antenna parameters part 3 - input impedance and vswr

Poynting Antennas (Pty) Ltd

13 April 2016 AntennaParameters Part 3 - Input Impedance and VSWR

ON THE PROPERTIES OF AN ANTENNA This is part of an internal document that that gives an overview of the properties of

antennas for non-engineers.

We have divided the document into different posts where we discus each of the

parameters:

- Frequency bands, gain and radiation pattern

- Polarisation

- Input Impedance and VSWR

- Port to port Isolation and Cross-polarisation

- Power Handling ability

- Antenna “Specmanship”

Where applicable we have added some videos explaining the properties discussed. You

will find a link to a PDF below.

Introduction An antenna is a device that converts energy from one form to another. When used in

transmit mode, currents in the coaxial cable (feeding the antenna) flow into the antenna

and the energy is converted to electromagnetic radiation which propagates into space.

When an antenna is used in receive mode, electromagnetic radiation interacts with the

antenna inducing currents into its components. These currents flow along the coaxial

cable connected to the antenna to a receiver.

In some ways the antenna is analogous to a speaker in a sound system. A speaker

converts electrical energy (from the wires powering the speaker) into sound energy which

we can detect using our ears. When operated in the opposite mode a microphone is

created. This device detects sound wave and converts them to electrical energy. An

antenna works with electromagnetic radiation and electric currents rather than sound and

electric currents.

An antenna is described by a number of attributes including frequency bands of

operation, gain, radiation pattern, polarisation, VSWR, input impedance, coupling, power

handling ability and so on.

This document describes each of these parameters.

1. Frequency bands of operation See part 1

2. Gain and radiation pattern See part 1

3. Polarisation See part 2



4. Input Impedance and VSWR The input impedance of an antenna per se is not usually reported directly in the brochure;

rather the antenna’s nominal impedance and its VSWR are given. The nominal

impedance is the impedance for which the antenna is (ideally) designed and the VSWR

can be “seen” as the antenna’s deviation from this value.

The VSWR (voltage standing wave ratio) is a parameter that is derived from the

antenna’s input impedance and the reported nominal impedance. One can view the

VSWR as “how far” the antennas input impedance is from the nominal impedance. If the

VSWR at a particular frequency is given as 1:1, then you can deduce that the antenna

input impedance is equal to the nominal impedance. The higher the VSWR the further the

antenna input impedance is from the nominal impedance. An example of a VSWR graph

for a 430 MHz antenna is given in Figure 1.

Figure 1: An example of a VSWR graph

4.1. So what? What relevance does the VSWR of an antenna have? Is it important?

It was mentioned earlier that the nominal impedance is the design target for the antenna

impedance. The electronics to which the antenna is to be attached has an input-impedance

equal to the nominal impedance. Ideally the antenna impedance and the electronic input-

impedance should be equal. If they are not, then some of the RF power is lost in the

system – it is actually reflected back to transmitter.

The graphs in Figure 2 show the power that is lost (reflected back to the transmitter) due

to an impedance mismatch between the antenna and the electronics. The mismatch in

impedance is given in the form of VSWR. Both graphs show the same data, but in

different units. The graph on the left shows the power lost in dB whilst the graph on the

right give the power lost as a percentage of input power.



Figure 2: Power lost due to VSWR (mismatch loss)

As an example, a VSWR of 6:1 corresponds to a loss of 3 dB (left graph) or 50% of the

input power (right graph). Similarly a VSWR of 3:1 corresponds to a loss of 1.25dB or

25% of the input power. At first this might sound like a ludicrously large amount of

power to waste and that a VSWR of 3:1 should be totally unacceptable. After all, how

can one just waste 25% of the input power? Let us examine this question by analysing the

losses in a typical radio link.

Radio frequency power is lost in many parts of a radio link. In order to get some idea of

the magnitude of these losses let us consider a 1km link operating at 2.4 GHz with a 500

mW transmitter at one and a receiver at the other end. Both transmitter and receiver have

5 dBi antennas. The table below gives a breakdown of the power lost at each stage of the

link and the power left for communicating data (in mW and dBm).

Power

lost (dB)

Power lost (%) Power left for

communicating (mW)

Power left

(dBm)

Transmitter 500 27

Connector 0.2 4.5 477.5 26.8

Cable (connecting transmitter

to antenna)

2 36.9 301 24.8

Antenna VSWR (3:1) 1.25 25 226 23.55

Transmit antenna gain (5) (316) 714 28.55

Power radiated into space 714 28.55

Power lost over 1km 100 99.999999989999992 0.000000071400085177 -71.45

Receiver antenna gain (5) (316) 0.000000225624269159 -66.45

Cable (connecting receiver to

antenna)

2 36.9 0.000000083255355319 -68.45

Power received 0.000000083255355319 -68.45

In the above analysis we transmitted 27 dBm and received -68 dBm. The minimum signal

strength that a typical Wi-Fi unit can successfully use for communication is about -100

dBm; so a signal strength of -68 dBm is good!

Also note that most of the power is lost in the space between the transmitter and receiver

(71 dB of the power radiated into space was lost or 99.999999989999992%). So, losing

25% (1.25 dB) of the power due to a VSWR of 3:1 is really small change and not a

substantial loss at all.



4.2. So why even report VSWR? A low VSWR is important in some technologies that make use of antennas, for example,

RFID (radio frequency identification). RFID works in the following manner. A RFID

reader transmits a signal towards a RFID tag. This signal is used to energise the RFID

chip. The chip then changes the load on the antenna in a predefined fashion such that

some of the incident signal is reflected back to the RFID reader (in a well-defined

pattern). The RFID reader receives this scattered signal (while it is still transmitting) and

uses it to ‘read the tag’. If the reader antenna has an unacceptable VSWR, then the signal

reflected back to the transmitter by the reader antenna will drown out the signal scattered

by the tag and the system will not work.

Jamming systems use antennas to transmit high power jamming signals towards a target.

If the antenna has a VSWR of, say, 3:1, then 25% of the power is reflected back to the

output stage amplifier. This reflected signal may have enough power to damage the

transmitting amplifier. Such a problem does not occur in low power applications like Wi-

Fi as the transmitter can easily handle these reflected powers. GSM base stations on the

other hand do need well matched antennas.

Good VSWR characteristics are still desirable in RF applications (such as Wi-Fi, GSM

handsets, alarm systems etc.,) as they give a good indication of the quality of the design

of the antenna – but VSWR on its own is not enough to discredit or condone an antenna.

antenna parameters part 3 - input impedance and vswr

Engineering