# Section 8 - Operating Practices & Procedures

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8.13 Adjusting the antenna

Note: The system may be set up by the instructor or as a group activity. The lowest transmit power giving a useable reading must be used and candidates should stand back from the antenna when radiating. The transmit power should not exceed that generated by a handheld transmitter. If this is unavoidable, or if concern is expressed regarding the safety aspects the ICNIRP (International Commission On Non-Ionising Radiation Protection) limits should be consulted. These are available on the National Radiological Protection Board website

As part of your practical assessments, it is necessary for you to demonstrate correctly matching an antenna to a transmitter. This will usually be a half-wave dipole set for the 2m band.

### Preparation

Before we begin, we should remember a few things first.

1. We must use the lowest amount of power possible to get the job done.

2. We will need an SWR meter, and we should revise its use. Re-read the appropriate page on using the SWR meter.

3. We will be transmitting, so we must follow the rules for station identification. When transmitting we should say something like "Mike Three Alpha Bravo Charlie, Mike Three Alpha Bravo Charlie Testing". This identifies our station and tells people what we are doing. If they know we are testing equipment they will not answer us expecting a chat!

4. We have transmitted so we must enter a record of the transmissions in the station log.

### Matching the Antenna

DO NOT TOUCH THE ANTENNA WHILE TRANSMITTING. SWITCH BACK TO RECEIVE BEFORE MAKING ANY ADJUSTMENTS.

1. Connect the antenna supplied to the transmitter if this has not already been done for you.

2. Select a clear frequency near the centre of the frequency band of interest.

3. Check the frequency is not in use and then take a measurement of the SWR. Use the lowest possible power setting that still allows you to take a reading correctly.

4. Shorten, or lengthen the antenna slightly, by about 10mm. If the antenna is a dipole make the same adjustment on both halves of the dipole.

5. Take another SWR reading. If it is higher than the first one then you are going the wrong way, so reverse the direction of your adjustments.

6. Once you have established if the antenna is longer or shorter than necessary, then continue to make small adjustments until the SWR is between 1.5:1 and 2:1. As you get closer, make smaller adjustments. If the SWR starts to climb again, then reverse your adjustment direction again and make smaller adjustments than before. If the antenna is a dipole remember to make the same adjustment to both halves before taking an SWR reading.

7. There is no point trying to obtain 1:1 SWR in this exercise. One the SWR is below 2:1, then show your tutor and get him to check it.

Tuning an antenna is a repetitive process of adjustment and measuring. There is no point in trying to hurry it.

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