Operating Procedures and Practice

Question File Number 24

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Receiver Facilities

RF and IF Gain Controls: Simple receivers for the broadcast band have one gain control only, this sets the level of audio gain. Communications receivers have other gain controls which work on stages in advance of the detector.

An RF gain control sets the gain ahead of the receiver mixer. Adjustment to the gain of the first stage in the receiver can assist reception in cases where front-end-overload may be bothersome. This occurs when trying to receive a weak signal adjacent in frequency to a very strong local signal.

An IF gain control gives an independent control over the amplification prior to the detector stage. Most of the amplification in a receiver takes place in the IF stages. There may be many IF stages and operator-gain-control can effect improved performance.

AGC Automatic Gain Control: Tuning a receiver from a weak signal to a very strong signal (and back again) calls for frequent adjustment to the receiver's gain control(s). This becomes tiresome and is a nuisance with a communications receiver when tuning across a band of frequencies. AGC does the gain changes automatically releiving the operator from this tiresome task.

HF signals fade and the received audio can change from loud to faint and back again at sometimes very fast intervals. This need to frequently adjust a gain control is also a nuisance and burdensome.

By sampling the strength of the signal being received (by rectifying it to produce a voltage) and by applying it to some of the amplifier stages, it is possible to automatically adjust the overall gain of a receiver. Tuning from a strong signal to a weak one, and the fading of a distant signal, will now have minimal effect on the level of audio heard from the speaker.

The signal-level sample for AGC applications may be taken from the detector or alternatively may be a rectified sample of the received audio. The AGC voltage is usually a DC voltage fed back to the IF amplifier stages where it controls the bias of the amplifiers.

S Meter

S Meter: This is usually a meter front-panel-mounted on a receiver and calibrated in signal strength units and dB. It varies as the signal fades. It is usually an electronic voltmeter measuring the AGC voltage. With a strong signal, the AGC level will be high. With a weak signal, there may be no AGC voltage at all.

RST reporting

As a absolute level measurement, an S-meter is generally unsatisfactory. It is useful for making relative measurements between different received signals. Read it with caution.

Noise blanker: Noise at HF is often of the impulse variety, short sharp spikes of noise that blank out reception. A noise blanker uses such spikes to form a gating signal in the path of the signal through the receiver. A noise spike then automatically mutes the receiver for the period of the noise spike. This makes reception more comfortable on the ears of the operator. The effectiveness of a noise blanker varies and depends on the type of noise and the signal levels being received.


Station Switching

PTT Push-To-Talk: The simple way to control the send/receive function on a transceiver is to use a pressel switch on the microphone. Pushing the switch is a simple and intuitive action when sending a voice transmission. Release the switch and the transceiver reverts to receiving incoming signals. The switch usually operates a relay inside the transceiver. The relay does all the switching changes needed to change from receive to send and back again.

VOX Voice-Operated-Relay or Voice-Operated-Transmit: This technique can be used to simulate duplex operation (i.e. telephone-type conversations) when operating phone on the HF bands. It is an extension of PTT operating. Just speak. A sample of the speech audio from the microphone is amplified and rectified to provide a DC control signal. That DC signal operates the relay which does the station send/receive switching.

A VOX system must have a fast attack, slow release characteristic to be sure that the first syllable of a spoken statement is not severely clipped, and to ensure that the relay does not clatter excessively in and out between the spoken words.

Break-in keying: This system uses the Morse key as the send/receive switch too. When using the key, on first key-down, the station changes to transmit. Stop using the key - and the station receives. The channel in use can be monitored during key-up periods when sending. Conversational-type contacts are possible.


Operating Techniques

RIT Receiver Incremental Tuning: A transceiver is usually a receiver and transmitter combination sharing a lot of common circuits - such as the various oscillators that determine its operating frequency. RIT provides a tuning facility so the receiver can be separately tuned for a few kHz each side of the transmit frequency, hence giving independent control over the receive frequency.

