The "Ugly Balun"

[Robb]

A little confusion about coax chokes/baluns.
After reading this link, some things fell into place.
BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!
I recommend reading the entire page...

Coax choke = 1:1 balun
But will it affect the present SWR/inpedance?

Although this link is a Ham site, the info is still relevant to CB antennas and CB operators. One of the points that confused me a bit, is that this article pointed out that beam antennas also need to have a choke/balun. I was told previously that if a beam has a gamma match on it - then a coax choke/balun isn't necessary.
Vertical and dipole antennas do require them.

Why does one need one? Well, to force all of the transmitted energy from your radio and off of the coax and mast - and keep this energy on the antenna where it belongs. Without it - you are losing efficiency and potentially making neigbors enemies. Although they do not cure all of these kinds of problems - making and installing one will assure that it can minimize TVI('TV interference') and RFI('radio frequency interference'). So long as your vertical antenna is isolated from the mast and building/using an 'ugly' and grounding your station - you have done all that you can to make things right for your neighbors and you.

The best and worst of this 'ugliness':
*Minimum 20 ft of coax is required for the choke/balun
*4 to 8 inch wide coil form can be used
*If using RG-58(thin coax); then use the 4 inch coil form or larger. If RG-8(thick coax); then the 8 inch coil form or larger.
*Keeping the choke/balun off or away from the masting itself is essential when mounting
*20 extra feet will contribute to greater line loss due to overall/net longer coax length
*There are no consistant method to make one; make one yourself using the guidelines!

The use of stacked toriod ferrites was discussed on QRZ - concerning the MFJ-915 RF Isolator:
http://forums.qrz.com/showthread.php?t=216977

The MFJ-915 review on eHam. Might be a usable alternative!
http://www.eham.net/reviews/detail/7780
http://www.mfjenterprises.com/Search.php?searchit=MFJ-915

 

[packrat]

A choke / balun has nothing to do with swr or signal. All an ugly balun sic (choke)will do is help keep stray rfi from getting back into your station on the coax shield.
You also dont need one unless you are experiencing rf in the shack.
It wont do a thing for your neighbors.
PR

 

[W5LZ]

I would suggest you go back and read the very first paragraph of that article again. That Coaxial 'choke' is not a balun. You can call your cat a dog, but I wouldn't quit buying cat food till you hear it barking, you know?
- 'Doc

 

[Lazybones1222]

All I can say is: ladder line.

 

[Sonwatcher]

All an ugly balun sic (choke)will do is help keep stray rfi from getting back into your station on the coax shield.

When I first made my Windom I experienced a lot of stray RF in the shack. The instructions for the Windom stated that if a person chose to use coax as a feedline this problem would occur. They stated to make an "Ugly Balun" . I did and it work excellent ! Stray RF was gone. They work great for getting rid of the stray RF on the feedline. At least it did in my case.

 

[Simon004]

the following is just "the way I see things", I've done no testing, just reading.

The "ugly balun" or coax choke is just a way to reduce CMC's. It is not a balun, and I agree, if you have no CMC's, it is pointless to add one to your antenna system.

Baluns can be designed so they can transform impedance, a coax choke cannot. Also, baluns can be designed to work over a wide range of frequencies, all of HF for eg. This is the thing I was surprised to learn, an air cored coax choke, or "ugly balun" cannot. There is a limited frequency range where a coax choke is effective. There is also a drop off of choking impedance, below, and above the frequency the choke is designed for. I can't take credit for finding this, it was posted on another forum, but it is interesting nonetheless.

I got this from this page Common-mode chokes

This fellow claims to have tested a wide range of coax chokes on different formers, and these are the results. For a choke to work you would want maximum impedance at the specific frequency of interest, and as it shows, impedance falls away both above and below the frequency you design it for.

Now the ham universe site that is saying that a minimum length of 20' of coax is needed maybe true for the best compromise for a multiband HF antenna, but going of the chart I posted above, it is not optimal for 27MHz.

As for whether a choke can be a substitute for a 1:1 balun, I think it can be, by presenting a high impedance on the outside of the screen of the coax at the feedpoint, this will force more current into the half of the antenna connected to the screen, and preventing that current from using the outside of the screen as part of the antenna. Just the same as a 1:1 balun does.

 

[Marconi]

I agree with Simon. The "ugly balun" may be named wrong, but I wouldn't argue that distinction. I also agree that the "ugly balun" is long enough for a multi-band antenna, but is too long if you just work 11 meters. The chart Simon posted looks about right for 10-11 meters instead.

However, I made a choke recently by adding 5' feet of RG8x coax to my feed line on my Sigma 4, and I saw resonance drop 36 channels. The bandwidth increased from 1 mhz to 1.4 mhz and that was good. I also have a comment in my notes that the noise on the antenna decreased as well.

