Choke Balun - a few questions..

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NE1C4NSC4N

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Hey folks, I hope everyone out there is having a good weekend. So now that winter is in full swing here in NS, I'm stuck inside at my workbench. I have been trying out some different ideas for making antenna's, but the one design I keep coming back to is the simple old 1/2 wave Dipole.

As I have read, I need to create an unbalanced feed point with a dipole, since it is a balanced antenna, and coax is unbalanced. So I am in the process of trying to find any kind of formula for creating said Balun. Here is a link to the page that I have based my "design" off of :

Martin E. Meserve - K7MEM - Center-Fed Half-Wave Dipole ( 3-30 MHz )

My Questions :

1) Is there a formula for making a choke balun?

2) If not, is the general info I have found about making one out of 8 to 10 loops of coax, with a diameter of around 8 to 10 inches the best way, or is there a more precise way of doing it, according to the antenna it self or what not..

Any input is much appreciated. Thanks in advance.
 

davedaver1

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1) The web page you cited has a decent description of how to make the choke balun (which, as he noted, is not much of a balun - it's really a choke and prevents your dipole from being a tripole and radiating off the coax and back into your shack!) An alternative to the coils of coax is to wind coax around a PVC pipe - and you can use that to make the center of the antenna. Here's a great article on how to make one: BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!

2) While it's not really critical, it's more about the length of the coax used to make the coils, about 24 feet would be fine, in whatever way you make the coils - wider loops bound together with tywraps or closely wound on a PVC pipe.

Make sure you weatherseal all the connections! Also, if you use tywraps you need to use the UV-stabilized ones. The white and un-UV-stabilized cheap ones will rot in a summer or two.

BTW - if you build the PVC-wound version, I would use coax seal on the exposed wires (electrical tape won't last long and doesn't seal well at the ends) and I would also use a PVC cap, which would help with the side-to-side strength where the elements are pulling in two directions. You wouldn't have to glue the cap on, just make sure it fits tightly and won't slide off while the antenna's bouncing in the wind!

Have fun!
 
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prcguy

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The ugly balun is not very effective compared to some other easy to make choke baluns using ferrite toroid cores. An ugly balun is also fairly narrow band where ferrite versions can be effective 80 through 10m.

A good starting point is 10 turns of small coax like RG-58 through an FT240-43 core or maybe a few less turns through an FT240-61 or other material.

You can use an antenna meter like the MFJ-259 to measure how effective an choke balun is by connecting the braid of one side of the balun to the antenna connector center pin and the other end of the balun to the meter ground. Instead of the meter showing a short it will show how much choking impedance the balun has up to 500ohms.

I like to see the meter off scale above 500ohms down to 160m so I know it will be much higher on 80m and up.
prcguy
 

LtDoc

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A choke is not a balun in any way. It's given that description, 'balun', because it can do something a balun can also do, get rid of unwanted currents (CMCs) on the outside of the coax feed line. It can't transform from a balanced to unbalanced state.
If you are only going to 'listen' with this antenna you really don't need a balun or a choke. Shouldn't hurt if you used them, but they just are not needed. That's up to you. If you are also going to use the antenna for transmitting then both a balun and a choke could certainly be useful... if you need them. You won't know that till you try it.
- 'Doc
 

prcguy

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An effective common mode choke on the feedline can isolate an antenna and feedline to the extent of forcing a balance on the antenna side. I would not consider an ugly balun effective enough to isolate enough to force a balance.

Ten turns of coax around an appropriate mix ferrite toroid where you double back on the toroid at 5 turns then reverse direction for the last 5 turns can be very effective, see:

http://kambing.ui.ac.id/onnopurbo/o...un, current choke, choke balun, 50 ohm 11.htm

prcguy
A choke is not a balun in any way. It's given that description, 'balun', because it can do something a balun can also do, get rid of unwanted currents (CMCs) on the outside of the coax feed line. It can't transform from a balanced to unbalanced state.
If you are only going to 'listen' with this antenna you really don't need a balun or a choke. Shouldn't hurt if you used them, but they just are not needed. That's up to you. If you are also going to use the antenna for transmitting then both a balun and a choke could certainly be useful... if you need them. You won't know that till you try it.
- 'Doc
 
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k3cfc

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Hey folks, I hope everyone out there is having a good weekend. So now that winter is in full swing here in NS, I'm stuck inside at my workbench. I have been trying out some different ideas for making antenna's, but the one design I keep coming back to is the simple old 1/2 wave Dipole.

As I have read, I need to create an unbalanced feed point with a dipole, since it is a balanced antenna, and coax is unbalanced. So I am in the process of trying to find any kind of formula for creating said Balun. Here is a link to the page that I have based my "design" off of :

Martin E. Meserve - K7MEM - Center-Fed Half-Wave Dipole ( 3-30 MHz )

My Questions :

1) Is there a formula for making a choke balun?

