btw the thick wire isnt good since you have a gap between the wire and the ferrite, just saying.

 

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Question: why do the large toroids from a switchting power supply heat up considerably?
when the resistance should be 0.02 Ohms?

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Some of the reasons ferrite toroids in power supplies heat up is that it takes energy to swing the magnetic field back and forth, and depending upon how the core is being used some or all of that magnetizing energy is lost.

In the case of a continuously changing magnetic field, eddy currents are induced in nearby conductors. Some ferrites have significant conductivity but the greatest part of the eddy current losses occur in the cross section of wires close to the surface of the magnetic core. That's why several parallel strands of fine wire or copper strips are often used instead of large gauge wire.

 

heliosh,

With that setup....you had better listen to papa.

But as he said, you will not like the results.

I wish I had more context of what you are trying to do.

If I understand right.......I think it is possible with only building the secondary and making it adjustable.

In this configuration, the air will be your primary.

But I need to understand exactly what you are doing.



And cores heat up due to inertia.

 

the only way to get an antenna that broad banded is to use a "voltage probe" ever wonder why a car radio am antenna is so short? it is a voltage probe.since at 1000 khx that antenna would have to be 150 meters for a half wave, and that would hit a lot of trees and overpasses, they use a voltage probe. not matched impedance at all. and most vooltage probe antennas have amplifiers, which will determine the bandpasss.

 

Some of the reasons ferrite toroids in power supplies heat up is that it takes energy to swing the magnetic field back and forth, and depending upon how the core is being used some or all of that magnetizing energy is lost.

In the case of a continuously changing magnetic field, eddy currents are induced in nearby conductors. Some ferrites have significant conductivity but the greatest part of the eddy current losses occur in the cross section of wires close to the surface of the magnetic core. That's why several parallel strands of fine wire or copper strips are often used instead of large gauge wire.

thats why you dont get the DC resistance, one of the reasons, besides that 0.02 ohms = absurd

 

Hi,

I'm trying to build a broadband transformer (1-300kHz) to convert an input impedance of ~20mOhm to 100 Ohm, that's 1:5000.

I thought I'd take a toroid and make 1:70 turns. But beyond 10 secondary turns there is no more gain in impedance/voltage.
I'm using an epcos N30 (µ=4300) toroid.

What am I doing wrong? Do I need a different core material? Do a need a different core type (binocular?)? Do i need more primary turns? is it even possible with a single transformer?


Thanks

You are on the right track.

With a loop antenna 10m diameter, this will have an inductance around 50uH and a radiation resistance varying from about 2.4x10^-14 ohms at 1kHz to 1.9x10^-5 at 300kHz. The real part of the loop impedance will be dominated by the conductor loss resistance, the DC resistance may be an approximation at 1kHz but you need to be aware of the skin depth, in copper it varies from about 2mm at 1kHz to about 0.1mm at 300kHz. I don't think that this is your problem though, as maximum power matching will not give you broadband operation.

You quote a value of radiation resistance of 2.4*10^-12 Ohm, this would correspond to a frequency of 3.2kHz, so I assume that you are testing around here. At 3.2kHz your loop has a reactance of about 1ohm. A simplified version of your circuit is

where the loop is represented by the loss resistance Rg (tiny) and the reactance XL (about j1), and the 100 ohm input impedance of the receiver is transformed back into RL (so it is 100/N^2). If you vary N to maximise the power in RL you find that N=10 gives you maximum power, surprisingly close to what you observed. (Of course this analysis is only correct if my assumption about your test frequency is correct) This limit exists because you haven't tuned out the reactance XL. You can get more power by tuning out XL, but this is not normally done if you want a broadband response.

There is a lot in the literature about VLF antennas and receiver design, you will learn a lot if you focus on first principles as you are doing.

If you have a look at http://traktoria.org/files/radio/an...vlf_receivers_with_air-core_loop_antennas.pdf you will find details of matching the antenna to the receiver on p15, take particular note of what they call the input turnover frequency (where |XL| = RL in my simple diagram above), and for broadband operation you operate above the turnover frequency, here the high pass response of the loop is cancelled by the low pass response of XL and RL.

So for a broadband response you would normally adjust N to give maximum response at the lowest frequency you are using.

 

Who worries about radiation resistance on a receive antenna.

This is not good advice, power is normally scarce at the receiver and it normally pays at least to be aware of how much you are discarding by poor antenna matching. It may be a design decision to be wasteful, but it is rare that it is overlooked.

Maybe if you work mainly at HF (say 2 - 20MHz) you can be more cavalier as here you are rarely receiver noise limited (i.e. improving your antenna matching brings in as much more noise as signal), but even there I would keep an eye on it.

Catching radio signals can be difficult, there's no point building bigger antennas than necessary in order to catch more signal just to waste it in a mismatch.

 

I've included a link to the work of James E. Storer who did some extensive theoretical analysis of the radiation resistance of thin wire loop antennas. Quite surprising results. The results can be "confirmed" by simulation of a loop antenna using an electromagnetic application such as MMANA. Be advised the download is around 4.2 MB. The original technical report was published in May 1955.

http://www.dtic.mil/dtic/tr/fulltext/u2/069089.pdf

 

This is an interesting subject. I remember a number of years ago the U.S Air Force was experimenting with real short antenna's about 6" long. They found that if they connected the short antenna directly to the base of a transistor amplifier very small signals would be amplified. Going one step further, I have seen schematic's that use some of the newer fet input op amps to be very effective rf untuned amplifiers for the AM broadcast band.
I once built a stand alone rf amplifier for the AM broadcast band, that used a tuned input and a emitter follower output to match the input impedance of my receiver. This stand alone used a short telescoping whip about a foot long. The results were really amazing to me. My workshop was in the basement below ground level, and one local AM station about 20 miles away gave me over 2v peak to peak on the tuned circuit. As I recall the first stage was a dual gate rf mosfet 40673. While I was looking for the rf mosfet# I found the OP amp I spoke of, it is TL081 series.