https://urgentcomm.com/wp-content/themes/ucm_child/assets/images/logo/footer-new-logo.png
  • Home
  • News
  • Multimedia
    • Back
    • Multimedia
    • Video
    • Podcasts
    • Galleries
    • IWCE’s Video Showcase
    • Product Guides
  • Commentary
    • Back
    • Commentary
    • Urgent Matters
    • View From The Top
    • All Things IWCE
    • Legal Matters
  • Resources
    • Back
    • Resources
    • Webinars
    • White Papers
    • Reprints & Reuse
  • IWCE
    • Back
    • IWCE
    • Conference
    • Special Events
    • Exhibitor Listings
    • Premier Partners
    • Floor Plan
    • Exhibiting Information
    • Register for IWCE
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Terms of Service
    • Privacy Statement
    • Cookie Policy
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • Mission Critical Technologies
    • TU-Auto
  • In the field
    • Back
    • In the field
    • Broadband Push-to-X
    • Internet of Things
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Call Center/Command
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Network Tech
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Operations
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Regulations
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • Organizations
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
Urgent Communications
  • NEWSLETTER
  • Home
  • News
  • Multimedia
    • Back
    • Video
    • Podcasts
    • Omdia Crit Comms Circle Podcast
    • Galleries
    • IWCE’s Video Showcase
    • Product Guides
  • Commentary
    • Back
    • All Things IWCE
    • Urgent Matters
    • View From The Top
    • Legal Matters
  • Resources
    • Back
    • Webinars
    • White Papers
    • Reprints & Reuse
    • UC eZines
    • Sponsored content
  • IWCE
    • Back
    • Conference
    • Why Attend
    • Exhibitor Listing
    • Floor Plan
    • Exhibiting Information
    • Join the Event Mailing List
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Cookie Policy
    • Terms of Service
    • Privacy Statement
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • TU-Auto
  • newsletter
  • In the field
    • Back
    • Internet of Things
    • Broadband Push-to-X
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Cybersecurity
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
acc.com

Call Center/Command


Understanding standing wave ratios

Understanding standing wave ratios

The term standing wave ratio, or SWR sometimes called voltage standing wave ratio, or VSWR frequently is a subject of discussions/arguments among communication
  • Written by Urgent Communications Administrator
  • 1st September 2006

The term standing wave ratio, or SWR — sometimes called voltage standing wave ratio, or VSWR — frequently is a subject of discussions/arguments among communication technicians, hams and others involved in the radio communications field. The easiest way to explain SWR is through an example.

In Figure 1, a transmitter is connected to an antenna through a 100-foot length of transmission line. Let’s suppose in this instance that the transmission line is loss-less (though no such circumstance exists in the real world.) Further suppose that the antenna is not properly matched to the 50 ohm system and that 25% of the RF power is reflected back down the line from the antenna toward the transmitter. With the in-line directional wattmeter connected between the transmitter and the transmission line, the wattmeter indicates 100 W forward power and 25 W reflected power. In Figure 1, Pf represents the forward traveling wave (forward power) and Pr represents the reflected traveling wave (reflected power).

The voltages of the forward and reflected waves will combine at different phase angles along the length of the transmission line. A phasor analysis will reveal that voltage maxima (antinodes) and voltage minima (nodes) exist along the length of the transmission line. Figure 2 shows the representation of the standing wave pattern on the transmission line. The VSWR figure is derived from the ratio of the voltage at the maxima to the voltage at the minima. So all we have to do is calculate the voltage of the forward wave and the voltage of the reflected wave and express the sum and difference of these two voltages as a ratio. This is the standing wave ratio. The calculations are as follows:

First, from the power and impedance we can calculate the voltage using Equation 1. Since the impedance is 50 Ω and the power in the forward wave is 100 W, the voltage of the forward wave is 70.71 (see Equation 2). The voltage of the reflected wave is calculated in the same manner, and the result is 35.35 V. The VSWR is computed as the ratio of the sum (maxima) to the difference (minima) of the two voltages, or 3:1, as shown in Equation 3.

