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 ********.

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




.... Phil