Thursday, 26 June 2014

Using The KØLR 'Antenna Meter' on 630m

If you plan to become active on Canada's new 630m ham band, one of the first things you're probably thinking about is an antenna and how to make it resonant. Without the proper instruments, determining antenna resonance below the broadcast band can sometimes require a combination of good luck and black magic!

One of the most useful pieces of gear for me has been a simple 'antenna meter' designed by Lyle Koehler, KØLR, who was very active at one time on the 185KHz 'lowfer' band. Although the original circuit was designed to cover a lower frequency range, it can easily be modified to cover the 2200m and 630m bands.

The KØLR LF Transmitter

The device is actually a low power (~1/2w) VFO-controlled transmitter designed to look into a 50 ohm antenna system. Since most LF antennas are matched to 50 ohms, either through a separate matching transformer or by tapping up from the bottom of the antenna's loading coil, the 50 ohm feedline from the antenna system is simply connected to the antenna meter's output and the VFO tuned for maximum deflection of the meter. The resonant frequency is then directly read from the calibrated scale of the antenna meter's dial. With most backyard LF antennas having a very narrow bandwidth along with a high-Q, the meter peak is quite sharp and easy to find. Look for a peak meter reading between 50-70ma as you sweep the VFO across the antenna. Normal current values will be very low until resonance is hit.

KØLR's original article gives more details and can be found here.

The 470pf capacitor (C in my diagram) determines the basic frequency range. To modify the circuit for dual range (both 2200m and 630m), I added a SPDT switch at pin 1 of the IC so that I could switch the value of "C". For coverage down to and below 136KHz, I used ~ 725pf (a 680pf + 47pf in parallel) while high end coverage up past 500KHz required just 250pf (220pf + 27pf in parallel). These values should get you very close to the required ranges and once installed it is an easy task to calibrate the tuning scale by listening to the signal on a receiver or measuring it with a frequency counter. I also chose to install the 1M 'modulation' resistor that gives the signal a distinctive FSK note.

The meter will quickly tell you if your antenna is too high in frequency (capacitive) or if it is too low (inductive) so that the proper corrective measures may be taken to resonate it within the 630m band. Once resonant, attention can then be paid to bringing the impedance match as close as possible to 50 ohms and a 1:1 match. Depending on your method of impedance matching, there may be some interaction between tuning for resonance and impedance matching but a perfectly resonant 50 ohm system can be achieved with a little juggling.

Although not updated for several years, KØLR's website contains much good information regarding simple backyard LF antenna systems (both transmitting and receiving), preamps and weak signal receiving techniques. It is well worth a close look if you are designing a new 630m station.

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