Licensed RF Antenna System - A beginner's troubleshooting guide.
Updated: Feb 26, 2022
This blog is for anyone interacting with remote water or wastewater antenna systems. My goal of this guide is to connect the dots and remove the mystery to these systems. Help you to identify common failures which create common communication outages.
Written by: Phillip A. Yancey - BCI Technologies Sales and Support Manager.
BCI is a GE MDS Channel Partner for FL, GA, AL, MS, NC, SC, TN
Review of antenna system hardware.
Yagi Antenna (#1) - A directional antenna made of several short rods mounted across an insulating support used in transmitting or receiving a narrow band of frequencies.
There are multiple styles of directional antennas which can vary depending on frequency.
Some low band frequency antennas require tuning prior to installation.
Antenna Analyzer is required for best-case tuning.
Some Yagi's may be turned vertical or horizontally to change EIRP.
Weather Proofing (#2) - Sealing exposed coax connectors to prevent moisture and dust and adding UV protection.
Cover coax connector with a 15" strip of tightly applied rubber splicing tape.
Wrap around connectors starting 1' before and after connectors.
Overlap previous wrap by 50% to ensure good seal.
Cover splicing tape with PVC electrical tape for UV and additional protective jacking.
Butterfly Hangers (#3) - A common way to mount coax is by using a standard butterfly hanger with grommet kit.
There are universal mounts.
Branded mounts for specific tower also exist.
These support the weight of the coax and can also help with grounding.
A grommet is used to cover and protects the coax from the hanger.
Coax Feedlines (#4) - The feedline is also called the transmission line and is the conduit between radio and antenna.
Common coax sizes include 1/2" (LMR-600), 7/8"(LMR-900), and 1-5/8" (LMR-1200)
LMR-XXX is a Times Microwave Systems brand similar to TWS-XXX a Terawave brand coax.
Coax sizes should be determined by length and frequency which impacts the attenuation loss.
Common connectors for Feedlines are N-Type or DIN Type.
When buying a remote feedline consider buying a pre-made, pre-terminated cable to simplify replacements.
Coax has bend radius restrictions which are pointed out in red circles. The top circle shows two bends one acceptable and the other not.
Bends and breaks will show up on analyzers with Time Domain Reflectometer (TDR) tests.
These increase losses, reflection quirks and impedance mismatches which translate to higher VSWR.
Feedline Grounding (#5) - Feedline grounding is when we usually clamp to the outer copper conductor on the feedline directly.
These grounds should be run directly into the outside earth ground and not panel ground.
More common on longer feedlines or AP feedlines.
Jumper Cable (#6) - Used to connect Radio to DC block/Feedline.
Consider using Ultra-Flex (UF) coax which provide a greater bend radius. Common sizes are LMR-240/ UF or LMR-400 / UF.
Consider Right-angle connectors for tight spots and female adaptors for direct feedline connections or existing jumper extension.
Bulkhead connectors are also available for specific applications where you might want to mount into a panel wall.
The size of jumper coax is not the same as a feedline since it would be difficult and bulky as shown in the photo above. Here you can see the feedline being brought through the cabinet and terminating inside the enclosure.
This has made it very difficult to work in the panel along with putting the point of lighting block inside the cabinet.
When testing with a Bird meter or Analyzer always start from the jumper to rule out individual components.
DC Block Lighting Arrestor (#7) - Found between the Feedline and Jumper. This is common for remote coax grounding and blocks energy from passing into the Radio device.
Intended for direct earth ground.
Commonly installed inside or though panel.
Most RF systems operate on a 50-ohm load. When an improper "Polyphaser" or DC Block is used, such as a 75 ohm, we have an impedance mismatch.
Equipment and Antenna is labeled and since protection is directional this is important.
Top connector is bulkhead style and can be mounted though panel so that feedline connects outside panel not inside.
Most common connector type is N Female (N/FM) for polyphasers but is not limited to only this.
There is no easy way to test a DC Block the simplest method is using an antenna analyzer with the DC block in inline or, the swap with good method.
Common Connector Types (#8) - This is a simple list of common connectors you're going to find in the field. Don't worry if you've mis-matched an adaptor there are converters available but remember every connection adds loss.
N Male connectors will be most common on feedlines and to the polyphaser side of a jumper. Alternatively, the Polyphaser side will be N Female.
TNC Male connector is most common for radio side of jumper. Alternatively, the equipment side will be TNC Female.
SMA Connectors will be most common on Cellular and Wi-Fi devices.
A plug style connector refers to a Male connector and a Jack style connector refers to a Female connector.
Determining antenna system performance.
What is VSWR?
Voltage standing wave ratio (VSWR) refers to the voltage standing wave ratio. VSWR is the ratio between the maximum voltage and the minimum voltage on a line (antenna) and is an expression of Return loss.
What is the ideal value of VSWR?
