A Dip Meter with Digital Display
There's nothing like a dip meter for tuning antennas. You can stop squinting at that Jaded old dial after building this meter with digital display!
by Larry Cicchinelli, K3PT0
One of the main difficulties I have had over the years in building my dipole antennas is that of cutting them to the correct length. This article describes a dip meter with a 3-digit digital display which I designed and built to aid me in solving this problem.
I have a grid dip meter which I built from a kit and which, for quite a while, worked quite well. One of its shortcomings was that of alignment. Even though the printed frequency chart was relatively accurate, it still needed calibrating for each band. Since I was trying to decide on my next construction project anyway, I decided to build a solid-state replacement for it.
The device is a dip meter with a 3-digit frequency display. The analog portion of the circuit consists of an FET oscillator, voltage doubling detector, dc offset circuit and amplifier. The digital portion of the circuit consists of a high-impedance buffer, prescaler, counter, display driver, LED display and control circuit.
The dip meter has four distinct functional blocks each of which I will describe in detail.
The RF oscillator is a standard
873 Wedgewood Dr Lansdale, PA 19446
Colpitis using a common junction FET, Ql, as the active element. Its range is about 1.7 to 45 MHz. A 200-pF tuning capacitor was selected because I wanted a 2:1 tuning range. In order to get a 2:1 frequency range the capacitor must have a 4:1 range. The sum of the minimum capacitance of the variable, the capacitors across the inductor, and the strays must therefore be in the order of about 70 pf. The values of LI were determined experimentally by winding the coils and observing the lower and upper frequency values. The coils I made cover the following frequency ranges: 1.7-3.1 MHz, 2.8-5.9 MHz, 5.6-11.9 MHz, 9.720.7 MHz, and 19-45 MHz. Because I experimentally determined the coil sizes and am using old coil forms of unknown parentage, I've not provided winding details. Experiment!
I mounted the tapped capacitors inside the coil forms so that their values could be different for each band if required. The frequency spread of the lowest band is less than 2:1 because the tapped capacitor values are larger than for the other bands. The frequency spread of the highest band is greater than 2:1 because its capacitors are smaller.
The analog display circuit begins with a voltage doubler for the detector in order to get higher sensitivity. It drives a dc offset circuit, U1A. R1 is used to insert a variable offset which will get subtracted from the detector voltage. This allows the variable gain stage, U1B, to be more sensitive to variations in the detector output voltage. Q10 was added to U1C in order to allow its output to get closer to ground. The resistor in series with the meter should be chosen so that the current is limited to a safe value. For example, if a 1-mA meter is used, the resistor should be 8.2 k.
The prescaler begins with a high-impedance buffer and amplifier, Q2 and Q3. If you are going to use the meter for the entire frequency range described, care should be taken in the layout of both the oscillator and this buffer/amplifier circuit. The digital portion of the prescaler is a divide-by-100 circuit consisting of 2 divide-by-10 devices, LT2 and U3. The devices I used were selected because they were available. Any other similar devices may be used as long as the reset circuit is compatible. Q5 is a level translator, shifting the 5-V signal to 9 V.
The first part of the digital display block is the oscillator circuit of U1C, which is used to create the gate time for the frequency counter. R3 adjusts the oscillator to a frequency of 500 Hz, yielding a 1-ms gate. The best way to set this frequency is to listen for the dip meter output on a communications receiver and adjust R3 until the display agrees with the receiver. Once this calibration has been made for one of the bands, all the bands are calibrated.
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