# Interference

• posted

I recall reading that if an AM radio is tuned to frequency x plus or minus the heterodyne frequency a radio tuned to frequency x will be blotted out. Is this true? If so what is the heterodyne frequency? Will it work with FM?

• posted

"Roger Dewhurst"

** Get two AM portables and try it.

** 455kHz.

** The antennas would have to be held close together.

..... Phil

• posted

Yes, but the radios would normally need to be fairly close and it will only work on some frequencies.

IF is usually around 455kHz.

Get a radio and put it on a blank frequency near 1000 kHz. Tune the second radio (held nearby) between about 1400 & 1500 kHz.

As local oscillator = tuned frequency + 455kHz you'll hear a swooshing noise as you tune past.

If it's tuned to a weak station you'll hear a beat note which will change in pitch when the second set is tuned.

Yes. Same principle except the IF is 10.7 MHz and local oscillator is 10.7 MHz above the dial frequency.

If you are in an area where TV channel 4 or 5 is used, you can blot out either picture or sound with an FM radio because these TV channels are near/in the FM broadcast band and the receiver's local oscillator frequency.

• posted

"Peter Parker"

** Errr - IME, the local oscillators of FM broadcast band receivers is set 10.7MHz below the received frequencies.

Likely dates from when oscillators struggled to even make it to 100MHz - using valves like the 12AT7 for example.

..... Phil

• posted

Yep, you're right.

Dial needed to be around 92 MHz to interfere with ch5 TV!

Another advantage of low side is that the LO won't interfere with the aircraft band above 108 MHz.

• posted

"Peter Parker"

** Ch5 was taken off air way back in the early day of FM broadcasting in Australia.
** Err - the choice of using a local oscillator ( LO) above or below an incoming carrier mostly relates to the "image response" frequencies. In the case of FM broadcasting, the chance of interference to FM listeners from images was worse if the LO was above the carrier.

For below, the image frequency range is between 66.1 and 86.6 MHz.

For above, the image frequency range is between 108.9 and 129.4 MHz.

The latter band is used for air traffic control and navigation beacons - so it is possible for broadcast FM receivers situated near airports or under flight paths etc to be swamped with interference if the LO was on the high side.

..... Phil

• posted

Unless you live in Bunbury (WA) where FM radio remains severely restricted due to high power TV transmitters on both ch3 & ch5.