RF isolation, cheap and easy Isolation may be best for eliminating AM broadcast signals and induced electrical hum in some communications base station and microwave facilities and where various users of communications equipment share tower space.
There are times when the signal from the local broadcast station gets into your station equipment. You don’t want it for reasons other than not liking the program content. Your equipment may be in an electrical substation where the power line magnetic field renders your lower frequencies unusable. If so, one or more of the dc/RF isolators shown in Photo 1 below may be for you.
Local broadcast stations use an amplitude-modulated (AM) transmitter connected to a vertical antenna with a minimum of 120 buried radials for a ground plane. The radiated field has a strong ground wave extending as far as several miles from the antenna. If your tower is within the influence of a broadcast tower or electrical substation, you should expect some of this energy to appear in the ground system of your station. Communications towers at a height nearly equal to the quarter wavelength of the AM station frequency not only influence the radiation pattern of the broadcast station, but also provide considerable energy in the ground return of the tower.
Consider the example shown in Figure 1 below right. A tower grounded at its base without other connections will not bother anyone other than to produce pattern disturbance. However, a closed current loop is provided when conductive appendages such as microwave dishes, VHF, UHF and 800MHz/900MHz antennas are attached. Cables leaving the tower at some elevation usually are attached to the electronic equipment, thus providing another current path separate from the tower ground. Often the shunt current is of sufficient magnitude that it interferes with the station ground.
A large percentage of communications facilities use shielded cable grounded at both ends or coax cable to interconnect voice and data circuits to a radio or multiplex channels. This cable often carries current derived from the AM broadcast station’s signal–current that will modulate your signal and that represents a form of interference.
Multipoint grounding, or the “ground everything” concept, will not eliminate this problem, it will merely reduce it to some lesser level. One of the simplest methods of interference reduction is to isolate the affected equipment from these ground currents. Research at the Washington State Patrol has resulted in the development of dc isolators. Figure 2 above is a schematic diagram of the 150MHz-170MHz dc/RF isolator that consists of two tightly coupled tuned circuits. To minimize insertion loss, coax cable is used instead of wound coils. Wound coils are cumbersome, and with wound coils, the necessary high voltages cannot be tolerated and low insertion loss cannot be achieved. The cable length shown in Figure 2 is optimum for both maximum bandwidth and minimum insertion loss. The cable’s electrical length is shortened by adding capacitance for a low “Q” quarter wavelength. (See also Photo 2 on page 32.)
Note that the polarity of the input and output connections does not follow conventional thinking about common ground. Air-dielectric variable capacitors are used not only for resonating the cable but also for handling the RF current from 110W base stations. APC-type variable capacitors normally have the rotor contactor isolated from the mounting screws, making them ideal for the isolated port, but the other capacitor needs to have its contactor grounded. Fixed capacitors may be used if their RF current capacities are adequate. A plot of attenuation vs. frequency yields a straight line from well below 100MHz to well above 200MHz, so it is unnecessary to show it.
The 150MHz, 450MHz and 750MHz-1,000MHz isolators are built in Bud CU142 C cast aluminum boxes with covers. Connectors are “user’s choice,” although type N fittings are preferred. An essential requirement of construction is that the insulated port connector must be completely insulated from the aluminum box by an insulating sheet with nylon screws for fasteners. The insulator used is 0.020″ thick fiberglass cut to fit over the ring of the type N chassis connector. A more suitable material can be used for the insulator when necessary. The hole for the connector should be 5/8″ in diameter to clear the ring. Drilling and tapping the aluminum box for 6-32 screws is the recommended method of attachment for the coax connectors.
Dc/RF isolators for the 450MHz band can be built with either a 3-1/2″ coax line (vp = 0.66) with 10pF capacitors similar to the 150MHz design or parallel lines in a box as shown in Figure 3 on page 34 and Photo 3 on page 34. The parallel transmission line design has insufficient capacitance provided by the insulated chassis connector to the box. To overcome this deficiency, a transmission line of an electrical halfwave is added to transfer the isolated shield RF current to ground, which accounts for the third conductor seen in the photograph. For applications subject to vibration, a stand-off insulator may be added near the ends.
