Point to Point Telecom, LLC
Specializing in Engineering of Radio Frequency Systems & Equipment
Antenna System Considerations
General considerations - In any radio communications system, the antenna, feedline, jumpers and connectors [Antenna System] are the most important elements. Mainly, because the antenna system can deteriorate over time, with little notice until the system only operates marginally. This is particularly true with the master station, as it impacts all remote station communications. This is why we recommend using only the very best antenna and Heliax cable at the master station. Generally, the health of the remote stations can be monitored by the master by reviewing & maintaining records of the Receiver Signal Strength Indicator [RSSI] levels, and any changes over time. Also, some indication of the master can also be noticed by reviewing RSSI trends that are shared with ALL remote stations. Fore example, if over a year 6 of 22 stations show a decline in RSSI, then it is likely the remotes are at fault. If all 22 stations show an almost identical decline in RSSI, then I would check the master antenna system.
Typical Problem - You would not believe this unless you witness it yourself, but two drops of water inside a coax connector is all that is necessary to partially or completely disable a radio link at any frequency, but particularly at 900 MHz. or higher. The most common problems with station antennas are moisture intrusion, excessive feedline loss, shorted/open connections at a coax connector, and broken connectors or bad jumpers, in that order.
1. Moisture Intrusion
can occur at the antenna, through the feedline/cable jacket, or at a
connector. Inexpensive or poorly designed antennas will allow
outside air to enter the interior of the antenna, and with the thermal
shift of night to day and day to night the moisture in the air
condensates, and accumulates. If the connector is on the bottom of
the antenna, the moisture will accumulate and eventually enter the
connector through the center pin opening, causing partial or complete
system failure. The most common problem area for moisture is
within the outdoor connectors, at the base of the antenna, or
where the flexible jumper meets the Heliax cable. This issue is
so important, we have dedicated an entire page [HERE]
describing the different methods commonly used to weatherproof the
connectors. Just remember, you need to weather seal and provide
thermal resistance, so the connector cannot form condensation inside.
All it takes is 1 to 3 drops of water to take your system from full
performance to just barely working.
2. Excessive Feedline Loss occurs mostly with coaxial cables with a braided shield under the outside plastic jacket, like RG-8, LMR400, unlike the solid copper shield used with Heliax type cables. What typically occurs over time is a small amount of outside air enters the space between the outside jacket and the center insulator, where the wire braid resides. This can occur through the jacket, or at either end. This air [with moisture] will over time corrode the individual strands of wire causing them to become insulated and lose conductivity with the other strands of wire. Even cable like LMR400 with an aluminum foil under the wire shield is no better as the aluminum is either coated or corrodes at a faster rate than the wires. In addition, the center insulator is often times a low-loss foam, which will absorb moisture and any mineral in the moisture. This contamination over time will cause changes in loss and impedance of the cable, which adds to the overall cable loss. Manufacturers of braided cable suggest the useful life is only 5 to 8 years, depending upon weather and exposure. It is clear to see why Heliax cable is superior, as it uses a solid copper shield and many installation have lasted more than 40 years. The cable TV industry has used coaxial cable with a solid aluminum shield like Heliax for 50 years with great success. However, in the land mobile industry, aluminum shielded cable has be largely rejected due to the soft aluminum causing connectors to become loose or short, and cause an intermittent connection.
3. Shorted/Open Connectors usually occur from improper assembly or bending the cable too close to the connector. Even Heliax cables have issues with connectors failing. It is not unusual for a Heliax connector to short if the cable is bent too close to the connector body. The Heliax cable MUST REMAIN STRAIGHT for at least 6 inches, measured from the back of the connector. This is why it is not recommended to connect the Heliax directly to the antenna, but use a 16" flexible jumper to connect the two. Tower climbers and installers may inadvertently shift or bend a Heliax cable, while moving about on a tower. This is enough to short a Heliax connector attached directly to an antenna. The reason this occurs is the center conductor is stiffer than the outside copper shield, so when the jacket is bent close to the connector, the center conductor bends the center pin assembly sideways and it shorts against the inside shell.
Coaxial cables with stranded shields should use high quality crimp connectors purchased directly from master distributors like Tessco, Talley Communications, and Mouser. We prefer the SILVER-PLATED connectors made by RF Industries.
4. Broken Connectors or Bad Jumpers - Avoid broken connectors by using short [12" to 24"] flexible jumper cables between your MAS radio and your feedline. We like using the smaller RG-55, RG-142, OR RG-58 coax cable for indoor use, in that order. We suggest using crimp connectors always, for higher reliability. Otherwise, if you connect your LMR400 [1/2"] cable directly to the radio, you risk breaking off the connector on the radio. At the antenna or other outdoor use, we suggest using RG-214 for short jumpers. This is a higher quality cable than LMR400, and has significantly more attenuation, but for short lengths is far more rugged and reliable. Keep a few spare indoor jumpers around for testing, should you suspect a problem with a cable.