Split Frequency Operating: A transceiver is usually a receiver and transmitter combination which shares a lot of common circuits - such as the various oscillators that determine its operating frequency. There are occasions when separation of the send and receive frequencies is desirable - to receive on one frequency but to transmit on another. An obvious example is when a Novice grade operator is receiving a station outside the Novice segment of the band but transmits inside the Novice segment.

Pileup: Loose colloquial jargon used by radio amateurs to indicate the congestion that can occur when many stations suddenly call and try to work the same station, usually a station in some rare DX location. Discipline is needed to minimise this problem. General Operating Procedures

Station Optimising

ALC Automatic Level Control: Just as we had AGC in a receiver, this is a similar thing for transmitters, usually for the linear amplifiers used in SSB transmitters. Its purpose is to prevent over-driving the linear amplifier stages especially the final amplifier.

It may also permit the peaks of an SSB signal to be limited in amplitude to enable an increase in the mean output power of the transmitter to improve the relative signal level at a distant receiver. This function can also involve processing the audio in the transmitter, known as compression.

SWR Bridge: This device has been discussed elsewhere. HF Station Transmission Lines Operating adjustments should be made to the Antenna Tuner for minimum reflected power indication on the SWR bridge. Appropriate antenna and transmission line adjustments should be made during installation for the same purpose.


VHF Repeater Working

A VHF (or UHF) repeater is a receiver and a transmitter connected together and sited on a hill-top or other high point - to get extended coverage.

VHF/UHF Repeater

In this diagram, the repeater receiver (Rx) audio output is passed to the transmitter (Tx).

The Rx and Tx can share a common antenna. The receive and transmit signals are directed to the appropriate places by the duplexer. This is a collection of high-Q tuned circuits, a passive device acting as filters for the repeater input and output signals.

The control detects a received carrier and switches the transmitter on - until the received carrier disappears when it then switches the transmitter off. So the push-to-talk switch in the mobile station also turns the repeater transmitter on and off for talk-through operating. The repeater receiver squelch is used to provide the transmitter send/receive function.

The frequency difference in this example is 600 kHz between the repeater receive and transmit frequencies. This is the standard split for repeaters operating in the 146 to 148 MHz band: i.e. it is plus 600 kHz above 147 MHz, and minus 600 kHz on or below 147 MHz. (The NZART CallBook gives details of the bandplans adopted in New Zealand and lists the frequencies and locations of amateur radio repeaters. Call Book)

UHF repeaters operating in the 430 to 440 MHz band use a 5 MHz split.

The carrier-operated switch at the repeater receiver may fail to operate when an input signal gets weak. When mobile stations are operating through the repeater, if a mobile moves into an area with little-or-no signal, the repeater may drop out, there being insufficient signal to hold the repeater receiver open.

The carrier-operated switch at the repeater receiver is similar to the squelch operation in an FM receiver. FM receivers are very noisy in the absence of an input signal. To make life comfortable for operators monitoring FM communications channels, a squelch mutes the receiver loudspeaker in the absence of an incoming signal. The squelch opens when a signal is received and the signal's audio is then heard from the speaker.

Repeater Networks

New Zealand radio amateurs have built and installed 2-metre band (144 - 148 MHz) repeaters to provide most of the country with local area coverage.

The National System on the 70 cm band (430 to 440 MHz) is a chain of linked repeaters. These provide communication along the length of the country. Refer to the NZART CallBook for maps and other details about the operation of the National System. Call Book)


Further details about NZART can be obtained from the web at  NZART. There are several categories of NZART membership which include Transmitting and Non-Transmitting. Anyone interested in radio can join. E-mail enquiries to  nzart@nzart.org.nz will bring details about NZART Membership. On-line details about  Joining NZART - Membership has advantages.
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Compiled Sun Nov 28 2010 at 8:41:44pm

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