How can the same FM station appear at two different spots on the dial?

[micky]

How can the same FM station appear at two different spots on the dial?

To the person who complained recently that I was off topic, I'm sorry.
This is not about any repair it would be feasible to make. It's only
about electronics.

Where I live there are two FM radio stations, 88.1 which is only a few
miles away, and 88.5 which is 40 or 50 miles away.

Right now, only my expensive KLM radio plus any car radio gets the
second one well, but I've had some cheap radios that do almost as well.

I lose track of which radios those are, so I'll start tuning at 88.1 and
tune up very gradually. After a period of silence, when I get above
88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.

How is that happening? I know about harmonics, but that doesn't apply,
does it?



P.S. This means 88.5 doesn't come in at all. I've tried stretching
out the power cord, which on the cheap radios is usually the antenna.
Sometimes that helps but on most of the radios, 88.5 won't come in at
all.

P.P.S. 88.1 is WYPR Baltimore. 88.5 is WAMU in DC. Sometimes they
play the same thing, like during the top of the hour news, Diane Rehm,
etc. although WAMU is on a 5 or 10 second delay most of the time.
Because the topic and the voices can be the same it means I can't tell
for a while if I've gotten 88.5 or just another 'instance' of 88.1.

[dave]

Wild Ass Guess here; you are hearing a Translator station rebroadcasting
the main station's programming. These mini-stations fill in nulls or
shadows. check fccinfo.com

[dave]

On 10/27/2014 09:32 AM, micky wrote:

Here's a list of FM licenses. Translators have a 3 digit number in the
callsign. If it's a flaky receiver you may be interfering with aircraft.

http://fccinfo.com/CMDProFacLookup.php?sCurrentService=FM&tabSearchType=Within+Search&ArchiveRecords=N&sKilometers=100&sLatitude=39-17-25&sLongitude=76-36-45&sPlace=Baltimore

[Steve Crow]

On Mon, 27 Oct 2014, micky wrote:

> How can the same FM station appear at two different spots on the dial?

--snip--

> I lose track of which radios those are, so I'll start tuning at 88.1 and
> tune up very gradually. After a period of silence, when I get above
> 88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.
>
> How is that happening? I know about harmonics, but that doesn't apply,
> does it?

My initial thought was that perhaps the station is operating a
"translator" (repeater) on 88.7-ish, but a search of the FCC database
doesn't turn up anything. There are two translators licensed to Maryland
on 88.7 MHz: W204BA in Oakland and W204CL in Lexington Park. Both belong
to Grace Missionary Church (d/b/a Grace Christian School). Some Googling
shows those affiliated with a small religious radio network, but it's
possible they could be re-transmitting 88.1 for some reason. Both
transmitters are fairly low power, as is typical of translators (250 and
55 watts, respectively), and given the distance (2-3 hours away) I doubt
there would be much overlap in coverage area, if any.

So... that possibility fairly well eliminated, I think the best bet is to
zip off an e-mail to the station and ask what's going on. Looks like
those are public radio stations, and my experience has been that the
engineers at those types of facilities are typically pretty helpful when
it comes to resolving reception concerns and addressing technical
questions. If you do that, I'd be curious what you dig up.

Since you are able to reproduce the behavior on multiple radios, I doubt
it's a problem with the receivers.

[David Platt]

In article <v7ss4allgf4oosl9r...@4ax.com>,

micky <NONONO...@bigfoot.com> wrote:
>How can the same FM station appear at two different spots on the dial?
>
>To the person who complained recently that I was off topic, I'm sorry.
>This is not about any repair it would be feasible to make. It's only
>about electronics.
>
>Where I live there are two FM radio stations, 88.1 which is only a few
>miles away, and 88.5 which is 40 or 50 miles away.
>
>Right now, only my expensive KLM radio plus any car radio gets the
>second one well, but I've had some cheap radios that do almost as well.
>
>I lose track of which radios those are, so I'll start tuning at 88.1 and
>tune up very gradually. After a period of silence, when I get above
>88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.
>
>How is that happening? I know about harmonics, but that doesn't apply,
>does it?

There are at least three ways in which you can end up with a strong FM station
at two locations on the dial.

(1) As somebody else suggested, it might be a "translator" - a second
transmitter carrying the same program material on a different
channel. Translators are sometimes used to "fill in" a station's
service footprint - e.g. to provide service to an area on the far
side of a mountain from the primary transmitter.

(2) Image. FM receivers are almost always superheterodyne
receiver... they have a local oscillator which is tuned either
above, or below, the station's frequency by a fixed amount (most
commonly 10.7 MHz). "Mixing" of the station frequency and the
local oscillator create an "intermediate frequency" signal at
(e.g.) 10.7 MHz which is then filtered, amplified, and decoded.

This architecture can cause a station to "reappear" on the dial,
if you're tuned away from it by twice the intermediate frequency
(e.g. by 21.4 MHz) - a second "image" of the station appears on
the dial. Good FM receivers have enough selectivity built into
their "front end" to keep this problem to a minimum - the tuner
"filters out" the station at the image frequency efficiently
enough, before mixing with the local oscillator, to keep it from
"reappearing" or interfering with a desired station (image
rejection is often 90-100 decibels, if I recall correctly).

(3) Intermodulation. If you have two strong stations nearby, their
signals can mix (either in the receiver front end, or elsewhere)
and create "spurious" signals located on either side of their true
locations on the dial. These spur signals will often be noisy and
distorted.

What you're describing doesn't sound like an image problem (#2)
because the second "copy" of 88.1 is so close to it on the dial. It
might be intermodulation, or the 88.1 station may have a translator
off in the distance.

Due to recent consolidation of radio-station ownership (both
commercial service and "noncommercial" FM), the signal at 88.7/88.8
might be a formerly-independent station in another market, which has
been "bought up" by the ownership of 88.1 and is now simply
rebroadcasting its signal.

