RF Feedback
From Freeradio
RF feedback is when a station's own Transmitter causes Interference to the studio equipment.
On FM stations, this tends to manifest itself as an annoying background hum on the station's audio.
The severity of RF feedback is much greater at higher Transmitter powers. But there are ways of reducing or eliminating RF feedback.
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Position of the Antenna
Obviously, the further away the Antenna (and preferably the Transmitter) is from the studio, the less RF feedback will be encountered. This is another reason (apart from increased range) why the antenna should be as high as possible.
At the same time, the antenna should be kept as far away from your neighbours as reasonably possible. If you interfere with their audio equipment, or with their radio reception, they won't thank you for it!
Keep the audio leads short and straight
Avoid excessively long and coiled-up audio leads - these act as an antenna to radio waves.
Screening of the audio equipment
All studio equipment should be screened, especially the audio leads. A good (low resistance, preferably short) path to earth of the case of all equipment, will also help considerably.
Metal cased equipment tends to be well-screened, plastic cased equipment may be less so.
Covering the studio with wire mesh (placing the equipment in a "Faraday cage") would be an effective, if rather drastic, answer to the problem.
Equipment prone to RF feedback
Microphones, due to the low signal level and long, often slightly coiled, lead.
Headphones can sometimes introduce RF feedback to the audio equipment, again pecause of the long and often coiled lead.
Turntable cartridges, due to the low output level and the fact that they contain a coil.
Tape heads, as they are an electromagnetic transducer which may pick up strong radio waves.
Un-screened audio leads - avoid these like Yersinia Pestis!
Interference suppression using electronics
Wrapping the ends of audio leads once around a ferrite ring will provide a simple, if relatively expensive, means of reducing or eliminating RF feedback. This is especially important on the lead from the studio to the Transmitter.
Safety Warning: Internal modifications to mains-powered equipment may run the risk of electrocution, and/or invalidate the manufacturer's guarantee.
A capacitor (approx 100pF) across any audio inputs will reduce RF feedback by short-circuiting any radio waves to ground.
A small inductor (single turn on a ferrite bead) in series with any audio inputs will provide a high-impedance path to radio frequencies, while letting through audio unimpeded.