2) If not, is the general info I have found about making one out of 8 to 10 loops of coax, with a diameter of around 8 to 10 inches the best way, or is there a more precise way of doing it, according to the antenna it self or what not..

Any input is much appreciated. Thanks in advance.
Here is something you may be interested in.

http://www.earchi.org/92011endfedfiles/Endfed6_40.pdf
 

k3cfc

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Hey folks, I hope everyone out there is having a good weekend. So now that winter is in full swing here in NS, I'm stuck inside at my workbench. I have been trying out some different ideas for making antenna's, but the one design I keep coming back to is the simple old 1/2 wave Dipole.

As I have read, I need to create an unbalanced feed point with a dipole, since it is a balanced antenna, and coax is unbalanced. So I am in the process of trying to find any kind of formula for creating said Balun. Here is a link to the page that I have based my "design" off of :

Martin E. Meserve - K7MEM - Center-Fed Half-Wave Dipole ( 3-30 MHz )

My Questions :

1) Is there a formula for making a choke balun?

2) If not, is the general info I have found about making one out of 8 to 10 loops of coax, with a diameter of around 8 to 10 inches the best way, or is there a more precise way of doing it, according to the antenna it self or what not..

Any input is much appreciated. Thanks in advance.
Here is another one you may be interested in.


Hope this helps
Windom Antenna Home Page, and Handbook
 

NE1C4NSC4N

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Hey guys, Thanks a bunch for the great replies! And the great links too, I have Google'ed around a lot, and never came across any of those pages.

As LtDoc mentioned, I won't be transmitting with this antenna, since I am not a Licensed HAM, yet at least :) I was actually going to make this Dipole for my ADS-B receiver, since the antenna that came with it, for one is some sort of dual band, and secondly a magnet mount, so I thought a small dipole would be more fitting to be up on my mast. I also thought about a 1/4 wave GP, but making a dipole is a whole lot easier, materials wise. I can pretty much make one stop at the hardware store and I'm all set.

Do any of you guys have any advice, or words of wisdom for working with ADS-B. Any suggestions for better ideas than a dipole for this setup? Obviously buying an antenna is my surest option, which is my plan, in a few months, but for now I want to tinker around with this.

I also am planning on making a dipole, for the 340.00 Mhz area of the spectrum. Any thoughts, ideas or whatever on a dipole in this area, also for listen only.

Thanks again guys, I appreciate you all taking the time to share all this great info! Cheers. :D

Ian
 

k3td

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A choke is not a balun in any way. It's given that description, 'balun', because it can do something a balun can also do, get rid of unwanted currents (CMCs) on the outside of the coax feed line. It can't transform from a balanced to unbalanced state.
If you are only going to 'listen' with this antenna you really don't need a balun or a choke. Shouldn't hurt if you used them, but they just are not needed. That's up to you. If you are also going to use the antenna for transmitting then both a balun and a choke could certainly be useful... if you need them. You won't know that till you try it.
- 'Doc
This description is a keeper 'Doc!
 

nanZor

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Let's put some actual numbers on both ferrite, and most importantly air-wound chokes should you decide to use them in a pinch:

Common-mode chokes

Personally I always use a feedpoint choke - one reason is to maintain the directionality of the antenna without having the coax braid distort the pattern. Although as mentioned before, sometimes this isn't a major criteria for general purpose listening.

Yet the 2nd biggest reason I use them is to help prevent noise INgress - that is rf interference and other junk generated in the shack from traveling UP to the feedpoint in common-mode, and right back down into the receiver via the coax internal differential mode.
 
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N1BHH

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Hey folks, I hope everyone out there is having a good weekend. So now that winter is in full swing here in NS, I'm stuck inside at my workbench. I have been trying out some different ideas for making antenna's, but the one design I keep coming back to is the simple old 1/2 wave Dipole.

As I have read, I need to create an unbalanced feed point with a dipole, since it is a balanced antenna, and coax is unbalanced. So I am in the process of trying to find any kind of formula for creating said Balun. Here is a link to the page that I have based my "design" off of :

Martin E. Meserve - K7MEM - Center-Fed Half-Wave Dipole ( 3-30 MHz )

My Questions :

1) Is there a formula for making a choke balun?

2) If not, is the general info I have found about making one out of 8 to 10 loops of coax, with a diameter of around 8 to 10 inches the best way, or is there a more precise way of doing it, according to the antenna it self or what not..

