[Test-Equipment] FS: Heath IM4180 FM deviation meter
Brooke Clarke
brooke at pacific.net
Tue Apr 22 16:37:34 EDT 2008
Hi WBob:
The PRC-25, PRC-77 and many other military VHF low band radios used a 150 Hz
tone, same idea as the PL tones on commercial and ham radios. But these are
susceptible to a type of jamming called Squelch Capture. For more on that see:
http://www.prc68.com/I/Audio.shtml#SC
Maybe that's what you are remembering? In Squelch Capture jamming all that's
needed is a carrier that's a few dB more powerful than the authorized user.
Because of the FM capture effect the weaker signal is totally suppressed and so
the receiver does not open the tone squelch.
The later military radios no longer use PL type tones because of this problem.
It's also why most mil radios have a squelch disable switch, to check for
jamming.
Have Fun,
Brooke Clarke
http://www.prc68.com/P/Prod.html Products I make and sell
http://www.prc68.com/Alpha.shtml All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
wolfbob wrote:
> You just reminded me of a curious phenomenon that occurs with FM radios.
> The squelch circuit usually operates on a thing called quieting. This is
> where the noise in the very high gain and limiting IF stages is
> monitored and it only takes the smallest signal to bring the noise level
> down a few dB. The drop is used to open the squelch. The receiver has a
> special filter that looks outside of the normal voice audio band pass
> for this noise as obviously your yapping voice could be considered by
> most circuits and people as noise so the circuit is usually looking
> around 20-25 KHz. Normally 5KHz of deviation will not have any voice
> energy to speak of in this band, although you might have noticed some
> over deviated folk on the local repeater actually squelching out on lowd
> places in their speech. This may have been the process that Sheldon
> observed.
>
> BTW and all jammers are forbidden to read this...You can shut down the
> local repeater by transmitting a modulating tone in this squelch
> bandpass. You don't need PL or anything and as long as you are strong,
> not capturing but strong enough not to be desensed too far, the repeater
> will remain squelched and off the air.
>
> WBob, WB6JPI
> \
>
> ----- Original Message ----- From: "Brooke Clarke" <brooke at pacific.net>
> To: "Discussion of Electronic Test Equipment"
> <test-equipment at mailman.qth.net>
> Sent: Tuesday, April 22, 2008 10:18 AM
> Subject: Re: [Test-Equipment] FS: Heath IM4180 FM deviation meter
>
>
>> Hi Sheldon:
>>
>> Thanks, the Agilent 7 Hints for signal generator has the table listing
>> the magic Bessell numbers.
>>
>> A couple of points on FM.
>> 1) Spread Spectrum can be thought of as the occupied bandwidth
>> exceeding the base bandwidth. For example: Broadcast FM has plus AND
>> minus 75 kHz deviation for a total of 150 kHz compared to a music
>> signal of 15 kHz (10:1). This is similar to C-band satellite TV where
>> the occupied bandwidth is around 36 MHz and the base band TV is under
>> 5 MHz (8:1).
>>
>> Military VHF low band (30 to 88 MHz) radios have 25 kHz channel
>> spacing and when used on the 6 meter ham band (5 kHz channel spacing)
>> the mil radio will sound weaker.
>>
>> The single channel Magnavox StealthCom spread spectrum radio occupies
>> the band of about 30 to 150 MHz for a voice signal with a bandwidth of
>> about 3 kHz (40,000:1). Conventional radios and television sets are
>> not aware of it's signal. Another way to think of spread spectrum is
>> process or modulation gain. In this case the range of a handheld
>> StealthCom radio far exceeds that of other conventional handheld radios.
>>
>> The Black Box (still don't know what it's for) has a pulsed signal
>> whose bandwidth exceeds that of any conventional receiver and so they
>> can't hear it.
>> http://www.prc68.com/I/BlkBox.shtml
>>
>> When Howard listened on a scanner that was the problem.
>>
>> 2) Capture Effect
>> The key difference between AM and FM related to what happens when two
>> signals on the same channel are received. In an AM receiver you will
>> hear both signals usually one weaker than the other. But in an FM
>> radio when the weaker signal is more than a few dB lower than the
>> stronger signal it will not be heard. This is why aircraft radios are
>> AM, i.e. so that a weak signal will be heard and not blocked out.
>>
>> Have Fun,
>>
>> Brooke Clarke, N6GCE
>> http://www.prc68.com/P/Prod.html Products I make and sell
>> http://www.prc68.com/Alpha.shtml All my web pages listed based on
>> html name
>> http://www.PRC68.com
>> http://www.precisionclock.com
>> http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
>>
>>
>> Sheldon Daitch wrote:
>>
>>> Yes,
>>>
>>> the indicated power does remain constant, but if one is specifically
>>> looking
>>> at the power in the envelope at the carrier frequency, that power
>>> level does
>>> change.