Transmitter power exceeding 100W should use this design. Lower power levels can safely incorporate the tuned coax style. Tuning of the coupled lines is outlined below. The insertion loss of either design is 0.05dB with a bandwidth of 200MHz.
The 750MHz-1,000MHz (Figure 4 and Photo 4 above) and 1,850MHz-2,300MHz (Figure 5 and Photo 5 on page 38) isolators rely on a fixed capacitor, as formed by the surface area of the coax connector, insulator and box, for the shield current. This fabricated capacitor is sufficient at these frequencies to produce a low impedance path for the shield current. Additional capacity may be added if the situation so dictates.
The 1,850MHz-2,300MHz device is constructed in a Pomona No. 2417 cast aluminum box. To use this box, all of the blue paint must be removed from around the coax connectors and from the lid grooves. The design of these isolators requires that the coupled lines be a quarter wavelength long inside the box. The primary factors in dimensions are probe length (which determines the frequency) and spacing (which determines bandwidth and insertion loss). If the coupled lines prove to be too short, add some solder to the end. All measurements must be made with the cover in place.
Adjustments are best accomplished using a tracking generator and spectrum analyzer. The length of the coupled lines and their spacing is extremely critical to bandwidth and linearity. Overcoupling (wires too close together) increases insertion loss at the middle frequency range. Insufficient coupling not only reduces bandwidth but also increases insertion loss. If narrower bandwidth is desired, tune for minimum insertion loss at the desired frequency. In all cases, ensure that the termination impedance is correct for adjustments. It is best practice to insert a 6dB-to-10dB attenuator at each input and output connector during measurements and adjustments.
For a 6GHz or higher frequency microwave antenna connection, the following method has proved successful. Isolation may be realized at the pressure window by using the electrical insulation provided by the mylar glued to the window face plate. Look at the pressure window and determine whether the mylar is installed on one side. If so, it may be used. Re-install with nylon screws for fasteners.
If the pressure window has a clear piece sandwiched between plated disks, remove the window for the following treatment. Place a single layer of heavy-duty polypropylene carton sealing tape, Permacel No. P-919 or the equivalent, on one surface. Carefully cut the tape over the mounting holes so that the nylon screws will not fold over the tape on the face plate as shown in Photo 6 above. Install with the tape facing the choke flange and nylon screw fasteners. It has been determined that this tape does not contribute significant additional loss. Check the installation for continuity between waveguide connections. (There should not be any.)
The object of this procedure is to dc/RF-isolate all transmission lines from the equipment by not allowing current supplied by the tower to return to ground through the baseband cable, audio lines, data ports and the like. Although some lightning protection may be realized by this method, it is not the intended result.
Here is an example of how successful this method can be. A 5kw, 1,280kHz AM broadcast station one-half mile from a large microwave installation with four 6GHz microwave antennas and three VHF antennas on a 200-foot tower affected 32 digital data ports and all microwave basebands, sending the 1,280kHz AM signal at 15dB below test tone in all directions. The level increased dramatically when test cables were attached to the baseband for measurement. By treating all antenna cables with dc/RF isolators as described and by removing a common ground wire located at the bottom of the racks, tower ground current was removed from the equipment, thus reducing the AM signal by 69dB at the communications equipment.
Here is a second example. Two county agencies shared the same power supply. Each had 2GHz microwave radios with multiplex in separate rows of equipment and interconnecting baseband cables. The ground loop consisted of the transmission line of radio No. 1 going up the tower leg with coax connected to the antenna, providing a ground at that point. The other county agency was on the opposite side of the tower with its transmission line going from radio No. 2 to antenna No. 2. The silicone rectifier (SCR)-regulated battery charger in the middle of the loop caused a magnetic field to induce 120Hz interference plus all of the harmonics associated with SCR switching directly into the cabling. Installing 2GHz isolators in the transmission lines close to the radios eliminated the problem.
You may recall a similar situation in your own systems. There may be installations where communications equipment of various users share tower space. Isolation may be the only way sharing is possible.
Several of these devices may be kept on hand for troubleshooting. It is much easier to isolate an antenna transmission line than it is to treat 672 voice-grade lines by process of elimination.