[whit3rd]

On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
> In article <v7ss4allgf4oosl9r...@4ax.com>,
> micky <NONONO...@bigfoot.com> wrote:
> >How can the same FM station appear at two different spots on the dial?

> (3) Intermodulation. If you have two strong stations nearby, their
> signals can mix (either in the receiver front end, or elsewhere)
> and create "spurious" signals located on either side of their true
> locations on the dial. These spur signals will often be noisy and
> distorted.

This sounds very likely; if it is due to front-end nonlinearity, it's
possible to test/treat it by inserting an attenuator between the FM antenna
and the receiver (assuming the receiver has a plug-in antenna).
Lower the signal level, and the spurious response should go away.

Alternately, one can attenuate (filter) either the interfering FM station or the (presumably
AM) difference-frequency station: this can be done with a lossy antenna+load
placed near your radio, so can apply without access to antenna terminals.

[Phil Allison]

David Platt wrote:

>
> (2) Image. FM receivers are almost always superheterodyne
> receiver... they have a local oscillator which is tuned either
> above, or below, the station's frequency by a fixed amount (most
> commonly 10.7 MHz). "Mixing" of the station frequency and the
> local oscillator create an "intermediate frequency" signal at
> (e.g.) 10.7 MHz which is then filtered, amplified, and decoded.
>
> This architecture can cause a station to "reappear" on the dial,
> if you're tuned away from it by twice the intermediate frequency
> (e.g. by 21.4 MHz) - a second "image" of the station appears on
> the dial.

** Not really possible since the FM band is only 20MHz wide.

For a low side local osc:

88.1-10.7 = 77.4 = lowest local osc f

77.4+20.0 = 97.4 = highest local osc f

97.4+10.7 = 108.1 = higher f than any station.

> (3) Intermodulation. If you have two strong stations nearby, their
> signals can mix (either in the receiver front end, or elsewhere)
> and create "spurious" signals located on either side of their true
> locations on the dial. These spur signals will often be noisy and
> distorted.

** Plus contain the audio modulation of both signals.

Be a real pain if the two FM carriers differed by 10.7MHz ...

FYI:

The 2nd harmonic of strong carriers can intermod with the 2nd harmonic of the local osc to produce a new signal on the dial.

In this case, the FM deviation is doubled so may be distorted by the detector.


... Phil

[dave]

I used to work on Radio Row in Houston. One day the FCC came to visit
KILT FM 100.1 because they were causing squeals on the aeronautical
band. It wasn't any of the station's pro gear making the interference;
it was an old console FM receiver in the station lobby. 100.1 + 21.4 =
121.5. Radio row was on the direct approach to Hobby Airport or this old
mis-aligned radio would have never been busted.

[Michael Black]

I thought that was some of the basis of the ban on electronic devices on
airplanes.

Certainly there is folklore that when AM/FM transistor portables became
cheap and available, suddenly people were using them on airplanes, and
that did or could have caused interference, precisely because the local
oscillator radiated and in the aircraft band.

It's murky whether that was the specific cause of the rule or not, and
probably made murkier since it's been forty years since I read about this.

Michael

[Paul Drahn]

If you have the AFC on, the station will pop up at different dial
locations depending on which direction you are tuning. At least my old
portable does.

Paul

[Michael Black]

I was thinking along that line, except thinking of pointing out that for
whatever reasons, not great selectivity or a noisy synthesizer, a station
can be heard on more than one frequency. But, I can't recall that
happening when there's an adjacent station, then the first station being
received further up.

If that second station wasn't there, AFC is a good suggestion, and
something we might not think of much anymore, with so many fm receivers
digitally tuned. But I'd think it would "lock" to the statino further up,
that presumably is stronger at that point than the first station.

Michael

[Arfa Daily]

>
> I used to work on Radio Row in Houston. One day the FCC came to visit KILT
> FM 100.1 because they were causing squeals on the aeronautical band. It
> wasn't any of the station's pro gear making the interference; it was an
> old console FM receiver in the station lobby. 100.1 + 21.4 = 121.5. Radio
> row was on the direct approach to Hobby Airport or this old mis-aligned
> radio would have never been busted.
>

I can understand the authorities getting a bit ansty about that one. 121.5
is the international aeronautical VHF distress frequency ... :-)

Arfa

[micky]

I think I've noticed this too.

But the AFC wasn't on, because that would have made it almost impossible
to get a weak station like 88.5.

Well, I'm calling it weak because most radios won't get it, but Wikip
says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
(also ERF? It didnt' say.) So maybe I'm calling it weak because it
farther away, in DC, not Baltimore where I live, but actually, there are
places north of here, farther from DC, the Westminster, Md. area, where
88.5 comes in well and 88.1 barely comes in. A friend moved to
Finksberg and she had to change to 88.5.

But maybe the FCC makes them arrange their antennas so that in the city
of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1.
But the frequencies are different, and there's no Baltimore 88.5, so why
would 88.5's antennas have to avoid the populous part of Baltimore, or
any part?

[Arfa Daily]

<snip>

>
> Well, I'm calling it weak because most radios won't get it, but Wikip
> says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
> (also ERF? It didnt' say.)

ERP - Effective Radiated Power ? The 'real' transmitter output multiplied
by the 'gain' of the transmitting antenna.

> So maybe I'm calling it weak because it
> farther away, in DC, not Baltimore where I live,

There are many many factors that affect the propagation of a VHF signal over
a lower frequency one, some of which will degrade that signal, and others of
which can, under the right conditions, enhance it. VHF signal reception is a
lottery, once you are outside the designed service area of the station.

but actually, there are
> places north of here, farther from DC, the Westminster, Md. area, where
> 88.5 comes in well and 88.1 barely comes in. A friend moved to
> Finksberg and she had to change to 88.5.
>
> But maybe the FCC makes them arrange their antennas so that in the city
> of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1.
> But the frequencies are different, and there's no Baltimore 88.5, so why
> would 88.5's antennas have to avoid the populous part of Baltimore, or
> any part?

