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If plagued by local QRM, lack of space or tall masts for a full-sized 80 or 40m dipole, the magnetic loop antenna may prove to be a useful alternative. It is relatively simple to make, does not depend on a good ground and operates well at low heights over short, medium and long distances. The down side is its narrow bandwidth when compared to a half -wave dipole tuned to the same frequency. Re-tuning will be necessary to cover a whole HF band. A problem with Faraday coupled loops is the susceptibility of the tuning capacitor to hand capacity effects. Based on the Travel Loop Antenna Tuner by John PD7MAA 1, this design, similar to the successful US Army Loop (aka, the Kenneth Patterson Loop tuner), uses capacitors to tune and match to the 50Ω output transceiver.
The alternative capacitor arrangement of this design is easier to implement than the Patterson design, employing two twin gang 500+500 pF variable capacitors configured as split stator capacitors. Although a single length of conductor for the loop can be used to cover more than one band there will be a variation in efficiency across the bands. For maximum efficiency on all the 80-20m bands, different lengths of coax or thick stranded wire are required but the loop circumference must not exceed 0.3λ. The US Army Loop antenna was designed by Kenneth H Patterson, working for the Department of the Army, US Army Limited War Laboratory, Aberdeen Proving Ground Maryland, which he first described in Electronics (Aug 21 1967). This magnetic loop antenna was developed for South East Asia to boost MF & HF signals covering the 2-5 MHz range to work out of narrow valleys and heavy forests.
It was primarily designed for compactness being just 12 ft wide, portability, and to be quickly assembled or dismantled and packed way in a small space. The image above shows the basic design of the US Army Loop. The octagonal loop antenna was constructed with 5 ft sides made from 1.75 inch diameter aluminium tube with gold plated ends. The gold plating ensured that overall losses were less than 0.1 ohms and ensured that the loop’s efficiency was high and be comparable to that of a full-sized dipole. The loop’s basic tuning and matching circuitry employed two high voltage twin gang 468pF variable air-spaced capacitors as shown in the simplified diagram. To cover the entire 2-5 MHz frequency range, an extra 450pF was switched across both C1a and C1b for coarse tuning. For improved fine tuning, capacitors C2a and C2b were provided with a 12-way switch to select one of twelve high grade mica capacitors (750 pF - 8250 pF) for additional impedance matching. Three Capacitive Couplings for Small Transmitting Mag Loops Mag loops can take on a variety of shapes e.g. circular, octagonal and hexagonal. The most efficient shape is circular because it has the largest area for a given perimeter size, i.e. maximum capture area for signal. In each of the coupling methods shown below, the capacitors connected across the ends of the loop are used to resonate the antenna and the other capacitor matches the low impedance of the loop to the transceiver.
There is some interaction with the resonating capacitors so that tuning and matching is an iterative adjustment process in order to minimise the SWR and maximise the loop's current. Figure 1 - Capacitors C1 & C2 resonate the loop and equally share the high voltage produced as a result of the loop’s high “Q”, but at the expense of reducing the effective tuning capacitance by a 1/2. C3 provides the matching of the loop’s low impedance to the transceiver’s 50Ω. This is easiest to implement using two twin gang variable capacitors i.e. the two sections of a single gang capacitor wired as a split stator for C1 and C2 and C3 formed similarly. Figure 2 - Capacitors C5-C6-C7 act as a voltage divider, this arrangement increases the voltage rating of the combined tuning capacitor, but at the expense of reducing the effective tuning capacitance by 1/3. C7 is provided to match the loop to the transceiver. Capacitor C7 must not be ganged with C5 & C6. Figure 3 - A single capacitor is used to tune the loop and has to withstand the loop’s very high voltages. Each of the ganged coupling capacitors, which are connected in series with the coax, share half the loop's high voltage. Although the diagrams above show single capacitors, for high power loops these are often two capacitors series connected to double the voltage rating to avoid flashover between the capacitor’s plates. Even at modest QRP power levels, voltages in excess of 1000 volts can be present across the tuning capacitor and matching capacitor. For that reason, the QRP Loop Tuner is built using a plastic case for a maximum power of 20 watts. Source: https://rsars.files.wordpress.com/2013/01/qrp-loop-tuner-80-20m-g8ode-iss-1-32.pdf
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