FM Harmonics?

[trusty]

I've long been an advocate of expanding the FM broadcast band down to at least 76.1, similar to where the Japanese band begins, so some AM stations could move to the new band and relieve the AM band

- until I started DXing in that area the other day.

I have a Sony ICF-SW7600G that has an FM band 76-108. One day I noticed I was getting faint signals between 77.9 and 85.3 mHz - exactly 21.4 mHz below the same broadcast station. 99.3 came in at both 99.3 and 77.9; 106.7 came in at 106.7 and 85.3
I tried this indoors and out - same result (I don't have another radio that will go to 76.0, so I can't compare radios.).

A couple of these stations are over 50 miles away; The closest is 8 miles away.

I checked and found the Japanese band is 76.0 - 90.0, so it wouldn't be affected by "harmonics"(?). Most of the FM bands are not 21.4 mHz wide (108 - 88 = 20).

Has anyone else experienced this? (If the 21.4 "harmonic" is prevalent, there goes my theory about expanding the FM band.)

 

[boiseengineer]

21.4 divided by 2 = 10.7 the IF frequency of your radio. It's all in your radio.

 

[Schroedingers Cat]

21.4 divided by 2 = 10.7 the IF frequency of your radio. It's all in your radio.

If you live in a place that has a strong station on 107.9, you've known about it for years. Many early FM portables would receive the image on 86.5 MHz, as the radios tuned down that far. And if you ever had a TV sound radio, you've heard FM stations all over the TV channel 4 through 6 sound region. It's actually called an image, not a harmonic. Radios with good image rejection won't get them unless the station is really strong. Most FM radios today don't tune down much further than 87.7, so you won't hear ay images. 40 dB image rejection, typical of AM radios, isn't very good. A 40 dB image rejection on FM would mean a city grade signal would be easily detectable on an open space in the 66.5 to 86.5 region.

 

[Big 121]

It's actually called an image, not a harmonic.

In the communications industry, the measurement for this is called "IF rejection". It's circuit-generated , and not dependent
on a signal at the antenna terminals. For maybe $2.50 Sony could have added a higher-Q IF section to deal with it.
This is one tradeoff in an otherwise decent radio at a decent price.

 

[Schroedingers Cat]

In the communications industry, the measurement for this is called "IF rejection". It's circuit-generated , and not dependent
on a signal at the antenna terminals. For maybe $2.50 Sony could have added a higher-Q IF section to deal with it.
This is one tradeoff in an otherwise decent radio at a decent price.

Though they may be connected by rejecting any off received frequency signal, the IF rejection figure refers to the ability of the receiver to reject two strong signals about 10.7 MHz apart (10.6 or 10.8 MHz frequency difference in practice in this country) mixing together and coming in all over the band. Often heard during tropo events, when a strong signal from another area comes in much stronger than usual.

 

[Schroedingers Cat]

Usually, two signals 10.6 or 10.8 MHz are not in the same area. Some of the strong signals that mixed together and came in all over the band were:

WFBE 95.1 (local) and 265000 watt then WOOD-FM 105.7 (tropo). At the time, WFBE didn't use compression, and during soft classical numbers, only WOOD-FM modulation would be heard all over the band.

WGMZ 107.9 (local) and WWJ-FM 97.1, both using Bonneville Beautiful Music on reel to reel tapes. They weren't in sync, so a long echo effect could be heard all over the band when tropo brought in WWJ-FM stronger than usual.

 

[rfry]

Image Rejection

I have a Sony ICF-2002 that I bought in the early 1980s (still working), as well as a Sony ICF-SW-7600GR that I bought around 1995 (still working).

Both of those receivers specify dual conversion for AM broadcast, which should provide very good "image" rejection.

An AM broadcast station on 1440 kHz located about 6 miles from me, licensed to the city where I live, broadcasts with different powers and directional patterns day and night, with the major lobe in my direction day and night.

The image of 1440 kHz for a 455 kHz IF used in a receiver is [1440 - (2 x 455)] kHz = 530 kHz.

My newer ICF-SW-7600GR when tuned to 530 kHz produces a very listenable output of the program audio of that local 1440 kHz station, day and night.

My older ICF-2002 does not. The only output from the ICF-2002 for these conditions is r-f noise (no useful audio from the local station on 1440 kHz, or from any other broadcast station).