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ISOTRON 40/20 
SO EASY!
 
WHAT IS IN A TUNER?
 
Many stations use Antenna Tuners. From Amateur to AM Broadcast stations. What is this device made of? How efficient are they?
 
There are several variations of Tuners. However, there are 3 designs that are quite common. The "L" network, the "pi" network and the "T" network.
 
The "L" type tuner is quite simple. From the output of the transmitter you have a capacitor directly across to ground. After the capacitor there is an inductor (coil) in series to the antenna. Using the reactances of these components you can match an antenna to a transmitter where the antenna is not 50 ohms. Also you can match an antenna that is not resonant.
 
The math concerning the reactances can be quite complex. To keep it basic, the larger the C value indicates the antenna is of a higher impedance (Z) than the transmitter. The coil is used to cancel the reactance so the entire antenna system is resonant. The smaller the C the opposite occurs. The results also depends on the frequency of the transmitter.
 
A "pi" network is the same as the "L", with a capacitor added to ground after the coil. This type of tuner has more range and can handle larger mis-matches. Usually the capacitors are variable and the coil is switchable by the operator.
 
The "L" network is popular in the automatic tuners. Normally you can hear relays switching rapidly on the automatic tuners. It is selecting through a variety of "L" networks to make the match. This circuit is quite efficient and offers little loss to a mis-matched antenna when matched.
 
The "pi" network will do much the same, but you mainly use 3 components that are variable. Usually the operator does the adjusting. The circuit is also quite efficient.
 
When higher powers are used, the "L" and "pi" networks can get a bit large physically. Therefore, a "T" network or match is incorporated.
 
The circuit has 2 capacitors in series between transmitter and antenna. An Inductor is between the capacitors and connects to ground. With 2 reasonably sized (physically) variable capacitors and a variable coil, You can match a very wide range of values.
 
The "T" match is popular for the higher power tuners. The disadvantage is, it can have up to a 28% loss to the antenna. This is usually overcome with the higher power.
 
Tuners are an important device to have for any station. It actually matches the mis-match. It is not fooling the transmitter as some have thought. Used reasonably, the tuner will make your station more versatile and affective.
 
73,
Ralph WD0EJA
 
11-15
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