TUNING THE T-NETWORK MATCHBOX
[ Efficiently ]
THERE ARE TWO PROBLEMS WHICH CAN OCCUR WHEN TUNING A T-MATCHBOX:
- You will find more than one, sometimes many combination(s) of settings for the 2 capacitors and the inductor which give you the minimum SWR.
- Only one combination is the most efficient. All other combinations of settings result in unnecessary losses inside the matchbox.
FAILURE TO TUNE THESE MATCHBOXES PROPERLY
MAY RESULT IN AS MUCH AS 50% LOSS
OF POWER WITHIN THE MATCHBOX !
Since several different combinations of settings of C-in, C-out, and the Inductor can result in a low SWR, it is important to have some general guidelines to follow:
- Most of the power lost in the matchbox (converted to heat) is lost in the inductor.
- Finding the position which gives you a good match with minimum inductance reduces loss and increases the amount power transfered to the antenna.
- Sometimes minimum inductance is when the turns counter is at "0" but sometimes minimum is when the counter is at its highest value; it depends on your make of tuner.
- If it is not clearly stated in the manual, remove the cover of your matchbox and look.
- For maximum efficiency, the Output Capacitor (ANT side) should be as high as possible, while still giving you a good match (low SWR).
- Maximum Capacitance is not always Full-Scale on the front panel indicator. Sometimes Maximum Capacitance is when the dial is at "0". DON'T GUESS. THIS IS VERY IMPORTANT.
- If it is not clearly stated in the manual, then you should remove the cover of your matchbox and look.
I STRONGLY URGE YOU TO CHECK THESE POINTS BEFORE USING THE MATCHBOX.
SHORT TUNING INSTRUCTIONS
- Input Capacitor (TX Side): 75% of MAX CAPACITANCE
- Output Capacitor (ANT Side): MAXIMUM CAPACITANCE
- Roller Inductor (or Switch): MINIMUM INDUCTANCE
PLEASE BE 100% SURE THAT YOU KNOW WHICH END
OF THE SCALE IS C-MAXIMUM AND WHICH END IS L-MINIMUM.
- Tune the combination of L and C-IN for lowest SWR.
- Once that point is found, slightly reduce C-OUT, re-tune C-IN and L for lowest SWR.
- Continue until you find the point where reducing C-OUT no longer helps reduce the SRE.
- Increase the capacitance of C-OUT a tiny bit, and tweak C-IN and the Inductor for minimum SWR
- RECORD YOUR SETTINGS ON PAPER
REMEMBER, MAXIMUM POWER TO THE ANTENNA
IS WHEN C-OUT IS AT THE HIGHEST LEVEL
THAT STILL ALLOWS TUNING C-IN AND THE INDUCTOR
FOR MINIMUM SWR.
FULL TUNING INSTRUCTIONS
- Input Capacitor (TX Side): 75% of MAX CAPACITANCE
- Output Capacitor (ANT Side): MAXIMUM CAPACITANCE*
- Roller Inductor (or Switch): MINIMUM INDUCTANCE**
* CAUTION: You MUST choose MAXIMUM capacitance for C-OUT - not maximum on the front panel scale. SOMETIMES MAXIMUM is when the front panel dial says "0" and sometimes it is when it says "10". To be sure, remove the cover and LOOK. Make sure you do this right. I have repeated what I wrote above because THIS IS VERY IMPORTANT!
** CAUTION: You MUST choose MINIMUM inductance. SOMETIMES MINIMUM is at "0" and sometimes it is at "the highest number". To be sure, remove the cover and look at the inductor. One end of the inductor will have a wire connected to it, the other end does not. MINIMUM is when the roller is at the end which has a wire connected to it.
- START with the Inductor.
- Use an antenna analyzer if you have one; if not, then apply a low amount of power (5 to 10w).
- Tune the Inductor for minimum SWR.
- NOTE: if you do not have any dip in the SWR, then rotate the Input Capacitors 25% and then adjust the Inductor for minimum SWR.
- If it does not find a dip, rotate this capacitor the other direction.
- If it still does not find a dip in SWR, rotate the OUTPUT Capacitor to 75% of max and try again all of the above again.
- Once minimum SWR is found with the Inductor, continue with the Input Capacitor.
- Tune the C-In for minimum SWR. Note whether you are tuning lower or higher on the front panel scale.
- After finding the lowest SWR, re-tune the L slightly, making sure it is set for the lowest SWR.
- Next slightly move the INPUT CAPACITOR farther in the direction you moved it before. This will cause the SWR to go up.
- Now re-tune the inductor for minimum SWR. Determine if it is lower or higher than the previous setting of C-IN.
- If it is lower, then move C-IN silightly farher in the same direction. But if the SWR is now higher, move C-IN back in the other direction, just past its previous point, and re-dip the Indictor for minimum SWR.
- Once again, if the SWR is lower, continue moving the capacitor in that direction in tiny increments, each time adjusting the Inductor for minimum SWR.
- Determine the setting where the SWR is lowest when adjusting just C-IN and the Inductor.
- FINALLY, note the level of minimum SWR and then reduce the capacitance of the OUTPUT CAPACITOR slightly.
- Then re-tune the inductor for minimum SWR.
- Also tweak the C-IN for minimum SWR.
- If the SWR is now lower than before tuning C-OUT, then reduce (or increase) its capacitance slightly again and repeat the tuning steps for the Inductor and C-IN.
- THE GOAL IS TO FIND THE POINT OF LOWEST SWR WHILE C-OUT IS AT THE HIGHEST POSSIBLE CAPACITANCE.
- WHEN FINISHED, RECORD YOUR SETTINGS ON PAPER.
ALTHOUGH THIS SOUNDS COMPLEX, YOU WILL FIND IT VERY EASY
AFTER YOU HAVE DONE THIS 3 OR 4 TIMES.