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Old July 22nd 06, 01:49 AM posted to rec.radio.shortwave,rec.radio.swap
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Default How to convert a domestic analog FM radio to VHF ?


jakdedert wrote:

Germany, for one....

jak


That's weird. I remember seeing an ad in a German magazine (from
Germany) for a multi-band portable that covered Air, police and other
PS bands. It was one of those walkie-talkie looking things with an
analog dial.

--
Stephanie Weil
New York City, NY


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Old July 22nd 06, 04:46 AM posted to rec.radio.shortwave,rec.radio.swap
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Default How to convert a domestic analog FM radio to VHF ?

Stephanie Weil wrote:
jakdedert wrote:

Germany, for one....

jak


That's weird. I remember seeing an ad in a German magazine (from
Germany) for a multi-band portable that covered Air, police and other
PS bands. It was one of those walkie-talkie looking things with an
analog dial.

--
Stephanie Weil
New York City, NY


If you had the ad today, you could probably find the disclaimer "For
Export Only" somewhere on it.

You can buy them all over (at least during the 80's when I was last
there); but they all had that wording somewhere on the ad or packaging.

"Nur fur Export"

jak

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Old July 22nd 06, 05:07 AM posted to rec.radio.shortwave,rec.radio.swap
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Default How to convert a domestic analog FM radio to VHF ?


Brenda Ann wrote:
wrote in message
ups.com...

Brenda Ann wrote:
"Slow Code" wrote in message
nk.net...
"Ridgh" wrote in
ups.com:

Hello,
I need to convert a domestic analog FM receiver to the 108-140 MHz
band.
I've bought a Sony ICF-S10MK2 receiver and found that the oscillator
tank has a 4 turns of 22 gauge. I've turned them apart, so the
distance
between the turns will be a little greater than the factory adjusted
coil, and could receive up to about 115MHz, no more.
Is this the way to do it? Maybe to cut one turn will be better?
I'm not stucked to this receiver, I'd like to know if other cheap
receivers may be converted, and if this is the way, or if I have to
adjust anything else. I have all the gear needed, oscilloscopes,
counters etc. - but no much knowledge on this particular field.
I'll be very thankful for your help.
Ridgh


You might be able to get it higher with more inductor adjustments, I
don't
know the circuit. Maybe have to add some capacitance to the tank.

Here's your real problem though. Broadcast FM is wideband. FM
communications 135-175 MHz are narrow band. That means more modifying
is
needed to the receiver in order to hear the narrow band FM better.

It would be easier just to buy a cheap VHF receiver. Talk to the
people
in rec.radio.shortwave for ideas on cheap plastic receivers. They're
experts.

Couple problems with your answer. Let's take them one at a time:

You might be able to get it higher with more inductor adjustments, I
don't
know the circuit. Maybe have to add some capacitance to the tank.

To raise the frequency of resonance, you would decrease either inductance
or
capacitance, or both. I.E. you would remove turns from a coil, or use a
smaller series or parallel capacitor. I have successfully modified a GE
Superadio II to receive as high as 155MHz by removing a turn from each of
the RF, Antenna and Oscillator coils and realigning for proper tracking.

Here's your real problem though. Broadcast FM is wideband. FM
communications 135-175 MHz are narrow band. That means more modifying
is
needed to the receiver in order to hear the narrow band FM better.

True, that broadcast FM is wideband (150KHz total, 75KHz deviation), but
most of the frequencies the OP wishes to listen to are in the aircraft
comms
band, and are not FM at all, but fairly broad band AM (IIRC, 12KHz).
These
can be heard by slope detection on an FM radio, and the discriminator (or
other detector circuit) can be detuned a bit to allow for reception with
the
majority of the radio's circuits tuned to the center frequency of the
carrier. The detector circuit could be modified to detect only AM (and
therefor slope detect FM) fairly easily, actually by removing some
components and wiring around them, but overall this is not absolutely
necessary.


I believe you have the slope detection application backwards, i.e. it
allows FM to be demodulated on a radio set up for AM demod.

A slope detection FM demod puts the FM signal through a filter that is
not centered properly, i.e. the signal's frequencies are on the slope
of the filter, not the passband. Then as the frequency changes, the
attenuation of the signal also changes, which effectively has turned FM
into AM. Then you rectify and low pass filter, as you would do with an
AM signal.

Note also that the FM demodulator rejects AM by the use of a limiter.
Thus FM radios reject AM quiet well, but the opposite is not true, i.e.
FM can be demodulated by an AM demodulator that is tuned off center a
bit. High end FM radios will spec their AM rejection:
http://www.fanfare.com/fta100-specifications.html
The AM rejection in this application relates to the ability to reject
atmospheric effects on the FM signal strength, but it would reject
broadcast AM too if somehow the AM was on an FM broadcast frequency.


You are correct, but 'slope' detection also works to detect an AM signal on
a discriminator or ratio detector. By tuning slightly off frequency (or, by
tuning the detector circuit slightly off frequency from the center carrier..
i.e. detection on the slope of the modulated signal) you can receive AM
modulated signals. As stated, I have successfully converted an SRII to
receive up to 155 MHz, including the 108-132~ AM aircraft band. Perhaps if
the signal from these AM aircraft stations (and the aircraft themselves)
were of a higher amplitude, then the limiter would affect them.. i.e. if you
were close to your local airport's control tower. Even a stock standard
AM/FM radio (not one with an RF amp, those have too much image rejection)
will receive aircraft comms between 108 and 129.4 MHz due to various mixing
products if the signal is closeby, and this with no modification at all.




I hate to be argumentative, but I just don't agree with you here.
Check out this link. Just how are you slope detecting with your ratio
detector?

http://www.radio-electronics.com/info/receivers/fm_demod/fm_demodulation.php
-----
Slope detection
The very simplest form of FM demodulation is known as slope detection
or demodulation. It simply uses a tuned circuit that is tuned to a
frequency slightly offset from the carrier of the signal. As the
frequency of the signal varies up and down in frequency according to
its modulation, so the signal moves up and down the slope of the tuned
circuit. This causes the amplitude of the signal to vary in line with
the frequency variations. In fact at this point the signal has both
frequency and amplitude variations. The final stage in the process is
to demodulate the amplitude modulation and this can be achieved using a
simple diode circuit. One of the most obvious disadvantages of this
simple approach is the fact that both amplitude and frequency
variations in the incoming signal appear at the output. However the
amplitude variations can be removed by placing a limiter before the
detector. Additionally the circuit is not particularly efficient as it
operates down the slope of the tuned circuit. It is also unlikely to be
particularly linear, especially if it is operated close to the resonant
point to minimise the signal loss.

Ratio and Foster-Seeley detectors
When circuits employing discrete components were more widely sued, the
Ratio and Foster-Seeley detectors were widely used. Of these the ratio
detector was the most popular as it offers a better level of amplitude
modulation rejection of amplitude modulation. This enables it to
provide a greater level of noise immunity as most noise is amplitude
noise, and it also enables the circuit to operate satisfactorily with
lower levels of limiting in the preceding IF stages of the receiver.
--------------



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