ANTENNA TUNER with SWR Meter

By: S. Rama Mohan Rao, VU2RM

    After building several RM-96 SSB transceivers it is observed that in some sets quite a bit of spurious output is there. So to suppress and to see that these output frequencies are not radiated, it became a necessity to use an antenna tuner. Having built an antenna tuner then to use this, it is essential to have a SWR meter without which it is not possible to tune the tuner. Therefore, ideas roamed around books for a tuner using cheap and easily obtainable parts etc., and the outcome is the circuit, is described now. 

    Since RM-96 gives a power output of about 30 watts on 40 M band. It is described to have an SWR meter that will respond to power outputs as Low as 2 watts on 40M. Therefore, the QRP SWR meter as described by W1FB is followed using the parts available for us here. The circuit is as follows. It is also accompanied with antenna tuner. The construction can be done on a general purpose PCB about 1-1/2 Inch Square. The components are arranged prior to soldering systematically so that placement of T1 is at; the top and equidistant from trimmers Cl and C and like wise the 27 ohms, diodes etc, are on either side. 

    Then make a L shaped aluminum plate (see Fig. ) Mount the VU meter on the front panel one side next mount the 10Kohm linear potentiometer, next to the SPDT switch and mount the PCB behind the meter mount the SO 239 or any other type of coaxial sockets on the front panel or of the back side, one for TX and the other for antenna. 

Adjustments are done as follows:

The SWR= FORWARD + REVERSE 
FORWARD - REVERSE 

Suppose the forward reading is reflected is 3, 
Then SWR=(10+3)/(10-3)= 1.9; 
so SWR=I: 1.9 

NOW about the Antenna Tuner:
   
Since RM-96 is giving maximum of 30 to 40 watts output, most of us using inverted "V" antenna whose characteristic impedance is supposed to be 50 ohms. It is estimated that with a properly matched antenna the RF voltage at the output socket is as follows. 

Antenna Cable impedance is 50 ohms. Power output estimated is 40 watts. Then the voltage is approximately.

 V = square root of (PR) = 44 volts (approx.), for 75 ohms it is 55 volts. Even if mismatch is there between feeder and feed point, it is thought that this voltage does not exceed :L00 volts. Therefore, [ expected the PVC TUNING CAPACITORS would survive without internal arcing. Therefore, I wanted to use. After these thoughts and ideas the following circuit is used and survived. 

Inter connections are made of 50 ohms coaxial with appropriate plug and sockets. Switch on the RX, tune to some signals if heard or otherwise listen to the background electrical noise etc. Tune both tuning capacitors of Antenna Tuner i.e., C and Ca, Cb for maximum noise as heard on the RX speaker. This is the initial setting of the capacitors. Now keep the carrier of the TX on and press P-I-I-. Keep the SWR meter in forward position, there will be some indication of the meter. If it is too much, turn the sensitivity control back for full scale reading of 10. Now through the SWR switch to REF position, the meter will show some reading. Now quickly turn the knobs of C, Ca, Cb, capacitors for minimum reading on the REF meter, then back the switch to F position and see that meter will have the maximum forward reading. If not adjust the :10 K pot for full scale reading of and adjust Ca and Cb for minimum reading on the reflected reading.

NOTE: Do not keep the PIT continuously on during these tests. The IRF may get hot and go QRT, Press the PIT for short periods. This way you tune the Antenna Tuner. You can leave the SWR meter in line and watch the REF power now and then OK. The whole SWR meter cum ANTENNA TUNER is built.

Components list:

1. C1, C2 :10 pf yellow Trimmers, Philips.
2. C half of PVC 2J gang.
3. Ca, Cb PVC 2J gang both sections are used.
4. L 12 turns of 18 SWG enameled copper wire close wound over 1 inch dia former.
  T1 Sec: 60 turns 30 SWG, Pri:2 turns of 22 SWG on Belgaum HFA-
  T2- (3/4 inch OD) over the secondary at the center.