Without seeing a published map of the station's service area, it's
impossible to say. However, something as simple as a tall building in the
direction of the transmitting site, can be enough to cast a 'radio shadow'
across a large swathe of territory on the 'downstream' side

Arfa


>> Michael
>

[Ian Jackson]

In message <4fbddf88-87c7-4b00...@googlegroups.com>, Phil
Allison <palli...@gmail.com> writes

>

>
>
> ** Not really possible since the FM band is only 20MHz wide.
>
> For a low side local osc:
>
> 88.1-10.7 = 77.4 = lowest local osc f
>
> 77.4+20.0 = 97.4 = highest local osc f
>
> 97.4+10.7 = 108.1 = higher f than any station.

I'm pretty sure that the local oscillator nearly always runs 10.7MHz
HIGHER than the radio signal.
>

>

--
Ian

[amdx]

I recently had a station on 107.5 also have a signal at around 87.xx,
don't know exactly I was using an analog radio. I almost called the
station, but waited until the next day and the lower frequency signal
was gone. I know it was just a single day event because I listen daily
to a transmission at 87.5Mhz.

Mikek

---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

[amdx]

I had a local situation at 94.5MHz interfering with 94.3MHz. The
94.3MHz station is an out of town station and signal strength is weaker.
The interference was on all my radios. I called the Radio station
engineer and he suggested the engineer from the out of town station
probably put me up to making the call, this was not true. From the
conversation, I think he had got a lot of calls about the interference,
but he assured my the station was in compliance with FCC Reg's. It was
Hip Hop vs O'Reilly back then. It went on that way for years until the
station changed from Hip Hop to some other format, then the interference
went away.
Today I can't even find a semi local 94.3 MHz station.

[William Sommerwerck]

"Ian Jackson" wrote in message news:0iUda0GN...@g3ohx.demon.co.uk...

> I'm pretty sure that the local oscillator nearly always runs 10.7MHz
> HIGHER than the radio signal.

Precisely If it were lower, you'd greatly increase the possibility of images.

[Michael Black]

Isn't it more precisely, that by putting the LO higher, the image falls
where fewer strong signals are?

You don't want images to be below the FM broadcast band, then you end up
with TV stations 2 through 6. But above the FM broadcast band, you get a
decent stretch of aero band, amateur radio, public service, weather.
Channel 7 doesn't start until somewhere above all that.

Michael

[Ian Jackson]

In message <m2rf4b$1av$1...@dont-email.me>, William Sommerwerck
<grizzle...@comcast.net> writes

Even with the local oscillator above the station frequency, if you're
near an airport, you can get the air traffic control traffic (120MHz +/-
quite a lot) breaking through - especially if the planes are passing
more-or-less overhead. My kitchen radio gets hit when it's tuned to
97.3MHz, by out-bound flights which have just taken off from London
Heathrow, on around 118.7MHz. But, of course, it all depends on the
'front end' selectivity of the radio. However, this doesn't explain the
OP's problem - which indeed does sound as if it's simply that the same
program being carried by more than one transmitter.
--
Ian

[William Sommerwerck]

"Michael Black" wrote in message
news:alpine.LNX.2.02.1...@darkstar.example.org...

On Wed, 29 Oct 2014, William Sommerwerck wrote:

>> Precisely. If it were lower, you'd greatly increase the possibility of

>> images.

> Isn't it more precisely, that by putting the LO higher, the image falls
> where fewer strong signals are?

That would depend on band allocations and transmitter power. I'm thinking of
images from within the FM band.

Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if
the LO is above the incoming signal, images would come from stations above
107.9MHz (outside the band). If the LO were below the incoming signal, you
could have in-band images starting at 98.9MHz.

[Ian Jackson]

In message <m2ri8g$e6a$1...@dont-email.me>, William Sommerwerck
<grizzle...@comcast.net> writes

No. Taking the upper band edge as 108, 108 - 2x10.7 = 86.6 (well below
the lower band edge). This is within old US TV channel 6 - and as you
tune lower, you will hit channel 5.
--
Ian

[Phil Allison]

Ian Jackson wrote:
> >
> >
> > ** Not really possible since the FM band is only 20MHz wide.
> >
> > For a low side local osc:
> >
> > 88.1-10.7 = 77.4 = lowest local osc f
> >
> > 77.4+20.0 = 97.4 = highest local osc f
> >
> > 97.4+10.7 = 108.1 = higher f than any station.
>
> I'm pretty sure that the local oscillator nearly always runs 10.7MHz
> HIGHER than the radio signal.
>

** In the examples I have checked ( both tube and SS ), it was always 10.7MHz lower.

Makes the LO more stable is one reason.


.... Phil

[Phil Allison]

William Sommerwerck wrote:



> Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if
> the LO is above the incoming signal, images would come from stations above
> 107.9MHz (outside the band). If the LO were below the incoming signal, you
> could have in-band images starting at 98.9MHz.

** Nonsense.

Long as a particular band has less width than double the IF frequency, no in-band images will occur.


.... Phil

[Ian Jackson]

In message <bc626660-d1bb-45a0...@googlegroups.com>, Phil
Allison <palli...@gmail.com> writes

> Ian Jackson wrote:
>> >
>> >
>> > ** Not really possible since the FM band is only 20MHz wide.
>> >
>> > For a low side local osc:
>> >
>> > 88.1-10.7 = 77.4 = lowest local osc f
>> >
>> > 77.4+20.0 = 97.4 = highest local osc f
>> >
>> > 97.4+10.7 = 108.1 = higher f than any station.
>>
>> I'm pretty sure that the local oscillator nearly always runs 10.7MHz
>> HIGHER than the radio signal.
>>
>
> ** In the examples I have checked ( both tube and SS ), it was always
>10.7MHz lower.

Then don't you get a lot of image trouble from the two TV channels below
the FM band?

>
> Makes the LO more stable is one reason.

I would think the benefit would be pretty marginal.