Most technicians use a directional in-line wattmeter to make forward and reflected power measurements in order to determine transmitter output power and reflected power. With some experience, the technician can get a quick mental evaluation of the degree of match or mismatch by comparing the forward and reflected power levels. Basically, the technician is comparing the percentage of reflected power to forward power.

In the example illustrated in Figure 1, the forward power is 100 W and the reflected power is 25 W. Thus, the percentage of reflected-to-forward power is 25%. Experienced technicians know that a reflected power of 25% translates to an SWR of 3:1. If we let r represent the ratio of reflected power to the forward power, we can use Equation 4 to calculate the SWR. Using the example in Figure 1, we find that SWR also is 3:1 (see Equation 5).

The term return loss often is used to indicate the degree of match or mismatch instead of the term SWR. To understand return loss, refer back to Figure 1. The forward power is 100 W, and the reflected power is 25 W. The return loss is simply the expression in decibels of the ratio of the forward power to the reflected power and is calculated by the formula in Equation 6.

Thus, a return loss of 6 dB is equivalent to an SWR of 3:1. It also is possible to convert SWR directly to return loss using Equation 7 on page 60. Table 1 provides conversions between SWR and return loss for several SWR ratios.

In the example of Figure 1, the line loss was theoretically zero — but this never happens in the real world. Let’s look back at Figure 1 and see what happens if the transmission line loss is 3 dB at the given length and frequency of operation. This changes things quite a bit. The power output from the transmitter (Pf) is still 100 W. However, since the line loss is 3 dB, the forward power arriving at the antenna is only 50 W (down 3 dB). Now the mismatch at the antenna causes 25% of the forward power to be reflected back toward the transmitter. Thus, 12.5 W of power is reflected back down the line. In traveling back down the line, this reflected power is attenuated by 3 dB. This means that the reflected power measured at the transmitter is only 6.25 W.

Using all of this information, we can analyze the results in the following manner. First, the difference between the forward and reflected power readings at the transmitter is 100 W minus 6.25 W for a net power of 93.75 W. The SWR at the transmitter is 1.67:1, and the SWR at the antenna is still 3:1. Thus, the loss of the transmission line makes the SWR measured at the transmitter look much better than it really is at the antenna.

Remember, too, that the actual power delivered to the load is the difference between the forward power and reflected power readings on the wattmeter. Now, at the transmitter, the difference between the two readings is 93.75 W, while at the antenna it is 37.5 W.

Because the line loss was 3 dB (representing a 50% loss), the net power delivered to the antenna should be 50% of the net power delivered into the transmission line at the transmitter. Calculations indicate that the net power delivered to the antenna is only 40% of the net power input to the transmission line. Thus, the loss in the transmission line is 60%, or approximately 4 dB. The normal (matched-line) loss of the transmission line is 3 dB. The additional 1 dB of loss is caused by the mismatch at the antenna. Equation 8 can be used to determine the additional line loss caused by a mismatch at the antenna.

When you measure the SWR (reflected and forward power readings) at the transmitter, remember that the line loss between the transmitter and antenna will mask the true SWR that exists at the antenna. The greater the transmission line loss, the greater this masking effect.

In fact, if the normal matched-line loss of the transmission line is 3 dB, the maximum SWR seen at the transmitter would be just slightly more than 3:1 — even for a worst-case mismatch at the antenna. SWR increases the line loss above the nominal matched-line conditions. When choosing a transmission line, use the best (read minimum loss) cable that your budget and other practical considerations will allow.

Until next time — stay tuned!

Table 1
Return Loss (dB) VSWR
10 1.92
12 1.67
14 1.5
16 1.37
18 1.28
20 1.2
Tags: Call Center/Command content

