HF Station Arrangements

Question File Number 15



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Station Components

Amateur radio stations range from the very simple to the vary elaborate and complex. Some of the common elements are considered here. This block diagram is typical of the High Frequency equipment used in an amateur station.

The Transceiver

HFrig

This is the centre-piece of the station and where most things happen! It contains both transmitter and receiver. These functions are treated elsewhere in this Study Guide.

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The Linear Amplifier

This is switched in to provide a stronger transmitted signal at times of difficult conditions. Not an essential item and not all radio amateurs use them or find them to be necessary. It provides an amplified version of the signal fed into its input. The term linear means that the output signal is a replica of the waveform of the signal fed into its input - except that the amplitude of it is greater.

The Low Pass Filter

This device is considered elsewhere in the Study Notes. Interference It is designed to prevent the passing of frequencies above 30 MHz (the limit of HF and where VHF begins) from the transmitter to the antenna. It is good practice to have this item in use but it may not always be required. Many modern transceivers are already fitted with such a filter.

S W R Bridge

This little box (Standing Wave Ratio bridge - or meter) does two things. It gives a measure of the transmitter output power level. It also gives an indication of how well the antenna is working. If the feeder to the antenna is damaged or the antenna itself is faulty, a glance at this meter will indicate a problem. This device is explained at Measuring. See too Transmission Lines

The Antenna Switch

Only two positions are shown in this diagram. The switch changes between the external antenna and the dummy load (used for testing). In practice, the Antenna Switch may have many positions and be used for selecting between various antennas as well as the dummy load. It is general practice to use a multi-element beam antenna for operating at 14 MHz and above, and to use a wire antenna on frequencies below 14 MHz, but there are no hard and fast rules.

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The Antenna Tuner

This name is not strictly correct. This device takes the impedance seen looking down the antenna feedline and corrects it for correct match to the output impedance of the transmitter. This device is treated elsewhere in this Study Guide.Transmission Lines and Meters And Measuring

The Dummy Antenna (Dummy Load)

The purpose of this device is to allow you to carry out adjustments to your transmitter without actually transmitting a signal on the air. It is usually a collection of carbon resistors in a can - for shielding. The can may be filled with transformer oil to assist cooling.

It is important to know the power rating for your dummy load. The time that you can use it with a high-power signal may be very short before overheating causes it to be severely damaged. Know your ratings and observe them!

The Dummy Antenna should be connected to your antenna switch as one of your antennas. The device simulates an antenna in all respects except that it does not radiate. It usually has a 50 ohm impedance with a low SWR of 1 to 1. Refer to Antennas

A Practical Unit

Sometimes an SWR Bridge, an Antenna Tuner, Antenna Switch and a Dummy Load, are all combined into the one box.

SWRdemo

Sometimes the two SWR meters are built into one instrument - with cross-needles. The crossing point of the two needles can be read directly as the SWR value off a separate scale on the face of the meter, while each separate needle indicates the forward and reflected power on its own arc-scale. An example is in the photograph.

The operation of the SWR device is explained at Measuring .



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Compiled Sun Nov 28 2010 at 8:41:44pm


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