While I'm sure there are exceptions, regardless of the frequency they're
receiving, there are probably very few 'normal' radios or TV sets etc
where the LO runs below the tuned frequency.
>

>
>

--
Ian

[Phil Allison]

> Ian Jackson wrote:
>
> > ** In the examples I have checked ( both tube and SS ), it was always
> >10.7MHz lower.
>
> Then don't you get a lot of image trouble from the two TV channels below
> the FM band?

** Who is this "you" - white man ?

The tube FM receiver was made in USA ( mono, 75uS de-emphasis) and used a 12AT7 local oscillator - barely able to run at 100MHz.






.... Phil

[Phil Allison]

micky wrote:

> How can the same FM station appear at two different spots on the dial?
>

> Where I live there are two FM radio stations, 88.1 which is only a few
> miles away, and 88.5 which is 40 or 50 miles away.
>
> Right now, only my expensive KLM radio plus any car radio gets the
> second one well, but I've had some cheap radios that do almost as well.
>
> I lose track of which radios those are, so I'll start tuning at 88.1 and
> tune up very gradually. After a period of silence, when I get above
> 88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.

** What make and model radio is doing this ??

Does it have a TDA7000 IC inside, by any chance ??

Those have an internal IF frequency of only 70KHz and image rejection is by purest magic.

FYI:

If this problem exists on only one radio, it must be the fault of that radio.

FYI 2

Your post is 99% incomprehensible drivel.



... Phil

[micky]

On Wed, 29 Oct 2014 09:30:32 -0000, "Arfa Daily"
<arfa....@ntlworld.com> wrote:

>
><snip>
>
>
>>
>> Well, I'm calling it weak because most radios won't get it, but Wikip
>> says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
>> (also ERF? It didnt' say.)
>
>
>ERP - Effective Radiated Power ?

Oh, yeah. P... F... closely related (just add a curved line to the F.)

>The 'real' transmitter output multiplied
>by the 'gain' of the transmitting antenna.

Thanks.

[micky]

Not surprising. The Hip Hop people are a bunch of trouble-makers.

From the miscellaneous drawer, that same expensive KLM radio that gets
88.5 well all the way from DC got 88.1 well too, not surprising since
it's a Baltmore station and that's where I am.

But about 6 months ago, there started to quite a bit of static (FM
static? Maybe I should listen to it again. Anyhow, it was hard to
listen to.) on the local station on the expensive radio. But all**
the other much cheaper radios continue to get the local station just
fine. So sometimes it pays to be cheap.

** I keep buying radios from the 60's and 70's at hamfests, looking for
one that will get 88.1, 88.5 and 101.1

> Today I can't even find a semi local 94.3 MHz station.

LOL
>
> Mikek
>
>

[micky]

On Wed, 29 Oct 2014 22:09:28 -0400, micky <NONONO...@bigfoot.com>
wrote:

>
>** I keep buying radios from the 60's and 70's at hamfests, looking for
>one that will get 88.1, 88.5 and 101.1

Not 101.1. 90.1, C-Span radio, which I guess I've lost interest in.
It's boring as all get out during the committee hearings, and the 7AM
program used to be great, but it's been discovered by the wackos.
Weekends, especailly evenings and nights, can be great. BookTV very
good. It or they had a long series about every president and another
series about every first lady, and their playing of the LBJ tapes was
enlightening (I'd wondered for decades if he really was pro-civil
rights or if his votes as senator were the real LBJ. It was the
first.)

[josephkk]

Nonsense, it would just change their location. Right into the VHF low TV
broadcast band (at least in the US).

?-)

[Ian Jackson]

In message <7ei35ads3e3n7bhuo...@4ax.com>, josephkk
<joseph_...@sbcglobal.net> writes

You don't change the mechanism for the interference, but because the two
TV channel allocations are where they are, if either was used in your
area you would certainly increase the possibility of interference from
them. However, as I've said, I think it's unusual for the LO to be on
the low side (probably for exactly this reason).
--
Ian

[Phil Allison]

Ian Jackson wrote:

> >
> You don't change the mechanism for the interference, but because the two
> TV channel allocations are where they are, if either was used in your
> area you would certainly increase the possibility of interference from
> them. However, as I've said, I think it's unusual for the LO to be on
> the low side (probably for exactly this reason).

** A man who prefers his ignorant opinions to facts is a complete fool:

The 6AQ8 along with the 12AT7 were the most common tubes used for LOs in FM tuners from the early 1950s onwards.

http://www.r-type.org/exhib/aav0008.htm

They were invariably used as low side oscillators.

Were not several TV channels tucked right under the FM band back then ?

All FM tuners have "image rejection" and benefit from "capture effect".

The former ranges from -40dB to -80dB while the latter ranges from 1 to 3dB.

So any image signal would be at least 100 to 10,000 times weaker than a good signal on the FM band - and it took only a 40% amplitude difference to make the stronger signal *completely* swamp the FM discriminator.




.... Phil

[Michael A. Terrell]

amdx wrote:
>
> I had a local situation at 94.5MHz interfering with 94.3MHz. The
> 94.3MHz station is an out of town station and signal strength is weaker.
> The interference was on all my radios. I called the Radio station
> engineer and he suggested the engineer from the out of town station
> probably put me up to making the call, this was not true. From the
> conversation, I think he had got a lot of calls about the interference,
> but he assured my the station was in compliance with FCC Reg's. It was
> Hip Hop vs O'Reilly back then. It went on that way for years until the
> station changed from Hip Hop to some other format, then the interference
> went away.
> Today I can't even find a semi local 94.3 MHz station.

Adjacent channel interference like that is caused by the IF bandwidth
and the skirt. The IF transformers aren't brick wall, the amplitude
drops away slowly outside the desired bandwidth. That allows a local
station to be strong enough to cause problems. AFC can make it worse, by
pulling the L.O. towards the stronger station.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

[William Sommerwerck]

"micky" wrote in message news:fq435a9mkm4cv1lni...@4ax.com...

