In the October 2000 issue of BE Radio, this column discussed parasitics and touched on several aspects of parasitic reradiation that are of concern to radio engineers. I mentioned reradiation from power line towers, undesired parasitic reradiation from towers that were part of an array and briefly discussed desired reradiation from parasitic towers. This month, I will look at the theory of controlled parasitic reradiators and possible directional arrays using parasitic elements.
When a vertical conductor is placed in an RF field, a current will be developed in it. The magnitude depends on the length of the conductor. As the conductor length approaches the resonant length, the induced current increases. Because there is current flow, a voltage will be induced and small I2R losses will occur, which can cause heating in some cases. The remainder of the induced power is reradiated at a different phase from the original signal from the driven tower.
We already know the unfortunate, and sometimes unexpected, effects produced by cell and similar towers on operating directional antenna systems. However, we can turn this reradiation effect to our own use in some limited cases.
A simple parasitic array
The difference between a driven and a parasitic tower is that there is no connection between the transmitter and the antenna, and the only power that it radiates is that which is induced in it by the surrounding RF field. But if no critical constraints are placed on the system, and it is properly designed and adjusted, there appears to be no reason why such an array should not be used for limited purposes. If a simple pattern resembling a figure eight or a broad suppressed radiation direction is required, it is possible that a parasitic radiator will meet the coverage requirements. Unfortunately, no one has a licensed parasitic array in use in the US at this time.
When a single vertical radiator is fed with RF, it normally radiates more or less uniformly in a circular pattern, unless there is a reradiating conductor in its field. In the driven directional-antenna system one or more driven radiators combine vectors to produce the desired pattern.
In some instances, the driven element is used to draw power from the array and reinsert it into the total radiated power. These negative towers exhibit a negative resistance when measured in an operating array. But they exhibit normal positive resistance characteristics when measured as individual nonenergized towers.
In this discussion, we'll assume the use of a 90 degree base insulated tower because it simplifies calculations. If a similar, but grounded tower is placed about a quarter wavelength away in any direction, a radiation increase in that direction will occur. There will also be a decrease in radiation in various other directions. The pattern shape produced depends on several factors. With a grounded tower located as described above, patterns similar to those in Figure 2 will be produced.
If the shape requirements of the area to be covered are not too complicated, it is possible to produce a usable pattern to cover a desired area. Very deep nulls cannot be obtained, and the range of possible patterns is quite restricted. Nevertheless, for international broadcasting organizations operating in countries that do not have the high broadcast engineering standards imposed by the FCC, parasitic arrays could be very acceptable.
A typical installation can be derived by locating the parasitic tower on the side of the driven tower towards the desired covered area and properly matching the driven tower base impedance to the transmission line with a TEE or L network. Figure 2 shows the ground layout of such a station.
Such an antenna system would be quite economical. No expensive phasor is required, nor is a coaxial RF drive transmission line to the second tower, and maintenance requirements on ATU equipment are reduced.
In actual construction a few specific changes should be made. For instance, an insulated base tower slightly shorter than 90° would normally be used. In such an array design, limited pattern adjustment is easier if the height of the grounded parasitic tower, somewhere around 85 degrees, has approximately zero reactance at the carrier frequency. It should be noted that, although the parasitic tower is said to be ungrounded, it is actually grounded through a phasing reactor.
A simple tuned circuit at the base of the parasitic tower makes limited pattern adjustment easier and smoother. When the base operating impedance of the driven tower is determined, the pattern can be adjusted and simple pattern shape adjustments made.
Once the tower characteristics are known, the pattern can be calculated using vector addition and, within certain limits, can be adjusted as desired. It simplifies calculations if a driven tower base current of 1 amp is used with zero phase angle, and parasitic tower current will depend on self impedance and distance from the driver tower. Great care is required when planning and constructing a parasitic array because when an array is built it sometimes differs a little from the original theoretical plan. If a major mistake is made in construction or design, there is not much leeway for adjustment with a parasitic array.
Directional antenna systems
Most engineers have become accustomed to seeing phasors in directional antenna systems. An operating two tower array can be built without a phasor using only the phase differences produced by the varying transmission line lengths. I recall one such system operating at 1.4MW using the different line lengths and a tap down the transmitter output to obtain a simple semi-cardioid pattern. The administration operating the system did not require a proof such as the one required by the FCC. Sufficient field strength measurements at specified locations to prove coverage were all that was required to satisfy the client. Skywave radiation was not considered.
It becomes immediately apparent that there was no means of adjusting the pattern as operating values changed. But the original specification did not require any tight pattern control. However, it was blown up before such matters required attention.
The technique of controlling unwanted reradiation within antenna fields draws quite heavily upon parasitic radiator and coaxial line theory. A future column will deal with detuning parasitic radiators, and it will be seen that the electrical characteristics of open and shorted coaxial lines play a large part in such operations.
E-mail John at: email@example.com.