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Author Topic: Balun Loss- Mismatch & Reactive Loads  (Read 7935 times)
W8JI
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« Reply #15 on: October 17, 2009, 03:16:48 AM »

Owen,

I'm afraid you will confuse people.

You seem to be saying someone has to apply power until the box gets no higher in temperature. That is not correct.

While you do have to apply enough power and time for temperature to reach a significant peak and stay there before declining, power is not applied until the device or insulated box's inside air gets no hotter. You will also find if a resistor is placed inside the balun and you match the heat peak, so long as the starting point is the same and applied power is enough to greatly surpass any thermal leakage over the measurement time period (the box has to be well insulated and dissipation reasonable) the results will be very close. You can test for that by seeing if a known power change and re-measurement process produces the same approximate efficiency.

If someone simply applies power until the device gets no hotter, they will either be measuring equilibrium in heat dissipation to the outside world or curie temperature of the core material. I'm sure you didn't mean what it sounded like you said.

Tom
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WX7G
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« Reply #16 on: October 17, 2009, 06:08:27 AM »

You ask about the increased losses of a 4:1 current balun when used with a mismatched load including a reactive load.

Let us take the case of a 4:1 current balun having two 100 ohm t-lines and two ferrite cores. Driving a 200 ohm balanced resistive load with a 100 watt source, the balun output voltage from either output termimal to the input side shield or common is 71 volts. The common-mode current seen by each ferrite core is 71/Z, where Z is the core impedance at that frequency.

Now let's drive an 800 ohm load. The voltage from balun output-to-input is 141 volts. The common-mode current in each ferrite core is 141/Z. The core loss has quadrupled.

How about a 200 ohm reactive load? The core common-mode current is the same as for 200 ohm resistive load, given a 50 ohm source. Note that no real power is delivered to this load.

But how about for a real amateur radio situation where the load might be 200 +j200 ohms and a tuner is adjusted to deliver 100 watts into this load? In this situation 100 watts of real power is delivered to the 200 ohm resistve part of the load. The load current is 0.7 amps. The magnitude of the load impedance is 282 ohms. Therefore the load voltage is 200 volts. Each balun half has 100 volts. The common-mode current for each balun half is 100/Z whereas with the matched 200 ohm load it was 71/Z. The core loss has doubled.

So far I have ignored the effect of the balun t-line length and the balun t-line losses. For low impedance loads the ferrite losses are lower while the t-line losses are higher.

How to design a balun for a wide impedance range? As far as the ferrite goes, we want a high common-mode impedance so as to reduce the common-mode current and therefore the ferrite loss. Given a spec for load impedance (2000 -j2000 ohms for example) and TX power a balun can be designed to handle it. It will then work well at a more 'normal' load such as 200 +j0 ohms.

So, rather than design a balun for a nice matched load and hope it works well into a huge mismatch, it should be designed to work well into the highest expected impedance.

It would be great if the industry got together and came up with a uniform method for specifying baluns. We could then intelligently chose the balun for the application rather than viewing them as a black box. Perhaps this would spawn a balun spec war where the numbers were driven at the expense of an unspecified parameter.
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W6RMK
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« Reply #17 on: October 17, 2009, 07:24:19 AM »

If by "the industry" you mean the RF components industry, they DO have a standard way of rating things (typically given as dB loss and a maximum power handling), and the test processes to back it up. You'll get a certificate of conformance along with the packing slip, etc.  Look at MiniCircuits datasheets for examples.  And, I'm sure that when DoD buys 4:1 balun from Harris for their green radios, it has copious testing and data.

If by "the industry" you mean ham radio suppliers, I don't see it happening.  A lot of ham products are basically "productized" versions of something that someone originally put together in their garage or derived from a historical design in a handbook.  The mfr of the product has neither the budget nor the facilities to do the testing, and hams, as a group, tend not to pay anything extra for products which have actual test data.
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WX7G
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« Reply #18 on: October 17, 2009, 08:44:09 AM »

Mini-circuits specifies baluns for matched impedances. They do not specify common-mode impedance. We need more than this to determine how the balun will operate with a high mismatch.

I see amateur baluns specified like so: 4:1 balun, 5 kW, 160-10 meters. From this the most I can assume is that it will run close to some thermal limit (curie temp, package temp limit) when running 5 kW continuous into a 200 ohm load. I can then calculate that the CM voltage is 500 volts and that the balun handles the resulting CM current from 1.8 to 29.7 MHz.  

How about with a 2000 ohm load? For 500 volts CM we can run 500 watts. Wow. Our 5 kW balun is now a 500 watt balun. And we would like the balun CM impedane to be substantially higher - perhaps 10,000 ohms. I'm thinking that it would be great if a balun was offered that would run 1.5 kW CW into such a load and provide a CM impedance of 10,000 ohms.
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HFRF
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« Reply #19 on: October 17, 2009, 09:58:04 AM »

People just keep making stuff up on this site.  None of you so called experts have actually done any real design work with ceramic cores, because if you did you wouldn't be making all these ridiculous statements.

Why would any of you propose all these stupid heat tests when the manufacturers have done them decades ago.  You must never have seen them.

Example:

Thermal Conductivity

-2 material 10.8 mW/cm^3 degree C
-8 material 29.9 "
-26 material 42.6 "
ferrite material 10 "

Thermal resistance can be derived from the above and core specs. You can also rearrange the equation and solve for dissipation.

Also, manufacturers have to drill a hole to insert a thermal couple into the center of the core since the internal core temperature can be up to 40 degrees hotter than the outer surface.

Apparently you experts never heard of any of this info.


Also, you can calculate the
B(flux)= E(10)^8/(4.4)(cm^2)(Turns)(Freq)

The manufacturer publishes the core loss for any particular material based upon the B flux though the core.

