RadCom April 2024, Vol. 100, No. 4

24 April 2024 Technical Design Notes LF-band hardware Back in the year 1996, radio amateurs got their first low-frequency band allocation, and were allowed to transmit up to 1W effective radiated power (ERP) in the band 71.6kHz to 74.4kHz. Back then, there was nothing in the books or magazines about operating at such low frequencies, so we all had to do basic research and experimentation on how to get going. All the equipment had to be home-constructed, and most of the technical details needed were buried in history, with articles from the pre-Internet days almost lost, apart from a few books on the subject. It was an interesting time; the Internet had just started, and an LF reflector was set up to facilitate discussion on all technical and operating matters, and for the setting up of skeds. Several years later, we lost the 73kHz band, but we gained the two bands that we have now: first 135.7kHz to 137.8kHz, with the same ERP, and then a few years later 472kHz to 479kHz at 5W equivalent isotropic radiated power (EIRP). There has been little technical information published since those early days of getting going at LF/MF, so we’ll have a re-cap here of that development work, with some pointers on how to get operational on those bands. Antennas The antenna is almost certainly the most difficult part of operating at LF or MF. A frequency of 137kHz corresponds to a wavelength of more than 2.2km, so anything we can put up in a garden is going to be electrically very small. There are two types of antenna to consider: the loop and the vertical. If space is restricted, small loops end up appreciably less efficient than verticals, but if you have a paddock or an orchard, then a physically- large (although still ‘electrically-small’) loop might just prove useful, especially at 475kHz. However, most of us aren’t in that position so here we’ll just look at the vertical antenna. An electrically-small vertical antenna looks capacitive, so has to be tuned to resonance with a loading coil that has to resonate with that capacitance. There has been much discussion over the placement of the coil, at the base or mid-way up. Base loading has the advantage of needing less inductance, as it has the whole of the vertical radiator to resonate with. An elevated loading coil needs more inductance, is mechanically more difficult to arrange, but can offer a slight improvement in efficiency. Base loading is by far the most popular, especially considering the sizes of typical loading coils. In a vertical electrically-short radiator, the RF current tapers linearly from a maximum at the base (the top of the loading coil) to the top of the radiator where the current falls to zero. This taper means that the effective height of the vertical radiator is exactly half of its physical length. To improve matters, it is necessary to add capacitance to the top of the radiator in the form of a ‘capacity hat’. This can consist of as may horizontal wires as possible and as long as possible. If the capacity hat were ‘infinite’, its capacitance would swamp that of the vertical itself and force current in that to be constant, with the result the effective height becomes the same as the physical height. In practice some compromise is needed, so the effective height ends up somewhere in the region of 0.6 to 0.9 of physical length for most practical amateur-radio installations. Why is effective height so important? We’ll come to that shortly. Increasing the antenna capacitance in other ways, for example by using multiple wires in the vertical radiator and the capacity hat, and using thicker conductors, all helps to lower the value of the loading inductance needed. Loading coils To get a feel for the size of loading coil needed, a good starting value is to assume 7pF per metre of wire in the vertical element, and 6pF per metre of wire in the top-hat. The capacitance is affected when multiple wires are run in parallel, and so the value is only ‘rule-of-thumb’, but can give a guide to planning things. Here at G4JNT, in a small suburban garden, I can manage 7m height and a longitudinal span of around 12m for the top hat. The garden is narrow, so the capacity hat has to follow its length. I use two wires spaced by 100mm for the vertical element, and three wires spaced 150mm for the top hat. This is shown diagrammatically in Figure 1 , with a photograph of the current configuration in Figure 2 . From the rule of thumb above, an estimate of 3 wires * 6pF/m * 12m + 2 wires * 7pF/m * 7m suggests that the capacitance could be around 310pF. With parallel conductors making up the total of wire used for the calculation, coupling between them lowers the actual value somewhat, but by how much is very much a case of ‘suck it and see’. Proximity to local obstructions also changes things, sometimes in unpredictable ways. In practice, the measured capacitance to the radial system (which we’ll come to in a minute) was actually measured as 210pF. Had the conductor spacing on the vertical and top hat been wider, an increase in capacitance would have been expected, but even with this arrangement, it would be unlikely to get much closer to the original estimate. At 73kHz, 210pF needed an inductance of 22.6mH to resonate. We can use Wheeler’s formula to get an approximation of the coil dimensions: L( m H) = N 2 D 2 (0.46D + G) where FIGURE 1: Essential elements of an LF vertical antenna.