Help! Picking up radio waves through my boom!
 

We're using a Rode NTG-1 with a Beachtek DXA-6 on the HV20 and we regularly get interference from radio signals, actual radio stations that become clearly audible and make the sound unuseable. How can I fix this?

At first it seemed to be the XLR cable so I replaced that with one that I was told would fix it but it didn't.

This is usually caused by using an unshielded cable - the XLR should have a shield and be fine. The output cable from the Beachtek is probably unshielded and could be the culprit.
You could try putting an extension on this cable - changing its length so it doesn't act as an antenna for the radio station.

How close are you to the radio station?

What brand of cable did you use originally, and what brand was the second cable?

It sounds like you are hearing just one radio station, and it is fairly clear.

To solve this, we need to eliminate each item, one by one.

I understand that you have:

Rode NTG-1

One XLR cable (what length, what brand or type of cable? Please give us what is printed on the cable.)

One Beachtek DXA-6 with a short cable to the camera.

Canon HV-20.


I would try the following:

1. In the same location where you are having the radio interference, I would try a quality brand of XLR cables. Borrow some XLR cables of different qualities/brands and most importantly length

2. Try a different microphone.

3. Try using the original microphone and cable into another XLR recording device, such as another camera. You may be able to get some help at a radio or television station, or a dealer of audio/visual equipment.

4. Try using a short extension cable (shielded) for the DXA-6 to camera connection.

5. Try unplugging the XLR cable from the DXA-6 to determine if you still get the interference without the microphone and cable.

6. Try removing the battery from the DXA-6.

7. Try setting the other XLR input of the DXA-6 to the "Line" position.

If you are within a short distance of the radio station's transmitter, then this may be a difficult problem to solve. If so, try your setup a few kilometers away from your current location.

The problem could be in the microphone itself. I had a Schoeps which would pick up a radio station in Midtown Manhattan and the only way to get rid of it was to change mics. As far as I remember the mic didn't exhibit any problems when tested elsewhere. Sometimes it is a design issue sometimes it is just that particular mic.

Thanks a lot for all your great feedback thus far! I still get the interference even without the mic attached, the interference does seem to disappear when I also unplug the xlr cable. We're also nowhere near the radio stations we hear.

The XLR cable should be shielded, although I'm not sure (I specifically asked the store whether the cable had any sort of isolation to prevent the interference I was getting and they assured me it did).

For the brands of the cables, I have to check, I'll post when I have more info/tests, thanks guys!

I think the beachtek is mis-wired. its using the ground as a hot and amplfing the RF its picking up on the sheild. so the sheild is instead acting as a antennea, and doing its job very well as such. get the beachtek replaced and let them know what the problem is. its also possible the mic is mis-wired. I bought a K-tek CF boom pole and the internal cable has 2 & 3 reversed between the mic end and the bottom. re-did the mic end and all is better.

Try different positions of the "ground lift switch" on the Beachtek. You may also have bad solder connection in the Beach or corrosion in the XLR socket itself that forms a rectifier and turns it into a little crystal radio receiver

It's not the XLR cable. It's most likely "pin 1 error" in the Beachtek, as it almost certainly was in Daniel's Shoeps mic.

Pin 1 error is this: The XLR cable shield is terminated inside the device's cover/chassis on the common ground area of an internal circuit board. The circuit board is grounded to the cover by a pigtail lead. This is fine at audio frequencies, which is what the engineers were thinking about, but at R.F. frequencies the impedance of the pigtail is significant and it becomes an internal antenna broadcasting the interference to susceptible electronics inside.

A defect like Steve mentioned could pick up an AM signal too. Either way, I'd call Beachtek about it.

Thanks for all the great feedback everyone. I think I found the problem, as someone posted here it seems the problem only occurs when the XLR cable is partially coiled or heaped together as it becomes an antenna then. Not sure yet, but we just did a scene shoot and I tried to keep the cable nicely stretched out at all times and we had no problems.

I'll be surprised if that doesn't turn out to be a fluke.

An unbalanced device lurks in the shadows (Halloween or not!)
 

I second David's assertion. The coiled/uncoiled XLR testing phenomenon suffers the cause and effect syndrome. Yes, it seemed to "fix" the problem, but the culprit is still lurking in the shadows. Uncoiling the ungrounded cable problem masked your problem because a coiled cable makes a better inductor (antenna). But make no mistake, the unbalanced problem will reappear and give a nasty bite when you least expect it. Perhaps it will be a problem with a mysterious power hum. Or maybe loud clicks from a solenoid operation. And maybe it's going to be a walkie talkie or wireless lavalier. But if you choose not to find the root cause, your enemy will be waiting...

Happy hunting, Michael

Check your cable with a VOM. Does pin #1 connect to the XLR shell at each end of the cable?

If not, try to get one that does and see what happens. It may help.

Regards,

Ty Ford