XLR cable acting as antenna?
 

I was setting up to shoot a video interview on a mountain top in the middle of Montreal, but received a loud radio signal in my headphones. When I tried to change the frequency on my wireless mic system, I realized that neither the receiver nor the transmitter were turned on (the receiver's battery pack was even off). When I pulled the XLR cable, the radio signal was silenced.

I turned around and realized that I was in relatively close proximity to a radio transmitter antenna (see Log In | Facebook). However, the short 12" cable to my shotgun mic captured no such radio transmission.

I ended up using a $30 voice recorder, but for future reference I'd like to know if there's a special shielded XLR cable I can get that won't act as an antenna?


J.

Re: XLR cable acting as antenna?
 

Jacques,
I have had similar problems when recording in certain locations. I use Shure A15RF RF attenuators to remove any RF interference. Somewhat pricey, but always works.

Regards,
Jerry

Re: XLR cable acting as antenna?
 

The proper link for the image is http://a7.sphotos.ak.fbcdn.net/hphot..._1526231_n.jpg


J.

Re: XLR cable acting as antenna?
 

Just to get the complete picture, what was the XLR cable plugged into?

Re: XLR cable acting as antenna?
 

It's a 6-foot cable going from a non-powered wireless receiver (no battery in it) to a video camera's XLR input. It's a balanced cable.

A second 12-inch balanced cable connecting a shotgun mic to the camera's other XLR input received no radio intererence.


J.

Re: XLR cable acting as antenna?
 

I'd bet the transmitter was an AM radio one - did you work out what station it was? This is very common and the problem with the XLR cable isn't that the screening is bad, it's that the screening, a few inches long, is a antenna - high powered radio, long bit of wire, and then the first thing inside your electronics are semi-conductor devices which do a marvellous job of converting the RF back into audio. Exactly the same process as the war time cats whisker radio sets. The power output from hill top broadcast stations can be really high, and any length of 'aerial' can 'collect' the energy and your equipment accidentally detects it. AM RF filtering is rarely very good on modern kit, because it's rarely a problem. FM breakthrough is less common, and rarely produces recognisable audio as normally a discriminator is required.

You can try all sorts of things - ferrite rings with the audio cable wrapped through it a few turns work pretty well - but few people carry them in the toolbox!

Re: XLR cable acting as antenna?
 

For all my new XLR cables I only use the Neutrik EMC XLR connectors.

They connect pin-1 to the shell via a ring capacitor - this gives a good shield through the shell and also prevents hum and buzz when you touch the connector.

They also have a ferrite ring on pin one and "teeth" on the female connector to make a good shield connection with the male.

I won't use anything else now.


I also have in my kit XLR in-line RF filters made by Canford Audio.

Re: XLR cable acting as antenna?
 

It's a Sennheiser receiver. Not the same frequency range as AM. Besides, as I mentioned, the receiver was not powered. No batteries were in it.


J.

Re: XLR cable acting as antenna?
 

Thanks. I make my own cables using Neutrik connectors. Where can I find ring those capacitors and ferrite connectors? What rating?

Yes, I've been looking at those. Thanks.


J.

Re: XLR cable acting as antenna?
 

AM frequencies are the ones most often picked up by long cable runs. MANY years ago, I "played" bass guitar in a punk band. I had 50' of TS guitar cable in my setup (with a gain box between the instrument and my amp, of course!) and we ALWAYS had issues with picking up the All Oldies station KY58 at 580KHz.

Paul was discussing the cables acting as an antenna INDEPENDENT of the wireless system.

Re: XLR cable acting as antenna?
 

Right, sorry Paul. I thought he meant the wireless mic transmitter, but he was referring to the big-ass antenna looming over the treetops.

I suspected it was the long-ish (6-foot) cable acting as an antenna, because my 12-inch shotgun mic cable didn`t pick up anything. And that antenna must have been no more than 100-200 feet from me (see picture).

I'll add a few filters to my kit. I'd still like to know about the parts to DIY, since I usually make my own cables.

Thanks everyone!

J.

Re: XLR cable acting as antenna?
 

The capacitors and ferrite rings are an integral part of the Neutrik EMC XLR.

Full details are HERE.

Drawing with explanation is here.

Re: XLR cable acting as antenna?
 

Thanks! I'll see if my local electronics supply store can get them.


J.

Re: XLR cable acting as antenna?
 

To add to the other info here, I just wanted to remind those interested that balanced audio is an ACTIVE system.

A balanced cable by itself is just a long wire. And ALL long wires are antennas by their very nature.

In a balanced system, what eliminates "common mode" hum and noise isn't exclusively the shielding or the connections of the cable, but the way that two separate signal legs are forced out of phase at the sending end, then inverted at the reception end such that inducted noise is phase canceled and so DAMPENED when the system is active.

The fact that the sending or receiving unit was powered off - means you've DISABLED the very process that supresses the noise in a balanced system.

Before you go nuts trying to add chokes or further shielding, next time try turning ON the equipment and see if its working properly when it's functioning as it's designed.

FWIW.

Re: XLR cable acting as antenna?
 

Sorry, but I respectfully disagree.

If an input is balanced, then the input is balanced. Both sides, (+) and (-) polarity, should (by definition) have identical input impedance. Therefore, any equal signal presented to the (+) and (-) inputs should cancel out and not get past the input stage. Even if the cable is open at the far end, the input is still balanced.

But we have to wonder whether the output of the Sennheiser receiver is perfectly balanced, "somewhat" balanced, or unbalanced. If the receiver output wiring is unbalanced (even if it's turned off), that will unbalance the entire circuit on that mic input channel (of the mixer) and that could make that channel much more susceptible to RF pickup.

Also, if the Sennheiser has an active output (rather than an audio transformer) then its output impedance will be much lower when it's turned on. That might, indeed, tend to swamp out some RF that's getting into the system at that point.

Unfortunately, a lot of equipment is not designed with strong RF fields in mind. For example, if pin 1 on the input XLR connector is connected to the nearest point of the chassis, then any RF picked up on the shield of the mic cable will be directed immediately to the chassis and won't cause much trouble. But if pin 1 connects to a wire (hookup wire, harness, whatever) that runs for several inches inside the case, before actually reaching ground, then that internal wire will act like a short antenna to re-radiate any RF that was picked up on the shield of the mic cable.

Many ICs (namely those used for the mic input) are pretty good at common-mode rejection within the audio band, but are not nearly as good when the common-mode signal is at RF frequencies, or a very high voltage. This is one instance where transformer inputs on the mic channels will make a big improvement.

Like ground loops, RF overload can sometimes be tricky to cure.