Magnetic Frame Antennas for VHF
The “magnetic frame antenna” for radio amateurs is used primarily in the upper amateur HF range of 6 – 20 meters and in the CB frequency range of 27 MHz. It is for use in places like those that exist under urban conditions where is not always possible to use a full-size or shortened antenna for such frequencies. Alas, it is often that we do not have not enough space and for the installation of a full-size VHF antenna! But, as experience has shown, the magnetic loop antenna can be used in the VHF range under such conditions
The Basic Configuration
As is known, the basic magnetic frame antenna configuration consists of one turn of a large conductor with a length of 0.1 wavelength on the frequency of use. The basic loop antenna is illustrated in Figure 1. Because of the small length of this type of loop antenna, it reacts primarily to the magnetic field component of the electromagnetic wave.
The directional diagram of the loop is shown in Figure 2. However, if the size of the loop is reduced even more, the directional pattern becomes more circular. The advantage of the magnetic loop over other antennas is that it is less sensitive to noise because of its characteristic reaction to the magnetic field of the electromagnetic wave. This classifies the loop as an anti-noise antenna since the atmospheric and industrial generated interferences are mainly electrostatic in nature, therefore the loop is far less sensitive to this type of noise. The magnetic loop allows reception in locations where the electrical component is absorbed, such as in concrete buildings and caves.
The single-turn un-tuned magnetic loop antenna (Figure 1) has very low input impedance and a very large reactive element which is why the un-tuned magnetic loop is not used very often. The basic loop as illustrated in Figure 3 is the version that is used most often. The tuned magnetic loop represents the circuit tuned to resonance on the desired frequency. The electric field is concentrated inside the capacitor. The magnetic field is generated around the single-turn loop which in this case is the antenna radiating system. The symmetry of construction and therefore symmetry of destabilizing factors tend to cause the interference to cancel out. The radio ground is not necessary for operation of a magnetic loop as is required for other types of antennas, vertical, dipoles, etc.
This lack of a grounding allows the loop to be used as a mobile antenna as well as in conditions where it is necessary to eliminate the grounding influence. The Q of the loop as well as the efficiency of the loop depends on the quality of the capacitor used, the quality of construction and method of matching to the loop. It is understandable that the Q of a loop cannot be constant, as there are many destabilizing factors, especially in the VHF range where the influence of any detuning factors are the greatest and most unpredictable. Therefore, in the VHF range, matching of the magnetic loop antenna becomes more difficult.
Matching Methods
There are two common methods of magnetic loop’s matching. One method is by using an RF amplifier with high impedance balanced input for the receiving magnetic loop antenna. For transmitting, matching is achieved through a coupling loop of one turn in close proximity to the loop antenna.
In my experiments, several loops were constructed for the 90-200 MHz range. The loops were etched on printed circuit boards with the perimeter of the loop at 0.1 wavelength. The etched antenna element width was 5mm (click for metrics conversion chart). Matching of the loop was accomplished by the use of link coupling with the link equal to 1/3rd of the perimeter of the loop.
Two coupling loops were tested. One was made by etching the loop in one corner of the printed circuit board forming the loop, with the loop width being equal to the width of the antenna as shown in Figure 4a.
The other coupling link was constructed out of a length of 1mm wire equal to 1/3 the length of the loop. This loop was located in the center of the loop antenna with the capability of moving the loop in order to improve the matching (Figure 4b). Testing showed that the input impedance of the loop varied with the location of the coupling loop and the pass-band also changed with the location of the coupling loop. Further testing showed that the pass-band of a loop on 100 MHz could be varied from 2 MHz to 10 MHz and within 5-20 MHz on 200 MHz with correct matching to 50-ohm cable. The printed circuit link coupling allowed a pass-band of 5 MHz on 100 MHz and 15 MHz on 200 MHz. The result of these tests proved that the loop can be used successfully for reception for FM VHF radio and that the loop antenna can be made to fit inside the receiver.
The loop antenna as described here also can be made to receive a single TV channel, particularly if a preamplifier is connected right to the coupling loop. A variable capacitor of 2-15pF enables tuning of the loop to 20 MHz on each side of a center frequency of 150 MHz, that is, up to 170 MHz and down to 130 MHz.
The use of a higher value capacitor will allow the tuning of the loop to a lower frequency range, but the Q factor is lowered, the pass-band becomes larger and the symmetry of a frequency characteristic is altered as demonstrated in the following frequency characteristic charts.
