[HCARC] Baluns, Ununs, Gamma Match, Stubs
Gary and Arlene Johnson
qltfnish at omniglobal.net
Sun Dec 9 20:32:16 EST 2012
Bill,
I think I saw that article also. Of course sometimes I don't know what they
are saying.
Gary J
N5BAA
----- Original Message -----
From: "Bill Tynan" <billandmattie at windstream.net>
To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>; "Kerry Sandstrom"
<kerryk5ks at hughes.net>
Cc: <hcarc at mailman.qth.net>
Sent: Sunday, December 09, 2012 6:51 PM
Subject: Re: [HCARC] Baluns, Ununs, Gamma Match, Stubs
> 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
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> -----
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>>>
>>
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>
>
>
> -----
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