[BC] Re: RF interference problem

[Mark Humphrey]

The problem can certainly be cured -- but as usual, the big question is: How
much time and money are you willing to spend?

Screening the room(s) and filtering any lines that pass through the screen
would be very effective, but this process is more expensive and disruptive
if done after the facility has been constructed.

Former classical station WFLN here in Philadelphia succeeded in shielding
their multitrack music recording studio as part of a 1988 remodeling project
at their studio/transmitter site in Roxborough, a very dense RF
environment.  They specified grounded steel IAC doors, installed copper
screening behind wall panels and windows, and added EMI filters on all power
lines coming into the studio and control room -- and these measures made a
big improvement.

I once worked at an FM station on the 17th floor of an office building in
downtown Rochester, NY with a beautiful view of the Pinnacle Hill antenna
farm, about a mile away.  Due to local airspace restrictions, the stations
there have relatively short towers, so our studio caught an abnormally high
dose of main lobe radiation.   With three 316 kW high-band VHF TV stations
and a couple of megawatt U's, sync buzz in the audio was a pesky problem,
but with care, we were able to get this under control.

A couple of techniques we used in addition to ferrite beads and bypass caps:

One of our prod studios had a Tascam Model 15 console with unbalanced ins
and outs -- a BAD situation in a high-RF environment -- but the board had
been pulled from a defunct studio that one of the station owners was
involved with, so we had to try and make it work. In addition to improving
the console grounding, I replaced all of the cheap Tascam interconnect
cables with RG-59.  The braided shield of this coax is much more effective
at rejecting RF than the thin twisted shield of most low grade RCA cables,
and made a big improvement.

We also had aluminum screens installed on the studio windows (screwed to the
metal frames inside the building).  Before-and-after spectrum analyzer
measurements showed these screens attenuated the UHF signals in the studios
by at least 20 dB.

The first step would be to identify the actual source(s) of interference,
then you can develop a plan of attack.  The "swirling rumble" hum you
described might be TV vertical sweep (which is not quite 60 cycles) beating
with another source of power line hum, so I tend to think the TV stations
are involved.

Mark


On 1/23/06, RadioPower.org <webmaster at radiopower.org> wrote:
>
> In my unprofessional opinion, I'm not optimistic
> that this RF problem can be cured.
>