> From the miscellaneous drawer, that same expensive KLM radio
> that gets 88.5 well all the way from DC got 88.1 well too, not
> surprising since it's a Baltmore station and that's where I am.

I assume you mean KLH. KLM is an airline.

Sammy Davis Jr once did a print ad for KLH in which he said "I used to think
KLH was an airline".

[William Sommerwerck]

"Phil Allison" wrote in message
news:1e547992-e524-4256...@googlegroups.com...

Did you actually read what I wrote? The second sentence says that.


In the third sentence, I said "If the LO were //below// the incoming
signal..."

Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

I wasn't the one who brought up the point about having the LO below the
incoming frequency.

[William Sommerwerck]

"Phil Allison" wrote in message

news:035d039d-5d91-45ad...@googlegroups.com...

> The 6AQ8 along with the 12AT7 were the most common tubes used for LOs
> in FM tuners from the early 1950s onwards.
> http://www.r-type.org/exhib/aav0008.htm

> They were invariably used as low-side oscillators.

Fascinating. I never knew this.

[Ian Jackson]

In message <035d039d-5d91-45ad...@googlegroups.com>, Phil
Allison <palli...@gmail.com> writes

> Ian Jackson wrote:
>
>> >
>> You don't change the mechanism for the interference, but because the two
>> TV channel allocations are where they are, if either was used in your
>> area you would certainly increase the possibility of interference from
>> them. However, as I've said, I think it's unusual for the LO to be on
>> the low side (probably for exactly this reason).
>
>
>** A man who prefers his ignorant opinions to facts is a complete fool:
>
>The 6AQ8 along with the 12AT7 were the most common tubes used for LOs
>in FM tuners from the early 1950s onwards.
>
>http://www.r-type.org/exhib/aav0008.htm
>
>They were invariably used as low side oscillators.

The fact that some what might now be considered 'highly desirable
collectibles' had low-side LOs doesn't mean it became a standard.

>
>Were not several TV channels tucked right under the FM band back then ?

Not the present FM band. However, in the USA FM started life between 42
to 50MHz* but this was essentially experimental. After the war, it was
allocated the present band (87.8–107.9 MHz).
*Now the analogue TV IF range - which otherwise would have been Channel
1 - and hence TV starts at Channel 2.

>
>All FM tuners have "image rejection" and benefit from "capture effect".
>
>The former ranges from -40dB to -80dB while the latter ranges from 1 to 3dB.
>
>So any image signal would be at least 100 to 10,000 times weaker than a
>good signal on the FM band - and it took only a 40% amplitude
>difference to make the stronger signal *completely* swamp the FM
>discriminator.
>

But that doesn't stop London Heathrow ATC (AM, of course) breaking
through on 97.3MHz (at least on my kitchen radio)!
--
Ian

[amdx]

The station is part of a group, and their 94.5 was interfering with a
station at 94.3 playing O'Reilly while their programing had Limbaugh on.

> From the miscellaneous drawer, that same expensive KLM radio that gets
> 88.5 well all the way from DC got 88.1 well too, not surprising since
> it's a Baltmore station and that's where I am.
>
> But about 6 months ago, there started to quite a bit of static (FM
> static? Maybe I should listen to it again. Anyhow, it was hard to
> listen to.) on the local station on the expensive radio. But all**
> the other much cheaper radios continue to get the local station just
> fine. So sometimes it pays to be cheap.
>
> ** I keep buying radios from the 60's and 70's at hamfests, looking for
> one that will get 88.1, 88.5 and 101.1
>
>
>> Today I can't even find a semi local 94.3 MHz station.
>
> LOL

Tried my truck radio today, I got 94.3MHz, JOY FM a religious
station. Covers the AL./FL. line near Dothan. Poor signal though.
Mikek

[David Platt]

> I had a local situation at 94.5MHz interfering with 94.3MHz. The
>94.3MHz station is an out of town station and signal strength is weaker.
>The interference was on all my radios. I called the Radio station
>engineer and he suggested the engineer from the out of town station
>probably put me up to making the call, this was not true. From the
>conversation, I think he had got a lot of calls about the interference,
>but he assured my the station was in compliance with FCC Reg's. It was
>Hip Hop vs O'Reilly back then. It went on that way for years until the
>station changed from Hip Hop to some other format, then the interference
>went away.

The station engineer might have been telling the strict truth... it
would have taken a spectrum analyzer or modulation meter to be sure.

Commercial FM is generally allowed a +/- 75 kHz carrier deviation.
Due to the way FM works, and due to the fact that the station is
transmitting a stereo subcarrier (centered on 38 kHz, with its own
sidebands going out as much as 15 kHz on either side), the FM
station's actual RF "footprint" can easily have significant energy 120
kHz on either side of its nominal carrier frequency. That's more than
half-way out to the "alternate" channel center, 200 kHz away. If the
station tends to run "loud" (highly compressed audio, cranked all the
way up) then the "wide footprint" is likely to be present much or most
of the time.

Things can be even worse these days, since many stations are also
transmitting in-band/on-channel digital subcarriers which go out even
further.

A lot of FM radios/receivers have fairly "broad" intermediate-
frequency filters... e.g. one or two crystal filters with 220 kHz or
even 250 kHz bandwidth. Such broad receptivity lets almost all of the
"desired" station's signal in... and that's good for low-distortion
stereo reception since you get the whole stereo subcarrier.
Unfortunately, if there's a strong signal on the "alternate" channel
(200 kHz away), that signal's outer sidebands will end up getting
through the filter, and will probably affect the stereo subcarrier and
increase distortion or "break through" into audibility. If you're
trying to tune in a weak, distant signal that's on an "adjacent"
channel to a strong local (100 kHz away) the problem is even worse.

There are ways to work around this:

- Use an FM tuner which has a narrower IF bandwidth. Better tuners
often have a wide/narrow switch setting, with the narrow setting
using different (or more) crystal filters with reduced bandwidth -
200, 180, 150, or even 110 kHz.

