How to Combine Signal Transmission Lines

Updated on January 8, 2018
tamarawilhite profile image

Tamara Wilhite is a technical writer, industrial engineer, mother of 2, and a published sci-fi and horror author.

Introduction

A sudden change in impedance in a transmission line creates an impedance bump or SWR. You lose power as a result, when part of the power reflects back as a standard wave and interferes with the transmission. And the transmitter may not like the impedance bump, since it is dealing with 100 watts out and 50 or more watts back. If the SWR (standing wave ratio) is too high, the outgoing signal plus the SWR can burn out your transistors.

Then how can you then make the transition combining signal transmission lines, say from a 50 ohm line to a 72 ohm line, without this problem? The solution is changing the impedance slowly to prevent an SWR reflection. There are several ways to accomplish this.

How to Combine Signal Transmission Lines with Limited SWR

A classic solution for preventing SWR when combining signal transmission lines is the quarter wave power divider.

In the case of a 50 ohm line, there are two 75 ohm lines to split the power forwarded one to two 50 ohm loads. These power dividers can combine the input of two amplifiers, combine two separate antennas or combine the output of two different amplifiers. If the loads aren’t well matched, you can add a 100 ohm balancing resistor to offset the difference.

The quarter wave power divider results in well balanced reception across 10% of the bandwidth while it is usable with degraded performance over 20% of the bandwidth. When you combine signals from two wideband antennas like log periodic antennas, you need a good impedance match across a wide range of frequencies. Tapered lines become even more valuable in this case at minimizing SWR. How could you do that?


The ideal way to handle a wide frequency range with minimal SWR is to taper the diameter of the central conductor. If you can’t do that, you can taper the shield by filing it down so that the taper is at least a quarter wave of the lowest frequency on which you intend to use the power divider.

For a six inch or less line, you can take out the central conductor from the line. The hole shrinks some. If you’ve pulled out a nineteen gauge wire, for example, you could push a 24-gauge wire in. You have to work fast before the conductor returns to its old shape. However, you end up with a smaller center conductor. For 19 gauge (0.040”) to 24 gauge (0.022”) wire, this process turns a 50 ohm coax to a 75 ohm one.

Tapered line power divider diagram courtesy of Kent Britain, WA5VJB, used with his permission
Tapered line power divider diagram courtesy of Kent Britain, WA5VJB, used with his permission | Source
This Quad Patch antenna has the tapered power divider as part of the PCB antenna printed circuitry.
This Quad Patch antenna has the tapered power divider as part of the PCB antenna printed circuitry. | Source

Another option is buying pre-made antennas like the one show here that already come with the tapered power divider so you only have to hook up the antenna and go. The quad patch antenna shown here is one such example. The line generates 100 ohms from each antenna to the central line. That handles 50 ohms at the broad ends before tapering down toward the middle where it delivers a 100 ohms load. Where all the signals meet, it delivers a combined 50 ohm signal.

Another option is phase matching lines using phase matching mixers or frequency mixers; this is the better choice when you’re working with very high frequencies.

References

Kent Britain, WA5VJB, was interviewed as a source of information in this article, along with other sources. He is also the designer of the quad patch antennas shown here.

Questions & Answers

    Comments

    Submit a Comment

    No comments yet.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, turbofuture.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://turbofuture.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)