RF Attenuation

RF, which stands for Radio Frequency, is the very high frequency (well above human hearing) electromagnetic signal (not sound) which carries the sound signal we hear on AM or FM radios as well as the sound and pictures that we see on TV. Electromagnetic signals do many other things. From lower to higher frequency, the electromagnetic spectrum goes like this:

LOW

AM radio (~ 1MHz)

long wave radio

short wave radio

Citizen's Band

TV channels 2 to 6 (42MHZ to 88MHz)

FM radio (88MHz to 107MHz)

aircraft communication

TV channels 7 to 14 (high VHF)

computer clocks (25MHz to 500MHz... and climbing)

UHF

radar

portable phones

microwave

infrared

visible light

ultraviolet light

x-rays

gamma rays

HIGH

We are, literally, continuously bathed in low level electromagnetic radiation. Some of it carries sound information, much of it does not.

It is possible for audio (sound) electronics to pick up and, in some cases, decode the sound information carried by electromagnetic waves. When that happens your guitar or bass may channel sound intended for radio receivers into your amplifier head where it may be heard quite clearly.

This is most likely to happen if you are playing near radio station transmitting towers. It may also happen if you play near an amateur (ham) radio station. Or you may hear R2D2-like goings on if you are playing near a computer and so on. When it interferes with what we are doing it is called RFI - Radio Frequency Interference - and is seldom welcome.

HOW TO GET RID OF IT?

with one or more of the many techniques that have been developed since the invention of the radio.

simple trapping with 1 or 2 capacitors

Passive Instruments

at your pickup selector switch (in 2 vol. instruments with switches) or

at the outer ungrounded lug of the vol. pot of a Strat* type harness or

at either outer ungrounded lug of a J-Bass* type harness

and

at the hot lug of the output jack:

Solder a small value capacitor from "hot" to ground keeping the lead of the capacitor nearest the "hot" lug as short as possible (1/4" or less but taking care not to overheat the capacitor).

 

Capacitor values: 47pf, 68pf, maybe as much as 100pf. Two of these along the signal path of the guitar may barely affect the highest treble note of a guitar. They will however look like very low value resistors to an incoming Radio Frequency signal. They will "shunt" the RF signal to ground.

Unbuffered Active Instruments

Cut the input lead that goes from the Pickup Selector switch, or the Blend Pot to the electronics 3/4" from the point at which it enters the first electronics module.

 

Very carefully strip a little more than 1/16" insulation from both cut ends. Thread one FM ferrite bead - the one with the ink marks - (first) and one AM bead (second) on the wire stub nearest to the electronics module.

 

Now solder the two cut ends back together and, if your soldering technique allows it, solder a 47pf capacitor from this rejoining point to ground. Keep the capacitor leads short, especially the one nearest the ferrite beads.