ADVANCED LESSON 57logomidi2.gif

             
LEARNING  OBJECTIVES and NOTES
EMC - Routes of entry into TV and Radio 2
7a.3 Understand that mast-head amplifiers are frequently wide band devices and can suffer from cross-modulation and overload (causing intermodulation and blocking), and may also overload the TV.
In weak signal areas it is common for TVs and FM radios to be fitted with mast-head preamplifiers. These are wideband amplifiers (often covering 100 to 1000MHz) and amateur signals can cause cross-modulation and signal overload. This will cause intermodulation and signal blocking.
The solution is to fit a high gain antenna to the TV and remove the pre-amplifier.
7a.4 Recall that amateur transmissions can enter audio stages via long speaker leads or other interconnections.
Understand that any p-n junction within an electronic device can rectify unwanted RF.
Hi- Fi speaker leads also make excellent dipole antenna, passing on the signal to any transistor based circuit where the p-n junction acts as a diode rectifying the signal to audio which can be heard on the speakers. AM will be clearly understood. SSB will be heard, but less intelligible.  FM will also be audible as clicks at the start and end of transmission, but may be missed.
One way to overcome this form of interference is to wrap part of the the speaker lead around a ferrite ring. This will increase the resistance at radio frequencies and reduce the signal level. It will not effect the audio frequencies in the speaker lead.
7a.5 Recall that passive intermodulation products can be caused by corroded contacts in any metalwork, including transmitting and receiving antennas, supports and guttering.
Corroded contacts in antennas, towers, metal guttering and any nearby metalwork can act as diodes which will produce intermodulation products.  Strong signals are mixed producing intermodulation products. These can be broadcast, causing interference.
If this is suspected, metalwork should be cleaned and reassembled. Try to avoid connecting dissimilar metals together.
Passive intermodulation is often called the rusty bolt effect!
7a.6 Understand that ghosting is caused by external reflections and does not normally indicate a fault in the TV receiver.

Ghosting is a situation where a strong, normal, picture exists on a TV  and  at the same time as weaker "ghost" image.
This is caused by the signal arriving from two directions, but out of phase. For example one signal may come directly from the transmitter and the other reflected from a nearby building.

This is not a fault  of the TV receiver or caused by amateur interference. One method of fixing the problem is to try moving the antenna to point in a slightly different direction. Where an indoor antenna is the cause, try using an outdoor antenna.

With the introduction of digital TV, ghosting does not occur although the effects of multiple signal paths may affect the pixels making up the picture.