the antenna/filter and the other is tuning the LO (Local Oscillator) at the same time. This is necessary because the job of the local oscillator creates the IF (Intermediate Frequency) fed to the next stage.

This IF is a fixed frequency. That is, it is always the same no matter what the tuned frequency of the first stage happens to be. The LO does its job with a mixer. A mixer is a circuit that combines the LO and the output from the antenna resulting in signals at the sum and difference of the two frequencies due to destructive and constructive interference. This is where "het", short for heterodyne, comes from.

The most common IF chosen for broadcast receivers is 455KHz, so the LO's tuning is contrived such that there will be a signal at 455KHz no matter where in the tuning range the dial is adjusted. This has the great advantage of being able to make the following stages highly optimized for that one frequency. There can be more than one IF frequency to improve various performance parameters.

Along with the desired IF signal there can be one or more image signals. An image is a signal caused by a frequency other than the one of interest that also falls out at the IF frequency because the difference between it and the LO equals the IF. Image rejection is an important performance parameter for a receiver. For the various IR schemes to operate, the stages must be properly tuned.
I already know all that.

The bottom line is that changing the resonant frequency of a loopstick in a receiver designed to use one will change the receive frequency—but it will also run* all the careful design that depends on the original resonant frequency range of the loopstick. It will never be a very good receiver. You could rewind the loopstick and replace the running capacitors to make it works on a different band, but you will also need to ensure the LO is operating as expected and the various filters are functional in that range.
* ruin

my hypothesis was that changing the L of the bar antenna would alter the resonance frequency, no need to change C. unless there is an optimum ratio of L to C? do you know about that?

I don't see how anything about the LO would need to change or why the filter(s) would become dysfunctional. can you elaborate on that?
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
Well, you are coupling (by overlapping the coils) that small coils value of inductance in parallel into the total tuning circuit. So even if you don't have a galvanic connection, you have an equivalent electromagnetic one.
nah. it's a different circuit.
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
OK, what circuit is it then? It's not a magic circuit if it actually tunes SW signals.
the wrapped wire is connected to itself in a closed loop along with a long random wire, not to the tuning capacitor. and what I was trying to accomplish at first was entirely different, simply adding L to the bar antenna with an inductor on the side that's not inductively coupled to the bar.
 

nsaspook

Joined Aug 27, 2009
12,778
the wrapped wire is connected to itself in a closed loop along with a long random wire, not to the tuning capacitor. and what I was trying to accomplish at first was entirely different, simply adding L to the bar antenna with an inductor on the side that's not inductively coupled to the bar.
That's what I said. It doesn't need to be physicality connected to the tuning capacitor to be electromagnetically coupled to the tuning capacitor/LO circuit.
1699990915161.png
The transistor (Q7) was the LO and Mixer in most of the old circuits. L4 is the antenna.
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
it's not the circuit you showed in the first picture. I can't identify it in the second picture. you asked "what circuit is it", it's a closed loop wrapped around the bar antenna and not connected to anything else but a random wire.

got any idea why it works? I don't see how it should do anything. yeah it's an inductor inductively coupled to the bar antenna, but so?? does it change L of the bar antenna? how is a random wire supposed to inject RF frequency into it when the current doesn't circulate between it and the closed loop? how does SW frequency RF from the random wire get picked up by the bar antenna, which is not tuned to anything in that frequency range?
 

nsaspook

Joined Aug 27, 2009
12,778
it's not the circuit you showed in the first picture. I can't identify it in the second picture. you asked "what circuit is it", it's a closed loop wrapped around the bar antenna and not connected to anything else but a random wire.

got any idea why it works? I don't see how it should do anything. yeah it's an inductor inductively coupled to the bar antenna, but so?? does it change L of the bar antenna? how is a random wire supposed to inject RF frequency into it when the current doesn't circulate between it and the closed loop? how does SW frequency RF from the random wire get picked up by the bar antenna, which is not tuned to anything in that frequency range?
The flux changes in the ferrite from the extra coil are induced (by magnetic coupling instead of a current path) by the external RF from the random wire and are coupled to the rest of the circuit. Yes, it changes the L of the antenna bar and re-tunes directly and indirectly via stray RF fields. That's why only strong AM stations get received with the tight wrap extra coil and you noticed other changes .
 

nsaspook

Joined Aug 27, 2009
12,778
how does a random wire introduce RF into a closed loop? or any frequency for that matter. even DC.
Do you understand EM or antenna theory where we can have energy coupling with EM fields? Explain and draw exactly what you mean by a closed loop because I think we have a disconnected here in terminology. You say the circuit is working, I'm giving you a possible valid explanation of why. Is it THE explanation of why, I don't know, because of incomplete information on the original circuit, modifications to that original circuit, measurements of values and frequency response changes from those modifications.
 
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