EMRFD Message Archive 11849

Message Date From Subject
11849 2015-10-25 17:18:57 Phil Sittner FT Transformer power capacity

I’m confused about which material is suitable for RF impedance matching transformers.

Amidon provides the following guidelines:

Type 43  EMI/RFI suppression 20-250Mhz

Type 61  suppress noise

Type 75  attenuators

Type 77  Transformers

Type F     Power conversion transformers

Type K     Line transformers for baluns


The application is impedance matching for a magnetic loop antenna and the source is a 100 watt transmitter. From the Amidon guidelines I suspect the appropriate core would be type 77, or F, or K. Most impedance matching I’ve done, or seen done, has used type 43. And how does one determine the minimum size core? Any and all responses will be most appreciated.





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11850 2015-10-25 19:49:04 kerrypwr Re: FT Transformer power capacity
The starting point is the Fair-Rite catalogue which can be downloaded from their website.

Each material is described in detail; the graphs of u' & u" against frequency should be examined.

u' is the property that "creates" inductance; the frequency range where u' is fairly constant is the range that makes good transformers.

u" "creates" loss, the resistive component of the complex impedance of a winding on the material.  The frequency range over which u" is high is the range where the material makes a good choke or EMI suppressor.

There are no hard-&-fast boundaries to these frequency ranges in practice; the curves don't usually change abruptly.

In a well-coupled transformer the loss due to u" is not important, at least at low power; #43 makes good transmission-line transformers at HF and beyond because its u' of 800 requires relatively-few turns (ie a short winding length) for a given inductance/impedance and the close coupling between the windings means that there is little net flux in the ferrite.

At the other end of the scale, if #43 is used in a high-power unun, say the common 9:1 device, inter-winding coupling is not good, ie there is a lot of leakage inductance, and there is a lot of flux induced in the core.

This flux is subject to u" and there is consequent heating and sometimes destruction of the core.

In these devices, #61 or an iron-powder is less subject to this effect; the downside is that a longer winding length is required because of the low u' but, as these devices are generally used at the LF end of HF, this is not necessarily a problem.

You will have to trade-off u' and u"; find a material that has high u' and low u" at your frequency of operation.

Remember that you require a certain minimum inductance for the shortest winding of your transformer so consider u' in that regard.

I have no expertise in calculating power-handling; I'm a "bench" amateur and a watt is QRO to me.  :)

Maximum flux density is given by;

B = (E.10^2)/(4.44.A.N.F) where:

B is the maximum flux density (gauss);
E is the applied rms voltage
A is the core cross section area (cm^2) (0.133 for an FT50-43);
N is the number of turns and
F is the frequency (MHz).

It is possible to calculate this from the ferrite data; try the Philips transformer design guide where I think you'll find that information and a great deal more.

Search for Philips Application Notes ECO6907 and ECO7213; if you can't find them, let me know.

Also search for Using Ferrites To Suppress EMI by Carole Parker; it is a very good overview of ferrite properties.

Kerry VK2TIL.
11851 2015-10-26 07:56:56 Andy Cutland Re: FT Transformer power capacity
Hi Phil,

I've only ever seen mag loops with variable capacitors to bring the loop to resonance ?

I am interested to see what you are trying to do here.

De Andy

11852 2015-10-26 08:15:42 Phil Sittner Re: FT Transformer power capacity



The loop is resonated conventionally with a vacuum variable capacitor. The feed method differs in that the main loop is made from 6” wide aluminum flashing material (4’6” rectangular) and a parallel wire is used as a transformer winding. Two turns of this winding run through a t50-43 core and the coax has a 3 turn winding on the other side of the core. I get swr readings of less than 1.5:1 across the entire range using this configuration. The conventional 20% primary loop presents dismal performance and experiments with loop sizes up to 50% prove inferior to the full loop arrangement.


Jerry Sevick’s “Transmission Line Transformers” suggests using type Q1 or K5 core material for impedance matching transformers (power) and goes on to suggest that cores of 1” are sufficient for 200-300 watts and recommends 1.5” for 1000-2000 watt applications. These cores have a permeability of 125 and 290, respectively. It should be also noted that the “Toroid King” offers a FT240-52, permeability of 325, and likely is the product I will use.


What are your thoughts

11853 2015-10-26 09:05:39 winston376 Re: FT Transformer power capacity

Been there done that....

Consulted with Sevick about this application many years ago.

Based on my work on this, the best you can hope for is to get a maximum amount of power to the radiating element  and in your case, that is to match the transmitter/transmission line  impedance to the coupling loop and radiating loop as a system. 

For a constant loop size and feed configuration, your loop impedance will vary significantly from the TX / TL impedance  value, even if you have 'tuned' your capacitor to resonance at the varying frequencies.  The remaining potential solution is to have a variable impedance matching device between the TL and the coupling loop at the antenna proper.  This can be done manually or with an automatic matching unit such as an SGC-239 or other from LDG. 

That being said,  the size of the loop will determine the effective antenna efficiency and ultimately,  the measured  field level that you will actually be producing at a point in the far field. 

Hope this helps.

11854 2015-10-26 14:10:40 kerrypwr Re: FT Transformer power capacity
Indiana General Q1 is much the same as Fair-Rite/Amidon #61; I don't know about K5 but a quick search suggests that is a discontinued TDK product.

It sounds as if your transformer is a flux-coupled/conventional type, not a transmission-line type; if so, the u" of the material is important for power-handling.

I mentioned #61 for flux-coupled transformers in my earlier post; it works in that application because, as may be seen from the Fair-Rite data, u" is low at low frequencies so that there is low resistive/heating loss in the core.

The data for #52 material shows low u" (taken as less than 10) up to 4 MHz; the value increases rapidly thereafter.  I think that this material would get hot at medium power and frequencies more than 4 MHz; it does have high saturation flux density and can withstand high temperatures so it may be OK in your application.

A safer alternative with regard to flux density might be #61 although the lower u' would mean lower winding inductance and, perhaps, more leakage inductance.

Electronics, like many other fields, is often a game of trade-offs but, as a Steinbeck character once said, "You gotta pay to learn things".

You might consider making two transformers, one with #52 and one with #61, and see what the differences are; I, for one, would be very interested in the results.

Kerry VK2TIL.

11855 2015-10-26 16:36:55 Phil Sittner Re: FT Transformer power capacity

Kerry, Dave, Andy and others


I placed an order for some FT140-61(Al=150), FT240-52(also shown as type ‘K’, Al=330), and FT240-61 (Al=170). The suspicion is that very little real difference will be noted and the FT140 core will prove to work as well as the other two. I’ll make some measurements and pass them along. Magnetics are an even bigger mystery than I suspected. No wonder the early work with switch mode power supplies was filled with smoke and fire.





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