1:1 or 6:1 Balun for Flat Top dipole

[VK2OMD]

How does RF radiated from the coax cause interference?

The closer a radiating conductor is to another conductor (eg house wiring, TV feedline, computer cabling etc) the greater the energy coupled to the other conductor. This is not a linear relationship, and not simply square law as often believed, but in the near field zone the field strength may fall by as much as the fourth power of distance... so keeping distance between radiators and conductors subject to interference is simply good practice.

A feed line may be much closer to these other conductors than the nominal radiator (dipole or whatever), so the extent to which the feed line radiates and its proximity increase the risk of interference.

One of the benefits of a balun that reduces common mode current along the feed line is reduction of radiation from the feed line and reduction of the risk of EMC issues.

Owen

[VK2HRX]

Some in this thread are proponents of current baluns. For the benefit of those like me who are still learning can you briefly explain when a voltage balun would be better to instal.

[VK2AAH]

Iain,

What Owen said! Thanks Owen...

Cheers


Richard
VK2AAH

[VK2OMD]

Some in this thread are proponents of current baluns. For the benefit of those like me who are still learning can you briefly explain when a voltage balun would be better to instal.

It is better if you want equal but opposite voltages on its terminals, irrespective of load impedance or symmetry... that is what it does. Note that it is capable of delivering different current from each terminal (in a three terminal model), wheras a current balun would try to force equal (but opposite direction) currents.

If you think that most applications are better served by a balun that tends to force equal but opposite currents irrespective of the voltage at each terminal (in a three terminal model), you are probably correct.

Why do I keep saying "three terminal model"? If you treat a balun as having only two output terminals that are truly independent of everything else (like ground), then the current out of one terminal MUST equal the current into the other terminal. Believe it or not, one of the balun gurus (SK) explained that a particular configuration "worked" for an isolated load (ie a load that could be fully represented with onty two terminals that are truly indpendent of everything else, including ground). Of course, you will recognise that if you have an "isolated load" under than defintion, you don't need a balun.

Anecdotal evidence is that some loads can be 'matched up' using an ATU and voltage balun, but substituting a current baun has been unsuccessful. One explanation if that the notional transformation ratio may differ, another is that voltage baluns tend to be quite lossy on very high differential impedance loads, and they tame the impedance excursions of antenna systems at the expense of efficiency. If you want this loss padding at high voltage / high impedance, a voltage balun may be better. Antenna systems with inbuilt loss to tame VSWR excursions are not new, they are often used to good effect where operated by unskilled persons.

Owen

[VK2HRX]

I found this informative. Should I believe it?

http://static.dxengineering.com/pdf/Cho ... 0Balun.pdf

Seems that curent balun is the appropritate one for all the cuircumstances whre I would use/need one.

[VK2OMD]

I found this informative. Should I believe it?

http://static.dxengineering.com/pdf/Cho ... 0Balun.pdf

Seems that curent balun is the appropritate one for all the cuircumstances whre I would use/need one.

This discussion is about baluns in antenna systems, and does not necessarily apply to baluns inside boxes of electronics were transmission lines might be electrically very short.

I have probably read that in the past, and I am not going to red pen it now... but its author has good credentials, not infallible (none of us are), but he understands the balun problem and is the designer of some of DXE's balun product.

I am not sure what you might do or not do, but people have used voltage baluns quite effectively on Yagis on VHF and above. Some kinds of voltage baluns are very easy to construct for short wavelengths, eg the traditional half wave 4:1 balun using say UT coax on 1296 is a very convenient construction, quite efficient, and given a quite symmetric load will provide fairly good current balance.

On the other hand, a voltage balun on an OCF dipole will do precious little to reduce common mode feed line current.

First step is to analyse the structure and decide wether it is more inclined fundamentally to force equal voltages or equal currents. The next step is how well it does what it does. The common mode impedance is a key metric, it is infinite for an ideal current balun, and zero for an ideal voltage balun.

So, should you ALWAYS use a current balun? Up to you, by I would not limit myself in that way, but I use current baluns more often than voltage baluns.