Any input is much appreciated. Thanks in advance.
If you are using this antenna for transmitting then an ugly balun will work, or a W2AU balun or any of the variety found at packetradio.com will do fine. As a receiving antenna, just hook the coax to the two wires. You could buy the Budwig HQ-1 for the coax connection, and hang it from the highest possible point and the legs as high as possible. That way you won't have coax weighing down the antenna.

budwig in Radio Communication | eBay
 

LtDoc

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"1) Is there a formula for making a choke balun?"
Unfortunately, no there isn't. At least not a -simple- one, there are too many variables. Take a look at the link in post #10, it will give you a fair idea of what's possible using specific items (coax/cores/etc.). Change one of those 'items' and the results change. If you can stay with the equipment/gear/'items' listed in that table with no trouble, you will at least get close to 'good enough'. That's about as 'simple' as it gets. If you want to learn how/why each 'item' was used, how the calculations are done, and then follow them, you certainly can. But it ain't -simple- no more, right? Oh well...
- 'Doc

(Or do like most people do. Try a number of coils of coax. Still got problems, add a few turns more. Quit when the problem goes away or you get tire of adding turns to the mess.)
 

USASAgencyman

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Off center fed long-wire application of this choke

Hello,

I am curious about the use of this method of matching my RG8X to an off center fed Windom long-wire antenna. I had planned on DIY'ing a 4:1 balun.

If it will work, I also wonder if the center conductor should feed the long or short leg of the antenna.

Thanks if you read this, many more thanks if you have time to respond.
Bruce


Bruce Hinton
 

prcguy

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Since the ugly balun keeps rearing its useless head I'll say again, they are not effective and have no as in zero resistive properties at any frequency and are only reactive, which at best will reflect common mode currents on your coax back up the coax to the source to bounce back again and again. On the other hand a properly designed choke balun made with ferrite or other lossy material will absorb common mode currents and turn them into heat instead of reflecting them back to the source and over a fairly wide frequency range.

Please read the excellent link in hertzain's post # 10 in this thread where it gives examples of ugly balun dimensions and winding's for all HF bands at the bottom of the chart and also points out they have no resistive choking impedance and they are very narrow band at best. Compare that to other ferrite chokes listed on the chart and look for examples showing green shades indicating a very high impedance at the frequency of interest plus a black line showing its also resistive over the frequency of interest.

And for LtDoc's comment than a choke is not a balun, it certainly is when it has enough resistive choking impedance. Take a good 1:1 choke balun with 30dB or more isolation and it will force a very nice balance on the unbalance coax feeding it. Whatever happens on one side of the choke balun will be practically invisible on the other side when your talking 30dB or a 1,000:1 isolation ratio.

And I also disagree with the comment that a balun or choke is not needed for receiving only. I experienced this recently and to more of a degree than I ever expected. My main HF antenna has a transition from balanced line to coax and at one time I used a W2AU 1:1 balun at the window line/coax junction, which turns out to be a lousy choke and I had noticeable RF on my coax. I made a more effective choke using 10 turns of RG-142 Teflon coax around a large FT-240-43 core and it fixed the RF on the coax problem that the W2AU balun could not.

I recently designed an elaborate 1:1 common mode choke using some info from hertzian's link and also some testing with a vector network analyzer where I'm using four large 240 series cores of different permeability to cover different parts of the HF bands and the project was inspired by this product: CMC-230-5K - MyAntennas.com

After replacing my simple 10 turns of coax around a single core choke balun with the very elaborate version I noticed the noise floor on my HF radios spectrum display had gone from a continuously varying and bumpy one to a really flat noise floor across the entire HF band with just an occasional birdie or small noise bump, not counting legitimate signals of course.

Apparently I had a lot of RFI traveling up my coax to the antenna due to most of my antenna cables passing right by a computer, monitor, router and lots of switching power supplies. The really effective 1:1 choke balun made a very impressive reduction in my receive noise and I'm very happy. A good choke balun can improve your receive! On the other hand an ugly balun is not worth the cardboard or PVC its wound on.
prcguy



Here is some information on building a choke balun. at least this is what they call it.

BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!
 
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USASAgencyman

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I thought it was in the "too good to be, (completely) true" category

PRCGuy,

I will stay with plan-A and build a good 4:1 ferrite based balun. I realize I am with some posters from years ago, who were listening only, and nothing subtracts from nothing. But how could it hurt?

I, as a neophyte, think such a choke might be a good addition to a feed line, (not sure where!), if the basics are taken care of first, with a proper balun at the feed point of the antenna.

Back to one of my earlier questions though; I think I know which is the hot lead from my soon-to-be-built balun, but should it feed the long or short leg of the antenna? The off-center antenna feed part is not often brought up in these transformer discussions.

BTW, I am grateful that this is still an active group! This response is even more than I had hoped to learn so quickly.

One day I will contribute... :) :)

B.
 

prcguy

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Its very hard to tell which lead of a 4:1 balun is considered hot. If its a really good balun with lots of isolation it doesn't matter. If its a voltage balun then it might.

