Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
General Information:
The final amplifier pi-network is a matching network that matches the high
output impedance of the tube to the low impedance of the antenna. The circuit
covers 6 bands with a separate coil for the 10m band.
How To Adjust/Tune A Transmitter With A Pi-Network
Output:
The normal method for tuning a transmitter with a pi-network output circuit is
as follows:
1. Fully mesh the loading capacitor.
2. Apply some drive to the final amplifier and adjust the
plate tuning capacitor for maximum output, as
indicated on an output meter. (or tune for minumum plate current on the plate
current meter.)
3. Slightly open the loading capacitor and retune the plate tuning capacitor
for maximum output. (or minimum plate current, which should now be higher than
before at the minimum.) You may have to increase the drive a bit.
4. Continue this procedure until the output no longer increases. (or the plate
current at the minimum is the rated value). Be sure that you adjust the drive
for proper grid current, and do not exceed the maximum grid current.
Final Amplifier
Pi-Network
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
| Plate Tuning Capacitor |
| 10m Tank Coil |
| Main Tank Coil |
| Bandswitch-A |
| Loading Capacitor |
| Bandswitch-B |
| Additional Loading Capacitor |
| Antenna RF Choke |
| Antenna |
| Plate Tuning Capacitor: The plate tuning capacitor and loading capacitor are essentially in series and connected in parallel across the tank coil., with their junction connected to ground. They form a capacitive voltage divider connected across the tank coil. The antenna is connected in parallel with the loading capacitor. Since the value of the plate tuning capacitor is much smaller than that of the loading capacitor, the plate tuning capacitor has a much larger effect on the resonant frequency of the whole combination, and is used to tune the whole pi-network/antenna system to resonance. |
 |
| 10m Tank Coil: To obtain better performance, a separate coil is used for the 10m band. The main tank coil is completely shorted out on the 10m band. |
|
| Main Tank Coil: The 10m coil and the main tank coil are connected in series. They are connected in parallel with the series combination of the plate tuning capacitor and loading capacitor. The main tank coil is tapped at a variety of locations and the proper tap is selected by Bandswitch-A. |
 |
| Bandswitch-A: Bandswitch-A progressively shorts out more and more turns of the main tank coil to decrease the inductance. On 10m, the entire main tank coil is shorted out and only the 10m coil is used. It is very important that the bandswitch is wired so that it shorts out any unused turns! This prevents high voltage from building up due to the autotransformer action of the tank coil. The reactance of the shorted turns prevents any current from flowing through them, even though they are shorted. |
 |
| Loading Capacitor: The loading capacitor and plate tuning capacitorare in series (with their junction connected to ground) and are connected in parallel with the tank coils, forming a parallel resonant circuit that is tuned to resonance using the plate tuning capacitor. The two capacitors simultaneously form a capacitive voltage divider, with the antenna connected directly across the loading capacitor. By changing the value of the two capacitors, while keeping the circuit in resonance, the voltage across the two capacitors (and thus the antenna) can be varied. This allows the coupling to the antenna (loading) to be controlled. Keep in mind that with capacitors in series, the larger the capacitance the smaller the voltage across the capacitor. Thus, when the loading capacitor is set to maximum capacitance (fully meshed), the voltage across the antenna is a minimum, and the coupling to the antenna is a minimum. As the loading capacitor is decreased (unmeshed), the voltage and thus coupling to the antenna increase. Anytime the adjustment of the loading capacitor is changed, the circuit must be retuned to resonance with the plate tuning capacitor. |
 |
| Bandswitch-B: On the 80m and 40m bands, the 720pf variable loading capacitor does not have enough capacitance. To obtain the required capacitance, Bandswitch-B switches in an additional 300pf of capacitance in parallel with the loading capacitor., increasing the maximum capacity to 1020pf. |
 |
| Additional Loading Capacitor: On the 80m and 40m bands, the 720pf variable loading capacitor does not have enough capacitance. To obtain the required capacitance, Bandswitch-B switches in an additional 300pf of capacitance in parallel with the loading capacitor., increasing the maximum capacity to 1020pf. |
 |
| Antenna RF Choke: The antenna RF choke performs several important functions: 1. The loading capacitor and plate coupling capacitor are connected in series across the plate of the tube. The DC plate voltage is divided across the two capacitors, causing a DC voltage to appear across the loading capacitor and the antenna. The antenna RF choke shorts this voltage to ground, while preventing the flow of any RF. 2. Should the plate coupling capacitor short, the tube plate voltage would appear across the antenna, a very dangerous situation. With the antenna RF choke in place, the voltage would be shorted to ground, blowing the fuse in the plate voltage supply, alerting the operator to a problem. 3. Any static buildup up on the antenna is shorted to ground by the antenna RF choke. |
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| Antenna: The antenna is connected directly across the loading capacitor. The combination of the plate tuning capacitor and the plate loading capacitor can be adjusted to vary the voltage across the loading capacitor (and thus the antenna) to the optimum value. If the antenna is reactive, this can also be compensated for to some degree. |
 |
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Greg Latta's Electrical Engineering and Amateur Radio Pages
If you have any questions or
comments, you can send E-Mail to Dr. Greg Latta at
glatta@frostburg.edu