RadCom April 2024, Vol. 100, No. 4

Technical 26 April 2024 A vertical antenna has directionality, giving it a gain of times 3 over an isotropic radiator [2] , so +5dB needs to be applied to these loss figures to get the antenna gain in dBi. These are -43dBi at 73kHz, -33dBi at 137kHz, and -18dBi at 475kHz. Achieving the licensed 1W ERP for the lower frequencies would need well over 1kW with this antenna. Some operators on the band have significantly-higher antennas with larger capacity hats, and actually can reach the licensed maximum ERP using around 1kW of RF power on 137kHz. At 475kHz the licensed power is 5W, or 7dBW EIRP, and to achieve this at G4JNT requires a more-reasonable 300W. Loop antennas have a radiation resistance proportional to the square of the area of the loop, and therefore proportional to the fourth power of the diameter of a circle. This is why loops may be useful for larger installations, but are a non-starter for those of us with small gardens. High voltage This can be scary! A capacitance of 210pF at 137kHz has a reactance of 5500Ω. Applying Ohm’s law, 700W of RF power into a 220Ω load resistance results in an antenna current of 1.78A. This, through the reactance of 5500Ω, develops 9.8kV on the antenna, and a massive field around the loading coil. If metalwork that might spark is in the close vicinity, in the presence of flammable material like a plastic cabin, consequences like that shown in Figure 4 can happen. Several low-frequency operators have had fires; one needed the fire brigade to be called when a wooden shed was set on fire. Receivers For receive, the same antenna as that used for transmitting can be used, but in view of the large amount of externally-generated local noise in this part of the spectrum, many users have a separate receive antenna. This often consists of a small active loop that can be rotated to null out as much interference as possible. Other options commonly used are active whips (E-field probes) mounted some distance away from local noise- injection points, with their feeds very thoroughly decoupled to kill-off locally-generated interference. Interference cancellers, combining and nulling the interference input from two or more antennas, are also in use. When the bands first became available, there were very few commercial receivers that could receive at these frequencies, and certainly nothing that would transmit. Today’s receivers will usually go down at least to 100kHz, so nothing special is needed now. Back then, receiver converters were often used to up-convert LF to a few MHz for tuning on a conventional HF receiver. The converters were simple, often no more than a crystal oscillator and diode ring mixer but, in view of the capability of modern receivers, no further details are included here. Transmitter drive sources Transmitters are a different matter, and usually have to be constructed at home. Some SDR transmitters may offer a low-level output at 137kHz or 475kHz which may be amplified directly, but many don’t. Even if other radios will tune down there on receive, they will not transmit any power. There are a few options available to generate a low-level drive waveform. One is to convert from a higher frequency that a transmitter can deliver. As is done in a receive converter, a crystal oscillator driving a mixer will suffice, and several designs can be found in various publications [3] . Where a simple waveform is required, such as for on-off keyed CW, or a variable frequency fixed-amplitude data mode such FST4 or FTS4W, a dedicated RF source such as a direct digital synthesizer (DDS) can be used. Another alternative, used by a few operators, is a digital divider connected to the output of a transceiver. A 74HC390 device, for example, set to divide by 100, allows the transmitter to be run at 13.7MHz to give an output at 137kHz. Obviously, this is only suitable for CW, or constant-amplitude frequency-shift modes. If used for the latter, it must be noted that any frequency shift must to be generated at 100 times the wanted output shift. Power amplifiers 137kHz is in the same frequency range where switch-mode power-supply units operate, so right from the start many SMPSU components were put into use, particularly ferrites for transformers. If linear mode operation is adopted via a transverter, then an amplifier can be designed along exactly the same lines as any conventional HF MOSFET design, but with the ferrite type and number of turns adjusted for the power and frequency in use. With the very low cost of MOSFET devices and ferrites used in SMPSUs, high-power amplifiers can be built relatively cheaply. Switching-type MOSFETs are not ideally suited to linear amplifiers, but they can be persuaded to work with careful biasing, and we covered this is the March 2016 Design Notes . However, as most operation on these LF bands is using constant-amplitude modes like multi-frequency-shift keying and CW, there is little need for linear amplifiers. There are plenty of switch-mode amplifier designs in the literature, with power outputs ranging from tens of watts up into the kilowatt region. Kits are also available for several designs in the few-hundred-watt region. Another favourite is the highly-efficient class E design, such as the 400W design for MF [4] , which requires more effort to set up (an oscilloscope is mandatory) than a simple class-D switcher. The load resistance of the antenna is usually matched using a tapped transformer on the power-amplifier output, or a tap on the loading coil. This article has only provided a brief outline, and has skated over the surface of how to get going on 137kHz and 475kHz. But I hope the information and pointers to further details will inspire more operators to try these fascinating low frequencies where home construction and a bit of thought are needed, rather than the buy-and- plug-and-play operation of HF. This month’s Data column continues the LF/MF theme with a look at the digital modes in use at these frequencies. References [1] Much has been written in RadCom and elsewhere about PME, otherwise known as TNC mains earthing systems. In a PME system, there are potential safety issues with connecting the common earth/neutral conductor to an outside RF earth. In particular, high currents can flow if there were to be a neutral fault somewhere on the distribution cabling. But provided the bonding is done with a suitably-thick conductor it should be a safe option. If you are concerned about this, read the relevant publications available from the RSGB such as https://rsgb.org/main/files/2019/12/ UK-Earthing-Systems-And-RF-Earthing_Rev1.4.pdf and elsewhere. [2] The reflection of the vertical antenna in the ground plane makes it appear as an electrically-short dipole, which has a theoretical gain of 1.5 in free space. However, the power from the vertical antenna spreads out over half as much space than it does with a dipole in free space, so its directivity is doubled to three. [3] https://www.rsgbshop.org/acatalog/Online_ Catalogue_Low_Frequency_43.html [4] http://g4jnt.com/QRO_500kHz_PA_Breadboard.pdf FIGURE 4: This shows what can happen when 700W at 137kHz is mishandled. Here, small spark gaps were created when overlapping metal tape was used to seal the plastic cabin joints. In the vicinity of the fields around the loading coil, sparking at the joints of the tape set fire to the plastic.