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

[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.

 

[whit3rd]

On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:

In article <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@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.

 

[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.

 

[dave]

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



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.

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:

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.

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

 

[David Platt]

In article <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@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.

 

[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]

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 <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@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.

 

[Michael Black]

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 <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@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

 

[Paul Drahn]

On 10/27/2014 9:32 AM, micky 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?



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.

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]

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?

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.

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.

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]

On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black <et472@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?

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.

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.

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?

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]

<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-bf41-ee38a0c27334@googlegroups.com>, Phil
Allison <pallison49@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]

On 10/27/2014 3:48 PM, whit3rd wrote:

On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
In article <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@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 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

 

[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

 

[amdx]

On 10/29/2014 1:11 AM, micky wrote:

On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black <et472@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?

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.

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.

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?

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.

Mikek



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[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

 

[William Sommerwerck]

"Ian Jackson" wrote in message news:0iUda0GN0QUUFwOM@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.