I’m using a choke balun (9 turns of RG-213 on a 4” PVC pipe) for an m104c beam as one way to avoid interference. The last article I read says that the inability of the coiled coax to dissipate heat, translates into ‘warming-up’ of the cable, with a resulting change in impedance and an overall reduction of the power reaching the antenna. The author made an impasse comment regarding the uselessness of an air choke balun for yagis with gamma matching devices. 1) Are there any truths to these conclusions, or perhaps a ‘power threshold’ at which it would apply? 2) Are there discernible benefits to the use of an ugly balun with 10/11 meter yagis?...or is it just a waste of coax?
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ugly balun deja vu
- Thread starter Wire Dawg
- Start date
Look at the very last bar on bottom of this chart.
The one that says '5T RG213 on 4.25" air cored'.
That green band is the band that is most efficient at 27mhz. It is more efficient for removing CMC's because the impedance is at its highest point to resist the CMC's coming down your coax. Also notice that it calls for 5 turns of RG213; not 9 turns. A balun with 9 turns will be more effective at ~20mhz; not 27mhz. So all you have to do is unwind four turns off of your balun - and it will be as right as rain for 27mhz.
I have to think that the only 'power losses' with a coaxial choke is what would be lost because of the use of the length of coax made in making the choke. I can't see/understand why the losses from 10 - 20 feet of coax would make any significant difference at all. The 'power threshold' would be the same for the power limit of the coax used. If that RG-213 causes any power losses then I would have to think you would have to be using a hell'uva lot of it!
The benefit of using a coaxial choke is that it reduces the common mode currents (unwanted RF currents) on the -outside- of the coax, it doesn't do anything to what's on the -inside- of that coax. I don't think it would make any difference if it was used with a gamma match or any other kind of impedance matching device, or no matching device at all. A coaxial choke seldom does anything detrimental, it's just more feed line.
I haven't seen the article you're talking about so have no idea of it's purpose or intent. Going only by what you've said here, I think the author is either mistaken or being miss-quoted or miss-quoting something.
- 'Doc
The benefit of using a coaxial choke is that it reduces the common mode currents (unwanted RF currents) on the -outside- of the coax, it doesn't do anything to what's on the -inside- of that coax. I don't think it would make any difference if it was used with a gamma match or any other kind of impedance matching device, or no matching device at all. A coaxial choke seldom does anything detrimental, it's just more feed line.
I haven't seen the article you're talking about so have no idea of it's purpose or intent. Going only by what you've said here, I think the author is either mistaken or being miss-quoted or miss-quoting something.
- 'Doc
I think the problem is a misunderstanding/misuse of terms/names. That 'dirty balun' isn't a balun at all, it's a choke. It won't do any 'balancing' but it can help reduce any CMC on the outside of the coax feed line. It doesn't affect what's inside that coax at all. So any power dissipated (from 'TTT's article) is dealing with those CMCs on the outside of the choke, which is good, you wanna dissipate it before it get's back to the transmitter end of the feed line.
- 'Doc
- 'Doc
It is comforting to know that the actual effect of the choke is not voodoo. Robb: the chart is really nice, and I wish I had it before I put the antenna up on the tower. As it is, it is going to have to wait until maintenance may be needed, as it is too much work climbing and taking the beam down/up. Will try to decipher the article from wavrider. Thanks to all.
B
BOOTY MONSTER
Guest
Booty witha Gamma if it is adjusted correctly there will be 50 ohm's impedance or X=0 so there should be no CMC on the coax.
Key word is IF adjusted correctly
B
BOOTY MONSTER
Guest
silly me . i thought CMC's were typically (but not always) caused by lack of ground plane ..... not by lack of perfect antenna tuning .
since a tapped coil or loop can typically tune CB antennas to 50 ohms , how can a gamma tuned antenna not have CMC's but the others can ? how does the gamma stop CMC's ?
since a tapped coil or loop can typically tune CB antennas to 50 ohms , how can a gamma tuned antenna not have CMC's but the others can ? how does the gamma stop CMC's ?
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... and it will be as right as rain for 27mhz.
nice chart,.... I don't think I have ever seen that one before
CMC's are typically caused by connecting an unbalanced feedline (coax) to a balanced antenna or to an antenna with inadequate groundplane (again lack of balance or symmetry). Since a gamma fed yagi is not a balanced antenna nor does it require a groundplane of any sort there should be no CMC's present.
And as 'wavrider' pointed out, if the gamma is adjusted -correctly- there -shouldn't- be any CMCs present. That's a big 'if', and certainly doesn't mean a gamma match is 'fool proof' by any means, or any other type of impedance matching device.
- 'Doc
(Do/did I think you weren't aware of that? No, but it doesn't hurt repeating, others can certainly get that idea.)
- 'Doc
(Do/did I think you weren't aware of that? No, but it doesn't hurt repeating, others can certainly get that idea.)
Booty I was on the way out the door to work when I replied so I did not go into detail.
Looks like CK and Doc posted what I left out.
Looks like CK and Doc posted what I left out.