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Old 23rd Feb 2021, 8:24 pm   #1
regenfreak
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Default Radiation resistance and gain of ferrite antenna

Forgive my ignorance, I have been looking for the mathematical relationship between the parameters for a ferrite antenna bar. It is a long-winded story; I have been measuring the AL and effective permeability of different ferrite materials and the effect of geometry on their Q and efficiency. I am looking for the relationship between:

1. Q factor
2. cross-section area of the ferrite core
3. length of the ferrite bar
4. radiation resistance
5. ferrite antenna gain
6.effective permeability

So far I can only find this ferrite gain equation (first equation voltage gain UA):

http://www.c-max-time.com/tech/antenna.php#01c

I find above equation odd because it is independent of the length of the ferrite bar. Any pointer for relevant literature would be appreciated.

Is high radiation resistance is a good or bad thing
What is the radiation resistance in plain language?

What is the significance of effective permeability?

For the lumped model of a ferrite antenna in parallel with a capacitor, the total resistance can be approximated by series resistance:

Total resistance = Dc resistance of the windings+ radiation resistance+ ferrite core loss equivalent resistance

Q = Z/total resistance


As the cross-sectional area of the ferrite bar increases, both core losses and radiation resistance increase, therefore the Q drops. This web site outlines the equations for above three type of resistance:

https://www.st-andrews.ac.uk/~www_pa...rt7/page5.html

This sentence from the article really confuses me:

Here is the ferrite’s ‘effective’ relative magnetic permeability. This depends upon the choice of material and the size and shape of the rod. (This shape dependence is because some of the magnetic field ‘escapes’ from the rod away from the coil.) For frequencies of a few hundred kilohertz we can obtain ferrites which provide values in the range from around 100 to around 10,000. Taking the example of an we can see that using the ferrite can increase the antenna’s radiation resistance by a factor of a million! Hence the ferrite can have a dramatic effect in improving the antenna’s efficiency.

To add to my confusion, I have found this paper who dismisses traditional theory as completely wrong:

http://g3rbj.co.uk/wp-content/upload...e-Antennas.pdf

PS the attached photo is the super ferrite antenna that is made of 75 ferrite bars in a diameter of 1 feet foam core. Each bar is 20cm long and 1cm diameter. It uses Litz wire of 500 threads of 0.1mm fine wires. I have been measuring the Q, AL, effective permeability and the results contradict most common expectations.
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Old 23rd Feb 2021, 8:49 pm   #2
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Default Re: Radiation resistance and gain of ferrite antenna

What are you trying to use it for?

Also, your first link isn't talking about radiation resistance as far as I can see.

Radiation resistance for a transmitting aerial is the apparent resistor that the transmitter must drive into to launch em waves. Everything else is getting in its way. The ideal aerial would look like a resistor where all the power lost in it is going into making the radiation.

Ferrite materials tend to be lossy so do not make good tranmitting aerials. The loss manifests as extra resistance but in this case that resistance is generating heat.
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Old 23rd Feb 2021, 10:06 pm   #3
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Default Re: Radiation resistance and gain of ferrite antenna

This is also known as Ferrite Sleeve Loop (FSL) antenna for MW Dxing:

https://www.youtube.com/user/DXerGary/videos

You place it near a MW receiver and it will allow you to tune to distance stations. The idea was linked to some pseudo science by the guy who came up with the name.

There is no quantitative measurements to prove that the bigger the ferrite sleeve loop, the more gain the inductively coupled receiver has. I measured the quality factor Q of my giant 75 loop sticks (around a foam core of 1 feet diameter )resonating with a 100pF silver mica capacitor (Q=10,000) , it is around Q=200 at 1MHz and Q=2000 at 100KHz. My giant ferrite antenna has inductance of L=226 microH, AL = 1199 nH per square turns. It uses standard MW Mn-Zn ferrite ferrite sticks.

I have never seen any actual antenna gain or Q measurements for FSL.

I am interested in the radiation resistance in the context of a receiver. The second link mentions radiation resistance. I think the equation for the first link may be incorrect but i cannot find any other literature on the gain of a ferrite antenna.

