[HCARC] Baluns, Ununs, Gamma Match, Stubs
Bill Tynan
billandmattie at windstream.net
Sun Dec 9 19:51:09 EST 2012
Gary:
I cam across a very interesting balun design recently. I think it was in
QEX.
You run your coax to a common T connecter. There you attach two runs of
coax. One is 1/4 wavelength long and goes to one side of a 1/2 wavelength
dipole antenna. The other is 3/4 wavelength long and goes to the other half
of the dipole.
The delay on the longer piece produces a 180 degree phase shift to the
other, exactly what you want to feed a balanced 1/2 antenna. No lossy
ferrites etc.
The only problem is that it's a single band antenna. However it might work
on the third harmonic such as 40 meter antenna used on 15 meters.
Of course, to work properly, the two lengths of coax must to cut very
accurately.
73,
Bill, W3XO/5
----- Original Message -----
From: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>
To: "Kerry Sandstrom" <kerryk5ks at hughes.net>
Cc: <hcarc at mailman.qth.net>
Sent: Sunday, December 09, 2012 2:10 PM
Subject: Re: [HCARC] Baluns, Ununs, Gamma Match, Stubs
> "Yes, these are examples of people (manufacturers) creating new names to
> describe old things to confuse users."
>
> And us newbies (Little Pistols) are VERY EASILY confused.
>
> Will have to re-read your response multiple times digest it fully. It's
> why I have been a proponent of the "smart/experienced" members of the club
> coming up with some sample Baluns, Unun's, etc that are known to work and
> that can be copied by others wanting to build somehing vs buy something.
> I have almost decided to buy something and do some destructive
> investigation to see what it's components are. However, 1) I don't know
> which to buy, and 2) in all likelihood they have removed all of the
> identifying numbers on the parts.
>
> I am very much in favor of simplicity and KISS.
>
> Gary J
> N5BAA
> HCARC Secretary 2013
>
>
> ----- Original Message -----
> From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
> To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>;
> <hcarc at mailman.qth.net>
> Sent: Sunday, December 09, 2012 1:39 PM
> Subject: Re: [HCARC] Baluns, Ununs, Gamma Match, Stubs
>
>
>> 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
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> -----
>> No virus found in this message.
>> Checked by AVG - www.avg.com
>> Version: 2013.0.2793 / Virus Database: 2634/5928 - Release Date: 11/30/12
>> Internal Virus Database is out of date.
>>
>
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