What entities operate at 78.0 MHz FM in the US?

Question

My cheap RF FM signal detector finds a very strong signal in several NE states at precisely 78.0 MHz. I confirmed the number with a weak signal generator. I also have a cheap FM receiver which transmits an unloaded signal at 78 MHz to the receiver, but otherwise there is no signal/station audible through the FM receiver.

FCC allocation for FM seems to stop below 88 MHz. There is some discussion of TV stations in this range. But it is specifically this signal which puzzles me, as it is very prominent and is not claimed by any entity (local TV stations, e.g.) that I can find online.

This must be very well known, considering the power usage involved, and I am probably looking in the wrong places.

Answer 1

It's not unusual for inexpensive receivers (and sometime good ones) to show phantom signals, often generated internally. Try disconnecting the antenna to see if it is really coming from the outside.

It is also very common to receive various signals where they're not expected, generated from a myriad of electronic devices in the vicinity. For example my weather station generates several signals across the HF and VHF frequencies. The signals are usually weak and localized; go a short distance and they change.

Answer 2

Try the US frequency spectrum allocation chart in that area: -

Image from with full size image here.

Of course, if your radio's intermediate frequency (IF) is 10.5 MHz and, you are trying to tune below the 88 to 108 MHz band, you might just pull-in a signal that is actually in the upper half of the 88 to 108 MHz band. An IF of 10.5 MHz is just over half of the 20 MHz width of the broadcast band (and intentionally so) but, because "mixing" produces the desired difference frequency of 10.5 MHz, what you are detecting might easily be a phantom at 99 MHz i.e. 78 MHz plus 10.5 MHz plus another 10.5 MHz.

Notice that to receive two broadcast frequencies when tuned to 108 MHz, the upper frequency would need to be 109 MHz and just outside the broadcast band. All intentional of course. So, what's happening at or close-to 99 MHz?

Answer 3

Television changed

Back in the day, 78.0 MHz would be smack in the middle of North American TV channel 5. Televisions were granted 6 MHz bands, with Channel 5 being 76-82 MHz and Channel 6 being 82-88 MHz. (and with TV audio being at the top of band, and FM, analog FM radios could pick up Channel 6's audio track at 87.75 MHz).

However, when TV went digital in the 2000s, the concept of "channels" was abstracted/virtualized. This separates the familiar "channel number" to which you are accustomed; from the frequency spectrum. A TV station is told "You get channel 44 for marketing purposes, and you are licensed to transmit on 76-82 MHz".

That, plus digital's better resistance to cross-channel interference, allowed a great re-shuffling of the frequency spectrum allocated to television, with about half the spectrum deleted and reassigned to other uses. (e.g. Channel 44's traditional spectrum).

When you set up your television (I mention this because so many people have cable and don't do this)... your digital TV does a "scan" of the airwaves, checking all known TV spectrum for signals. It encounters a signal at 78 MHz, and that signal says "Hi, we are TV channel 44". The TV remembers that, and if the user pushes "44" on their remote, the TV goes there (even though it's resting in the spectrum of Channel 5).

Traditional TV was transmitted uncompressed. Just as JPEG compresses images down to a tiny fraction of size as a TIFF, digital TV also compresses TV to a tiny fraction. Thus, even though the resolution is now much higher, the data stream is much smaller. In the original 6 MHz bandwidth, several channels can now fit. In most cases, TV stations use their entire bandwidth allocation, but run several "sub-channels": 44/1, 44/2, 44/3, etc. with varying resolution on each channel. Of course, some do not, which is why you might see a narrow band in a wide TV channel footprint. Again, your cable provider may remove this from your view by granting each of those sub-channels its own 3-digit channel.

Answer 4

78 MHz in the U.S. is within the bandwidth of OTA TV channel 5, but none of the carriers for that channel (DTV pilot, analog video or analog audio) fall right on that frequency. That would tend to indicate that it's not a broadcast signal.

In receivers that work over a wide frequency range, especially synthesized ones, it's common to have what are colloquially known as 'birdies' (also known as 'self-quieting frequencies'). These are signals that come from the receiver itself. The oscillators,synthesizer and mixers cause strong signals to be heard on various frequencies, often some nice round number harmonically related to the reference oscillator. The signal will usually sound like a very strong unmodulated carrier.

Pretty much any inexpensive synthesized receiver that covers more than a small frequency band (say under an octave, such as with an FM radio) is going to have this problem. It's a well known problem in police scanners.

Some programmable two way radios have a 'Reference Frequency' setting. If a birdie falls on a frequency you need to use, this setting shifts the reference oscillator a bit (and compensated for it in the synthesis) so the birdie falls on some other unused frequency.