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Old 24th Feb 2021, 9:32 am   #4
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Default Re: Radiation resistance and gain of ferrite antenna

Transmitters are trying to get maximum energy efficiency, so the ability to drive the radiation resistance efficiency is everything. While there is reciprocity with aerials, receivers can often sacrifice some energy efficiency for signal levels that work better with their input.

I would not expect an FSL to have huge gain. Aerial “gain” is all about the unevenness of the reception pattern as no magic is involved, i.e. it can only capture the energy that is there. A physically bigger aerial will likely capture more signal so are we surprised that if you compare a tiny little ferrite rod aerial with a huge box of ferrite rods in a whopping great coil then the latter may do better?

The reason radiation resistance gets a big mention in the context of tiny aerials (relative to wavelength) is that tiny aerials suffer from very small values that for transmitters are a huge problem. Tiny loop aerials are the worst of all for the perhaps obvious reason that one side of a loop is very close to the opposite side so any currents flowing tend to be both almost equal and opposite, so do a fine job of cancelling out in terms of the em far-field they generate.

I think the reason people are attracted to trying to make them work is that there is no theoretical reason why they cannot be as good as any other aerial – it is entirely a matter of the practical details, which are really awkward.

Placing something conductive near another aerial always changes that aerials radiation pattern and hence its gain. Think of a yagi aerial where the central dipole is greatly affected by the nearby other elements.

But at the end of the day, tiny aerials will usually end up, after matching, with such a narrow bandwidth that you may not find the result so useful except perhaps for CW or narrow data modes.
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Old 24th Feb 2021, 10:19 am   #5
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
Originally Posted by regenfreak View Post
Is high radiation resistance is a good or bad thing
What is the radiation resistance in plain language?
High or low radiation resistance is good or bad depending on circumstances. You DO want some radiation resistance.

Zero or infinite radiation resistance would definitely be bad. Either one would entirely stop an antenna working.

They said energy could neither be created or destroyed, then they modified that to allow that mass and energy are equivalent. We're not concerned with high energy physics, so we can stick to just the original version.

So we can only change energy into one form or another.

Then entropy rears its ugly head. It's dead easy to turn most forms of energy into heat, but it is much more difficult and rather inefficient trying to turn heat back into other forms of energy. So all other forms of energy seem more valuable than heat.

Britain once had a problem with convicts. It could hang them, and that made them appear to go away, but it was thought a bit brutal. So someone thought up an alternative that didn't spill blood; Transportation! They would ship them to the far ends of the earth, so far it was thought they wouldn't be able to return. Both methods worked. One day you had a criminal, the next day you didn't.

We have the same thing with energy. We can dump it into randomness into heat using a resistor. It doesn't come back. Gone for good.

There is also a transportation option. We can feed the energy into a transmission line. If we do some short-term thinking, the line swallows the energy and nothing comes back if the source was matched to the line... at least nothing comes back for a while. If you made the line very long, it could be a long while.

So what does the transmission line look like to the transmitter if there hasn't been enough time for any reflection to get back from the far end?

We're back to our convicts again. As far as the transmitter can tell, the energy has gone. It cannot tell whether that's gone for good, or whether it's gone for a while. So for the time being, the line looks like a resistor to the transmitter.

If you try to shove energy into a capacitor or an inductor, it goes in, but it stays there, stored, and comes back out the moment you stop pushing. These are no good as energy disposal machines.

If I transmitted into an antenna, Some of the energy will be lost to heat, warming up parts of the antenna, some may be reflected back to the transmitter, and the rest is radiated in the form of an electromagnetic wave.

Antennae are usually resonant, so we can model this as an LC resonator. We can add some resistance (series or parallel can be calculated) to represent the losses in the antenna materials, and we can add some more resistance (series or parallel can be calculated) to represent the energy which is transported away as the electromagnetic wave. It has a whole universe to explore, it can get aborbed by many things, plenty will travel forever, very little might get reflected back and it could be a long wait.

So, radiation resistance is just that, a part of a model of an antenna. It's job is to represent the loss of energy from the system which is the transmitted EM wave.

There are many ways of modelling an antenna, and there are several ways of incorporating a radiation resistance component. What the value is depends on your choice of model.

Antenna loss resistance works similarly.

Everything that hits the antenna goes out as:
Losses (heat)
Transmission (what we want!)
Reflection back down the feeder.

There is nowhere else to go.

