March 6, 2022

“What is VSWR and how to adjust it”

  

VSWR

There is equipment available that will allow you to measure the VSWR of your antenna system.

Many hams have an inexpensive VNA, short for a Vector Network Analyzer which is great to see what your antenna’s swr is currently at your home QTH.

We will not be discussing those devices in great detail tonight (unless you have a question about one).

#1 Nano VNA and other types

MFJ259

A major problem with these devices are when attempting to adjust the antennas swr, they are useless when being in used in an RF environment to read an antenna’s swr, such as when up at a repeater/radio site. (antenna connection)

There are NO VNA’s that can be used in this environment due to the way these VNA’s are designed. (detail if asked)

VSWR…..the name pretty much tells you what we are going to be involved with, voltages being measured and the ratio between those voltages. (see formula below)

The instrument that I use to measure and adjust an antenna’s swr is with the use of a thru-line bi-directional RF watt meter.

#2 Bird Model 43 thru-line wattmeter (introduced 60 years ago)

model43

As you can see, there is an arrow on the frequency/wattage determining element.

These elements or “slugs” are how you can measure rf levels from 1 watt full scale to as much as 10,000 watts or more full scale accurately at different frequency ranges.

By connecting the antenna to be measured on one side of the meter and the transmitter to the other side, the arrow will be displaying the rf energy going to the antenna or the level of rf energy returning back down the coax to the transmitter….quickly and easily simply by rotating the element.

The following is the formula to determine the antenna’s SWR.

vswr4

#3 This example is my UHF mobile with 95 watts out and 11 watts reflected.

The antenna being measured is a 1/4 wave whip or “Spike” antenna.

vswr5

#4 What happens in the transmitter with High SWR?

vswr9

#5  What does it all mean?

reflected power

 

#6  Types of transmitter/PA VSWR protection

ACTIVE

#7  Circulators – Non-active protection.

How do they help the system?

Circulator

So…. what do we know now?

Final

Obviously a radio system’s VSWR is important for the proper operation of the station and to extend the life of the equipment involved.

Let’s hear from anyone that has had to tackle the job of adjusting their antennas swr. How did you do it, what equipment did you use and your results.

Using a Bird Model 43 thru-line watt meter is the way I have been measuring and adjusting antennas for over 49 years and it is the one piece of test equipment that you will buy once and it will serve you well for many years to come.

Whether you are checking your HF antenna system or a microwave antenna up in the upper Ghz range a thru-line wattmeter will get the job done quickly, easily and economically by simply changing the slugs needed.

DE WB6AMT

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