"The 'Tenna Dipper"

A Low Power Antenna Analyzer

and ATU Tuning Aid

Using just a few mw of output power, this circuit is used to find the 50 ohm resonance frequency of antennas or can be used to adjust an Antenna Tuning Unit for 50 ohm match, without generating QRM. The unit is powered by a 9 volt battery and a Frequency counter is required to know where the oscillator is tune to.

The output match indicator is a LED. The LED is off when the bridge is in balance and gets brighter when the bridge goes out of balance. Therefore, when using to adjust an ATU for 50 ohm match, one would adjust the ATU until the LED goes out. This circuit can be built on a small circuit board and uses little power, so is practical to take into the field

The " 'Tenna dipper" consists of a wide range voltage tuned oscillator, a 50 ohm resistive bridge and a sensitive defector circuit. Shown below is the circuit built on a 2" x 2" board, which is small enough to fit in an Altoids tin, along with a 9V battery.

The R/C oscillator section of a 74HC4046 PLL/VCO chip is used as the frequency source. The oscillator is tuned by the voltage on pin 9. A bank of four trimmer resistors is used to select tuning "bands" by the use of SIP pins and shorting blocks. DIP switches could be substituted. These trimmers can be used to preset the frequency to center it in one of four ham bands. The fine tune control is used the trim the frequency. If none of the jumpers are in place, the fine tune can be used to tune the oscillator over it's entire range, though with a single turn pot, it will be touchy.

With the 22K R3 value, the stable tuning range of the oscillator is about 2.5 to 22 MHz. To extend the stable tuning range, a second resistor, R11 is put in parallel with R3 and makes the stable tuning range about 7 to 40 MHz.

Here are the tuning ranges of the trimmers. The range of the fine tuning control varies with which trimmer is selected, but is generally a MHz or two.

Trimmer W/O R11 jumper with R11 jumper
V2 2.5 to 4.8 MHz 7 to 13 MHz
V3 5.0 to 8.5 MHz 13 to 21 MHz
V4 8.6 to 12.3 MHz 21 to 27 MHz
V5 12.5 to 22 MHz 27 to 40 MHz

The upper frequency limit of the oscillator depends on who made the chip. One's made by Phillips and Harris should be able to go to 40 MHz. Ones made by Fairchild will crap out about 20- 21 MHz. Harris parts (CD74HC4046AE) can be found at Digi-Key and Mouser. The value of R9 may have to be trimmed depending on the chip used.

The output of the VCO directly drives the bridge through a VHF Low Pass Filter and then a 51 ohm resistor. Directly driving the bridge from the VCO ensures a constant drive level, which is difficult to achieve using a wide band amp without complicated frequency compensation.

Because of the low level of drive supplied to the bridge, a step up transformer is connected across the bridge output to provide passive gain. The output of the transformer is further amplified by a darlington transistor amplifier consisting of Q1 and Q2. R9, a 1 Meg ohm resistor provides a little bias voltage, but not quite enough to turn the amplifier on. In this way, the LED will go off if there is no input signal, yet keeps it sensitive to small input signals from the bridge.

There is no need for DC rectification of the RF signal being amplified by the darlington array. The LED provides an effective visual indication of the current flowing in the collector. The larger the input signal, the brighter the LED. This eliminates the "dead zone" produced by a diode rectifier which makes finding an exact null of the bridge impassable to achieve. It is also possible to put a meter across R10, if one so desires.

In order to know what frequency the oscillator is tuned to, one must connect up a frequency counter. You might only need to use this once to set the trimmers for each ham band you want to use. Then you would use the fine control and spot the frequency with your receiver. If your trimming or finding the resonant frequency of an antenna, then you'd need a counter connected to find the dip frequency.

Operation:

For finding the resonant frequency of an antenna, simply connect the antenna lead to J1 and sweep the frequency until the led goes out. If the LED can not be made to completely go out, then there is some reactive component to the impedance seen by the bridge.

For adjusting an ATU, connect the bridge to the input of the ATU and set the frequency to where you wish to operate. Now adjust the ATU to make the LED go out.

Although the oscillator isn't real stable, this unit is also handy as a general purpose signal generator.

Printed circuit board layout:

Several Hams have recently requested a board layout, so here it is. Parts placement are shown in the diagram below. Part numbers don't match those used on the schematic in a number of places, but I think you can figure it out. I don't want to spend the time making them match at this time. This board uses a four pole DIP switch for selecting band trimmers instead of jumper pins, which is easier to use and no jumper blocks to loose. Note that there is one point to point jumer, indicated by the black line. This connects power to the tuning resistors so that the ground plane is not broken. Down load a pdf file with the to scale board layout by clicking HERE. The layout is a through board view, suitable for printing directly on toner transfer film.

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