KB6NU's Ham Radio Blog

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Use forward and reflected power to calculate SWR

Posted: 27 Sep 2016 11:17 AM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email

You can use a Bird 43 wattmeter to make forward and reflected power
measurements and then calculate the SWR.

This morning, I got the following e-mail from a friend of mine who is a
member of the University of Michigan Amateur Radio Club. He got a request
from aÂ*researcherÂ*at the university, who wrote:

We put archival acoustic and motion sensing tags on marine mammals. The
tags record for about a day and then release from the animals. A VHF
transmitter (transmitting in the 220 MHz range) is built into the tag,
along with the antenna. We then use directional antennas to home in on the
tag in the ocean for recovery and data offload.
I am looking for equipment on campus that I could use to tune the antennas
I am making and would like to be able to measure things like standing wave
ratio andÂ* transmission frequency.

My friend wrote back:

Unfortunately 220 MHz is a band that isntÂ*very popular, so there arent as
many tools available.Â* That means even if you find an HT or mobile unit
that will transmit in that band, you wont have very much luck finding an
SWR meter for it.Â*I think your best bet is an antenna analyzer.Â*


He then mentioned a couple of models that covered the 220 MHz band.

Next, he forwarded the e-mail exchange to me and another club member,
asking for any guidance that we might be able to give. The first thing that
occurred to me, of course, is questionÂ*T7C08 from the Technician Class
question pool. It asks, What instrument other than an SWR meter could you
use to determine if a feed line and antenna are properly matched?

The answer is a directional wattmeter. By making both forward and reflected
power measurements, you can calculate the SWR with the following equation:



IÂ*suggested that they make these measurements with a Bird 43
wattmeter,Â*with the appropriate plug-in. Bird even has an application note,
VSWR Measurements Using In-Line Power MetersÂ*on how to do Â*this.Â*This
application note not only discussesÂ*the theory behind this equation, but
also some of the practical aspects of making this measurement,
includingÂ*how the accuracy of a power meter affects the uncertainty of SWR
measurement.

This sounds like a very interesting application, and Im interested in
finding out more about it. Its a good example of how an amateur radio
license could be of use to young people interested in pursuing a career Â*in
science or technology, even if that career isnt electrical engineering.

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