Owen

[VK4WDM]

VK2MOD wrote:

That is generous Wayne, but also imposes your values of what makes an effective balun

We can have all the interesting technical discussion we like, but at the end of the day it is all about getting a signal from here to there and back that is readable both ways.

The balun I am sending him is a commercial one that I was using on my 40/15m dipole up to a couple of months ago. I have had hundreds of contacts from all over the world with that antenna so the balun is proven to be effective in the "real radio world". How it stacks up in the theoretical world is not really a concern

73

Wayne VK4WDM

[VK4WDM]

Vk5ZD wrote:

How does RF radiated from the coax cause interference? Surely all that happens is the radiation pattern gets modified a bit; not having a balun isn't going to generate any extra frequencies.

In my experience and humble opinion, common-mode currents flowing back down the coax are enemy No 1 (closely followed by earth loops). Whilst they may not directly cause interference, they can play havoc with antenna matching (and poor matching can cause interference), cause unstable SWR, cause malfunction of electronic antenna switches, and as I found very recently, mess with the function of digital interfaces

All my antennas have a coaxial choke (sometimes called an "ugly balun, but it's not a balun) or, and often and, ferrite beads at the feed point. A the function of a balun is to transform from an unbalanced to balanced situation. An effective balun can reduce common-mode currents but not stop them entirely - you need to choke them

It should be noted that the "Carolina Windom" which is specifically designed to have radiation from the feed line (yes, it says that in the manual) has a balun at the feed point and a choke at the end of the vertical section.

Isn't wonderful to be so passionate about hunks of ferrite and bits of wire - no wonder the rest of the world thinks hams are odd. We are

73

Wayne VK4WDM

[VK2OMD]

...
Isn't wonderful to be so passionate about hunks of ferrite and bits of wire - no wonder the rest of the world thinks hams are odd. We are

Yes, passion prevails over cold hard facts most of the time!

Owen

[VK5ZD]

Hi

... An example would be a G5RV which is designed to radiate from the feed line yet these are not renowned for causing interference.

Can you refer to anything written by its designer that supports the above proposition?
Owen

No; my mistake. Upon checking it was a reference I'd seen to using one on 160m by connecting the balanced lines together. i.e. nothing to do with the normal G5RV configuration.

A feed line may be much closer to these other conductors than the nominal radiator (dipole or whatever), so the extent to which the feed line radiates and its proximity increase the risk of interference.

Agreed. However, this is to do with the position of the radiating element (whether that's the antenna or the coax) rather than the fact that the coax is radiating. If you know the coax is going to radiate and route it with this in mind then there shouldn't be a problem. That said, I'm in the 'use a balun' camp. As has been suggested, a few ferrites or just winding the coax into a coil (the "ugly balun") is simple and relatively cheap.

[VK2OMD]

Hi

Can you refer to anything written by its designer that supports the above proposition?
Owen

No; my mistake. Upon checking it was a reference I'd seen to using one on 160m by connecting the balanced lines together. i.e. nothing to do with the normal G5RV configuration.

Yes, the use of the thing as a top loaded Marconi (and IIRC it was mentioned by Varney) is quite a different mode of operation.

When the feed line is used as a feed line, it should not radiate... though Varney's first article did not recommend a balun and so did not help to ensure lowest feed line radiation. He did recommend a balun in a later article.

We have learned a lot about baluns over the years, well some of us.

... That said, I'm in the 'use a balun' camp. As has been suggested, a few ferrites or just winding the coax into a coil (the "ugly balun") is simple and relatively cheap.

I wrote comment on the Ugly Balun at Re: Is This Balun Suitable For An OCF Dipole.

For those interested, compare the impedance characteristic of the Ugly Balun in the article above with the prediction of the LO1238 balun given earlier in this thread.

There are lots of balun designs around some innovative and work well, some are just innovative.

Tom Rauch has an article on steel wool baluns at http://www.w8ji.com/steel_wool_balun.htm .

I posted measurements of a "steel bolt balun', the article is no longer online, but above are the measurements of 10 turns on a 16mm steel bolt. This has lower choking impedance than if the bolt was removed! Again, compare it with the Zcm plots for the LO1238 balun earlier in the article.

