84 metre vertical loop with wideband 200 Ohm Balun:
A large resonant loop antenna, with a circumference of the order of the radio wavelength, has a gain pattern in free space that resembles a “figure eight” with maxima broadside along the axis of symmetry. Whereas the opposite gain pattern occurs for the small loop antenna with total length less than a tenth of the radio wavelength. It does not resonate with the radio wave and has a gain pattern with broad maxima in the same plane as the loop with two sharp deep nulls broadside along the axis of symmetry. The small loop antenna is also called a magnetic loop antenna because it is more sensitive to the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. Because of the long wavelength used for medium and long wave radio, the small loop antenna has a compact size compared to other antennas and may have multiple turns of wire with a total length still less than a tenth of a wavelength. The magnetic loop antenna is often used in portable AM radios, consisting of many turns of wire around a ferrite core.
Resonant loops are makeshift antennas and easily deployed, especially if there is no other antenna available. The terminals of the loop are connected to the inner conductor and outer shielding of the coaxial cable. A resonant loop has a circumference equal to the radio wavelength of the fundamental frequency and resonates at the harmonic frequencies as well. Loop antennas are not grounded and over moist earth they often make a suitable alternative to inverted-V dipoles. At the fundamental frequency, the loop antenna has maximum gain broadside and minimum gain in the plane of the loop, but the antenna pattern is elevated when close to the ground and more complicated with sidelobes for the harmonic frequencies. So a vertical loop at the fundamental frequency has an elevation pattern with high gain broadside at low angles on the horizon for long distance communications and also at higher elevation angles for short range communications. However if the loop antenna is at a height less than half a wavelength above the ground or sea water at the fundamental frequency then the antenna pattern will be elevated with less gain at low angles on the horizon. Also the ground affects the impedance of the antenna so that the SWR will be greater if the antenna is at a height less than half a wavelength above the ground, particularly if over a good conductor like sea water. The antenna pattern is not omni-directional for a loop antenna in free space but it has nulls in some directions depending on ground effects and what harmonic is being used and thus rotating the antenna in azimuth can improve the gain. Proximity to the ground actually makes the antenna pattern more omni-directional as does bending the vertical loop in an L or U shape around a perimeter fence for example. A bow and arrow with rope or fishing line attached to a blunt arrow tip can be used to hoist antenna wire over the branches of a tree. The loop antenna can be draped over a tree or the mast of a yacht in any convenient shape, although avoid coiling the wire because any intrinsic reactance in the antenna (capacitance or inductance) increases the SWR without an ATU. The vertical loop antenna has an antenna pattern with low angles on the horizon even when close to the ground, unlike the horizontal loop for which most of the antenna gain is directed vertically upwards.
For the Amateur radio channels, a loop antenna with perimeter length of 84 metres resonates on the fundamental frequency 3.5 MHz plus the harmonics 7, 10.7, 14.2, 21.1, 24.7 & 28.1 MHz. The impedance of a resonant loop antenna is usually above 100 Ohms depending on ground conductivity if at a height of less than half a wavelength. So a wideband 200 Ohm Balun (4:1 impedance transformer & 2:1 voltage transformer) will match the antenna to the 50 Ohm cable at the fundamental frequency and harmonics as well. See "How to make a wideband 200 Ohm Balun". Furthermore, the resonant loop behaves as a series RLC circuit when stepping up the resistance with Balun. This decreases the Q factor and increases the bandwidth (typically more than 200 KHz), producing a low SWR (<2:1) for most of the channels in the Amateur bands. Thus an ATU is not required on most Amateur bands with this antenna. On the other hand, a tuned loop that is shorter than the wavelength and brought to resonance with an ATU, using the variable capacitor across the terminals of the loop, has a very narrow bandwidth around the resonant frequency, especially when used with a Balun since it behaves like a parallel RLC circuit. Stepping up the resistance in a parallel RLC circuit has the opposite effect of increasing the Q factor, which is not a problem for a tuned loop, provided the bandwidth is greater than the channel size (3 KHz for SSB).