1:1 is the ideal VSWR value and indicates a reflected power is 0. This signifies that there is no power wastage or %99 perfect power through power. VSWR is commonly known as SWR with the primary difference being a ratio of voltage.
1 = 1 0% reflected power.
3 = 25% reflected power.
6 = 51% reflected power.
Why is SWR/VSWR important?
A high SWR indicates poor transmission-line efficiency and reflected energy. This can in turn damage the Radio transmitter and decrease its efficiency.
An Antenna Analyzer is the faster and easier way to test VSWR.
A bird 43 can also be used to test VSWR when using the conversion chart provided in manual.
Useful in antenna testing and tuning.
When testing VSWR we use a general pass or fail rule:
less than < 2.5 is a General Pass. This means that 18.4% power or -7.36 dB is being reflected back.
more than > 2.5 is a General Fail. This means that 25% power or -6 dB is being reflected back, or worse.
The lower number the better.
What is Return Loss (RL)?
Return loss is the difference in dB between the forward and reflected power. (Forward power - reflected power = Return loss). Return loss is related to SWR and increasing RL corresponds to a lower SWR.
What is the ideal value of Return loss (RL)?
It may seem a little backwards, but the higher the loss the more energy into the antenna system. So, the higher the RL value the better! A low RL indicates less energy going into our antenna which of course is bad.
0 dB = 100% energy reflected by antenna system
3 dB = 50% energy reflected by antenna system
10 dB = 10% energy reflected by antenna system
14 dB = 5% energy reflected by antenna system
20 dB = 1% energy reflected by antenna system
30 dB = 0.1% energy reflected by antenna system
Why is Return loss (RL) important?
A good return loss indicates how well our devices and lines are match. When this isn't true it tells us that discontinuities or impedance mismatches exist in the antenna system. A discontinuity may be caused by a bent or kinked cable or improper terminations or corroded jacks and plugs within coaxal lines. Impedance mismatch are usually component-to-component related including incorrect connectors, DC block specifications, and coaxal types.
A bird 43 is a simple way to identify return loss and Reflected power.
Most telemetry systems are 50-ohm systems and all components must match.
All connectors should be properly sealed and weatherproofed to prevent water intrusion and corrosion.
When testing Return loss (RL) we use a general pass or fail rule:
More than > 14 dB is a General Pass. This means that 3.98% energy is reflected by antenna system.
Less than < 13 is a General Fail. This means that 5% energy is being reflected by antenna system.
The higher number the better.
Testers and pass or fail testing.
Below we will cover two devices used by BCI and covering a basic pass-fail scale in device and antenna system testing. These devices average $600-900 and have trade off in simplicity and complexity.
Brid 43 - Directional Wattmeter
Measures forward power and reverse power on a 50 ohm system. Uses element whose frequency range encompass the range you're transmitting on and power level that is equal to or greater then output of device. This device does not usually do peak reading which means it's not fast enough for active device reading, so you must manually key the radio when using the tester.
The Bird 43 Watt meter is very versatile and cost effective. The bird meter can be used for a number of test but does 2 very simply. The two basic test of a bird meter are Forward power measurement and reflected power measurement. Keep in mind the Bird device does not generate power, so you must use the radio device to manually key the RF to perform a measurement test.
Must be manually keyed in radio device to generate power.
Does not measure SWR or active peak power.
Antenna system is defined as Load.
Radio device is defined as Transmitter.
Uses swappable slugs or elements shown in the center of the image with a directional arrow.
Elements/Slug for frequency range encompassing your frequency.
Element/Slug power should be equal or greater than device power.
Meter scaling is determined by element/slug's max power.
Element/Slug storage on left side, and another right side of meter.
Quick-change RF connectors show as N Type Female / Jack
The direction of the element determines what test is being performed.
When transporting or not using meter put arrow facing up or use blank element/slug.
Forward Power Measurement:
Radio or transmitter is outputting expected power.
Radio or transmitter is outputting less than expected power.
Move between different components to identify source of failure.
Verify equipment is configured to output expected power.
Reflected Power Measurement:
Radio or transmitter reflected power should be 0 or less then < 4%
Unlike with Return loss the lower the Reflected power the better.
More than > 4% reflected power.
RigExpert AA-1000 - Antenna Analyzer
The RigExpert is designed for testing, checking, turning or repairing antennas and antenna feedlines.
Generates own power for testing.
Does not measure forward power and should not be connected to a radio.
Setup uses frequency center and range to be configured.
Simple reflection test.
Always a positive number.
Measures Return loss (RL)
Result of impedance mismatch
Measures Time Domain Reflectometer (TDR)
Used for locating faults in transmission lines.
Requires coax type be configured in setup.
Software included for computer analysis of saved test.
Calibration hardware not included and required after frequency setup.
SWR Measurement: (Lower the better)
General Pass: < 2.5 (Green line)
General Fail: > 2.5 (Orange line)
Return Loss (RL): (Higher the better)
General Pass: > 14 dB (Green line)
General Fail: < 13 dB (Orange line)