I've been building excellent low power 4:1 baluns that are good from 1 to 50MHz similar to a kit sold by Elecraft. These will handle about 150w, are very small and only cost about $5 to make using a BN-43-7051 core available here: BN-43-7051 - Amidon with instructions here: http://www.elecraft.com/manual/E740061 BL1 4 to 1 Rev B.pdf

Its best to use Teflon wire but you can also strip the wire out of CAT5 cable as its insulation has good RF properties. Just cut two pairs of wires about a foot long, then wrap 3 turns through each hole and connect per instructions.

This type of balun has excellent common mode rejection and works great for offset center fed dipoles, etc.
prcguy

PRCGuy,

I will stay with plan-A and build a good 4:1 ferrite based balun. I realize I am with some posters from years ago, who were listening only, and nothing subtracts from nothing. But how could it hurt?

I, as a neophyte, think such a choke might be a good addition to a feed line, (not sure where!), if the basics are taken care of first, with a proper balun at the feed point of the antenna.

Back to one of my earlier questions though; I think I know which is the hot lead from my soon-to-be-built balun, but should it feed the long or short leg of the antenna? The off-center antenna feed part is not often brought up in these transformer discussions.

BTW, I am grateful that this is still an active group! This response is even more than I had hoped to learn so quickly.

One day I will contribute... :) :)

B.
 

nanZor

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I will stay with plan-A and build a good 4:1 ferrite based balun.
That's fine - BUT, since your off-center fed antenna is *extremely* unbalanced by design, that "balun" is doing nothing more than an impedance transformation basically - not much BALancing of currents will take place.

Thus, you will want a 1:1 current balun following it as well.

And, as noted before, the ugly-balun is just that: UGLY IN PERFORMANCE. Unfortunately, the simplicity of it all is tempting, but is a waste of coax. In many cases, it just moves the problem to another area. So use a broadband ferrite-based design, or if using coiled coax, one must be VERY careful about the total amount of winds, circumference, and size of coax - and all within a very limited bandwidth as seen in G3TXQ's chart.

The same issue exists today although most guys don't do this any more - wind their own "air-core" 4:1 / 9:1 baluns on pvc, usually with a bi or tri-filar wind of #14 or #12 wire, about 12-13 turns. Problem is, it is narrow in bandwidth, and at the higher end of the spectrum, has a lot of capacitive reactance. It was the only way to go before ferrite.

I, as a neophyte, think such a choke might be a good addition to a feed line, (not sure where!), if the basics are taken care of first, with a proper balun at the feed point of the antenna.
You think correctly! Place it right after the 4:1 "balun", which with your antenna especially, is only doing impedance transformation.

You'll note that the same issue exists with the scanner-antenna, the "OCFD vertical" for the vhf-uhf bands. The 75-300 ohm tv-balun is NOT doing much balancing at all! Merely impedance transformation since the OCFD is waaaay unbalanced to start with and too much for the balun to cope with as far as balancing goes. Thus, some put a few small clamp-on #43 ferrites on the feedline near the feedpoint. Same issue, just much higher in frequency. :)
 

prcguy

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Since most offset center fed dipoles will have some common mode current problems on the coax, at least one company has made good use of that by strategically placing an RF choke in the feedline so the vertical hanging feedline becomes a predictable and useful radiator at some frequencies.

RadioWorks does this and offers radiation pattern charts showing the low angle improvement on some of the higher HF bands. They seem to use about 10ft of feedline before the choke on their 40m version and about 20ft of feedline on their 80m version.

This trick would be frequency dependent and probably confined to the amateur bands, since that is what these specific antennas are designed for. If your offset center fed dipole is made to amateur band dimensions and you plan on listening to HF amateur stuff you might consider copying the RadioWorks idea and placing a 1:1 choke balun in a similar spot in the feedline.
prcguy
 

USASAgencyman

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I see I have only begun; (to be confused in new ways!)

A lot to learn from just this thread.

I hope to find a few good consistent signals inside of skip distance to use as a frame of reference as I listen and fool with this antenna, transformers and chokes, etc. I have a lot of experimenting to do just from the suggestions here.

Thanks for all the guidance!

My only actual radio activity is 2m and 70cm local and repeater, I have a couple of HTs and a small J type antenna on the roof that does a pretty good job, (surprisingly), of getting to the repeater I most often use, about 23 miles with the first five populated with giant Oak trees.

Looking forward to DX HF transceiver work, 160m-6m.

(Had quite a bit of antenna and general RF electronics in my short three and a third years in the Army Security Agency. Still love it. Something about using the right tricks to allow the Earth and my gear to reach out that appeals more than cell phone chatting around the world.)

Bruce
WW4ASA
 
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