Noisy AM reception at studio.

AM Radio discussion. Directional arrays are FUN!
Post Reply
Relay
Posts: 48
Joined: Fri May 28, 2010 9:59 am

Noisy AM reception at studio.

Post by Relay »

I am at an AM station that has studios located 3 miles from its 5Kw transmitter site. For some reason, the studio building has a very high level of what sounds like mains power interference that effects frequencies from about 700Khz down to about 530Khz. Our station is within this range and I am haveing real trouble getting a static free off-air feed for monitoring. At such a close range to the TX I thought the task would be a lot simpler than what it is. If I take a battery operated portable radio for a walk aroung the external roof top of the building the noise is extremely harsh and if I place the radio near the iron roof it increases. Recently I found a quite spot on our roof top and decided to mount a tuned loop antenna on a small metal mast and run rg58 feeder back to the wide band receiver in out CTA. To my dissappointment, the noise was still there and it is at an unacceptable level - so much so that we cannot monitor te station from the off-air feed.

If I take my portable radio and hold it very near to the loop antenna I can find the antenna's max reception point but the noise is still extremely high. Interestingly, if I disconnect the loop antenna feeder and then place the portable radion near the loop antenna there is very little noise, it is only when the feeder is connected againg that the noise returns. From this observation I think that the noise might be getting transmitted along the feeder from the equipment in the CTA. I've tried two different off-air receivers amd the same result is obtained.

The noise is very strong and it sounds like the sort of mains noise you get when you are near dirty high tension power insulators. Mains distribution in this area is underground at the front of the building. If I turn the mains off and allow the emergency plant to take the load there is no difference. Even during the short period after the mains fails leading up the the EPP taking the load, there was no change to this interference. The equipment in the CTA rack is protected with UPS equipment.

Does anyone have any good advice that might help with this - any comments would be appreciated.

User avatar
RGORJANCE
Posts: 1550
Joined: Mon Dec 31, 2007 7:48 am
Location: RACINE, WI

Re: Noisy AM reception at studio.

Post by RGORJANCE »

Somewhat similar problem was encountered about ten years ago. A 5kw station was being wiped out in a town about 5 miles SW of the site. There was a factory located in the town that seemed to be much noisier than the rest of the town. The hash was also traveling on the power lines over a very wide area. Turned out they (factory) had lost a ground on one of their large three phase motors. I had contacted the power company and they actually located the problem and helped resolve the issue.

You might start looking for any electric motors that are constantly operating that could be the source.

If anything else pops into the skull, I'll post it.

Good luck.

Fossil

User avatar
Deep Thought
Posts: 3513
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Noisy AM reception at studio.

Post by Deep Thought »

"The equipment in the CTA rack is protected with UPS equipment."

What kind of UPS equipment are you using? The usual computer backup supplies, being 'stepped square wave' devices, are notoriously noisy and unusable for audio or video equipment which expects a low-distortion sine wave for its AC power. Are your UPSs online (running off battery) all the time (that would be a true uninterruptible supply rather than a back-up) or do they switch on power mains fail?

Can you selectively turn off loads? Just switching to back up power isn't much of a test.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Relay
Posts: 48
Joined: Fri May 28, 2010 9:59 am

Re: Noisy AM reception at studio.

Post by Relay »

We have several UPS units in the CTA - the unit that supplies the off-air receiver is one that operates in standby mode - not an on-line unit but there are two other smaller units that supply the studios as well. Doing a selective shut down wont be easy because we have an FM service operated by a compeditor integrated within the same racks and it shares the same UPS unit. I would have to go through the process of getting permission for an outage etc but if this is the only way through, it will have to be done.
I might give our power supply company a call and see if they can assist in any way but I have read similar comments about UPS noise in systems like this.

NECRAT
Site Admin
Posts: 3179
Joined: Sat Nov 17, 2007 9:13 pm
Location: Taunton, MA
Contact:

Re: Noisy AM reception at studio.

Post by NECRAT »

Here is a way to make sure the noise is internal versus external. What happens if you try to pick up the station in the parking lot, using a DC source, a way from the building?

Do you hear the same noise there?
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

Relay
Posts: 48
Joined: Fri May 28, 2010 9:59 am

Re: Noisy AM reception at studio.

Post by Relay »

There is virtually no noise in the car park using a battery receiver. Likewise when I go for a walk across the building roof with a portable radio. Around the main part of the building the noise is "everywhere" but if I walk out on a section of roof that heads away from the main building the noise decreases to virtually nill. This is the place that I errercted the tuned loop antenna but as soon as I plug the feeder cable the noise is there.

ai4i
Posts: 85
Joined: Sun Nov 16, 2008 4:49 pm
Location: Area Code 305, 754, 786 & 954

Re: Noisy AM reception at studio.

Post by ai4i »

We assume you are using 100% shielded cable.
Why run RF down the line?
Why not place a receiver at the antenna and run audio back down?
_______________________________________
Ai4i is always on the trailing edge of technology

Baylink
Posts: 438
Joined: Wed Dec 15, 2010 3:21 pm

Re: Noisy AM reception at studio.

Post by Baylink »

And for what it's worth, everyone here should google the phrase "heroic quantities of ferrite". :-)

User avatar
KPJL FM
Posts: 613
Joined: Fri Nov 16, 2007 9:28 am
Location: planet Earth, 3rd rock from sun

Re: Noisy AM reception at studio.

Post by KPJL FM »

Ferrite is my middle name. Toriod is my confirmed name. :P

try this, get some of those larger (2-3") toroids and wind the tuner power power cord on it. Also try winding the coax on one. I know it sounds stupid to try to get RF off the shielded cable, but what's probably happening is the noise is induced by means of separate ground potentials. As a test, you could put your tuned loop on a PVC post and not bond the shield except at the tuner. Don't leave an antenna outdoors without ground bonding, however, as the first lightning storm will find it's way into the equipment.
Trim to fit, paint to match, tune for minimum smoke.

ai4i
Posts: 85
Joined: Sun Nov 16, 2008 4:49 pm
Location: Area Code 305, 754, 786 & 954

Re: Noisy AM reception at studio.

Post by ai4i »

Baylink wrote:...google the phrase "heroic quantities of ferrite". :-)
As such.
_______________________________________
Ai4i is always on the trailing edge of technology

Post Reply