Alternatively, a quarter wavelength of 75 Ohm cable can be used as a matching transformer to reduce the SWR below 2:1 for a load somewhat greater than 100 Ohms. When a vertical loop antenna is suspended less than half a wavelength above sea water, the impedance at the fundamental frequency is often less than 100 Ohms so a matching transformer is only required for the harmonic frequencies. So a loop antenna performs better with lower SWR over land than when mounted on a ship. For the marine channels, a loop antenna with perimeter length of 143 metres resonates on the fundamental frequency 2.1 MHz plus the harmonics 4.2, 6.3, 8.4, 12.5 & 16.6 MHz. EZNEC modeling with a 200 Ohm transformer that broadens the bandwidth of the resonant frequencies shows that this antenna covers most of the marine radio channels (voice ,DSC & weather Fax) without the need for an ATU over moist ground. I helped construct a 143 metre vertical loop antenna to receive the marine weather radio stations and we measured the SWR with a HF Analyst that does not have a tuning circuit and with a tiny output signal, is subject to broadband interference. So a HF Analyst meter can overestimate the SWR for broadband traveling wave antennas that use a load resistor to reduce the SWR while sacrificing antenna gain, but it measures SWR accurately for resonant standing wave antennas like dipoles and loops.
Try to keep the antenna wire away from conductive objects like an aluminium mast or metallic deck railing on a yacht that can detune the antenna and increase SWR. Trimming the length of the loop will tune it for resonance on the fundamental frequency plus the harmonics, if changing the shape of the loop alters the resonant frequency. Bare or insulated wire will work, but stranded wire with plastic insulation is preferable since stretching and flexing due to wind loading are major considerations. Also insulated wire protects people from the high voltages typical in standing wave antennas. Polyethylene insulation sheds water very well and helps keep the antenna up during winter storms. Insulated wire has a lower velocity actor than bare wire, so a resonant element will be physically smaller. If you use insulated wire, downsize the loop by about 1 or 2 percent from the full wavelength of the fundamental frequency. This can be done during installation, by trimming the length of the antenna for resonance while measuring the SWR. Some sort of lightning surge protector is also advisable if the antenna is not disconnected from the radio when not in use. Fuses do not provide adequate lightning protection.
QRP amateur radio relies on resonant antennas with a minimum power of only Watts to communicate by HF radio. Resonant standing wave antennas produce the highest gain with a narrow beamwidth for transmitting or receiving in a certain direction. Not only is using minimum transmitter power more energy efficient, it also avoids the health hazard of prolonged exposure to radiation close to an antenna. The recommended safe radiation limit above 10 MHz is 2 Watts per square metre. Therefore people should stay away from an antenna while transmitting with a minimum distance of at least 3 metres for 100 Watts, 9 metres for 1 KiloWatt and 30 metres for 10 KiloWatts, depending on the antenna beam pattern. You should not stand in front of a Microwave dish because of the narrow beam and higher frequencies, even with a power output as low as 20 Watts. A 1:1 choke Balun will prevent parallel currents in the outer shield of the coaxial cable and avoid RF feedback, RF burns and reduced antenna gain. An air core RF choke Balun is easily constructed by simply winding the coaxial cable in a coil with at least 10 turns of 15 cm diameter. Place the choke Balun at the location where the unbalanced coaxial cable connects to the antenna feedline and away from any conductive material.
"How to make a wideband 200 Ohm Balun": (courtesy of Newtek Electronics).
For wideband circuits a magnetic ferrite core is commonly used because the higher permeability of ferrite material will provide a higher inductance for a given number of turns and also provide tighter coupling. The type of ferrite chosen must exhibit low loss over the desired range of frequencies. The common rule for design of wideband transformers is that the reactance (XL) of a winding must not be less than four times the source impedance at the lowest frequency. "What about the effects of this at the high frequency end?" you may ask. Well luckily there is no cause for concern, as the effective permeability of the ferrite core material decreases with increasing frequency, thus reducing the inductance of the winding. With the proper selection of core material it is easy to make wideband transformers which cover one decade in frequency: ie. 2–30 MHz. Although the rule states that the winding should exhibit a minimum of four times the source impedance at the lowest frequency, it should be noted that if too many turns are wound on the core, troubles can arise at the high frequency end of our wideband transformer. It would be wrong to put too many turns on the primary and secondary windings.