Most Recent


  • NATE: Todd Schlekeway highlights organization's safety, legislative initiatives
    Todd Schlekeway, executive director of NATE: The Communications Infrastructure Contractors Association, discusses many of NATE’s planned activities for 2023, including a legislative visit to Capitol Hill in May, safety/training initiatives, and a broader release of the Vertical Freedom documentary that focuses on the lives of tower climbers in the communication arena.  
  • Cybercrime ecosystem spawns lucrative underground Gig Economy
    Over a 30-month period, cybercriminal gangs and threat groups posted more than 200,000 advertisements seeking workers with skills in software development, maintaining IT infrastructure, and designing fraudulent sites and email campaigns. The demand for technically skilled individuals continues, but it peaked during the coronavirus pandemic, with double the average job advertisements coming during March 2020, […]
  • FAA approves beyond-visual-line-of-sight (BVLOS) flights in North Dakota
    The unmanned aerial vehicle (UAV) avionics company uAvionix received Federal Aviation Administration approval to conduct advanced beyond visual line-of-sight (BVLOS) flights of small UAVs in North Dakota.  The flights will be conducted at the Northern Plains Unmanned Aerial Systems (UAS) Test Site (NPUASTS) in Grand Forks, one of seven FAA-run UAV test sites in the U.S., using […]
  • AT&T wireless growth keyed by FirstNet—now provides 24,000 agencies with 4.4 million connections
    AT&T this week reported that FirstNet ended 2022 supporting more than 24,000 public-safety agencies with “about” 4.4 million connections, including 377,000 connections that were added during the last three months of 2022—a total that represents more than half of the carrier’s post-paid wireless growth for the quarter. AT&T officials released these figures in conjunction with […]

Leave a comment Cancel reply

To leave a comment login with your Urgent Comms account:

Log in with your Urgent Comms account

Or alternatively provide your name, email address below:

Your email address will not be published. Required fields are marked *

Related Content

  • New Orleans-area 911 center inks multiyear APEX deal with Carbyne to replace call-handling system
  • Understanding standing wave ratios
    Newscan: Feds recover millions from pipeline ransom hackers, hint at U.S. Internet tactic
  • Cyber is the new Cold War, and AI is the arms race
  • Private wireless networks in the US start going public

Commentary


How 5G is making cities safer, smarter, and more efficient

26th January 2023

3GPP moves Release 18 freeze date to March 2024

18th January 2023

Do smart cities make safer cities?

  • 1
6th January 2023
view all

Events


UC Ezines


IWCE 2019 Wrap Up

13th May 2019
view all

Twitter


UrgentComm

Hytera parent cites financial health, but unable to make royalty payment to Motorola Solutions dlvr.it/ShlrlM

1st February 2023
UrgentComm

NATE: Todd Schlekeway highlights organization’s safety, legislative initiatives dlvr.it/ShljHj

1st February 2023
UrgentComm

Cybercrime ecosystem spawns lucrative underground Gig Economy dlvr.it/ShkKbf

31st January 2023
UrgentComm

FAA approves beyond-visual-line-of-sight (BVLOS) flights in North Dakota dlvr.it/ShgxHW

30th January 2023
UrgentComm

AT&T boasts of core ‘white box’ success in 5G, fiber push dlvr.it/Shgb4w

30th January 2023
UrgentComm

Spending American Rescue Plan Act funds: A primer for municipalities dlvr.it/ShgZ52

30th January 2023
UrgentComm

AT&T wireless growth keyed by FirstNet—now provides 24,000 agencies with 4.4 million connections dlvr.it/ShY5qH

27th January 2023
UrgentComm

Report: Remote work causing offices to empty, but walkable cities still in high demand dlvr.it/ShXM7Z

27th January 2023

Newsletter

Sign up for UrgentComm’s newsletters to receive regular news and information updates about Communications and Technology.

Expert Commentary

Learn from experts about the latest technology in automation, machine-learning, big data and cybersecurity.

Business Media

Find the latest videos and media from the market leaders.

Media Kit and Advertising

Want to reach our digital and print audiences? Learn more here.

DISCOVER MORE FROM INFORMA TECH

  • American City & County
  • IWCE
  • Light Reading
  • IOT World Today
  • Mission Critical Technologies
  • TU-Auto

WORKING WITH US

  • About Us
  • Contact Us
  • Events
  • Careers

FOLLOW Urgent Comms ON SOCIAL

  • Privacy
  • CCPA: “Do Not Sell My Data”
  • Cookie Policy
  • Terms
Copyright © 2023 Informa PLC. Informa PLC is registered in England and Wales with company number 8860726 whose registered and Head office is 5 Howick Place, London, SW1P 1WG.