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FM Shortspacing and Intermediate Frequency Separation.

R

RF101

The FCC allows shortspacing between stations less than the standard distance if (in simple terms) the primary station's contour does not overlap with the "adjacent" station's interfering contour: co-channel 20dB down, first-adjacent 6dB down, etc. I get that concept and studied lots of engineering proposals with different contour plots to affirm the rules. (There's an obvious limit to shortspacing, usually around an 8%-9% reduction in full-space distancing.)

What's puzzling is the IF protection minimum distance separation rule for 10.6/10.8 MHz , effectively making sure a nearby station does not overload the 10.7 internal IF circuitry of the listener's FM receiver. The puzzling part is that rule is "hard-and-fast", ergo no shortspacing exceptions are permitted with the IF rule.

Questions: With newer receivers and associated technology has there been any progress in reducing this IF overloading issue? If so, why hasn't some group submitted a rulemaking petition to ease off on the IF separation rule? This might be a way to open up a few metro areas to even more stations?
 
At least one consulting engineer of my acquaintance has raised the issue with staff, in particular the ridiculous requirement that translators have to follow the same IF spacing as 6 kW class A full-power FMs. No motion so far, sadly.
 
Correct. Under 99w is exempt... but there's still no reason to treat a 250-watt translator as though it's a 6kW class A.
 
Getting a CP and being allowed to broadcast are two different things. Reason,
LPFM'S and translators are secondary services. That means you must go off the air or fix any reception problems you cause with a new signal. Yes, there are many many old receivers still working. Those radios will have trashed reception if you.do this. Only go for this as a last resort. The new station might be buying new radios for many people. No kidding.
 
Last edited by a moderator:
fybush said:
At least one consulting engineer of my acquaintance has raised the issue with staff, in particular the ridiculous requirement that translators have to follow the same IF spacing as 6 kW class A full-power FMs. No motion so far, sadly.
Click to expand...
Of course, the first and second adjacent channel separation rules predate current receiver design by a handful of decades and could be revised.

My first FM radio in the later 50's had no AFC, and stations slobbered all over the dial on it. That was 60 years ago!

When Mexico's congress decided that AM radio was obsolete and that, to preserve employment and diversity of opinion, a route to migration onto the FM band was developed with essentially reduced the separation requirements for adjacents. That meant the Mexico became more like the rest of the Hemisphere to the south of them, allowing more stations on the dial in any market and closer allocations outside the market.

The result was that about 600 or so of the 800 AMs in Mexico moved to FM. The ones that stayed on AM were either in a couple of very big cities that had "too many" AMs to allow all to migrate or along the US border zone where they had to respect the separation and allocation of the Comisión Mixta agreement.
 
Two FM signals co-located or closely located on 2nd or 3rd adjacent channels are OK. But, two strong signals 10.6 or 10.8 MHz apart will cause some really nasty problems for many.
 
Flying_Dutchman said:
Two FM signals co-located or closely located on 2nd or 3rd adjacent channels are OK. But, two strong signals 10.6 or 10.8 MHz apart will cause some really nasty problems for many.
Click to expand...
With today's PLL radios, that's a thing of the past....it use to be with manually tuned radios in cars but the PLL models did away with that problem
 
1.. Most people aren't running out to buy new radios. 2. Their old radios still work and that's what they use. 3. You will have to go on Ebay to find PLL receivers. The chain stores don't have them because no one is buying them.
4. It's the new station's responsibility to buy new radios for everyone who has a reception problem. So, if I put a new station in a populated area, that can be expensive.

That's why I would only apply for such a frequency as a last resort.
 
Flying_Dutchman said:
Two FM signals co-located or closely located on 2nd or 3rd adjacent channels are OK. But, two strong signals 10.6 or 10.8 MHz apart will cause some really nasty problems for many.
Click to expand...
Local stations on third adjacents work find in much of the world, in fact.
 
2nd and 3rd adjacent channel spacing is OK on just about any radio made after 2000. It's fine on some receivers made in the 1970's as well.
It's the 10.6 and 10.8 IF frequencies that can be a pain.
And, PLL portable radios are battery hogs.
 
I understand the part about PLL and frequency-synthesized radio eliminating the adjaceny problem. I thought the IF stage was still a part of the equation - of course if you're only 100 kHz from the IF under a PLL arrangement then that aspect could go away as that is enough channel separation..
 
Flying_Dutchman said:
Getting a CP and being allowed to broadcast are two different things. Reason,
LPFM'S and translators are secondary services. That means you must go off the air or fix any reception problems you cause with a new signal. Yes, there are many many old receivers still working. Those radios will have trashed reception if you.do this. Only go for this as a last resort. The new station might be buying new radios for many people. No kidding.
Click to expand...
Even buying new radios won’t work. A translator had complaints that it was interfering with a class B from 50 miles away. The translator owner tried to resolve it by buying smartphones with the iHeartRadio app installed so the people who complained could listen to the station by streaming it. The FCC wasn’t satisfied and the translator had to go off the air.
 
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