[HCARC] Baluns, Ununs, Gamma Match, Stubs

[Kerry Sandstrom]

Hi Gary,

Yes, I've noticed how quiet things have been.  For a while I thought the 
reflector had stopped working, but then K5XA sent out a test message.

An antenna tuner does two things:  One is it adds reactance in the right 
sign to make the load purely resistive; and second it acts as an RF 
transformer to convert the the resistive load to the correct resistive load 
for the transmitter.  When you are tuning an antenna tuner, that is what you 
are doing, but neither you nor the tuner think about it that way!

A section of transmission line that is either open circuited or short 
circuited acts as a pure reactance.  Using a Smith chart, you can see how 
long to make the transmission line so it has the reactance you want. 
Quarter wave sections of transmission line act as transformers to change the 
impedance of the load without adding or reducing reactance.  Smith charts 
were designed to solve matching problems.  You can do it with equations, but 
they aren't pretty!

Reactance is either capacitive or inductive.  You add a capacitor or 
inductor to add reactance.  Impedance generally has a complex value, that 
is, Z (Impedance) = R (Resistance) +/- j X (Reactance), where j = square 
root of -1.  The value of reactance is either + or - depending on whether it 
is inductive or capacitive and whether you add it in series or parallel.

Microwave transmission lines are generally matched using 2 or 3 adjustable 
stubs spaced along and attached to a section of waveguide or transmission 
line.  At HF, waveguide and transmission lines are generally too long to be 
practical so we use lumped circuit elements, inductors and capacitors, but 
at VHF/UHF it is not unusual to see transmission line matching sections and 
stubs.

Believe it or not, it is easier to due it by trial  and error than to try to 
calculate and measure everything.  There are just too many unknown variables 
to really get close to the right answer.  Experience, both your own and 
others, will get you close enough to construct a matching section close 
enough to correct with adequate adjustment range.

A gamma match is an antenna tuner at the antenna instead of at the 
transmitter.  A dipole, for example, has an impedance depending on its 
length.  If it is in free space and if it is at resonance and if it is fed 
at its center, its impedance is 73 + j0 Ohms.  If it doesn't meet all those 
conditions it is R +jX Ohms where R and X are unknown and the sign of X is 
unknown.  What a gamma match does is tap the dipole element at a distance 
from the center where the real part of the antenna impedance is 50 Ohms ( to 
match the coax) and the reactive part is inductive.  The capacitor in the 
gamma match is of the opposite sign of the reactance in the dipole impedance 
so the result is a pure resistive impedance of 50 Ohms.  A T-match is just a 
balanced gamma match.  A hairpin match is a T-match using inductance.  Yes, 
these are examples of people (manufacturers) creating new names to describe 
old things to confuse users.

I figure out how long to make a gamma match by looking at what other people 
have used for similar antennas.  Same thing for the capacitance.  That is 
the "experience of others part".

Yes, you can calculate all these things.  I don't think the answer will be 
particularly close.  You can measure it also, and that answer should be OK, 
but then you still have to figure out how to adjust the series capacitor and 
gamma element length to get a match.  That will be still be by trial and 
error.  So, why go to the trouble of calculating/measuring if you still have 
to do trial and error?

No, I don't have an antenna analyzer and its not even on my wish list! 
Everything I've done from HF through microwaves, both ham and professional, 
has been trial and error.  With a little practice its not difficult at all. 
Note that I have simplified the description somewhat.  I don't think the 
most practical way is to eliminate the reactance in one step and then 
transform the impedance in a second step.  Its actually done together and is 
more difficult to actually calculate.

As far as the diference between an air core and a ferrite balun, the ferrite 
balun will be smaller and have less wire than an air core balun because of 
the magnetic properties of the ferrite material.  The disadvantage of the 
ferrite material is it can saturate and generate non-linear responses at 
high power levels and also, I believe, at high impedance levels.  PVC is not 
the same as air core, by the way.  There have been cases of people using 
construction plastic pipes for high power coil forms.  The material was 
lossy and when RF was applied, the electromagnetic fields from the coil 
heated  the plastic form and it melted.  I don't know how to tell the RF 
properties of construction material so I generally avoid using them.   In 
years past, people who used air core baluns usually used air wound coils 
with interlaced windings.  I think B&W and /or Airdux made special coils for 
use in baluns.  If you look at RF coils used in military and commercial and 
most ham gear many years ago, you will notice that they were all wound on 
ceramic forms.  Now we use plastic of various kinds - not because it is 
better - because its cheaper and lighter.

Have fun,

Kerry