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:
Construct a 50 ohms dummy load using 20 numbers of 1K i watt resistors in parallel (wattage of these resistors are chosen as per your requirement).
The resistors should be of carbon composition.
Now connect this dummy load to antenna jack of the SWR meter.
The other socket is connected to TX with a small length of 50 ohms coaxial cable.
Switch on the TX, insert carrier by turning the carrier balance control (1K preset).
That is now we are feeding the TX output, which is a continuous carrier into SWR meter and then to dummy load.
Keeping the switch in forward position, press PTT and adjust 10-K linear sensitivity control for full-scale deflection in the meter.
As Low as 5 watts should give full scale deflection.
Now turn the switch S:L to reflect position, the needle will not be at the same point but fall Low.
Now turn one of the trimmers, there should be variation in the reflected power meter reading.
If no variation is found try the other one and adjust it for minimum reading on the reflected position.
Now remember this trimmer.
Now interchange the antenna and TX socket connections i.e. the TX output coaxial will be connected to the socket that was earlier connected to dummy load.
The dummy load is connected to earlier TX socket.
Now the meter REF/FOR readings will inter change.
That is the meter shows forward power with S1 in reflected position and vice versa.
Adjust the same way as above, with the other trimmer for minimum reading in reflect i.e. in forward position.
Try once again, going back, i.e.
Dummy load to antenna socket and RF to TX.
See that you get complete null on the reflect position.
Redo all adjustments if adjusting 1O pf trimmers changes null.
That finishes your adjustments.
Connect all the cables as originally started.
If you are keen on knowing the SWR, a little more work is to be done on the SWR meter.
The VU meters are made for specific purpose and use, the calibration on the dial will not suit us.
So, remove the dial if you are dare to capable to open it and make another dial with 0-10 linear divisions and refit ion it.
Now alter the adjustments are over connect the antenna to the output socket.
Key the TX, adjust the meter for full scale deflection by means of the sensitivity control (10-K), the needle at 10th division in forward position.
Now through switch S1 to reflect position and key the TX, note the reading.
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.
|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.|