While testing 500 kHz antennas I needed a tool to measure the antenna impedance at resonance. First I modified a commercial HF antenna bridge to 500 kHz and although it could be used I found it difficult to tune to the correct frequency and stayed tuned there. Therefore I decided to build an antenna bridge for the range 500-505 kHz that provided a stable frequency.

The starting point was the VXO of the M0BMU transmitter. Jim uses a ceramic resonator VXO at 4 MHz, that is divided by 8 to get a 500 kHz signal. The maximum tuning range was more than sufficient, with a 5-50 pF variable capacitor it could be tuned from 495 to 510 kHz. I replaced the variable capacitor by a varicap (D1), actually just a 1N4007 rectifier diode, good for a tuning range of 500-505 kHz. When using another varicap diode or tuning range you might have to experiment with the value of C2.

A PNP/NPN buffer (T3/T4) at the divider output makes it a low impedance signal source.

Now I had a stable 500 kHz signal. But a square wave is, because of its harmonics, not suited to be used with a traditional (wideband) diode detector. Therefore I used a 455 kHz IF filter (F1), tuned to 500 kHz, as a harmonic filter. It ia placed between the bridge and the diode detector (D2). I used a 7 mm filter from TOKO (yellow dot, marked LPSC4200A92309) with the low end toward the bridge. It could easily be tuned to 500 kHz and had a bandwidth of about 5 kHz.

Trimmer R14 set the maximum current trough the meter and trimmer R12 gives a slight offset in order to get a nice sharp dip. To mention the obvious: R1 tunes the frequency and R10 is used to zero the bridge. I calibrated R10 just by measuring the resistance at different positions. As antenna losses are rather high at my QTH I used a 100 Ohm potmeter, but at other locations 50 Ohm might be more convenient.Frequency calibration was done by connecting the output to a frequency counter.

The antenna bridge is powered by a 9 V battery. Be aware that the power consumption is rather high (75 mA), mainly due to the PNP/NPN buffer. Therefore there are 2 switches: one that powers the oscillator (S1) and a second that powers the remainder of the circuitery (S2). The oscillator draws less than 10 mA, so S1 can be left on for a longer time to ensure the frequency stability while S2 is only switched on to perform a measurement.

Possible modifications:
- By replacing D1 by a 5-50 pF variable capacitor the tuning range can be extended to 495-510 kHz. However be aware that the bandwidth of F1 is only about 5 kHz, so the signal might be down at the edges.
- By putting a 50-500 pF variable capacitor in series with R10 untuned vertical antennas can be measured (antenna capacitance and loss resistance).