Care with Capacitors
It is emphasized here that the smaller size variable capacitors used in the VHF range work poorly. Switching of one capacitor by another of equal size and type usually causes a change of the loop antenna parameters due to the difference in the quality of the capacitors.
Certainly the most logical method for operation on a narrow bandwidth is by using two-sided printed circuit board material with the capacitor located on the opposite side from the loop side of the board (Figure 5). At the loop gap, located on the end of the loop, there are two 10mm squares (Figure 5 side a). On the side of the board directly opposite the gap, there is another strip of copper. This strip makes up the other part of the capacitor.
A portion of the capacitor on the back side (Figure 5 side b) of the board is made up of small 4x4mm squares, which at tuning to resonance, are then soldered to each other. When the point of resonance is reached, then cut off corners of the squares for fine tuning.
Transmission Tests
In spite of the difficulty of operating this type of antenna in the VHF range, especially with its small size and matching complexities, the antenna was then tested as a transmitting device. In the open areas, performance was equivalent to a 1/8-wavelength vertical whip. Using the loop inside a concrete and steel building, the performance was slightly worse than a 1/4-wavelength vertical whip.
During A/B tests of the whip and loop antennas inside a building, an immediate advantage of the loop over the whip was noticed. When using the whip some effort was required to find an optimum point of operation in the building. In both cases, the loop antenna and vertical whip were attached directly to a transmitter connector without use of any feedline. The tests were done on the amateur two-meter band.
With the whip antenna there was a high level of interference from the various equipment located inside the building such as computers and air conditioners. Using the loop, the optimum location for reception was much more easily found because receiving interference was lower due to the characteristics of the magnetic loop.
However, I cannot strongly recommend this antenna system for use as a transmitting device in the VHF range. Because of the radiation from the strong magnetic field generated by the loop, it will create internal feedback and oscillation in the transmitter stages. This is true when the loop is mounted directly on the transmitter output connector. With a cable connection to the transmitter, other types of antennas would be best. Matching a loop antenna is very difficult when using either a cable or direct transmitter connection.
Only when the transmitter is in an environment of high RF absorption, such as concrete and steel buildings and buildings made of other high conductance materials should the VHF loop be used. Also in an environment of high interference caused by other equipment, it would be preferable to use the magnetic loop antenna. Also, the directional characteristics of the loop would allow the directing of transmission from a concrete and steel building.
When operating the loop as a transmitting antenna it is necessary to give close attention to matching the antenna to the receiver and transmitter antenna connections. A receiver input and the transmitter output should have an identical input and output impedance. If they are not, when switching from transmission to reception, the received signal will be reduced significantly. Further experiments with the magnetic loop in the VHF range indicate that by using a length of coaxial cable of at least 10-15 cm, matching could be easily done for both receive and transmitting. (See Photos at top and left) In this case, the antenna is tuned by gradual shortening of the cable and by the selection of the correct size of the coupling loop.
When tuning up a loop as a transmitting antenna, it is best to use a field strength meter. It is possible to get excellent results in this manner. The field strength meter should be no closer than 4 meters to the loop antenna. Using an external VSWR meter will not allow tuning for optimal output if it involves the use of an added length of short cable for connection. When a longer length of cable is used, the VSWR meter can be used on transmitter side for tuning the magnetic loops on 2 meters.
It is Better to Receive
It is a better idea to use this type of an antenna with simple, sensitive receivers on VHF such as one assembled on one chip with a low intermediate frequency or in a super-regenerative receiver. The antenna, due to its frequency selective characteristics will allow avoidance of interference from nearby VHF/UHF ranges where there are powerful radio or TV transmitters. With the directional capabilities of the loop antenna, one can select the best receiving direction. Even with the nearness of the receiver and power supply board will change the characteristics of the antenna somewhat, but it will still be a very useable receive antenna.
TV Too!
The magnetic loop can be used for TV reception in areas with strong reflected “ghost” signals. Due to the small dimensions of the magnetic loop antenna it allows one to locate a spot where TV reception can be optimized. For the large antennas that are sensitive to the electrical component of the electromagnetic wave, they are more complicated and of course take up more space. Thus, in addition to its application for radios, using a tunable preamplifier, the magnetic loop may be used for TV reception and enable channel selection, if desired.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 26th May 2024