Balanced antennas 5d.1 Understand the difference between balanced and unbalanced antennas and that a balun should be used when feeding an HF dipole with coaxial cable (which is unbalanced).
Some parts of a radio and its antenna
and feeder are balanced. This means that equal currents exist in both
halves of the system. If we take a dipole for example it is
a balanced system. However, coaxial feeder is unbalanced. When a
balanced system is connected to an unbalanced system a device called a
Balun (short for balanced to unbalanced) is required. This is a type of transformer. It is important when connecting a dipole to coax that a 1:1 balun is placed at the junction.
Standing Wave Ratios (SWR) 5e.1 Recall that an SWR meter shows whether an antenna presents the correct match to the transmitter and is reflecting minimum power back to the transmitter.
One of
the most common pieces of equipment used by amateurs is an SWR meter.
This is connected between the TX/RX and the antenna or ATU. It shows
the power going out and the power coming back from the antenna. If the
antenna is properly matched there will be no power coming back and the
antenna will show a 1:1 SWR.
5e.2 Recall that a high SWR (measured at the transmitter) is an indication of a fault in the antenna or feeder (and not the transmitter). (Relate this to item 4b.5.)
If the SWR is 3:1 or higher then this
shows a problem with the antenna or feeder. It does not show there is a
problem with the transmitter. High SWRs can lead to damage to the
output transistors in modern transceivers. Most modern transceivers have protection circuits that reduce the power when the SWR rises above 3:1. As
discussed before in section 4b.5, a resonant dipole with a 50 Ohm
impedance connected to 50 Ohm coax with a 1:1 balun should be a perfect
match for a 50 Ohm antenna socket. This will show a 1:1 SWR.
Dummy loads 5f.1 Recall that a “dummy load” is a screened resistor connected instead of an antenna to allow the transmitter to be operated without radiating a signal.
There are two ways to test a transmitter:
Connect to a 50 Ohm
antenna and test it on air. This however can lead to interference to
other users and is antisocial as it blocks up part of RF spectrum which may be required by other amateurs.
Connect a dummy
load in place of the antenna. A dummy load consists of a 50 Ohm
resistor in a metal enclosure that prevents any RF radiation. This is a
better way of setting up a transmitter as the dummy load has a known
impedance of 50 Ohms across the HF bands and does not radiate a signal.
