LESSON 12

Interference to and from consumer devices, public relations,
intentional and unintentional interference

After receiving my amateur radio license I learned that there were two types of electronic construction qualities. There were the commercial quality and the consumer quality. Commercial construction is built with the most efficient overall operation in consideration while consumer quality construction focuses on cost and appearance.

Television sets and broadcast radios are built with consumer quality construction while two way radios use to be built with commercial quality. One of the big differences in the two qualities comes in the area of shielding against interference. Television sets and AM/FM radios are notorious for lack of protection against interference and thus any strong RF signal around them will interfere with their operation.

This problem has plagued ham radio operators sense the days when these consumer products were first constructed and sold to the general public. It has caused serious battles between hams and their neighbors which sometimes leads to hostilities.

One of the problems a strong RF signal can cause in a television set is something known as front end overload. This simply means that the early stages of the television input are not properly constructed to protect them against very strong RF signals and thus they become inoperative when a transmitter is operating nearby. The owner of the television receiver is responsible for eliminating the problem.

Front end overload can create a problem in almost any radio receiver. A television set is nothing more or less then a radio receiver. The definition of front end overload is: Interference caused by strong signals from a nearby source.

If a ham receives a complaint that their transmitter is causing problems with a telephone, CD player, or some other non RF operated device it is not the fault of the transmitter. The reason for such interference is the device was not equipped with adequate interference protection when manufactured. It is not the ham’s responsibility to eliminate such interference.

Cable television has done a lot to reduce interference problems caused by amateur radio transmitters. This is because the cable television transmission line is a coaxial cable (one conductor inside another separated by an insulator.) The outer conductor shields the inter conductor and when properly balanced the system is very efficient. Sometimes there are things that cause an imbalance in the system such as a break in the transmission line which can result in an amateur radio transmission interfering with the television set and sometimes the break can cause the transmission from the cable company to interfere with the amateur radio receiver.

If a ham receives a report that transmissions are causing splatter interference on nearby frequencies it should not be assumed the transmitter is ok. The transmitter should be checked for off frequency operation or spurious emissions (undesired signals being transmitted with the desired signal). Sometimes assistance from other more experienced hams in the area will be required.

One piece of equipment every ham shack should include is a dummy load. A dummy load is a resistor which turns most of the RF signal into heat and thus prevents interference when testing the transmitter and at the same time protects the transmitter from damage. Never transmit without a proper load on the transmitter output.

It should never be assumed that just because a dummy load is being used there is no RF radiation. I have personally communicated with stations over several miles while either I or they were using a dummy load. Some radiation will be present.

The ham frequencies are crowded and sometimes interference can be caused unintentionally. When an amateur radio operator is informed that transmissions from that station are causing interference to a QSO (communications) already in process the proper course of action is to properly identify and change frequencies. A ham operator is never allowed to deliberately interfere with any other transmission.

There are two organizations, RACES and ARES that have both been established by amateur radio operators to provide radio communications during emergency. They are both excellent organizations and well worth every amateur radio operator’s consideration to join. The provision of radio communications during emergencies is the one thing these two organizations have in common.

When operating with RACES, ARES, or any other organization it is always to be remembered that it is the FCC who establishes the rules and regulations of operation.

Sometimes during an emergency situation certain frequencies will be declared by the FCC for emergency operations only. Otherwise no frequency is considered to be for exclusive operation only. But there is one thing that should be remembered which almost seem a contradiction to that statement. If there is a distress operation in process or a distress call is given distress communications will always have priority.

1. T3D01
What should you do if you receive a report that your transmissions are causing splatter or interference on nearby frequencies?

A. Increase transmit power
B. Change mode of transmission
C. Report the interference to the equipment manufacturer
D. Check transmitter for off frequency operation or spurious emissions
~~

2. T3D02
Who is responsible for taking care of the interference if signals from your transmitter are causing front end overload in your neighbor's television receiver?

A. You alone are responsible, since your transmitter is causing the problem
B. Both you and the owner of the television receiver share the responsibility
C. The FCC must decide if you or the owner of the television receiver is responsible
D. The owner of the television receiver is responsible
~~
3. T3D03
What is the major cause of telephone interference?

A. The telephone wiring is inadequate
B. Tropospheric ducting at UHF frequencies
C. The telephone was not equipped with adequate interference protection when manufactured.
D. Improper location of the telephone in the home
~~
4. T3D04
What is the proper course of action if you unintentionally
interfere with another station?

A. Rotate your antenna slightly
B. Properly identify your station and move to a different
Frequency
C. Increase power
D. Change antenna polarization
~~
5. T3D05
When may you deliberately interfere with another station's
communications?

A. Only if the station is operating illegally
B. Only if the station begins transmitting on a frequency you are using
C. Never
D. You may cause deliberate interference because it can't be helped during crowded band conditions
~~
6. T3D06
Who has exclusive use of a specific frequency when the FCC has not declared a communication emergency?

A. Any net station that has traffic
B. The station first occupying the frequency
C. Individuals passing health and welfare communications
D. No station has exclusive use of any frequency
~~
7. T3D07
What effect might a break in a cable television transmission line have on amateur communications?

