100 metre vertical loop + wideband 800 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. The small loop 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 for the harmonic frequencies the antenna pattern is more complicated with sidelobes as well. 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 a vertical 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. An archer’s 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.
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.
HF antennas do not have to be resonant, if energy efficiency and getting the most out of available transmitter power is not the highest priority. When there is not enough space to construct a resonant half-wave dipole or grounded quarter-wave monopole (on a ship for example) then traveling wave antennas can be used instead that are wideband and more compact than standing wave antennas, although sacrificing the gain of standing wave antennas. The folded dipole has a radiation resistance at the resonant wavelength of 300 Ohms and has a broader bandwidth than the half-wave dipole. The tips of the dipole are folded back until they join opposite the feedpoint so that the total length is double that of the half-wave dipole and it forms a loop antenna with perimeter equal to one wavelength. The wires are parallel and the current must be divided between the two wires. Therefore the current in each is half but the total power has not changed so that the feed point resistance is quadrupled or 4 * 73 Ohms = 292 Ohms. If a terminating 300 Ohm resistor is used in the middle of a folded dipole, opposite the feed line, then it becomes a terminated folded dipole that operates as a wideband traveling wave antenna with low SWR across the HF spectrum, although the gain is much reduced compared to a resonant dipole. The terminating resistor absorbs an increasing portion of the RF power (either captured from the air or supplied by a transmitter) as the operating frequency nears the lower limit of the design range. So the terminated loop antenna is less efficient at the lower frequencies with reduced gain.
The T2FD (Tilted Terminated Folded Dipole) was developed by the US Navy in the late 1940s for use onboard ships as a compact wideband HF antenna that does not need a complex tuning device. The antenna is tilted to make the gain pattern roughly omni-directional, with the folded dipole sloping down at an angle of 20 to 40 degrees from horizontal. A typical T2FD is constructed with a width equal to about 1/3 of the lowest required wavelength. The exact length does not matter since it is a traveling wave antenna rather than a resonant standing wave antenna. To maintain an antenna pattern like that of a horizontal dipole rather than a loop antenna, the distance between the upper and lower conductors should be equal to 1/100 of the wavelength and is maintained by a number of insulating dowels. It is terminated in the middle of the upper conductor with a 300 Ohm non-inductive resistor, rated to safely absorb at least 1/3 of the applied transmitter RF power. The antenna feed line is in the middle of the lower conductor with an impedance of about 300 Ohms, using a standard 4:1 impedance transformer or 200 Ohm Balun. Note that an Off Centre Feed (OCF) increases the impedance and SWR of an antenna, so there should be equal lengths of wire between the feed and the terminating resistor to keep the antenna symmetrical. Otherwise the SWR increases so a balanced antenna should be symmetrical and if fed with an unbalanced line like coaxial cable, then an RF choke is sometimes required to prevent feeder line radiation from parallel currents in the outer shield of the coaxial cable and avoid RF feedback, RF burns and reduced antenna gain. The RF choke is also called a 1:1 Balun and can be easily made by wrapping at least 10 turns of coaxial cable around an iron powder, ferrite core or even a plastic pipe that is 10 cm in diameter for an air core RF choke. Place the choke Balun at the location where the unbalanced coaxial cable connects to the antenna feed line and away from any conductive material. 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.
A vertical loop antenna of perimeter length 100 metres that is terminated with a 800 Ohm non-inductive resistor opposite the feed point, and matched to the coaxial cable with a wideband 800 Ohm Balun, provides low SWR across the whole HF band. The 800 Ohm Balun is a 4:1 voltage (16:1 impedance) transformer with four times as many secondary turns as primary turns. See "How to make a wideband 800 Ohm Balun" to calculate exactly the number of turns required, given the type of magnetic toroid core. This antenna has a gain pattern exceeding that of the isotropic radiator for the higher frequencies (> 10 MHz) but with decreasing gain for the lower frequencies (< 10 MHz). The SWR is low (<2:1) over most of the HF band so it does not need an antenna tuner . Furthermore, the higher frequencies (> 10 MHz) have some gain at very low elevation angles (< 10 degrees) suitable for long distance communications, even when constructed close to the ground and it does not have to be suspended half a wavelength above the ground.
How to make a wideband 800 Ohm Balun:
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 and a quintufilar winding can be used to make a 800 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. Enameled copper wire (available from electronics shops) is normally used for winding Baluns.
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 800 Ohm load. From the wideband rule, the secondary winding should have an inductive reactance of not less than 3200 Ohms at 2 MHz. Calculating the required inductance to get 3200 Ohms at 2 MHz gives a result of 254.6uH. 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 255 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 medium power application (e.g. for Ham radio) we will decide on a ferrite toroid of 2.4 inch diameter and with permeability of type 43 material, which is listed as suitable for 1-50MHz. The AL value for a FT-240-43 is given as 1160.
AL is the core Inductance Factor of a core, and is expressed as nH / turn^2 (or mH/1000 turns):
1000 * L(uH) = N^2 * AL, where L = inductance in uH 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 255 uH will be 14.8. Since we require an impedance ratio of 16 to 1 in our example, the final transformer should have a turns ratio of 4 to 1. A toroid core cannot have partial turns so we cannot have a secondary of 14.8 turns and a primary with 3.7 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 16 and our primary to 4 turns, which preserves the 4 to 1 turns ratio which we require. Note that if we used the smaller low power 0.5 inch toroid core FT-50-43 then the AL value is listed as 523 or about half the AL value of the FT-240-43, consequently increasing the number of turns required with thinner 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 16:1 and the maximum impedance not exceeding 800 ohms.
FT-50-( ) 5 Watts maximum with #22 gauge wire.
FT-82-( ) 50 Watts maximum with #20 gauge wire.
FT-114-( ) 100 Watts maximum with #18 gauge wire.
FT-140-( ) 300 Watts maximum with #16 gauge wire.
FT-240-( ) 1000 Watts maximum with #14 gauge wire.
VK2STH (formerly VK2FADZ)
2nd September 2009