[KL7AA] Antenna Analyzer

[Jim Wiley]

After preparing my response, I decided to send this to the entire KL7AA 
list, and not just Adam.  If the rest of you are not interested, than 
that's what the "delete" key is for.   - Jim


Adam -

Congratulations on your interest in Ham Radio.   May I offer some 
perspective on antennas and using an antenna analyzer in particular.  I 
am an Amateur Extra class licensee, licensed since 1959, a member of the 
national question pool committee that prepares amateur exams, and the 
person in charge of the Anchorage VEC.

The first thing to remember about antennas is that ANY antenna is better 
than no antenna.  When I first started in ham radio I knew next to 
nothing about antennas, electronics in general, and how to start 
(anything).   In those days we built antennas by using diagrams and 
articles in amateur radio publications.  There was no such thing as an 
antenna analyzer.  But somehow, we managed to get on the air and have 
many successful contacts. How was this possible?

I built antennas by cutting wire according to published tables that 
said, in effect, for this band, cut the wire this long, attach some sort 
of feed line, hang it up in a tree (or use other available supports), 
connect the feedline to your transmitter, and get on the air.  It was 
just that simple.  No measurement equipment of any kind.  And guess what 
- it worked!   For something like a VHF antenna, the pictures showed 
what the finished product should look like, gave some dimensions, and 
you were on your own from there.  And guess what - it worked! (again).

Was any of this perfect?  It was not.  Could my antennas have been fine 
tuned to improve performance?  Of course.  But, in the final analysis,  
they worked, to a greater or lesser degree , and I managed hundreds of 
contacts with hams all around the area.

As years went by, I learned by doing, and by reading books. Lots of 
books.  I learned that, for the most part, the evil god SWR really 
didn't matter much, as long as a person could match the antenna to the 
transmitter somehow.   There was no detectable difference on the distant 
end if my antenna had perfect SWR of 1:1, or it was 3 to 1.  My 
transmitter had adjustable tuning networks that could correct for a 
certain degree of mismatch, and once the "tuning up" process was 
complete, my transmitter was happy as a clam.

Modern transmitters are a little different, but the principles are the 
same.  Instead of internal tuning adjustments to the final amplifier 
stage, modern transmitters use "antenna couplers" (also known as 
"antenna tuners") that are either internal the the set or as a separate 
external device.  The net result is the same, however.  Once the tuning 
process is complete, the transmitter "thinks" it is looking at a 
perfectly matched antenna, and all is well.

Why is some sort of antenna coupling or tuning system needed? For HF 
antennas, that is to say antennas operating in the 1.8 to 30 MHz range 
(the 160 through 10 meter bands), the bands are so wide (in terms of 
frequency excursion from one end of the band to the other) that no 
antenna can be perfectly matched across the band.  Technically, any 
antenna is perfectly matched at only one specific frequency, but  in 
real terms, all antennas have a defined bandwidth where the reflected 
power (SWR) is low enough to provide adequate power transfer without 
undue stress on the transmitter amplifier stages.  This bandwidth is 
generally that within which the SWR is 2:1 or lower.    For antennas 
intended to be used on the 160, 80, and 40 meter bands, that 2:1 limit 
represents only a relatively narrow portion of the entire band. If that 
same antenna is to be used successfully across the entire band, some 
sort of adjustable tuning network is  required.

However, before we get all hot and bothered about SWR (Standing Wave 
Ratio) it is important to understand that SWR */on the antenna 
feedline/* may be of little importance or concern.    If the system can 
be adjusted so that the transmitter itself is "happy" (feeding into what 
appears to the transmitter to be at or near to a 50-ohm resistive (not 
reactive) load), it will perform as intended, no "reflected" power will 
reach the transmitter, and no damage can occur.  Even if the SWR on the 
feed line is 3:1, 4:1, 5:1, or even higher, as long as the output tuning 
network or antenna coupling unit can be adjusted to make the SWR 
/*at*//*the transmitter antenna connection*/ appear to be 2:1 or better, 
all will be well.

This whole thing revolves around something called the "conjugate 
match".  This is a complex sounding name for an even more complex set of 
equations, and beyond the scope of one email.  But, it is easy to 
understand if you engage in a little research. Fortunately, there are 
several references available, many of which are written in "no math" 
language.

On the other hand, VHF and UHF operating presents a different set of 
conditions.  In the first place, the VHF and UHF bands are relatively 
narrow, when considered in terms of percentage change of frequency from 
one end of a band to the other.  An antenna that is properly tuned at 
the center point of a VHF band  will, generally speaking, present no 
problems from excessive SWR from one end of the band to the other.   
This "broad" statement is applicable to simple antennas such as dipoles, 
ground planes, j-poles, and similar types.  On the other hand, "gain" 
antennas such as multi-element Yagi arrays or stacked arrays of 
collinear or dipole radiators tend to be increasingly more narrow banded 
as the "gain" increases.  As a wise man once said, "There ain't no such 
thing as a free lunch" (Robt. A. Heinlein).

Rather than go into the details of gain antennas here (you would be 
reading for weeks), it is sufficient to say that if you build a VHF or 
UHF antenna such as a J-pole or ground plane by following the dimensions 
given in a published diagram or construction article,  you won't have 
problems, and that antenna will work pretty much as intended.

SWR for HF antenna systems is really sort of a "paper tiger", and really 
doesn't matter if you can get the entire antenna system set up 
properly.  On VHF and UHF the situation is somewhat different because 
feed line losses can become an important part of the design 
consideration, but even then, there ways to handle it.

So to summarize, don't let the lack of an antenna analyzer dissuade you 
from building your own antennas.  Doing so is a much cheaper way to get 
on the air, and you won't be so hesitant to tear one down and try 
something else if you  are into experimenting.  Having instruments 
available is nice, but by no means crucial.

All this being said, as it turns out, I happen to have an antenna 
analyzer.   It is a useful instrument, but I use it basically as a 
"confidence check" to see if what I just put up is (probably) working, 
or if something I have up quits working, then it is a good tool to use 
in figuring out where the problem lies.

If I am asked to advise a new ham what test equipment they should have, 
I tell them "If you can have only one piece of test equipment, get a 
good digital multimeter.  If you can afford more than on item, make the 
second thing an antenna analyzer".  But, against that, remember that I 
got on the air, operated for several years, and had thousands of  QSOs, 
all before I had even one piece of test gear.  Not even a multimeter!   
It can be done, and you can do it!

If you need additional assistance, I will be glad to help where I can.   
You can contact me by email as: kl7cc at arrl.net, or if you are a 
Anchorage ARC club member, via the KL7AA website as VE at kl7aa.net.

73

- Jim, KL7CC