KB6NU's Ham Radio Blog

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If my radio has an SWR meter, why do I need an antenna analyzer?

Posted: 23 Sep 2020 08:36 AM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email


On the Amateur/Ham Radio subreddit, a poster asks:

I’m new to HF and reading my FT-450D manual while waiting for the supplies
to build a dipole, and I have a question. If some HF radios show the SWR
while transmitting, what is the purpose of using a separate antenna
analyzer? I’m assuming that the analyzer provides more information or shows
the SWR of multiple bands. Is that correct? Thanks!

As usual on this subreddit, hes gotten some very good answers:

An antenna analyzer lets you tweak the design of your antenna right at the
antenna itself without connecting it to the radio and gives you instant
feedback if you need to lengthen or shorten the elements, the tuning stub,
etc. It is a much faster/easier process (if you are into building antennas)
than using your rig to do so. And yes, it gives you more info such as
impedance.


The SWR bridge in most rigs are not terribly accurate but good enough. In
Icom radios at least the circuit the SWR is derived from is actually the
fold-back circuit that reduces output when the SWR gets high to protect the
power transistors.

An analyzer does more than just SWR, it will tell you the resistive and
reactive components at a minimum. The better ones have graphing functions,
can sweep frequency ranges, do smith charts, measure 1/4 and 1/2 wave
stubs, coax velocity factor, interface with software running on a PC so you
can save and review etc. The RigExpert analyzers do all that stuff..
Invaluable if you like experimenting with antennas.


Some antenna analyzers can draw a nice graph of SWR versus frequency, so
without the hassle of taking a lot of readings you can say hmm, the swaaar
is low just below the amateur band, lets try making this antenna slightly
shorter
Heres an example of a fancy SWR vs. frequency display that some antenna
analyzers are capable of.

One aspect of this is that some antenna analyzers can measure complex
impedance.

A SWR of 2:0, in a 50 ohm system, could mean the antenna impedance is 25+j0
or 100+j0 or 50+j36 or any number of other impedances.

If you know the antenna impedance at a particular frequency then you can
calculate component values for a matching circuit.

My take on this issue is that antenna analyzers provide much more
functionality than the SWR bridges built into transceivers. Sure, you can
tune an antenna by making SWR reading on your transceiver, but its going to
take you a lot longer than it would with an antenna analyzer. Even if your
analyzer doesnt have a fancy display, you can easily change the operating
frequency, note the SWR reading, and determine if an antenna is too long or
too short. And, you dont have to run up and down stairs to make a
measurement on your transceiver.

I dont know about the accuracy. It would be interesting to compare the
reading from an antenna analyzer with the reading from a built-in SWR
meter. My guess is that the antenna analyzer would be a little more
accurate. After all, the purpose of the built-in SWR bridge is to protect
the rigs final transistors, and that being the case, the designers might be
a little more conservative about the readings.

I always advise students in my Tech classes to consider purchasing an
antenna analyzer, especially if theyre going to be playing around with
antennas. Not only is it a very practical thing to have, its an educational
tool as well. You learn a lot about antennas by tinkering with them and
noting the changes in measurements.

The post If my radio has an SWR meter, why do I need an antenna analyzer?
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