Tape Antennas
When either substitute or invisible antennas are installed either at a fixed location or at a temporary location, often there are problems with the antennas shifting frequency after installation. As a rule, the antennas are placed in an environment containing RF absorbing objects. The resonant frequency, in contrast to what the theoretical calculations indicate will shift down in frequency and retuning the antenna becomes necessary. Wire substitute antennas are particularly affected by the closeness of RF absorbing materials as well as the presence of the operator. This causes the resonant frequency to shift away from the desired operating frequency which causes the antenna to not work satisfactorily. As a result of all of these destabilizing factors, installation of one type of substitute antenna in different locations will have problems with achieving proper resonance on the desired frequency of operation.
To reduce the influence of the previously mentioned detuning factors on the antenna and to improve operation, an alternative choice is to use broadband tape asymmetrical radiating elements.
An asymmetrical vertical antenna made from aluminum foil used for wrapping food was tested. On one side of the foil, scotch tape 10 cm (3.9 in) wide was pasted. The antenna was located on the wall of a room. The ground plane of the antenna was made by using a length of foil 5 meters (16 feet) long, which was installed along the along the bottom of the wall of a room. The diagram of this antenna is shown in Figure 1. This antenna was constructed for primary operation on the 21 MHz amateur band. The length (L) of the antenna was 3.5 meters (11.5 feet). The input impedance and resonant frequency of the antenna was measured with an HF bridge and determined to be Z = 38 ohms, Rf = 19.2 MHz. A secondary resonance was located at 26.4 MHz and the input Z = 350 ohms. To resonate at 21 MHz, the top of the antenna was rolled up as shown in Figure 2. When the length was 3.1 meters (10 feet) the resonant frequency was raised to 21.1 MHz and the input Z was = 39 ohms. The secondary resonance was increased to 28.1 MHz and the input Z was 350 ohms.
During my experiments of installing this antenna in rooms of different locations, on the wall of a room, around RF absorbing objects and in open space, its resonant frequency varied by an insignificant amount. This indicates that the tape antenna can be used in many different locations with different conditions surrounding the antenna with minimal affect on the antenna. It also means that when installed, there will be only a minimum amount of tuning required to resonate the antenna to the proper frequency.
For feeding the antenna on both ranges of 21/28 MHz, it is possible to use a twisted line with a characteristic impedance of 130-160 ohms electrical length on quarter wave for upper antenna range. For a feed line with an impedance of 130-160 ohm, a length of regular ac power cord with small diameter conductors was used. The impedance of the cord was determined by the following method: 1) a length of power cord not less than 1 meter was connected to the RLC meter and left open on the opposite end and the capacitance of the line is measured. 2) next, the open ends of the line are shorted and the inductance is measured. With the capacitance and inductance of the length of line determined, the impedance can be determined with the formula below, which is standard.
Where Z = line impedance in ohms Given the Inductance (L) of a line in Henries, and the capacitance (C) of the line in Farads.
The above will define the impedance of the line being used in place of a normal feed line accurately enough for amateur radio use. In this case this will result in a very good impedance match for the antenna being used on both 21 and 28 MHz and also to match the coax and output of the standard 50-75 ohm impedance of the transceiver.
As experienced in actual practice for operation on a range of 28 MHz, it was found best to use an antenna with a length of 3.1 meters. In this case, the strength of receiving and transmitting signals from a tape antenna with a length of 3.1 meters versus a tape antenna length of 1.9 meters increased the signal by 1-1.5 dB. For feeding the antenna in this case, it is possible to use a twisted line with characteristic impedance of 130-160 ohms electrical length on quarter wave for the 28 MHz range.
The tape antennas can be tuned to a higher frequency by rolling up the antenna tape material and shortening it to the proper length. If necessary to operate on a lower frequency below the lowest resonance, it is possible to do this by making a cut in the foil for the addition of a loading coil as shown in Figure 3 and Photo 1. In my case, the antenna shown in this figure had a resonant frequency of 18.1 MHz. The input impedance was measured with the help of an antenna bridge and was measured at 38 ohms. This comes nearer to the theoretical input impedance of a vertical asymmetrical quarter-wave shortened antenna. The number of turns on a coil n with a length of 60 cm (24 in) was 14 turns. The second resonant frequency was at 25.2 MHz and the input impedance was 350 ohms. This antenna worked well on the amateur bands at 18 MHz and at 25.5 MHz.