Apparently you experts have never seen this stuff either.



Continue on and just keep making stuff up and try to reinvent the wheel.
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HFRF
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« Reply #20 on: October 17, 2009, 12:27:55 PM »

Also, where did you people ever learn that when something reaches thermal equilibrium, that everything included reaches the same temperature.  Thats what you are assuming and implying.  At equilibrium, there are different temperatures all over the surfaces and internally.  You need to measure the temperature at the place you want to measure heat (Q) to figure losses.

Also, to calculate loss in a core, you need to calc the B(flux) in each wire and add algebraically for total flux though the core.
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VK1OD
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« Reply #21 on: October 17, 2009, 01:24:21 PM »

Dave,

Your analysis of the 4:1 Guanella balun seems to be based on an ideal balun, isolated (ie not part of an antenna system) with a symmetric load.

Just considering that environment for a moment.

If the common mode impedance of each of the two chokes is very high, then the common mode current in each choke is approximately Vcm/Zcm. Vcm for one core is aproximately zero and for the other core is approximately equal to the unbalanced input differential voltage. One would expect that under those conditions, the losses in the two cores would be quite different. Copper losses would be almost identical.

But, it is my contention that tests of a balun between instruments on a bench is a shielded room do not directly indicate the behaviour in an antenna system. The common mode impedance of a current balun should be quite high, and core loss is very dependent on common mode current which is very much influenced by the entire antenna system.

The first analysis is what leads to the popular Rule 500, that common mode impedance of a current balun in an antenna system should be ten times differential Zo, or 500 ohms. Common mode current in an antenna system is probably independent of Zo, and so the rule is based on an independent quantity.

To give an example, if I erect identical half wave dipole antenna systems with W2DU style current balun near the feed point, but feed them with RG11 and RG8 respectively with equal power, the common mode current will be identical, though Zo of the second is just 66% of the first. Common mode current in this case is determined by the antenna / ground / common mode conductor geometry, and not at all by differential Zo.

Owen
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VK1OD
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« Reply #22 on: October 17, 2009, 01:47:01 PM »

Tom,

Lets look at a calorimetric example of two almost identical setups, a resistor dissipating 100W immersed in a beaker of 1 litre of liquid which is stirred.

The liquid in one beaker is water, and in the other is oil. All other things constant (eg ambient temperature, initial temperature, pressure etc).

The temperature of the liquids will not rise at the same rate, it depends on the specific heat capacity and density of the two liquids, and water more than twice that of oil, so the water will increase in temperature more slowly.

If one was to take the care to plot the temperature increase of each over a sufficient interval, and fit the data to an exponential curve, one will be able to show by extrapolation that the resistor power is the same. Otherwise, you need to wait for equilibrium whereupon both beakers will be constant temperature (no change in stored energy) at identical temperatures and so the power lost as heat to ambient must be the identical.

The energy stored as heat in a balun core is significant, and it takes a long time to heat up, thus saving the bacon of many an installation... but because it doesn't reach equilibrium.

Owen
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WX7G
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« Reply #23 on: October 17, 2009, 03:04:24 PM »

VK1OD: Yes! My analysis is based on a specific and limited case. This is where I like to begin an engineering analysis. Too many trees and we can't see the forest, you know.

My simplified, first-order analysis gives us a starting point. It allows is to see roughly how core losses are related to impedance. And the core loss I site - that core loss is  proportional to the square of current - is a simplification also. Core loss varies with the core material, flux swing, and frequency. The datasheet for the core material gives a more accurate number.

I will save my formula rich, all inclusive analysis for an engineering magazine or for QEX.
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W5DXP
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« Reply #24 on: October 18, 2009, 08:06:11 AM »

Has anyone ever just taken an average 4:1 voltage balun and run bench tests while varying the RL or RC load? Even a Zin vs Zload graph for an average 4:1 voltage balun would be helpful, e.g., what does an average 4:1 voltage balun do with a 1000 +/- j1000 ohm load at 2MHz, 10MHz, and 30MHz? Has anyone done that?
--
73, Cecil, w5dxp.com
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VK1OD
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« Reply #25 on: October 18, 2009, 05:50:52 PM »

Cecil asked if anyone had "just taken an average 4:1 voltage balun and run bench tests while varying the RL or RC load? "

The first question is what is an "average" 4:1 voltage balun. I think we need to be specific in measurement of a particular balun, and any of the legendary baluns would be a good subject for exploration.

IMHO a better approach is to characterise the balun using S Parameters, and to develop a tool that solves the network for an arbitrary load impedance.

At first, it might seem any of the analysis tools might do this, but it isn't as straight forward as that.

I have developed a spreadsheet that can be loaded with a table of 2 port S Parameters measured using a VNA. It will solve the network and produce graphs of expected performance. Now, getting S parameters for a 4:1 balun is a bit tricky, but if the balun is connected as a 1:4 unun, it can be measured using an ordinary 2 port VNA, and the 2 port parameters will give a very good approximation of its performance in the normal balun configuration.

A screen shot of the spreadsheet is at http://www.vk1od.net/lost/BalunSParm.png . The measurements shown are from a low end, ham grade VNA, and so measurement error can be seen (eg S21<>S12).

If someone wishes to measure a common balun in unun mode with a high end VNA, I could plug the S parameters into the spreadsheet and publish it so that folk could play with different load impedances and observed the performance changes.

This model does *not* include the effects of common mode current, but a voltage balun has low Zcm, and hence the dissipation caused by common mode current is low. Another way of stating it is that the common mode component of current balances and produces approximately zero net core flux. The same is *not* true of a current balun.

At the end of the day, ideal impedance transformation is not a high priority requirement for such a balun used with an ATU, but efficiency is a high priority.

Owen
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