Hum in am signal

[bobhuge]

Ok peeps way smarter than me.

I have a am station. It has a hum. The circuit goes from out building, through a dry Pair to the tower. The hum isn't in my building...clean at exit. Once I hook it to the dry Pair, it hums. When I disconnect it from the dry pair at the tower, I check the input from the dry pair and it hums...so I know it's not in the tower building.

I say it's in the pair from the phone company...am I wrong?

[Deleted User 15924]

Ok peeps way smarter than me.

I have a am station. It has a hum. The circuit goes from out building, through a dry Pair to the tower. The hum isn't in my building...clean at exit. Once I hook it to the dry Pair, it hums. When I disconnect it from the dry pair at the tower, I check the input from the dry pair and it hums...so I know it's not in the tower building.

I say it's in the pair from the phone company...am I wrong?

Three suggestions:

1. If your dry pair has a ground or shield tied down at both ends, lift one and see if it goes away.
2. Any chance some new power cable has been laid close to your dry cable and inducing?
3. Check power supplies for AC ripple.

These are the first three things that come to mind.

[audiophile]

There won't be shield, it's telco dry pair. The console/AudioProcessor shield can be floated at transformer.
There should be a 600:600 ohm transformer at each end of dry pair or else it will hum like crazy.

Like this:
Console---600:600---DryPair---600:600---AudioProcessor---Transmitter

[bobhuge]

I will check for that transformer. Thank you

[Colonel Flagg]

If you have line of sight from the tower to the studio, a microwave STL would eliminate all of these issues. One of our stations had a 42 mile microwave STL and it was trouble free.

[k8jd]

I have worked with remote controlled Two Way Radio base stations with telco links for 4 decades+ and even a high resistance leakage on one side of a DC pair can induce hum into the audio. A "split pair" with one wire from one twisted pair in a cable, to the other wire from a different pair connected in error on a cable connection block, same result hum.
We used to say "if you can't find the problem, it's the Telco's fault". That was often the case !

[Ben Zonia]

If you have line of sight from the tower to the studio, a microwave STL would eliminate all of these issues. One of our stations had a 42 mile microwave STL and it was trouble free.

Can you tell us any more details on that 42 mile path? Was it in Michigan? Unless it's at the top of on a 1000+ foot tower, I can't see a 42 mile path working without fadeouts, and that's probably not a clear Fresnel Zone, causing more path signal loss and fading. I can see maybe the transmit and receive antennas on mountaintops with nothing substantial in between, maybe in the Rockies, working well.

Rich studied a path between the WIOG tower in Indiantown, and in the highlands along US 23 South of Fenton, which was about that distance, and though it was LOS, and a pretty good path, the Fresnel Zone was partially blocked. Plus it's 102.5 MHz, not microwaves.

With Microwaves, it's all about the specific path, the elevation of the antennas at both ends, and the terrain profile.

[djb]

While off topic from the OPs post, I'm curious about this WIOG path between Indiantown and Fenton. (Reasoning?)

Also got me thinking about some other STLs; WCEN for example. 35.12 miles, according to points on Google Earth.

[Ben Zonia]

Colonel Flagg brought up a 42 mile STL link. Without details of the path, that doesn't tell us much.

The elevation of the WIOG site is 587 feet AMSL. The elevation of one tower site South of Fenton is 1151 feet AMSL, which is 564 feet higher than a spherical earth based on the AMSL of the WIOG site. The elevation of the path in between the two sites also enters into the prediction.

[Ben Zonia]

While off topic from the OPs post, I'm curious about this WIOG path between Indiantown and Fenton. (Reasoning?)

Also got me thinking about some other STLs; WCEN for example. 35.12 miles, according to points on Google Earth.

It looks to me like the STL to WCEN is at the top of the 400 foot WGER tower on Tittabawassee Road, and the receive is at the top of the nearly 1000 foot WCEN tower near Coleman. The terrain goes up gradually about 160 feet toward WCEN, so you could say the tower is effectively close to 1150 feet with respect to a spherical earth model at the elevation of the WGER tower.

It used to be that STL antennas were often mounted not that high on towers, but I see that there are now many at the top of very tall towers, apparently to maximize Fresnel Zone clearance and reduce fading. Line of Sight alone isn't good enough. This is probably due to faster computers and availability of Longley Rice software and detailed terrain and obstruction databases, including seasonal deciduous tree obstruction variations.