>>>
>>> Take a look at:
>>>
>>> http://www.cliftonlaboratories.com/bessel_nulls.htm
>>>
>>> for one discussion on carrier null.
>>>
>>> Shamelessly llifted from the website:
>>>
>>> The principle behind Bessel Nulls is that in a phase or frequency
>>> modulation system, the relationship of the carrier level, the modulating
>>> frequency and the deviation is defined by a mathematical relationship
>>> known as a Bessel function, named after their discoverer, the German
>>> mathematician Friedrich Bessel. Bessel lived nearly 100 years before
>>> FM was invented but his mathematical techniques have proven useful
>>> in describing many physical phenomena. For more details, see Agilent's
>>> Application Note 1390. At the moment, it may be found at
>>> http://cp.literature.agilent.com/litweb/pdf/5988-5677EN.pdf,
>>> but Agilent periodically rearranges its web site so this address may
>>> not last indefinitely.
>>>
>>> For our purpose, it is sufficient to understand that for certain values
>>> of deviation and modulating frequencies, the carrier will reduce to
>>> zero.
>>> More practically, a deep null, on the order of 30 or 40 dB can be seen
>>> as these values are approached.
>>>
>>>
>>> 73
>>>
>>> Sheldon
>>>
>>>
>>>
>>> Mike Manes wrote:
>>>
>>>> How arcane. Yes, the math works, but try looking at it with just an
>>>> RF power instrument: FM envelope amplitude is constant, and that's why
>>>> one can use a non-linear PA, like Class C.
>>>> 73 de Mike W5VSI
>>>>
>>>> James Duffer wrote:
>>>>
>>>>> While the provided answer to your question will suffice, actually
>>>>> Frequency Modulation (FM), the carrier does not change in
>>>>> frequency, the amplitude varies, this can be observed with a
>>>>> spectrum analyzer and proven mathmematically. The modulated wave
>>>>> has various side frequencies that vary in both frequency and
>>>>> amplitude depending on the modulating source. AM (Amplitude
>>>>> Modulation) the carrier is constant but the modulated composite
>>>>> varies in amplitude as a result of the combination of the carrier
>>>>> and side frequencies.
>>>>> Keep asking questions about things that interest you, and follow
>>>>> up with reference material.
>>>>> 73, Jim, wd4air> From: kc7hkp at comcast.net> To:
>>>>> dickburk at ix.netcom.com; test-equipment at mailman.qth.net> Subject:
>>>>> Re: [Test-Equipment] FS: Heath IM4180 FM deviation meter> Date:
>>>>> Tue, 22 Apr 2008 00:40:01 +0000> CC: > > Thanks You very Much
>>>>> Richard for answering Me !, I did not know what it was, I can only
>>>>> learn from books since elmer are not around much any more.> being
>>>>> handicapped in a wheelchair and stuck in my bedroom, I only have
>>>>> books or the net to learn> So Thanks again for taking the time to
>>>>> answer.> Instead of degrading me because i did not know.> George>
>>>>> --> KC7HKP > George Yazzolino > 14801 NE 20th Circle > Vancouver,
>>>>> Wa. USA ,98684 > Grid CN-85 > kc7hkp at arrl.net> > --------------
>>>>> Original message -------------- > From: "Richard Knoppow"
>>>>> <dickburk at ix.netcom.com> > >
>>>>> > > > ----- Original Message ----- > > From: > > To: > > >
>>>>> "Discussion of
>>>>> Electronic Test Equipment" > > > > Sent: Monday, April 21, 2008
>>>>> 1:25 PM > > Subject: Re: [Test-Equipment] FS: Heath IM4180 FM deviatio
>>>>
>>>>
>>>>
>>>> n > > meter > > > > > > > Hi Dick > > > What is FM Deviation ? > > >
>>>> I have not heard this word before in ham radio . > > > Sorry to
>>>> bother you > > > George > > > > > In case this is a serious
>>>> question... > > Deviation is the measure of the amount of frequency
>>>> > > modulation in an FM signal. It is superficially the amount > >
>>>> the carrier is changed in frequency when modulated. There > > are
>>>> several ways to measure the amount of FM but its > > possible to
>>>> make a fairly simple device to indicate it on a > > meter although
>>>> other methods are more accurate. > > FM transmission became popular
>>>> for the ham bands after >
>>>> > WW-2. It has pretty much disappeared on the HF bands > but is > >
>>>> quite widely used on 6 meters and above. > > The other reaction to
>>>> the question is because FM should > > be elementary knowledge. OTOH,
>>>> no one should be afraid to > > ask a question for fear of being
>>>> humiliated. > > May I suggest that a Google search is a good place to
>>>> > > start for definitions. > > > >
>>>> --- > > Richard Knoppow > > Los Angeles, CA, USA > >
>>>> dickburk at ix.netcom.com > > > >
>>>> ______________________________________________________________ > >
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