I don't have any ready Zcm plots for a voltage balun, but a good one will be almost zero. Might be something to do if it rains today!

Lots of baluns are purchased or built and deployed without any data on their expected properties. Random components are likely to give random results, and as I mentioned earlier, this is probably one of the factors that contributes to some anecdotal evidence that a balun didn't make a difference.

Owen

[VK2OMD]

Im setting up a 40m Inverted vee that i may wish to change to a flat top dipole at some point. i have been advised against using a 1:1 balun for a flat top dipole and to instead use a 6:1 as the input impedance of a flat top is around 300 ohms. However many flat top dipoles use a 1:1 balun. Im just being hit with conflicting information here. would like some clarification

You need to look for explanations. If someone advises you but cannot explain the reasons why, then you should have lower confidence in their advice.

If you are talking about use of an ordinary half wave dipole on one band centre fed with coax feed line, then system efficiency will usually be greatest when the feed line VSWR is lowest. The feed point resistance of a half wave dipole will range from about 40 ohms to about 80 ohms depending on its environment, height etc, and so to obtain minimum VSWR with 50 ohm feed line you would not want a 6:1 balun, a 1:1 current balun with high choking impedance would be my choice.

You can make a 1:1 current balun with high choking impedance with inexpensive parts from Jaycar, but it seems most hams elect to buy something ready made.

The attachment Clip 130.png is no longer available

Above is the predicted common mode impedance of 9t on a LO1238 core (about $4 for two), the cable could be RG174 or RG316 - but it MUST have copper centre conductors, not plated steel, or WB1702 speaker twin - but that cable is not suitable above 10MHz. Coax is best as it will give best Insertion VSWR, but heed the warning about steel cored conductors, they are a fast route to QRP^2 which is "doing even less with less" to twist a QRP slogan. Yes, the chart is an estimate, but I have built and measured 8t versions and I am confident that the 9t with crossover winding will be close to the graph. This design is specific to the components mentioned.

If you are not a 'maker', and lets face it, most hams aren't, Balun Designs in the US has some good products, one of the few sellers who publish some limited Zcm data for their current baluns, and which you could get here for around $100 give or take.

Owen

Well, I poo-pooed using the WB1702 speaker twin based on measurements of its transmission line attentuation (and it is not good), and a concern that these inexpensive PVC insulated conductors have a wide range of insulation quality (voltage withstand).

I have measured a sample of the Jaycar WB1702 and it was quite good insulation, a single conductor withstands 7kV peak to a knife edge, so the wire to wire withstand should be well over 14kV (a lot better than ordinary enamelled wire).

Above, the prototype balun on a Jaycar LO1238 ferrite core.

Above a thermal image of the balun after 5 minutes of steady 100W carrier at 80m on my G5RV with tuned feeders. It was 20° before the test, so 7° rise in free air is quite low. This is inexpensive 70° wire, that needs to be respected.

The worst case Transmission Loss with a 50+j0Ω load was 0.15dB. Though this is not very good transmission line, there is only 400mm of it in the balun.

Something for 'makers' who understand the value of current baluns. Wire and cores for 2 baluns are about $5 from Jaycar.

Owen

PS: a full descrition of this balun is given at http://owenduffy.net/blog/?p=713 , and that article may be updated from time to time.

[VK2AVR]

Great analysis as always Owen, it's great that you're finding good, cheap ways to make baluns using Jaycar items.

What would be amazing is a list of Jaycar ferrites and a description of what sort of balun (voltage/impedance transformer vs current choke) they are suited for.

[VK1UU]

VK2MOD wrote:

The balun I am sending him is a commercial one that I was using on my 40/15m dipole up to a couple of months ago. I have had hundreds of contacts from all over the world with that antenna so the balun is proven to be effective in the "real radio world". How it stacks up in the theoretical world is not really a concern

73

Wayne VK4WDM

Hundreds of contacts from all over the world only proves that it works. That's not to say it can't be improved or there is something out there that works a whole lot better.