A. A break cannot affect amateur communications
B. Harmonic radiation from the TV may cause the amateur
transmitter to transmit off-frequency
C. TV interference may result when the amateur station is
transmitting, or interference may occur to the amateur receiver
D. The broken cable may pick up very high voltages when the
amateur station is transmitting
~~
8. T3D08
What is the best way to reduce on the air interference when
testing your transmitter?

A. Use a short indoor antenna when testing
B. Use upper side band when testing
C. Use a dummy load when testing
D. Use a simplex frequency instead of a repeater frequency
~~
9. T3D09
What rules apply to your station when using amateur radio at the request of public service officials or at the scene of an emergency?

A. RACES
B. ARES
C. FCC
D. FEMA
~~
10. T3D10
What do RACES and ARES have in common?
A. They represent the two largest ham clubs in the United States

B. One handles road traffic, the other weather traffic
C. Neither may handle emergency traffic
D. Both organizations provide communications during emergencies
~~
11. T3D11
What is meant by receiver front-end overload?

A. Too much voltage from the power supply
B. Too much current from the power supply
C. Interference caused by strong signals from a nearby source
D. Interference caused by turning the volume up too high
~~


D D C B C D C C C D C


Modulation

Modulation is the process by which intelligence is placed on a radio signal. (Intelligence is the information which is to be transmitted, Voice, Teletype, Morse Code, Video, Control Tones, etc)

The fundamental frequency of an alternating current is the basic frequency and any multiples of that frequency are harmonics. If a transmitter is transmitting on 3.6 MHz the second harmonic would be 7.2 MHz, the third harmonic would be 10.8 and so on. Harmonics, while undesired when on the fundamental transmitting frequency, are present. They should be kept at very low levels.

Sidebands are frequencies are separate signals which are sent along with the fundamental frequency and are the result of the mixing of the fundamental frequency and another frequency or frequencies.

There are basically two types of modulation, Frequency Modulation (FM) and Amplitude Modulation (AM). FM is accomplished by varying the Frequency of the carrier (fundamental frequency).

FM moves the frequency in accordance with the volume of the signal being used to modulate the carrier frequency. So the loader the volume is the further the frequency of the carrier will shift. The frequency shift will move from the central frequency (carrier frequency) in one direction until the peek of the modulating signal is reached then it will move back to the central frequency as the modulating frequency moves back towards zero. Then the central frequency will shift in the other direction as the modulating frequency moves up in level.

If a 146 MHz signal is modulated by a 1 KHz tone at the maximum legal level (5 KHz) the following would be observed. 146,000,000 Hz will start to move up as the 1 KHz signal starts to move in the positive direction. Half way up the positive slope the 146,000,000 frequency will have changed to 146,002,500 Hz. At the peak of the 1 KHz tone the central frequency will have changed to 146,005,000 Hz and will again move back down passing the 2.5 KHz positive shift at half way down the positive slope to 146 MHz. Then as the modulating tone moves in the negative direction the 146 MHz signal will move down in frequency until it reaches 145,995000 Hz and then star back down. If the modulating signal’s volume is cut in half then the frequency shift of the 146 MHz signal will be 146,002,500 and 145,992,500 Hz. This means the carrier will move up in frequency to a maximum peak and back to 146 MHz then down to a maximum dip and back to 146 MHz 1,000 times every second. The receiver will turn this into a 1 KHz tone.

FM has many sidebands but the considerations of these sidebands are complex and not necessary for this discussion.

I am going to explain the process a little differently then most because, though it might be a little more difficult to understand at first, in the long run I believe it will make it easier to understand the whole picture.

With AM the carrier always remains the same. The modulating signal is mixed with the carrier to produce a carrier and two sidebands. If two different frequencies are mixed together in a nonlinear device they will produce two new frequencies that equal the sum of the two frequencies and the difference to the lower frequency from the upper frequency.

Back to the 146 MHz carrier being modulated with the 1,000 Hz tone. When these two frequencies are mixed together they will produce a new frequency of 146,001,000 Hz called upper sideband and another of 145,999,000 Hz called lower sideband. All three RF frequencies are transmitted at the same time. If the modulating frequency changes the sideband frequencies will change correspondent.

With standard AM when modulated at 100% the output power will go up to 150% of the output power of the carrier alone. Thus a 100 watt transmitter modulated at 100% will have a total output power of 150 watts. 25 watts will be in the upper sideband, 25 watts will be in the lower sideband, and 100 watts will be in the carrier. When the modulating signal becomes greater then 50 watts the transmitter will be over modulated and this will cause distortion of the audio signal. It will also cause the band width of the transmitted signal to be wider then it should be and cause splatter.

When the AM signal reaches the receiver it is turned back into audio in the detector. The detector is non linear and thus mixes the sideband and the carrier to produce an audio frequency. The more signal there is in the sidebands the greater the volume will result.

The description above is far above what is needed for the technician class license but I believe it will give you a good foundation to understanding what modulation is. This information will be very helpful is you upgrade to general class.