[KL7AA] Antenna Analyzer

Tue Apr 28 09:26:26 EDT 2015

Thanks Jim   Great explanation

Daniel Stevens KL7WM

In a message dated 4/28/2015 12:05:58 A.M. Pacific Daylight Time,  
jwiley at gci.net writes:

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

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