The narrower filters can eliminate a lot of adjacent- and
alternate-channel bleedover. The price is higher distortion
(especially in stereo) since the outer FM sidebands of the desired
station are also eliminated by the narrower filters.

- Use a directional FM antenna, and aim it in the direction which
gives the best results. This may be "aimed towards the desired
station" (increasing its relative strength), or "aimed at an angle
away from the undesired station" (to put the interfering station in
a "null" in the antenna's reception pattern).

[Phil Allison]

Michael Terrell wrote:
>
>
> Adjacent channel interference like that is caused by the IF bandwidth
> and the skirt. The IF transformers aren't brick wall, the amplitude
> drops away slowly outside the desired bandwidth.

** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.

That allows a local
> station to be strong enough to cause problems.

** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.


.... Phil

[Phil Allison]

William Sommerwerck wrote:

> "Phil Allison" wrote in message
> news:1e547992-e524-4256...@googlegroups.com...
> William Sommerwerck wrote:
>
> >> Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz,
> >> if
> >> the LO is above the incoming signal, images would come from stations above
> >> 107.9MHz (outside the band).
>
> >> If the LO were below the incoming signal, you could have in-band images
> >> starting at 98.9MHz.
>
> > ** Nonsense.
> > Long as a particular band has less width than double the IF frequency,
> > no in-band images will occur.
>
> Did you actually read what I wrote? The second sentence says that.

** It says NOTHING the sort - Wanker boy.

>
>
> In the third sentence, I said "If the LO were //below// the incoming
> signal..."
>
> Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

** Garbage.

With the LO at 77.4, the image is at 66.7

The FM broadcast band does not suffer from in-band images long as the IF is 10.7MHz of higher.


... Phil

[Phil Allison]

O Ian Jackson wrote:
> >> >
> >> You don't change the mechanism for the interference, but because the two
> >> TV channel allocations are where they are, if either was used in your
> >> area you would certainly increase the possibility of interference from
> >> them. However, as I've said, I think it's unusual for the LO to be on
> >> the low side (probably for exactly this reason).
> >
> >
> >** A man who prefers his ignorant opinions to facts is a complete fool:
> >
> >The 6AQ8 along with the 12AT7 were the most common tubes used for LOs
> >in FM tuners from the early 1950s onwards.
> >
> >http://www.r-type.org/exhib/aav0008.htm
> >
> >They were invariably used as low side oscillators.
>
> The fact that some what might now be considered 'highly desirable
> collectibles' had low-side LOs doesn't mean it became a standard.

** Never said it was "standard" - just quite common.

Proves you are wrong - sonny boy.

> >Were not several TV channels tucked right under the FM band back then ?
>
> Not the present FM band. However, in the USA FM started life between 42
> to 50MHz* but this was essentially experimental. After the war, it was

> allocated the present band (87.8-107.9 MHz).


** Huh? What is the relevance of that crap ?

ATC (AM, of course) breaking
> through on 97.3MHz (at least on my kitchen radio)!
> --

** So that is your only case?

Piss off fool.


.... Phil


> Ian

[William Sommerwerck]

"Phil Allison" wrote in message

news:ba56876f-a7d1-4673...@googlegroups.com...

William Sommerwerck wrote:

> "Phil Allison" wrote in message
> news:1e547992-e524-4256...@googlegroups.com...
> William Sommerwerck wrote:
>
> >> Given that the FM band is 20MHz, and twice 10.7 MHz is greater than
> >> 20MHz,
> >> if
> >> the LO is above the incoming signal, images would come from stations
> >> above
> >> 107.9MHz (outside the band).
>
> >> If the LO were below the incoming signal, you could have in-band images
> >> starting at 98.9MHz.

> > ** Nonsense.
> > Long as a particular band has less width than double the IF frequency,
> > no in-band images will occur.

> Did you actually read what I wrote? The second sentence says that.


** It says NOTHING the sort - Wanker boy.

It says exactly that. It's amazing that someone as intelligent as you can't
understand plain language.

> In the third sentence, I said "If the LO were //below// the incoming
> signal..."
> Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

** Garbage.

With the LO at 77.4, the image is at 66.7.

Uh... that's right.

[Phil Allison]

William Sommerwerck wrote:

> >
> > >> Given that the FM band is 20MHz, and twice 10.7 MHz is greater than
> > >> 20MHz,
> > >> if
> > >> the LO is above the incoming signal, images would come from stations
> > >> above
> > >> 107.9MHz (outside the band).
> >
> > >> If the LO were below the incoming signal, you could have in-band images
> > >> starting at 98.9MHz.
>
> > > ** Nonsense.
> > > Long as a particular band has less width than double the IF frequency,
> > > no in-band images will occur.
>
> > Did you actually read what I wrote? The second sentence says that.
>
>
> ** It says NOTHING the sort - Wanker boy.
>
> It says exactly that.

** FFS learn to read you autistic idiot.

MY post say there are NO in-band images at all - neither from high nor low side injection of the LO.

>>
> > In the third sentence, I said "If the LO were //below// the incoming
> > signal..."
> > Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?
>
> ** Garbage.
> With the LO at 77.4, the image is at 66.7.
>
> Uh... that's right.

** Wot a thick head.


.... Phil

[Michael A. Terrell]

More Philshit, as always. It's a damned good thing you didn't design
and build deep space telemetry equipment. Even business radios used
expensive crystal filters to reduce adjacent channel interference,
instead of 50 cent IF transformers. Digitally tuned FM receivers can
still receive an adjacent channel, but more than a channel away it
becomes quite distorted. Go back to hacking old stereos.

[Phil Allison]

Michael Terrell wrote:
> Phil Allison wrote:
> > Michael Terrell wrote:
> > >
> > >
> > > Adjacent channel interference like that is caused by the IF bandwidth
> > > and the skirt. The IF transformers aren't brick wall, the amplitude
> > > drops away slowly outside the desired bandwidth.
> >
> > ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.
> >
> > That allows a local
> > > station to be strong enough to cause problems.
> >
> > ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.
>
>
> More Philshit, as always. It's a damned good thing you didn't design
> and build deep space telemetry equipment.

