Search RFCafe.com                           
      More Than 17,000 Unique Pages
Please support me by ADVERTISING!
Serving a Pleasant Blend of Yesterday, Today, and Tomorrow™ Please Support My Advertisers!
   Formulas & Data
Electronics | RF
Mathematics
Mechanics | Physics
     AI-Generated
     Technical Data
Pioneers | Society
Companies | Parts
Principles | Assns


 About | Sitemap
Homepage Archive
        Resources
Articles, Forums Calculators, Radar
Magazines, Museum
Radio Service Data
Software, Videos
     Entertainment
Crosswords, Humor Cogitations, Podcast
Quotes, Quizzes
   Parts & Services
1000s of Listings
 Vintage Magazines
Electronics World
Popular Electronics
Radio & TV News
QST | Pop Science
Popular Mechanics
Radio-Craft
Radio-Electronics
Short Wave Craft
Electronics | OFA
Saturday Eve Post

Software: RF Cascade Workbook
RF Stencils Visio | RF Symbols Visio
RF Symbols Office | Cafe Press
Espresso Engineering Workbook

Aegis Power  |  Alliance Test
Centric RF  |  Empower RF
ISOTEC  |  Reactel  |  RFCT
San Fran Circuits

Innovative Power Products (IPP) RF Combiners / Dividers

Crane Aerospace Electronics Microwave Solutions

Innovative Power Products (IPP) Directional Couplers

Please Support RF Cafe by purchasing my  ridiculously low-priced products, all of which I created.

RF Cascade Workbook for Excel

RF & Electronics Symbols for Visio

RF & Electronics Symbols for Office

RF & Electronics Stencils for Visio

RF Workbench

T-Shirts, Mugs, Cups, Ball Caps, Mouse Pads

These Are Available for Free

Espresso Engineering Workbook™

Smith Chart™ for Excel

DC-70 GHz RF Cables - RF Cafe

VSWR Reduction by Matched Attenuator

By inserting a matched (nominal system impedance) attenuator in front of a mismatched load impedance, the mismatch "seen" at the input of the attenuator is improved by an amount equal to twice the value of attenuator. The explanation is simple.

VSWR Reduction by Matched Attenuator - RF CafeReturn loss is determined by the portion of the input signal that is reflected at the load (due to impedance mismatch) and returned to the source. A perfect load impedance (complex conjugate of the source impedance) would absorb 100% of the incident signal and therefore reflect 0% of it back to the source (return loss of ∞ dB).

For the sake of illustration, assume that the load is an open (or short) circuit, where 0% of the incident signal is absorbed by the load and 100% is reflected back to the source. The reflected signal would therefore have a return loss of 0 dB. Insert a 3 dB  attenuator in front of the load. Now the incident signal is referenced to the input of the attenuator.

As signal at the input of the attenuator will experience a 3 dB reduction in power by the time it reaches the load. That 3 dB less power will be 100% reflected by the load and experience another 3 dB reduction in power by the time is returns back to the input, for a total loss of 6 dB. The same principle applies for a load anywhere(§) between zero and infinite load impedance (short and open circuits, respectively).

Calculate the improved VSWR as follows. Note that by my convention the loss value is returned as a positive number, since the word "loss" implies the negative. If it were to be termed "return gain," then the result would be reported as a negative number. Equally qualified experts will disagree on whether return loss should take on a negative value or a positive value; the important thing is to keep the sign correct in your calculations; i.e., if you use a positive value, then subtract it, and vice versa.

  • Convert the load VSWR to load return loss per the following equation:

    VSWR to Return Loss Conversion Equation - RF Cafe

  • Add twice the attenuator value to RLLOAD:

    RLImproved = 2*Attenuator + RLLoad [dB]    (where RL=Return Loss)

  • Convert back to VSWR per the following equation: Return Loss to VSWR Conversion Equation - RF Cafe

Of course, the method can be reversed to predict the attenuator required to improve a load VSWR by a predetermined amount. To do so, calculate the desired return loss and subtract the known load return loss. Divide the answer by two to get the attenuator value needed.

Here is a JavaScript calculator for VSWR / Return Loss / Reflection Coefficient / Mismatch Error / Improvement

See the VSWR Calculator page.

§ Actually, the attenuator is only rated for its specified attenuation level when it is connected between two nominal impedances. Therefore, the attenuator will either have to be designed to closely match the two impedances at its input and output (source and load, respectively), or an adjustment will need to be made in the specified attenuation value to compensate for the mismatched load impedance.

 

Related Pages on RF Cafe

- VSWR - Return Loss - Γ  Conversions

- VSWR <--> Return Loss <--> Γ  Conversion Calculator

- VSWR Mismatch Errors

- VSWR Reduction by Matched Attenuator

DC-70 GHz RF Cables - RF Cafe




Windfreak Technologies Frequency Synthesizers - RF Cafe