A Tunable LowFER Test Transmitter
By Lyle Koehler, KØLR
A variable-frequency transmitter is often useful, especially when checking a new antenna for resonance. The diagram below shows a simple oscillator using a hex Schmitt trigger IC to drive the complementary-pair LowFER transmitter final.
Low-pass filtering has been deliberately left out of the circuit, except for a ferrite bead and capacitor to prevent interference to television and FM broadcast reception. This makes the output "flat" over the entire tuning range, which extends from approximately 150 to 300 kHz. The tuning range can be increased by reducing the value of the resistor in series with the frequency adjust pot from 10K to something like 4.7K. Changing the value of the 470 pF capacitor will shift the entire tuning range up or down. A larger capacitor lowers the frequency and a smaller capacitor raises it. A frequency counter or receiver can be used to calibrate a dial scale for the frequency adjust pot.
Connecting the 1 meg resistor between pin 4 and pin 1 of the 74HC14 will put a very low-frequency modulation on the signal to help in distinguishing it from "birdies" and power-line carriers if you want to run some receiving tests. The modulation will be a sort of frequency-shift keying; not pretty but distinctive. It won't show up in an AM receiver, however. For on-off keying, the 1 meg resistor can be replaced by a 1N4148 or other general-purpose diode. The polarity of the diode shouldn't matter. Decreasing the value of the capacitor in the low-frequency oscillator circuit will raise the modulation frequency. A modulation frequency in the audio range will make the signal detectable on an AM-only receiver.
When this transmitter is connected to a typical LowFER antenna system consisting of a vertical antenna with a series loading coil, the final amplifier current will be very low unless the transmitter is tuned to the resonant frequency of the antenna. This provides a simple test for antenna resonance. However, since the transmitter output is a square wave that is rich in harmonics (primarily odd harmonics), there may be smaller current peaks when the test transmitter is tuned to a subharmonic of the actual antenna resonant frequency. If you suspect that this is happening, try extending the tuning range of the transmitter by using a smaller value of capacitance in the oscillator circuit, and see if there is a larger current peak at the suspected harmonic. For example, if a current peak is observed at 150 kHz, check to see what happens at 300 and 450 kHz. It's very easy to underestimate the size of the loading coil needed for a LowFER antenna, so an actual resonance on a harmonic of the desired frequency is a distinct possibility.
With a high-Q loading coil, a good ground system, and no nearby trees or other objects to introduce losses to the antenna circuit, the DC current to the final amplifier should peak at more than 50 milliamps when the transmitter is operated from a 12-volt supply. If the current at resonance is much lower than 50 mA, it is either an indication of high series loss resistance or that the actual antenna resonance is on a harmonic of the transmitter frequency.
To make the circuit even simpler, you can connect the output of the 74HC14 "exciter" (pins 6, 8, 10 and 12) directly to the antenna through a DC blocking capacitor of about 0.22 uF. Not as much output as with the complementary-pair final, but it should be enough to let you detect the antenna resonance with a voltage probe or field strength meter.