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

** Nonsense.

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

.... Phil

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

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

** 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

** 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

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

Thanks.

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

LOL

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.)

images.

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

?-)

In message , josephkk 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

** 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.

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

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.

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

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.

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

--
Ian

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".

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.

Fascinating. I never knew this.

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.

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

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).

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

That allows a local

** 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

** It says NOTHING the sort - Wanker boy.

** 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