SWR Seeing Watts Reflected by Rick Hiller W5RH
SWR Seeing What s Reflected by Rick Hiller W5RH
Agenda The Antenna System What is SWR How it is developed How it is measured Why/How we use it
Handout (See last page of PDF)
The Antenna System Antenna - The Load Transmission Line - TL Transceiver The Source
Defacto Standards 50 ohm (nominal) coax is the most commonly used in Ham Radio RF Transmitter outputs designed for 50 ohms Antennas are commonly designed for 50 ohms
The Requirement (our desire) Maximum power transfer from the transmitter to the antenna (minimize losses) This is accomplished by having a matched system i.e. 50/50/50 ohms
The Transmission Line Also called Feed Line Passive Device Primarily, for getting RF power from an RF power source to an antenna Characteristic Zo Different types have differing loss numbers Different physical configurations
If I had one wish in Ham Radio it would be that both my antenna and transmission line would be totally lossless..we can get close, but no cigar. Neither the line or the antenna actually consumes power, they are simply passing it off to something else George Grammer, W1DF QST -- 1957.
Xmitter What happens? A N T E N N A For demonstration purposes, TL is many times longer than a typical Ham Station feed line
Xmitter Voltage is generated by the transmitter What happens? FORWARD POWER Zero Reflected A N T E N N A If the system is matched all power will be accepted by the antenna
Xmitter What happens? FORWARD POWER Reflected Power If the system is NOT matched some power will be reflected. A N T E N N A How much is reflected is dependent on the value of the mis-match
Xmitter What happens? Forward wave and reflected wave travel independently, but mix/add into a static condition called the Standing Wave A N T E N N A Creating an envelope, similar to AM modulation wave form.
Xmitter What happens? Standing Wave Envelope The waves will add at some places and subtract at others, producing mins and maxes It is almost as if the Reflected wave is modulating the forward wave A N T E N N A
SWR Demo Bessernet Web Note: URL is on the Handout
Standing Wave Envelope Maximum Minimum
Standing Wave Envelope Movie -- Similarities of Wave Behavior (Note: URL is on the Handout)
Standing Wave Ratio Maximum Envelope Voltage versus Minimum EnvelopeVoltage SWR = E max E min The differential of the max to the min is dependent on the size of the reflected voltage wave
Standing Wave Ratio If, the differential of the max to the min is dependent on the size of the reflected voltage wave. Then -- SWR can be used to indicate the quality of the match between the Transmission line and the antenna. SWR = E max E min = I max = Z Ant I min Z TL
SWR Cardinal Rules Always determined by the load Load vs Zo of line (which is constant) Does not change over the length of a lossless transmission line (theoretical) Will change over the length of a normal line SWR is greatest at the load
SWR Due to inherent line loss SWR will decrease as you approach the line input point Absolute and always greater than 1 SWR= Zo / Zl or Zl / Zo Singular never SWR s or s wers
How To Measure SWR? With an SWR Meter, of course!!! Power meter Forward / Reflected Cross Needle Meter VNA Vector Network Analyzer
Measuring SWR
Measuring SWR Bird
Crossed Needle Meter
VNA Based Meter
Antenna Analyzer Also use an antenna analyzer -- with caution Very low drive signal so.. High RF fields can distort the measurements Blame it on IZO BVARC Tech Articles (Note: URL is on the Handout)
Living in the Emperical World A few insights and caveats
SWR Excursions SWR Z 50ohm +/-j 1:1 50 0 2:1 100/25 36 3:1 150/16.6 58 4:1 200/12.5 75 6:1 300/8.3 102 10:1 500/5 142
Antenna System Philosophy The good thing is that Hams have bands of frequencies to use The bad thing is that hams have bands of frequencies to cover
Antenna load Z s Unless you are rich and famous and own a self tuning antenna system (ie Stepper IR) then you do not have a resonant or matched antenna across the Band. Your antenna generates a range of input feed impedances which can vary extremely.
80 Meter Dipole Feed Z Excursions 5.29 SWR 43 j86 ohms 3.95 SWR 79 +j88 ohms 1.18 SWR 59 +j1.3
Should we worry about low SWR? Should we worry about high SWR?
