Ruud Poeze wrote:

Ron Hardin schreef:

Tim Brown wrote:

In article ,
Ron Hardin wrote:

Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else?

The R8B hears it except with DSB synch detection, so there's some
relation between I and Q channels that cancels the hum.

If I offtune 1 kHz and use CW, the 120Hz lines move up 1 kHz.

Other stations don't have it.

On the other hand I don't hear it on WFAN on other receivers, hence
this query. But I don't get a good WFAN signal on other receivers
either. The R8B is running on DC.

10:11 PM - I'm listening to them on my R-2368 receiver and hear
absolutely no hum at all (60, 120 or 180 Hz). Your receiver has a 50 kHz
IF frequency. You sure your not getting an image in there 50 kHz away
from 660 kHz (like WOR at 710 kHz)?

Tim Brown

It's still there at 4:30am EST. It's not audible at all if you're hearing both
sidebands but obvious in quiet phone call pauses (no music, not fan noise).

Either receive it SSB, or synch detect one sideband, or offtune until the signal
almost distorts on AM, and I think you'll hear the hum.

It's some difference between the upper sideband and lower sideband that gets
modulated by hum; so if you don't hear them equally the hum starts to creep in,
and at some point becomes noticeable. (So propagation at night can make the hum
come and go if you're hearing both sidebands, by depressing one and raising the
other.)

Is there some form of stereo that produces sideband differences? WFAN is famous
for ground loops in its audio and maybe they left one plugged in to the stereo
difference channel or something.

It is more likely that the hum is provided by an electronic device
generating a signal almost on 660 kHz.
Stereo is not sideband split, and an AM TX cab never produce a different
signal in one of its sideband: except for bandwidt.
ruud

It's a mystery. I don't think it can be that though because it happens most
when WFAN is strong, whereas a local signal would show up more at some point
in its fading. Also a series of 120 Hz lines + harmonics show up on an audio spectrum
around the carrier when it's detuned in SSB by a kHz or so.

I have the feeling that they don't use straight AM transmitters anymore; something like
SSB transmitters with carrier reinserted, fed into a broadband final. The high power part
doesn't care what feeds it. Bandwidth limiting is done at audio frequencies, for instance.

Well I can't experiment again until WFAN is again audible, sometime tonight.

I can distinguish directionally very easily (I have an 8-element MW phased array) but
have been unable to do anything but null away WFAN, winding up with weak Cuban stations
rather than a surviving hum signal.
--
Ron Hardin


On the internet, nobody knows you're a jerk.