Bredhurst Receiving and Transmitting Society

Syllabus Sections:-

7a Routes of Entry into TV and Radio Sets

7a.1 Understand that amateur transmissions can be picked up by the intermediate frequency stages of TV and radio receivers and identify related amateur transmissions.

The IF stage frequency in a TV is often about 36 MHz +/- thus an amateur transmission on the 18MHz band could be doubled in the TV and present patterning on the TV screen or some other visual interference.

In an FM broadcast radio receiver the IF frequency is about 10.7MHz. This could then be susceptible to direct pickup from the 10MHz band and the third harmonic of the 3.5MHz band.

Understand that television receivers and most broadcast radio receivers employ superheterodyne circuits and recall some typical frequencies used in radio and television receivers; i.e. 470-854MHz TV r.f. 33-40MHz TV i.f., video baseband 0-5MHz Radio i.f's typically 455-500kHz and 10.7MHz.

If the intermediate stage is badly screened then you can get amateur radio transmissions break through into TV and radio receivers. Often manufacturers are not making equipment that has sufficient screening to take account of such amateur transmission in fact there can be no screening at all!

If you are operating using AM it is likely that the person using the TV or radio will be able to hear what are saying. If you are using CW then the breakthrough will be minimum and the person using the equipment would not be able resolve when it is. SSB will be less intelligible than AM.

There can also be pickup in the audio stages of TVs and radios where the first audio amplifier stage acts like a detector and demodulates your transmission.

The IF frequencies that are used in TVs and radios are generally 33 - 40MHz and whilst we do not have any transmission on those frequencies allocated of the amateur bands  it could be that the shere weight of RF could break through into the IF stage.

The UHF radio frequencies used for the for the channels 21 to 68 are between 470 to 854MHz. Whilst again there are no amateur transmissions  in those bands but high powered RF can "shock excite" the first RF amplifier in the TV tuner and cause cross modulation but virtue of shear breakthrough so we need to keep out any signals above and below the TV  bands but the use of high and low pass filters.

Further in a TV set all the video frequencies that come out of the video demodulator  will lie in the range 0 to 5MHz and this means that lower amateur bands eg 1.8MHZ and 3.5MHz signals could again break into the video amplifiers and that would cause coloured herring bone effect on the screen.    

THE CURE for TVs

Keep the RF signals out of the TV for all of those problems.

You would therefore suggest to the neighbour that as you work HF bands that they fit a HIGH pass filer at the aerial socket on the TV so that it would allow the high TV frequencies to pass into the UHF tuner. It is difficult to add screening to any TV as you have to go inside the TV and if anything went wrong the neighbour would think that it was your fault.

You cannot re-engineer the TV so you are reliant upon the manufacturers to do a good job in the first place.

You would have of course have checked that it was you signals causing the problem by reducing power to such an extent  that the problem went away - then fit the filers and increase power again to a level where no problem occurs with the TV.

THE CURE for radios

On radios break through can occur into the IF can occur - on Medium Wave the IF is is 455 to 500kHz and on the FM band it is usually 10.7MHz - BUT thankfully breakthrough is rare and see also below.

Understand the potential for second channel (image frequency) interference.

What is a problem is second channel interference ( image frequency ) where you get a transmission twice the IF removed from that which the neighbour is listening to, and that would be the offending frequency. It is difficult to remove second channel interference as it means that it is a fault of the receiver and what you would have to do is add further RF filtering - you cannot re-engineer a broadcast receiver all you can do is move frequency so that second channel IF frequency does not break through and interfere with the station that the neighbour wishes to receive.

7a.2 Recall that amateur transmissions can enter the RF stages and cause cross modulation and/or blocking.

Recall that cross modulation occurs when strong varying transmissions (e.g. AM, SSB or CW signals) impresses its own modulation on the wanted signal.

Recall that blocking (also known as desensitisation) occurs when strong constant transmissions (e.g. f.m. signals) cause the radio or television to be overloaded.

Crossmodulation in the RF stage of a TV or Broadcast Receiver.

When an RF stage of a broadcast radio is is presented with both :-

1. a very strong signal from local amateur transmission together with

2. a Broadcast signal

the amateur signal will very often be higher in amplitude than the broadcast signal the amateur signal modulates the broadcast signal or it can desensitised ( also known as blocking) the RF stage so that the broadcast signal cannot get through.

If the RF stages are presented with a strong local FM signal there is a constant carrier and the broadcast receiver may be desensitised ( blocked) as the all the while the FM transmission is there as the FM signal impresses it modulation on the wanted signal and overloads the receiver.

If the RF stages are presented with an AM signal then the modulation may be heard as the gain of the RF amplifier in the receiver will change as the AM carrier changes as the AM signal impresses it modulation on the wanted signal. 

If the RF stages are presented with a CW signal then they may hear a "ticking" in the receiver as the Cw signal impresses it modulation on the wanted signal.

The cure ?

Cross modulation can usually be prevented  by installing some form of HIGH pass filter to keep the amateur signals away from the TV or radio input if you have and aerial going to the radio but very often it is just a whip aerial.

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.

The mast-head amplifiers used by many who receive TV via an antenna are probably one of the most difficult areas of EMC to remedy. The broadband nature of the mast-head amplifier will readily suffer from overloading which in turn can overload the TV.

There can also be cross-modulation where the strong RF signal from your equipment enters the amplifier and then gives the result of varying the "gain" of the TV signal in sympathy with your RF signal.

7a.4 Recall that amateur transmissions can enter audio stages via long speaker leads or other interconnections.

Not only can signals enter equipment from antenna leads but also via speaker leads in fact any interconnecting cable between one part of equipment and another can and sometimes does act an an antenna bringing the unwanted signal from your RF transmission into audio equipment.

In addition to audio equipment an alarm system also has long leads interconnection items and these too can act as an antenna. Both audio and alarm systems are intended to amplify small electrical signals and can therefore contain high gain amplifiers which can be prone to interference from almost any RF source.

Understand that any p-n junction within an electronic device can rectify unwanted RF.

In the Intermediate course you were introduced to the crystal set. Click here to check back. The crystal set has a simple diode to rectify the RF signal. A diode as is explained in this course ( check here ) is a PN junction. Because the PN junction can rectify the RF in a crystal set it can and sometimes does in a TV or radio receiver.

7a.5 Recall that passive intermodulation products can be caused by corroded contacts in any metalwork, including transmitting and receiving antennas and supports and guttering.

The diode effect, also called the "rusty bolt effect" can occur and due to rectification give intermodulation products. This occurs when corroded piece of metal are linked together with rust or corrosion in none ferrous metals between in any metalwork, including transmitting and receiving antennas and supports and guttering.

7a.6 Understand that ghosting is caused by external reflections and does not normally indicate a fault in the TV receiver.

Some effects seen on a TV screen are NOT EMC problems, but your neighbour will still think you are the cause. The effect of a double image or "ghosting" on the TV screen does not normally indicate a TV fault but the TV is receiving two similar signals due to reflection off external items such as buildings and other large structures for instance gas storage tanks or power station chimneys.

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