Another way of looking at an antenna and the fields around it is as a transforming mechanism. Free space has an impedance: 377 (volts per metre) per (ampere per metre) which is the ratio of the field strengths of a mature electromagnetic wave as they settle into equilibrium.

Some people look at the dimensions of that value and say that the 'per metre' bits can be cancelled. So they get 377 volts per amp and they know another word for that, Ohms! So they say the impedance of free space is 377 Ohms.

Ever since that statement was made, people have been wondering where to stick the AVO prods to be able to measure it

You could view the radiation resistance of an antenna as being the 377 Ohms of free space transformed by the antenna and presented to the transmitter.

Radiation resistance has to appear real (bad pun) in order to make energy go away.

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Old 24th Feb 2021, 12:40 pm   #6
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Default Re: Radiation resistance and gain of ferrite antenna

Thank you gents for the insights.

Quote:
I would not expect an FSL to have huge gain. Aerial “gain” is all about the unevenness of the reception pattern as no magic is involved, i.e. it can only capture the energy that is there. A physically bigger aerial will likely capture more signal so are we surprised that if you compare a tiny little ferrite rod aerial with a huge box of ferrite rods in a whopping great coil then the latter may do better?
The surprising part is the Q of a massive 1-foot diameter FSL is no better than a single ferrite rod! I also think they don't work as good as they claim to be!!

Here are the measurements that I obtained using 0.56mm enamel wires. I connect the following coils in parallel with a high Q 100pF silver micra capacitor(Qc=10,000).



Q=200 for 75 Mn-Zn 20cm bars of ferrite FSL at 955khz L = 226mH

Q=50 for a 1m hexagonal loop antenna L = 162 microH

Q=125 for a single Mn-Zn 20cm bar L = 160microH

Q =526 for a single Ni-Zn R40C1 20cm bar L = 226microH


The overall Q of a LC tank circuit is:

1/Q = 1/Qc + 1/QL

The common belief is that the Qs of air variable capacitors are high which is not true. In fact the Qs of most aluminum plated capacitors are quite poor and about half of the valve of a high Q silver micra capacitor.

With two Ni-Zn R40C1 toroid stacked 5 X 26 X 8 mm stacked to 16 mm high, Litz 270/46 (0.04 mm), I got Q= 1200. This Ni-Zn R40C1 has created stir in the crystal radio community due to its extreme high Q. However for modern receiver, I have found that high Q is only part of the equation.

I compared the performance of the homebrew superhet 5-valve MW receiver with the following configurations:

1. 1m hexagonal loop
2. one single ferrite Mn-Zn bar
3. 25 ferrite bars in a cardboard tube (solid core)

In all three cases, the antennas track with the three point solutions of the RF LC front end. The 1 m hexagonal loop beats both the single and 25 ferrite bars in the number of weak stations it can receive despite of its poorer Q =50 at around 1MHz.. Surprise? You would expect the 25 ferrite bars antenna would have very high Q and would overload the Rf front end of a valve supehet but it isn't. I think it may be increased core losses which is translated into higher equivalent loss resistance in series with the LC tank.

Quote:
Tiny loop aerials are the worst of all for the perhaps obvious reason that one side of a loop is very close to the opposite side so any currents flowing tend to be both almost equal and opposite, so do a fine job of cancelling out in terms of the em far-field they generate.
If you read the third link in my first post, the author G3RBJ thinks the classic work by Snelling in Mullard Research Work is all wrong:

http://chiataimakro.vicp.cc:8880/%E5...tions,1969.pdf

Here is the homepage of G3RBJ:
https://g3rbj.co.uk/

Then ferrite antenna calculator in Coil 32 is based on the equation from G3RBJ:
https://coil32.net/online-calculator...alculator.html

There have been billions of valve and transistor radios manufactured using ferrite loop sticks, I am flabbergasted that I have not found a modern textbook provides a definitive theory applicable to ferrite antennas.

Quote:
Britain once had a problem with convicts. It could hang them, and that made them appear to go away, but it was thought a bit brutal. So someone thought up an alternative that didn't spill blood; Transportation! They would ship them to the far ends of the earth, so far it was thought they wouldn't be able to return. Both methods worked. One day you had a criminal, the next day you didn't.
I wonder if it was Paupa New Guinea or Isle of Man during the Victorian times?