I assume the choke helped, but by adding line length to the system I saw transformation at the feed point, and that changed the resonance of the antenna. I know that Simon suggested that the choke will not transform feed point impedance and he is right, but that is exactaly what I saw when adding length like a lot of the examples show. If you make your choke using the existing feed line---then Simon is right, you should not see a change in transformation.

My antenna SWR was pretty flat in the area around resonance in the CB band, so I wouldn't likey see this happen unless I did the SWR bandwidth curves. These bandwidth curve charts are in my album dated 10-01-09.

I took the antenna down before I could redo the choke using the feed line instead of adding line length to the system, so I will have to check this out further in the future. This is something to keep in mind if you add a choke that adds length to your antenna feed line and you see a change in the resonant match.

 

[Robb]

Using Simon's chart as the source; then 10/11 meters would be best served by using 10 turns with RG-58 on a 2 inch air core?
This would make the length of coax needed to make this choke approximately >6 ft long.
Did I get that right?

Or...
5 turns of RG213 on a 4.25 inch air core?
Is the RG213 equivalent to any RG-8 type coax?
If that is the case, I can use my existing RG-8/9913Flex and use just coil up enough to make it and not have to add another piece. Keeping it one continous piece.

Pi x radius(squared) = circumference
Since the radius of a 4.25 inch coil form is 2.125 - the squaring it will be 4.52 inches
Multiplying 4.52 inches by Pi(3.14159) will equal 14.19 inches - or 1.18 ft for each turn on the coil form.
Multiplying 1.18 ft by 5 turns = 5.91 feet of coax needed for the 4.25 inch air choke.
I trust my math was correct - or close...

 

[W5LZ]

Marconi
"I made a choke recently by adding 5' feet of RG8x coax to my feed line on my Sigma 4, and I saw resonance drop 36 channels. The bandwidth increased from 1 mhz to 1.4 mhz and that was good. I also have a comment in my notes that the noise on the antenna decreased as well."

Okay, but that length just happens to be real close to a 1/4 wave length depending on the VF of the coax used, and a resonance shift typically deals with things on the inside of the coax not the outside of it, which that choke only affects. It also makes me think that if changing the length of that feed line changed the performance of the antenna, then the antenna didn't have the same impedance as the rest of the antenna system. Wasn't matched very well. Especially since the length of the antenna didn't change.
---
Your next statement supports the above.

"I assume the choke helped, but by adding line length to the system I saw transformation at the feed point, and that changed the resonance of the antenna. I know that Simon suggested that the choke will not transform feed point impedance and he is right, but that is exactaly what I saw when adding length like a lot of the examples show."

So, the added feed line length made the difference, not the fact that it was in the 'form' of a choke, which would only have made a difference in what was happening on the outside of the feed line, CMCs. Right?
None of which disputes what you found, only the reason for it.
- 'Doc

 

[heyu]

Pi x radius(squared) = circumference
Since the radius of a 4.25 inch coil form is 2.125 - the squaring it will be 4.52 inches
Multiplying 4.52 inches by Pi(3.14159) will equal 14.19 inches - or 1.18 ft for each turn on the coil form.
Multiplying 1.18 ft by 5 turns = 5.91 feet of coax needed for the 4.25 inch air choke.
I trust my math was correct - or close...

circumference = pi x D

You are calculating area (square inches).

But, since you also want to add a bit to the diameter to account for the thickness of the coax, you're about close enough.

 

[Marconi]

Marconi
"I made a choke recently by adding 5' feet of RG8x coax to my feed line on my Sigma 4, and I saw resonance drop 36 channels. The bandwidth increased from 1 mhz to 1.4 mhz and that was good. I also have a comment in my notes that the noise on the antenna decreased as well."

Okay, but that length just happens to be real close to a 1/4 wave length depending on the VF of the coax used, and a resonance shift typically deals with things on the inside of the coax not the outside of it, which that choke only affects. It also makes me think that if changing the length of that feed line changed the performance of the antenna, then the antenna didn't have the same impedance as the rest of the antenna system. Wasn't matched very well. Especially since the length of the antenna didn't change.
---
Your next statement supports the above.

"I assume the choke helped, but by adding line length to the system I saw transformation at the feed point, and that changed the resonance of the antenna. I know that Simon suggested that the choke will not transform feed point impedance and he is right, but that is exactly what I saw when adding length like a lot of the examples show."

So, the added feed line length made the difference, not the fact that it was in the 'form' of a choke, which would only have made a difference in what was happening on the outside of the feed line, CMCs. Right?
None of which disputes what you found, only the reason for it.
- 'Doc

You're right 'Doc.

I always seem to have something off just a bit with my antenna systems, and it surprises me when I read about all of these guys that have perfect antenna systems after checking their SW-dubin-Rs (SWR) with their MFJ 249B/259B.

