Practicals page 3

A quick Reminder of the Practical’s covered on this Page :-

9) Demonstrate connecting a Radio transceiver to a Power supply, Antenna and Feeder.

10) Use a 1/2 wave dipole. Adjust the SWR until you find the lowest value. ( not while transmitting)

I am going to start with the SWR practical. SWR or more correctly VSWR stands for Voltage Standing Wave Ratio. Simply put, it is a measure of how well the Antenna is matched to the Signal being transmitted. This topic can become very complicated. I will try and keep it simple.

When we talk about Radio, We talk about Frequencies and Wavelength. A typical Radio wave follows the rough pattern of a Sine wave.

rf-wave

You can see in the above image the sine wave pattern. You can also see the marks indicating half wavelength and Full wavelength. The frequency of a Radio Wave is the number of Full waves in one second. One full wave in one second is one Hz. so a 144 Mhz Radio wave has 144 Million Full waves in one second. The Wavelength is connected to the frequency. It tells us how long in Meters the Radio Wave is. Low Frequencies have long wavelengths. High frequencies have short wavelengths. Both the wavelength and frequency can be worked out if you know one of them.

300,000,000 M/s
Wavelength = ———————–

Frequency

300,000,000 M/s

Frequency = ———————
Wavelength

The formulas above are used to calculate our missing value. Either wavelength or frequency. You will notice in both the value 300,000,000 M/s. This is the speed of light. The actual speed of light is slightly less but we use this value to make calculations easier. Why do we use the speed of light ? Radio waves are electromagnetic waves. All of which travel at the speed of light. ( unless something stops them ) So we the Wavelength and Frequency are closely connected to how fast the Radio wave travels. lets try an example :-

we are told that the 10 Meter band is open for contacts over large distances. What frequency do we set our radio to ?

300,000,000
Frequency = ————— = 30,000,000 Hz or 30 MHz
10 Meters

The 10 Meter Amateur band is 28 MHz to 30 Mhz. so we are correct !

Now we have another problem. We want to build an antenna for our 144 Mhz Radio. How long should the Antenna be ? i.e. its wavelength.
300,000,000
Wavelength = ——————– = 2.08 Meters.
144,000,000

The 2 Meter Amateur band is 144 MHz to 146 Mhz.

Now we have a little background, we can get to the practical. Measuring VSWR. Now we have or formula we can work out roughly how long an antenna should be. For this practical we are going to use the simplest and most popular antenna. The Half Wave Dipole.

halfwavedipole

The Diagram above should give you some idea of how a Half wave dipole looks and works. The two black lines are the Radiating Elements. They are the wires that send the Radio waves on their way or receive them. There is two Radiating elements both are 1/4 wave long. This makes the total length 1/2 wavelength. Hence the name.

So, we have our formula, we have some wire. Can we just work it out and cut it ? Unfortunately the answer is no. You may get close but equally you may not. It is important that Antennas match the Frequency that is being transmitted into them. Modern Radio equipment is designed to work at 50 Ohms. So it expects the antenna to present 50 Ohms. Moving away from it will damage the Radio Equipment ! Extended or high power use with this type of mismatch can cause fatal damage. Another issue is that we are using the speed of light in free space ( 300,000,000 M/s ) for our calculations. In reality Radio waves don’t travel this fast. In some materials Radio waves might travel 30 % slower. The difference between the speed of light in free space and the speed that a radio wave travels though something, Is called the Velocity Factor. This is given as a number. The Velocity Factor of RG58 Coax is 0.69. This is easy to work out. 0.69 is a decimal. Another way of writing it, is 69%.
So, in RG58 Coax the Radio wave travels at 69 % the speed of light. I.e 300,000,000 / 100 = 3,000,000 ( one percent ) X 69 = 207,000,000 M/s

We can also do, 300,000,000 * 0.69 = 207,000,000 M/s

The second way is simpler. I included both to give some background in how the calculation works.

How do we Measure to make sure its correct ?

We need to briefly introduce another topic here. Feeders. Feeders are the lengths of wire that connect an Antenna to the Radio. The Feeder and Antenna come together to form one system. We are going to use VSWR to measure the antenna system. ( feeder and antenna ) To do this we use a VSWR meter. Which is connected between the Radio and feeder.

SWR-Meter

The above image is a cross needle VSWR meter. There are other types. This is probably the most common for Radio Amateurs. In the Image you will see there is two needles and two scales. One side is “Forward” the other is ” reflected “. Between them is a third scale. We read this by looking at the position where the needles cross. This tell us our VSWR. A Value of 1:1 is what we are aiming for. We may not always reach it. On some bands we may have to settle for 1.5:1. It is best to stick below 2:1.

The practical tells us we have to adjust a half wave dipole for the best VSWR. When I do this practical I use telescopic whips. This makes it easier and can be set up quickly. I will explain the technique. We are going to use the 2 Meter band.

First, we transmit on 144 MHz and take a note of the VSWR. Next we Transmit on 146 MHz and make a note of the Frequency. Finally we transmit on 145 MHz and make a note of the frequency.

Remember while doing this to listen first and make sure the frequency is not in use, As well as making sure you are not on or too close to the band edges. Your instructor will help with this.

Now we need to adjust the antenna for the best VSWR, this should not be done while transmitting !

A general rule to help here is that, if the value at 144 MHz is lower than the value at 146 Mhz the antenna is too long. If the value at 146 MHz is lower than the value at 144 MHz, the antenna is too short. Why does this work ? As the frequency decreases the wavelength increases. and vice versa. So antennas for frequencies below the 2 Meter band are longer and above are shorter.

Now its trial and error. make short adjustments to both sides until you can get the lowest VSWR.

Comments are closed