Quote:
Antenna loss resistance works similarly.

Everything that hits the antenna goes out as:
Losses (heat)
Transmission (what we want!)
Reflection back down the feeder.
In the context of a ferrite bar, the fatter or longer the ferrite bars, the more magnetising heat losses. The efficiency of a ferrite bar does not go up infinitely as the length increases or cross-sectional area, i think the efficiency reach plautea when the length to diameter ratio is about 20 (for a 20cm long ferrite bar of 1cm diameter).

I started off the experiment with 5 ferrite bars, then 10 and 25 sticks..etc I was impressed the performance up to 25 sticks in solid cores. But I didn't see the FSL works that great as the number of sticks goes up to 45, 55 and now 75. Have you ever heard of the sunk cost fallacy? A guy got hooked on gambling after a small win and then he put more big bets and he loses in the end.
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Old 24th Feb 2021, 1:04 pm   #7
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Default Re: Radiation resistance and gain of ferrite antenna

I think you may be confusing Q with aerial gain.
They are not the same thing.
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Old 24th Feb 2021, 1:14 pm   #8
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Default Re: Radiation resistance and gain of ferrite antenna

I know Q and gain are not the same thing. That's why i have been looking for the mathematical relation between Gain = a function of variables like Q, frequency, ferrite geometry and properties. If you read G3RBJ's article, he has an equation for the induced voltage....

BBC TV white paper on ferrite by Poole. The methodology described here can be repeated with signal gen and a cheap spectrum analyzer(Meguro Loop test):

https://www.bbc.co.uk/rd/publications/whitepaper091

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Old 24th Feb 2021, 1:55 pm   #9
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Default Re: Radiation resistance and gain of ferrite antenna

It seems both the Q and relative permeability reach plateau around length-to-diameter ratio over 20 based on this 1952 RCA research paper:

http://www.philbrickarchive.org/ferr...1952%20ocr.pdf

The relative voltage gain correlates with Q but they are not the same of course. When i put two 6 inches length of or three Ni-Zn R40C1 bars in series (12 inches, 18 inches length), the Q starts to drop.

In the RCA article, it states "high electric resistivity" for ferrites being a good thing. I guess it means high radiation resistance. A LC tank resonance has high impedance, so the higher the radiation resistance of the ferrite, the better the impedance match and hence more energy transfer from the free space to the receiving antenna..correct me if i am wrong. The actual value of radiation resistance is very small (10 to the power of -4 ohms) so I dont know what it means.

g3rbj concludes
Quote:
When antenna engineers became interested in ferrites they presumably would not have been totally
convinced by the demagnetisation theory, and needed to make measurements at RF preferably of the far
field. But far-field measurements are difficult to do accurately and very time consuming, and so they turned
to the much easier near-field measurements, believing that these could be accurately related to the far-field
by well established theory for loop antennas. This theory seemed to be relevant since the accepted view
was that the ferrite antenna was a loop antenna, with the ferrite merely increasing the effective area. Their
near-field measurements confirmed the demagnetisation theory, and from then-on other researchers
accepted this without question.
But it now seems that the ferrite antenna is a magnetic dipole and the wire loop around it is merely a
method of detecting the magnetic field. The theory linking the far field to the near field of a loop was
therefore not applicable to ferrites antennas,

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Old 24th Feb 2021, 2:28 pm   #10
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Default Re: Radiation resistance and gain of ferrite antenna

OK, I think the term "gain" is causing confusion.
Being a passive structure an aerial doesn't have a gain at all, only a loss.

What is termed the gain of an aerial is a way of thinking about the effect of a non-uniform performance. Easiest understood for a transmitter - if the aerial sends all the em waves in one direction only then if you stand in the right place you see a lot more power than you might have expected, and this effect can be stated as a "gain" compared to what it might otherwise have been. It isn't an actual gain because the high em field in one direction was obtained by not sending em waves the other way.

The Q is the property of a resonant thing and is a measure of how undamped it is. In this context the Q helps by bigging up the tiny radiation resistance. Think of it like the power fed in goes through the small radiation resistance, is then stored and fed back through it thus getting a second bite, and so on - thus making it look bigger to the external circuit as the signal kind-of went through in many times.
But this comes at a price, because deviate the frequency a tiny bit and it stops working - in other works the bandwidth is low.

