After building the "clothesline reel" 1/2 wave end feed antenna, with integral "L" tuner and resistive SWR bridge, I found it difficult to find a good match using the built-in bridge. I also found it difficult to key the transmitter and make the adjustments at the same time. One really needs three hands for the operation, as the tuner will not sit still by it's self.
However, adjusting the tuner using the "Tenna Dipper" circuit was easy and a good definitive match could be easily found. The Tenna Dipper was something of overkill for what I needed though. What I needed was something which performed the same function, but was much smaller and lighter than the Tenna dipper, so that it could be justified being taken along on a long distance hiking trip, where weight and space was of great concern.
Since I simply needed to adjust or verify the match at one frequency on one band (7.040 MHz), it occurred to me that by using a crystal oscillator, the Tenna Dipper circuit could be greatly simplified and made to fit in a much smaller package. By using SMT parts and a small type 21/23 12 volt battery for power, the complete "tuner Dipper" fit into a mini-Altoids tin.
The circuit uses all common and inexpensive parts. QRP frequency crystals can be obtained from NorCal or AMQRP club web pages, for $3.00 each. If a socket for the crystal is used, the Tuner Dipper could be used on a number of different bands.
Operation is very simple. Connect the antenna tuner to the jack and power up the Tuner Dipper. Adjust the tuner until the LED goes out. You now have a 50 ohm match. Remove the Tuner dipper from the tuner and connect the rig to the tuner. Your now ready to operate.
Q1 is a classic Couplits crystal oscillator to provide the signal source. Q2 buffers the output of the oscillator to drive the resistor bridge. Resistors R5, R6 and R7 form three legs of a Wheatstone bridge. The tuner forms the fourth leg of the bridge.
When the bridge is un-balanced, a voltage is developed between the junctions of R5/R7 and R6/Rx (rx=tuner). This voltage is sensed by the transformer between these junctions. The transformer has a 1:5 turns ratio which supplies some passive voltage gain. The signal is further amplified by a high gain Darlington amplifier consisting of Q3 and Q4. Collector current flowing in Q3/Q4 while a signal is present is indicated by the LED in the collector circuit.
When the bridge becomes balanced, that is Rx (the tuner) is 51 ohms resistive, the voltage at R5/R6 and R6/Rx is identical. Therefore, a signal is no longer being amplified by Q3/4 and there is no collector current flowing, so the LED goes out, indicating a match.