2. Excessive Feedline Loss occurs mostly with coaxial cables with a braided shield under the outside plastic jacket, like RG-8, LMR400, unlike the solid copper shield used with Heliax type cables. What typically occurs over time is a small amount of outside air enters the space between the outside jacket and the center insulator, where the wire braid resides. This can occur through the jacket, or at either end. This air [with moisture] will over time corrode the individual strands of wire causing them to become insulated and lose conductivity with the other strands of wire. Even cable like LMR400 with an aluminum foil under the wire shield is no better as the aluminum is either coated or corrodes at a faster rate than the wires. In addition, the center insulator is often times a low-loss foam, which will absorb moisture and any mineral in the moisture. This contamination over time will cause changes in loss and impedance of the cable, which adds to the overall cable loss. Manufacturers of braided cable suggest the useful life is only 5 to 8 years, depending upon weather and exposure. It is clear to see why Heliax cable is superior, as it uses a solid copper shield and many installation have lasted more than 40 years. The cable TV industry has used coaxial cable with a solid aluminum shield like Heliax for 50 years with great success. However, in the land mobile industry, aluminum shielded cable has be largely rejected due to the soft aluminum causing connectors to become loose or short, and cause an intermittent connection.
3. Shorted/Open Connectors usually occur from improper assembly or bending the cable too close to the connector. Even Heliax cables have issues with connectors failing. It is not unusual for a Heliax connector to short if the cable is bent too close to the connector body. The Heliax cable MUST REMAIN STRAIGHT for at least 6 inches, measured from the back of the connector. This is why it is not recommended to connect the Heliax directly to the antenna, but use a 16" flexible jumper to connect the two. Tower climbers and installers may inadvertently shift or bend a Heliax cable, while moving about on a tower. This is enough to short a Heliax connector attached directly to an antenna. The reason this occurs is the center conductor is stiffer than the outside copper shield, so when the jacket is bent close to the connector, the center conductor bends the center pin assembly sideways and it shorts against the inside shell.
Coaxial cables with stranded shields should use high quality crimp connectors purchased directly from master distributors like Tessco, Talley Communications, and Mouser. We prefer the SILVER-PLATED connectors made by RF Industries.
4. Broken Connectors or Bad Jumpers - Avoid broken connectors by using short [12" to 24"] flexible jumper cables between your MAS radio and your feedline. We like using the smaller RG-55, RG-142, OR RG-58 coax cable for indoor use, in that order. We suggest using crimp connectors always, for higher reliability. Otherwise, if you connect your LMR400 [1/2"] cable directly to the radio, you risk breaking off the connector on the radio. At the antenna or other outdoor use, we suggest using RG-214 for short jumpers. This is a higher quality cable than LMR400, and has significantly more attenuation, but for short lengths is far more rugged and reliable. Keep a few spare indoor jumpers around for testing, should you suspect a problem with a cable.
From the Start - Following is a simple outline to follow when you first decide to design a MAS system.
1. The operating radius must be determined from the master station. If all of your remote stations are in one general direction from the master, then a bi-directional antenna should be selected, to concentrate the signal in that direction and to reject possible interference from the undesired direction. However, most master stations require a "Omni-directional' antenna, as remotes are scattered in all directions.
2. If the master is on a hill or a mountain, and all remote stations are in the area below, then an antenna with "Down-Tilt" or sometimes called "Null-Fill" may be required. Generally, this is not an issue unless the master station antenna is looking down more than 3 degrees to the majority of the remote stations. If this is the case, a Radio Frequency consultant should study the issue and suggest the best antenna for your unique application. Otherwise, the standard antenna that concentrates all the signal at the horizon is the best choice.
3. The height of the master antenna should be determined. If the master station is to operate in an area with trees, then the base of the antenna needs to be installed at least 20 feet above all trees, as any foliage will absorb both incoming and outgoing signal. Typically, the antenna would be mounted on a galvanized steel tower about 12 inches wide on each side, at the proper height. If the master station location is in an area sensitive to the appearance of a small tower, then the use of a monopole [like a flagpole] might be a better selection.
4. The antenna gain should be determined. If all of your remote stations are within 5 miles and the terrain is reasonably flat, the I would suggest a 3 or 4 dB gain antenna, only about 4 ft high. However, if you have even just a few remotes beyond the 5 mile radius, or one or more remotes in difficult terrain, then I would suggest a 10 dB gain antenna. This antenna will be 14 to 16 ft high, but will increase all inbound and outbound signal be 6 dB, or a factor of 4 to 1 improvement. The cost for the larger antenna can be 4 to 8 times greater that the smaller antenna.
5. The final consideration will be the size and makeup of the coaxial cable, often called the feedline. Heliax type cable is always the best choice, but it is not very flexible, and short flexible jumper cables at each end is highly recommended. For short runs [under 50 ft] of cable at remote sites, flexible braided shield cables such as LMR400 is a good choice. It will not last as long as Heliax, but is far easier to install and the need for a jumper at the antenna is eliminated.
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More to come!
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