To feed this antenna on both bands it is possible to use a twisted line by characteristic impedance 160 Ohm and built as described previously. The electrical length of the line is equal on the 25.5 MHz band.
Bandwidth of the antenna shown in Figure 3 with the variation of input impedance of the antenna magnitude was not less than 1.2 MHz on the band where the antenna had a low input impedance. On the band where the input impedance was high, the bandwidth was not less than 1 MHz. For more exact tuning of an antenna in the upper ranges of operation, it is possible to make saw tooth cuts in the upper part of the antenna. These cuts are shown in Figure 4 and Photo 2. This will cause the resonant frequency of the antenna to increase. These saw tooth cuts allow the frequency coverage of the antenna to be set very precisely in the upper ranges. However this will cause a small decrease in bandwidth. The sawtooth cuts should only be made in the top element of the antenna. No change of this nature should be made in the lower part of the antenna.
In Table 1, the lengths of an antenna L and the resonant frequency of the antenna is exhibited for the antenna illustrated in Figure 3.
Parameters of antenna from Figure 1 | Parameters of antenna from Figure 3 |
|
---|---|---|
L = 3.5m | L = 3.1m | |
Fr = 19.2 MHz | Fr = 21.1 MHz | Fr = 18.1 MHz |
Za = 36 Ohm | Za = 36 Ohm | Za = 38 Ohm |
BW = 1.2 MHz | BW = 1.2 MHz | BW = 1.2 MHz |
Fr = 26.4 MHz | Fr = 28.1 MHz | Fr = 25.2 MHz |
ZA = 350 Ohm | ZA = 350 Ohm | ZA = 350 Ohm |
BW = 1 MHz | BW = 1 MHz | BW = 1 MHz |
The feed line of the tape antenna can be made from a symmetrical 2-wire line with an impedance of 130-160 ohms. The length of the line is equal to a quarter-wave length in the upper frequency range of the antenna. This is shown in Figure 5. The 2-wire line is connected to a coaxial cable of 50-ohm impedance and with an electrical length greater than 20-30 % of the resonant wavelength on the lower range of operation.
The tape antenna can be built with the radiating element at the end of the foil ground plane in addition to being located in the center. It is desirable that the distance from the radiator to the edges of the tape ground be not less than 1.5 meters (4.9 feet). If the room has a lot of RF absorbing material, the antenna, and its radiating field must be placed as far away from them as possible.
A directional antenna that will work on 21/28 MHz may be built of tape although it will exhibit rather weak directional characteristics. This antenna configuration is illustrated in Figure 6. In this case passive reflector elements for appropriate ranges in 21 and 28 MHz are made from the same tape foil that the radiator is made from and the whole antenna can be placed on the walls of a building or home. It should be understood, however, that the construction of a tape foil directional antenna will not result in a highly effective directional antenna due to the compromises that are necessary in the construction of this type of antenna. Some directional characteristics will be present which is helpful in reducing interference.
In Figure 7 the simplified version of a directional tape antenna is exhibited which is possible to use as a beam antenna for operation on 21/28 MHz and 18/25 MHz. In this case, the length of a grounded foil receives at a bit greater than 5 meters. When employing the beam version, the best placement is around the window or similar spot where the antenna will be subjected to the least amount of RF absorbing objects.
The experiments with tape antennas described have shown that the construction of invisible tape antennas capable of operating two amateur bands is possible while more than two may not be feasible without retuning. However, It is easy to tune such an antenna for operation on multiple bands by merely rolling up part of the radiating element. For the lower frequencies, a loading coil can be used to load up the tape element. By experimenting with this type of antenna from various familiar locations, it becomes easier to install the antenna in many other fixed locations as experience is gained with each installation. The antenna can be placed in a fixed unnoticeable location on the wall of a room in a hotel, behind cabinets or curtains. This technique is very convenient when traveling. Bear in mind, the use of this type of antenna does not allow for permanent outdoor installation, as the aluminum foil will deteriorate rapidly due to the weather.
The principles given here are presented to describe the construction of the tape antenna for use primarily on the lower bands. However, other higher frequency ranges, such as the VHF can also make use of this type of antenna.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 28th May 2024