So this high Q is a bad thing for an aerial. But for a crap aerial it helps make it less crap if you can tolerate the consequences.

As I said, a receiver may have a different agenda. Maybe it is more interested in voltage than power at the input. Now we have some wiggle room because we can trade voltage for power and this might give better performance, and the Q helps us do that. I think this is why we see ferrite rod aerials in receivers but very rarely in transmitters.
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Old 24th Feb 2021, 3:12 pm   #11
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
OK, I think the term "gain" is causing confusion.
Being a passive structure an aerial doesn't have a gain at all, only a loss.
you are right. I should call it relative voltage or induced signal strength.

g3rbj toys with the idea of gain in equation 7.9 (Grod) for a ferrite stick:

http://g3rbj.co.uk/wp-content/upload...e-Antennas.pdf

I think a masochist would be naturally interested in antenna theory. Any attempt to understand the theory will be rewarded with a good beatings of the head. I found above article hard to understand.

Quote:
The Q is the property of a resonant thing and is a measure of how undamped it is. In this context the Q helps by bigging up the tiny radiation resistance. Think of it like the power fed in goes through the small radiation resistance, is then stored and fed back through it thus getting a second bite, and so on - thus making it look bigger to the external circuit as the signal kind-of went through in many times.
But this comes at a price, because deviate the frequency a tiny bit and it stops working - in other works the bandwidth is low.
The effect of radiation resistance bugs and confuses me. Thevein's lumped element model of a LC tank consists of radiation resistance in series with the ferrite core loss resistance and DC resistance of the coil and ferrite core
combination. "Bigging" the radiation resistance would degrade the Q, dont you think?

This is from wiki:

Quote:
In a receiving antenna, the radiation resistance represents the source resistance of the antenna as a (Thevenin equivalent) source of power. Due to electromagnetic reciprocity, an antenna has the same radiation resistance when receiving radio waves as when transmitting. If the antenna is connected to an electrical load such as a radio receiver, the power received from radio waves striking the antenna is divided proportionally between the radiation resistance and loss resistance of the antenna and the load resistance.[7][8] The power dissipated in the radiation resistance is due to radio waves reradiated (scattered) by the antenna.[7][8] Maximum power is delivered to the receiver when it is impedance matched to the antenna. If the antenna is lossless, half the power absorbed by the antenna is delivered to the receiver, the other half is reradiated.[7][8]
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Old 24th Feb 2021, 3:28 pm   #12
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From wiki:
Quote:
The power consumed by radiation resistance is converted to radio waves, the desired function of the antenna, while the power consumed by loss resistance is converted to heat, representing a waste of transmitter power.[2] So for minimum power loss it is desirable that the radiation resistance be much greater than the loss resistance. The ratio of the radiation resistance to the total feedpoint resistance is equal to the efficiency of the antenna.
It makes sense for a transmitter but for a receiving ferrite loop stick; the power for radiation resistance very small but is large compared with the incoming signal (-50 to -100 dBm?) from the free space.
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Old 24th Feb 2021, 4:30 pm   #13
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Default Re: Radiation resistance and gain of ferrite antenna

You do not want world record setting Q from an antenna.

Q is a measurement of the loss of energy from a resonator

You WANT to lose energy from an antenna

You just have a preference that the loss is into the radiation of an electromagnetic wave and not into the warmth of the hardware. You can treat reception as being reciprocal in most cases.

So your model of your antenna WILL contain resistance. Your Q should not be terribly high. The art lies in the partitioning of it between radiation and thermal losses.

Some years ago designs for copper loops a few feet in diameter (no ferrites, just a resonating capacitor) popped up in magazines and journals. some of them used straight lengths of plumbing type copper pipe and 135 degree Yorkshire fittings to make an octagon. Careful analysis showed that more oomph went into the 16 soldered joints than got radiated. The loop resistance (ESR) was small, but the radiation resistance was smaller. The sliding contacts in variable capacitors put the tin lid on it.

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Old 24th Feb 2021, 7:05 pm   #14
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Quote:
You do not want world record setting Q from an antenna.

Q is a measurement of the loss of energy from a resonator

You WANT to lose energy from an antenna

You just have a preference that the loss is into the radiation of an electromagnetic wave and not into the warmth of the hardware. You can treat reception as being reciprocal in most cases.

