Canadian (from VE2AAY) Advanced Qualification pool, section L16

Subelement L16

FM Repeaters.

Section L16

A-5-6-1

If the signals of two repeater transmitters mix together in one or both of their final amplifiers and unwanted signals at the sum and difference frequencies of the original signals are generated and radiated, what is this called?

A.
Amplifier desensitization
Correct Answer
B.
Intermodulation interference
C.
Neutralization
D.
Adjacent channel interference

A-5-6-1

Intermodulation is the unwanted mixing of two or more signals that produce new signals (products). The fundamental or harmonic energy from strong nearby transmitters intermix to create intermodulation products. Mixing can take place in the front-end of the affected receiver, in the Power Amplifier of one of the transmitters or through "external rectification or passive intermodulation" (in some electronic device or some corroded junction between two metals acting as a diode mixer).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A-5-6-2

How does intermodulation interference between two repeater transmitters usually occur?

Correct Answer
A.
When they are in close proximity and the signals mix in one or both of their final amplifiers
B.
When the signals are reflected in phase by aircraft passing overhead
C.
When they are in close proximity and the signals cause feedback in one or both of their final amplifiers
D.
When the signals are reflected out of phase by aircraft passing overhead

A-5-6-2

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A-5-6-3

How can intermodulation interference between two repeater transmitters in close proximity often be reduced or eliminated?

A.
By using a Class C final amplifier with high driving power
Correct Answer
B.
By installing a terminated circulator or ferrite isolator in the transmission line to the transmitter and duplexer
C.
By installing a low-pass filter in the antenna transmission line
D.
By installing a high-pass filter in the antenna transmission line

A-5-6-3

A circulator is usually a three-port device; any energy coming in one port is forwarded in one direction only, towards the next port. An isolator is a circulator with one port terminated into a dummy load. With the transmitter on port 1, the antenna on port 2 and a dummy load on port 3, RF from the transmitter is routed to the antenna but any outside energy picked-up by the antenna is diverted to the dummy load and thus cannot enter the Power Amplifier where it might lead to intermodulation. Magnetized ferrite material is used in the fabrication of circulators.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A-5-6-4

If a receiver tuned to 146.70 MHz receives an intermodulation product signal whenever a nearby transmitter transmits on 146.52, what are the two most likely frequencies for the other interfering signal?

A.
146.88 MHz and 146.34 MHz
B.
146.01 MHz and 147.30 MHz
C.
73.35 MHz and 239.40 MHz
Correct Answer
D.
146.34 MHz and 146.61 MHz

A-5-6-4

Third Order Intermodulation products where the second harmonic of one signal mixes with the fundamental frequency of another are the most troublesome because of the relative frequency proximity of the involved signals: filtering them out is difficult. Given two signals, F1 and F2, the Third Order IMD product can be computed as (a) "twice F1 minus F2" or (b) "twice F2 minus F1". In this example, IMD = 146.70, F1 = 146.52 ; solving for F2 in case "a" is (F1 times 2), minus IMD ; solving for F2 in case "b" is (IMD + F1), divided by 2.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A-5-6-7

Which type of filter would be best to use in a 2-metre repeater duplexer?

A.
A DSP filter
B.
An L-C filter
C.
A crystal filter
Correct Answer
D.
A cavity filter

A-5-6-7

In the context of a repeater installation, a duplexer is a specialized filter which allows operating the receiver and transmitter simultaneously on the same antenna. The duplexer is built with four or more quarter-wavelength cavity resonators. The duplexer provides isolation ( 90 dB or more on 2 m ) between the receive and transmit paths at the expense of insertion loss.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A-5-6-11

Intermodulation interference products are not typically associated with which of the following:

Correct Answer
A.
intermediate frequency stage
B.
final amplifier stage
C.
receiver frontend
D.
passive intermodulation

A-5-6-11

"Undesired mixing of two or more frequencies in a non-linear device which produces additional sum and difference frequencies" (ARRL RFI Book). Final amplifier stage: signals from nearby transmitters may find their way into a power amplifier creating intermodulation. Receiver front-end: strong offending signals may enter the RF amplifier or mixer and create intermodulation. Passive intermodulation (PIM), sometimes called "rusty bolt effect": passive devices subjected to strong signals cause the problem; cables, antennas, connectors, dissimilar metals, etc. due to oxidation, corrosion, metal flakes, dirty mating surfaces or poor contact.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Log in

- or -

- or -