In a wideband transformer it is best to wind the core by using a "multi-filar" type of winding. The bundle of wires can be wound on the core at one time by laying the wires "side-by-side" or by twisting the wires together. For example, a trifilar winding can be used to make a 200 Ohm Balun. The advantages are twofold: all wires are of equal length and the inter-winding coupling is improved. The disadvantage is that the turns ratio must always be a whole number (1, 2, 3, 4 etc), which means that the impedance ratio will be 1, 4, 9, 16 etc. Where you require a fractional ratio (1.5, 2.8 etc) you must use a "standard" winding technique, where the windings are placed separately on the core. This "standard" winding always produces a narrower bandwidth than the multi-filar technique.
Suppose that we want to design a wideband transformer for use between 2 MHz and 30 MHz and it has to match a 50 Ohm source to a 200 load. From the wideband rule the secondary winding should have an inductive reactance of not less than 800 Ohms at 2 MHz. Calculating the required inductance to get 800 Ohms at 2 MHz gives a result of 63.7 uH. Similarly the primary winding would need an inductive reactance of 200 ohms which works out to be 15.9 uH. Since these are minimum values let's round them off to 64 uH and 16 uH. Now let's work out the turns required for a ferrite core of suitable material for the frequency range. Assuming this a low power application (e.g. receiver or very low power interstage transformer in a transmitter) we will decide on a ferrite toroid of 0.5 inch diameter and with permeability of type 43 material, which is listed as suitable for 1-50MHz. The AL value for a FT-50-43 is given as 523.
AL is the core Inductance Factor of a core, and is expressed as nH / turn^2 (or mH/1000 turns):
L(nH) = N^2 * AL, where L = inductance in nH for N turns.
Note ^2 denotes to the power of 2 so doubling the turns produces four times the inductance.
Using this formula the turns required to give 64 uH will be 11. Since we require an impedance ratio of 4 to 1 in our example, the final transformer should have a turns ratio of 2 to 1. A toroid core cannot have partial turns so we cannot have a secondary of 11 turns and a primary with 5.5 turns. Our original rule stated that a wideband winding should have a minimum of four times the load impedance, so we are able to increase our calculated secondary turns to 12 and our primary to 6 turns, which preserves the 2 to 1 turns ratio which we require. Note that if we used the larger 2.4 inch toroid core FT-240-43 then the AL value is listed as 1160 or about double the AL value of the FT-50-43, consequently reducing the number of turns required with thicker wire. These magnetic toroid cores are available from https://www.amidoncorp.com
Temperature rise can be the result of using too small a size wire for the current involved as well as magnetic action within the core. If the operating temperature (ambient + temp rise) is greater than 60 degC when used intermittently, or more than 40 degC if used continuously, a larger size core and/or larger gauge wire should be selected. The following table is presented as a guide to RF power handling capabilities for wideband transformers operating in the 1.8-30 MHz range using impedance ratios not exceeding 4:1 and the maximum impedance not exceeding 300 ohms.
FT-50-( ) 5 - 10 Watts maximum with #22 gauge wire.
FT-82-( ) 50 - 75 Watts maximum with #20 gauge wire.
FT-114-( ) 100 - 150 Watts maximum with #18 gauge wire..
FT-140-( ) 300 - 400 Watts maximum with #16 gauge wire.
FT-240-( ) 1000 - 1500 Watts maximum with #14 gauge wire.
P.S. Can anyone tell me what radio frequency they use for telepathy ? It must be a low frequency using the ground wave because it propagates into the basement, railway tunnels and even into RF shielded Faraday enclosures.