Should we worry about low SWR? YES Should we worry about high SWR? Yes
SWR is best at the Xmitter X M I T T E R A N T
Mis-matched loss graph A d d I T I O N A I L O S S
SWR At Antenna 4 2.25 SWR By example Reference Sheet
It s all math
Transmission Line Apps TL Details Dan McGuire AC6LA Provides a Smith Chart on a computer ARRL TLW Dean Straw ARRL Antenna Book CD
SWR Vs. Reflected E & P
Loss -- db vs Watts 100 watt reference db watts(loss) 0 = 0-1 = 10-2 = 37-3 = 50 half power -4 = 40-5 = 69-6 = 75 1 S Unit -10 = 90
Where should I put my ATU? If you want to minimize loss, place the ATU at the antenna feedpoint. What if I can t?
In-line matching No Matching Xmitter High SWR at xmitter Reduced power output A N T E N N A
In-line matching Matched Xmitter SWR 1:1 ATU SWR 4:1 A N T E N N A Matched TL losses between xmitter and ATU Additional losses in TL between ATU and Antenna
Xmitter In-line matching SWR 1:1 Matched ATU SWR 4:1 Reflected power is re-reflected at the ATU Circulating current between ATU and antenna All power, except for losses, makes it to the antenna A N T E N N A
Re-reflected Power Video Standing Up for Standing Waves (Note: URL is on the Handout)
Mis-matched loss graph A d d I T I O N A I L O S S
Loss -- db vs Watts 100 watt reference db watts(loss) 0 = 0-1 = 10-2 = 37-3 = 50 half power -4 = 40-5 = 69-6 = 75 1 S Unit -10 = 90
Summary Understand what your SWR measurement is trying to tell you. Use the published TL specifications to establish a base line for losses. Further you knowledge of antennas and TL s by reading more and using the free tools available to you. Get hold of the ARVN Video SWR etc.
Summary Use the appropriate coax for the situation. Use the best that you can afford. In general -- The length of transmission line that is recommenred in the length that gets you from your transceiver to tyour antenna not 1/2wl, not 1/wl, etc..
Summary Remember the antenna and TL and matching network are a system. Better Antenna Systems mean more contacts.. More contacts mean more fun...and that is what this hobby is all about!!!!!!!
The End SWR Seeing Watts Reflected Rick Hiller W5RH
SWR Seeing Watts Reflected HamCom June 12, 13 2015 by W5RH Rick Hiller rhiller@sdicgm.com Terms and Definitions Incident (Forward) Power the power delivered to the feed line input by the source/generator/transmitter Reflected Power the portion of the incident (forward) power, sent back toward the generator from the load, as a result of the mismatch between the line characteristic Zo and the load Z. Standing Wave the wave created on a transmission line, being not terminated in its characteristic impedance, by the sum of the incident and reflected voltage waves. VSWR (SWR) -- Voltage Standing Wave Ratio the mathematical ratio of the standing wave s voltage max to voltage min. It is an easily made measurement that is used as a reference to define the quality of the line to load match. ================================= Reference Articles and Publications *The Why s of Transmission Lines Series George Grammer QST Jan, Feb. March 1965 *Understanding SWR By Example Darrin Walraven K5DVW QST November 2006 *SWR, Reflected Power What Do They Mean Joel Hallas, W1ZR, QST June 2011 *Reflections (I, II and III) Walt Maxwell, World Radio Books Publishing *Transmission Lines, Antennas and Wave Guides King, Mimno and Wing 1945 US Army *ARRL Antenna Handbook ARRL Publishing -- Any printing from 1939 to 2007 * Web Sites * SWR DEMO -- http://www.bessernet.com/ereflecto/tutorialframeset.htm * Bell Labs Movie -- https://www.youtube.com/watch?v=dovunoxly1k * Standing Waves Video --http://arvideonews.com/hrn/hrn_episode_0199.html * TL Details Download -- http://www.ac6la.com/tldetails.html * BVARC Articles -- http://www.bvarc.org/index.php?page=tech