** Desperate liars resort to abuse when they have no case.

> Even business radios used
> expensive crystal filters to reduce adjacent channel interference,

** Not one bit relevant to *wide band* FM broadcast receivers.

> instead of 50 cent IF transformers.

** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band.

> Digitally tuned FM receivers can still receive an adjacent channel,

** More irrelevance, it matters not how the LO is tuned.

> but more than a channel away it
> becomes quite distorted.

** Tuned FM detectors are like that.

>Go back to hacking old stereos.

** Go back to washing dunny floors, you pathetic ass.



.... Phil

[Michael A. Terrell]

Phil Allison wrote:
>
> Michael Terrell wrote:
> > Phil Allison wrote:
> > > Michael Terrell wrote:
> > > >
> > > >
> > > > Adjacent channel interference like that is caused by the IF bandwidth
> > > > and the skirt. The IF transformers aren't brick wall, the amplitude
> > > > drops away slowly outside the desired bandwidth.
> > >
> > > ** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.
> > >
> > > That allows a local
> > > > station to be strong enough to cause problems.
> > >
> > > ** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.
> >
> >
> > More Philshit, as always. It's a damned good thing you didn't design
> > and build deep space telemetry equipment.
>
> ** Desperate liars resort to abuse when they have no case.

DING! DING! DING! Whenever you have no clue you start your 'Angry
Dumbass Dance', and call people a liar. All you do is prove what a fool
you are.

> > Even business radios used
> > expensive crystal filters to reduce adjacent channel interference,
>
> ** Not one bit relevant to *wide band* FM broadcast receivers.

Only if you have the I.Q. of a rusty doorknob.

> > instead of 50 cent IF transformers.
>
> ** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band.

Define sharp. If it is too sharp, it causes distortion in the
recovered signal. Each transformer has an insertion loss in the IF
stages. FM radios use just enough tuned circuits to get barely
acceptable performance. Even the cheap Murata ceramic filters have a
sloppy skirt. The only advantage is that they don't need aligned during
manufacturing.


Cram your bullshit and look at it on a network analyzer. Oh, that's
right. You have no real test equipment, just junk from a '70s TV shop.
You want to talk wideband? One of the Telemetry products we
manufactured had an IF range from 1 KHz to 20 MHz bandwidth at the -3 dB
points. They had to be aligned on a network analyzer, or with a
calibrated sweep generator to achieve the proper skirt.

> > Digitally tuned FM receivers can still receive an adjacent channel,
>
> ** More irrelevance, it matters not how the LO is tuned.

Sigh. More of your stupidity. Digital has no AFC, so it can't be
pulled off of center.

> > but more than a channel away it
> > becomes quite distorted.
>
> ** Tuned FM detectors are like that.
>
> >Go back to hacking old stereos.
>
> ** Go back to washing dunny floors, you pathetic ass.

Whatever the hell that crap means, but I guess that you've heard it
all your life from people around you. Read a damned book on receiver
design. Analog IF bandwidth is specified at the -3 dB points. That
wouldn't be possible without a skirt. Brick wall requires a FIR filter
or another digital filter that processes a digitized input. The design
I worked on digitized to 50 to 890 MHz range for a 70 MHz IF. That was
followed by a pair of FIR filters for the IF and another pair for the
output bandwidth.

[dave]

On 10/28/2014 08:40 AM, Michael Black wrote:
> On Tue, 28 Oct 2014, dave wrote:
>
>> On 10/27/2014 01:48 PM, whit3rd wrote:
>>> On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
>>>> In article <v7ss4allgf4oosl9r...@4ax.com>,

>>>> micky <NONONO...@bigfoot.com> wrote:
>>>>> How can the same FM station appear at two different spots on the dial?
>>>

>>>> (3) Intermodulation. If you have two strong stations nearby, their
>>>> signals can mix (either in the receiver front end, or elsewhere)
>>>> and create "spurious" signals located on either side of their true
>>>> locations on the dial. These spur signals will often be noisy and
>>>> distorted.
>>>
>>> This sounds very likely; if it is due to front-end nonlinearity, it's
>>> possible to test/treat it by inserting an attenuator between the FM
>>> antenna
>>> and the receiver (assuming the receiver has a plug-in antenna).
>>> Lower the signal level, and the spurious response should go away.
>>>
>>> Alternately, one can attenuate (filter) either the interfering FM
>>> station or the (presumably
>>> AM) difference-frequency station: this can be done with a lossy
>>> antenna+load
>>> placed near your radio, so can apply without access to antenna
>>> terminals.
>>>
>>
>> I used to work on Radio Row in Houston. One day the FCC came to visit
>> KILT FM 100.1 because they were causing squeals on the aeronautical
>> band. It wasn't any of the station's pro gear making the interference;
>> it was an old console FM receiver in the station lobby. 100.1 + 21.4 =
>> 121.5. Radio row was on the direct approach to Hobby Airport or this
>> old mis-aligned radio would have never been busted.
>>
> I thought that was some of the basis of the ban on electronic devices on
> airplanes.
>
> Certainly there is folklore that when AM/FM transistor portables became
> cheap and available, suddenly people were using them on airplanes, and
> that did or could have caused interference, precisely because the local
> oscillator radiated and in the aircraft band.
>
> It's murky whether that was the specific cause of the rule or not, and
> probably made murkier since it's been forty years since I read about this.
>
> Michael
>

Cheap consumer radios are using pretty exotic methods to reduce the mass
and the current draw these days. Any oscillators are deep into the chipset.