You can see the problems I have in my Antenna Work Sheets posted in my album where I hardly ever get things perfect. And, when I do see a little SWR dip in my meter or BW curve---I figure I'm as close as I will ever get.

"AS MUCH AS I TRY, I'M JUST LUCKY THAT MOTHER NATURE PROVIDES THAT I DON'T HAVE TO GET MY RADIO SYSTEMS PERFECT IN ORDER TO WORK"​

 

[Robb]

Still a bit confused - but trying...

From the Common-mode chokes page:

"...Amateur Radio (G3TXQ) - Common-mode chokes
The following chart presents the results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz. Amateur frequency allocations are indicated approximately by the vertical grey bands.

The colours of the bars indicate the magnitude of the CM (common-mode) impedance; however, depending on the style of choke and the type of ferrite material used for the core, that impedance might be mostly Resistive, mostly Reactive, or somewhere in between. The black bars at the bottom of the coloured bars indicate the range of frequencies over which the choke impedance is predominantly Resistive - that is Rs>Xs. No black bars are shown for the air-cored chokes because their impedance is almost entirely Reactive apart from a very small band of frequencies around resonance.

Reactive chokes have the disadvantage that they can "resonate" with a CM impedance path that is also reactive but of opposite sign - in some cases actually increasing the CM current flow rather than choking it; Resistive chokes have the disadvantage that if they have insufficient impedance to reduce the CM current to a very low value, there may be significant core heating.

Aim to choose a choke which has a high impedance and is Resistive over the frequency range of interest.

I hope to add more data as I make further measurements..." -G3TXQ

(Bold emphasis was mine)

He noted that the ferrite choke was better in that the resistance was non-reactive. He also pointed out that air-core chokes could worsen the CMC problem...


I'm trying to make this work the first time around! I cannot understand why some comments made said that sometimes a choke was necessary for the same kind of antenna.
If using an A99 or an Imax; wouldn't this dictate a consistent need for a choke due to the consistent design of these kinds of antennas?
Why would one of these antennas need a choke - and another doesn't?

To keep the resistance consistent, it appears that the MFJ-915 choke has its act together.
Would this device insure no CMC's due to its resistive behavior across 1-30mhz? Otherwise, the air core 4.25 inch/5 turn RG-8 coax might be hit or miss.

If it sounds like I'm a bit confused about which to choose and how to do this right the first time - you would be right...

Look at the resistive lower bar of the ferrite chokes on the upper part of the graph for the 10 meter range. They seem to be resistive - consistently - throughout the entire range. Or close to it. Reactance takes up only a small portion of that scale. However, the air core chokes are purely reactive.
Radio is a kind of rocket science, in that there is a certain amount of predictability if all of the controls/criteria are in place - right?
So - why bother with a hit-or-miss air choke when a purely resistive ferrite choke will insure positive/favorable results?

What advantage/disadvantage would be found for using the MFJ-915 choke?

As a guess based upon this chart, a FT240-61 ferrite core with 8 turns of RG-58 coax (thirteenth bar from the top of the chart) will keep any heating/RF loss to a minimum.
Can I use 9 turns to move/change/shift the scale upward slightly - so that it is closer to >8K ohm impedance for 10/11 meters?
But this also means adding another piece of coax between the antenna and the piece of RG-8/9913Flex coax that I am using presently - does this change the overall effect?
I am trying to learn and understand this subject from my perspective and the gear I am using. Not to forget what opinions and foundations I read here from those who contribute here.
Thanks!

 

[packrat]

You do not need a choke if you are not getting CMC / rf back into the shack. Is your mic biting you when you talk? Are you tearing up the phones, tv, stereo?

PR

 

[Robb]

You do not need a choke if you are not getting CMC / rf back into the shack. Is your mic biting you when you talk? Are you tearing up the phones, tv, stereo?

PR

Yup.
I had to replace the computer monitor recently. Motherboard developed a glitch that it never had before. All happened within a relatively short period of time. I think that CMC's may have done them in. New radio w/100 watt output. Computer speakers all of the time. I never put my mouth on my D104 TUPS-9 so I don't know about that one.

It looks as though I will have to go with the 5 turns of RG-8 coax on a 4.25 inch air core choke if I cannot find any specs on the MFJ-915 choke!
:bdh:

 

[Sonwatcher]

Not sure what specs you're looking for but this comes from the MFJ site-

The MFJ-915 is made up of 50 ferrite core beads placed on a 13-inch length of RG-303 coaxial
cable. The coax and SO-239 connectors have Teflon insulation for maximum insulation and life
of the product. The RF Isolator is enclosed in a Schedule 40 PVC pipe for strength and
protection.

http://www.mfjenterprises.com/man/pdf/MFJ-915.pdf