[dave]

On 10/28/2014 11:11 PM, micky wrote:
> On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black <et...@ncf.ca> wrote:
>
>> On Tue, 28 Oct 2014, Paul Drahn wrote:
>>

>>> On 10/27/2014 9:32 AM, micky wrote:
>>>> How can the same FM station appear at two different spots on the dial?
>>>>

>> I was thinking along that line, except thinking of pointing out that for
>> whatever reasons, not great selectivity or a noisy synthesizer, a station
>> can be heard on more than one frequency. But, I can't recall that
>> happening when there's an adjacent station, then the first station being
>> received further up.
>>
>> If that second station wasn't there, AFC is a good suggestion, and
>> something we might not think of much anymore, with so many fm receivers
>> digitally tuned. But I'd think it would "lock" to the statino further up,
>> that presumably is stronger at that point than the first station.
>>
>> Michael
>
The fcc.info link I sent lets you search the FCC database in a friendly
gui fashion

[dave]

On 10/29/2014 02:30 AM, Arfa Daily wrote:
>
> <snip>

>
>
>>
>> Well, I'm calling it weak because most radios won't get it, but Wikip
>> says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
>> (also ERF? It didnt' say.)
>
>

> ERP - Effective Radiated Power ? The 'real' transmitter output

> multiplied by the 'gain' of the transmitting antenna.
>
>

>> So maybe I'm calling it weak because it
>> farther away, in DC, not Baltimore where I live,
>
>

> There are many many factors that affect the propagation of a VHF signal
> over a lower frequency one, some of which will degrade that signal, and
> others of which can, under the right conditions, enhance it. VHF signal
> reception is a lottery, once you are outside the designed service area
> of the station.
>
>

> but actually, there are
>> places north of here, farther from DC, the Westminster, Md. area, where
>> 88.5 comes in well and 88.1 barely comes in. A friend moved to
>> Finksberg and she had to change to 88.5.
>>
>> But maybe the FCC makes them arrange their antennas so that in the city
>> of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1.
>> But the frequencies are different, and there's no Baltimore 88.5, so why
>> would 88.5's antennas have to avoid the populous part of Baltimore, or
>> any part?
>
>

> Without seeing a published map of the station's service area, it's
> impossible to say. However, something as simple as a tall building in
> the direction of the transmitting site, can be enough to cast a 'radio
> shadow' across a large swathe of territory on the 'downstream' side
>
> Arfa
>
>
>>> Michael
>>

An obstruction in the energy field creates Fresnel Zones on the side of
the obstruction opposite the antenna. These are alternating peaks higher
than normal and dips lower than normal. The distance between the peaks
and nulls (Fresnel Zones) is determined by the distance between the
antenna and the obstruction.

[chuck]

I have a Denon tuner where the narrow ceramic filters were not narrow
enough to weed out the station I wanted to eliminate. I bought
narrower filters from Digikey which resolved my problem. Chuck

[Phil Allison]

David Platt wrote:

>
> Commercial FM is generally allowed a +/- 75 kHz carrier deviation.
> Due to the way FM works, and due to the fact that the station is
> transmitting a stereo subcarrier (centered on 38 kHz, with its own
> sidebands going out as much as 15 kHz on either side),

** It's worth pointing out that there is only ONE carrier for an FM broadcast signal. At any instant in time, there is only one frequency to deal with and one signal voltage coming from the detector.

With FM stereo, the detector's output includes supersonic signals up to 50 KHz or so. The supersonic stuff provides the L-R difference signal.

> A lot of FM radios/receivers have fairly "broad" intermediate-
> frequency filters... e.g. one or two crystal filters with 220 kHz or
> even 250 kHz bandwidth.

** Crystal filters are a tad expensive for an FM radio - so designers make do with tuned transformers and Ceramic filters in the 10.7MHz amplifier stages.

Even a budget FM tuner will have at least one or two of each along with a tuned RF stage to provide good out of band and nearby signal rejection for normal use. FM DXing is NOT normal use.

> Such broad receptivity lets almost all of the
> "desired" station's signal in... and that's good for low-distortion
> stereo reception since you get the whole stereo subcarrier.
> Unfortunately, if there's a strong signal on the "alternate" channel
> (200 kHz away), that signal's outer sidebands will end up getting
> through the filter, and will probably affect the stereo subcarrier and
> increase distortion or "break through" into audibility.

** There are never two, strong FM signals separated by 200KHz - authorities govern frequency allocations on the band so as to prevent this.

> If you're
> trying to tune in a weak, distant signal that's on an "adjacent"
> channel to a strong local (100 kHz away) the problem is even worse.

** Only mad FM DXers have that issue.

> There are ways to work around this:
>
> - Use an FM tuner which has a narrower IF bandwidth. Better tuners
> often have a wide/narrow switch setting, with the narrow setting
> using different (or more) crystal filters with reduced bandwidth -
> 200, 180, 150, or even 110 kHz.

** Yep - mad FM DXers sometimes do that.

>
> - Use a directional FM antenna, and aim it in the direction which
> gives the best results. This may be "aimed towards the desired
> station" (increasing its relative strength), or "aimed at an angle
> away from the undesired station" (to put the interfering station in
> a "null" in the antenna's reception pattern).

** Sure - an antenna rotator with a high gain Yagi on top is what every home needs. Pure bollocks.

The most common FM antenna is the TV antenna on the roof or maybe a dipole parked inside the roof cavity.




... Phil

[Tim R]

On Wednesday, October 29, 2014 10:23:21 PM UTC-4, micky wrote:
> On Wed, 29 Oct 2014 22:09:28 -0400, micky <NONONO...@bigfoot.com>
> wrote:
>
> >
> >** I keep buying radios from the 60's and 70's at hamfests, looking for
> >one that will get 88.1, 88.5 and 101.1
>
>
> Not 101.1. 90.1, C-Span radio, which I guess I've lost interest in.
> It's boring as all get out during the committee hearings, and the 7AM
> program used to be great, but it's been discovered by the wackos.

I can't stand C-span though they have some good programming. The problem is that fake synthesized brass keyboard they use for the theme music. How hard would it have been to buy a CD of a real brass ensemble playing it? That piece has been recorded many times.