So your model of your antenna WILL contain resistance. Your Q should not be terribly high. The art lies in the partitioning of it between radiation and thermal losses.
Thanks. This is really thought-provoking and like a knock-out punch to the notation that the higher the Q the better the resonator.

Quote:
Some years ago designs for copper loops a few feet in diameter (no ferrites, just a resonating capacitor) popped up in magazines and journals. some of them used straight lengths of plumbing type copper pipe and 135 degree Yorkshire fittings to make an octagon. Careful analysis showed that more oomph went into the 16 soldered joints than got radiated. The loop resistance (ESR) was small, but the radiation resistance was smaller. The sliding contacts in variable capacitors put the tin lid on it.
If you ever try to measure high Q inductors (Q >1000), you will notice that crocodile clips and leads are ruthless Q-killers causing large insertion losses. Contact switches create massive insertion losses, it is the magic of skin effect of RF on the surfaces of conductors.

Below is the primary coil of my large tesla coil. To reduce heating and skin effect losses, I use 6mm copper tubes and 200A automobile battery cable and thick copper 2cm braids as earth cable. The peak RF ground current pulses are over 1000A based on software simulation. The resonance frequency is 150KHz. The copper tubes and the 200A cable get hot after a few minutes of operation. If the contact clip between the primary tapping in the photo is not good, you would get like scary arcing between the turns of primary coil. It happened to me a few times and it could set off fire.
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Old 24th Feb 2021, 7:38 pm   #15
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
knock-out punch to the notation that the higher the Q the better the resonator
I think I still see some confusion here. We are talking about aerials, not resonators. When aerials are described as resonant it does not mean like a highly tuned LC circuit. The ideal aerial looks like a resistor that is all radiation resistance. That is practically the opposite of a "resonator". The half wave dipole, a resonant aerial, comes close to this if cut to the right length.

As I said, all this Q stuff is part of the desperate attempt to make an almost useless aerial - the small loop - actually work as an aerial. Think of the LC resonant aspect as the ATU that is desperately trying to match an aerial with almost zero radiation resistance to the driving circuit.
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Old 24th Feb 2021, 8:04 pm   #16
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Quote:
I think I still see some confusion here. We are talking about aerials, not resonators. When aerials are described as resonant it does not mean like a highly tuned LC circuit. The ideal aerial looks like a resistor that is all radiation resistance. That is practically the opposite of a "resonator". The half wave dipole, a resonant aerial, comes close to this if cut to the right length.

As I said, all this Q stuff is part of the desperate attempt to make an almost useless aerial - the small loop - actually work as an aerial. Think of the LC resonant aspect as the ATU that is desperately trying to match an aerial with almost zero radiation resistance to the driving circuit.
Thanks for steering my boat in the right direction. As a result of your comments, I am looking at chapter 4 of the book "Impedance Matching" by Alexandra Schure. The beauty of this book is the maths is not so intimating.
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Old 24th Feb 2021, 11:27 pm   #17
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Default Re: Radiation resistance and gain of ferrite antenna

For the connaisseurs of RF witchcraft, radioheads or masochists, here is a free textbook that cost £200 on Amazon uk:

Antenna Handbook .VOLUME II ANTENNA THEORY Edited by Lo and Lee,1993

Page 64; section 7 Ferrite loop antenna, radiation resistance:

https://epdf.pub/antenna-handbook.html

Have to register for free. It is the most modern antenna textbook including microstrips that is free!!

Last edited by regenfreak; 24th Feb 2021 at 11:34 pm.
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Old 25th Feb 2021, 1:07 am   #18
Al (astral highway)
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Default Re: Radiation resistance and gain of ferrite antenna

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Originally Posted by regenfreak View Post
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Below is the primary coil of my large tesla coil. To reduce heating and skin effect losses, I use 6mm copper tubes and 200A automobile battery cable and thick copper 2cm braids as earth cable. The peak RF ground current pulses are over 1000A based on software simulation. The resonance frequency is 150KHz. The copper tubes and the 200A cable get hot after a few minutes of operation. If the contact clip between the primary tapping in the photo is not good, you would get like scary arcing between the turns of primary coil. It happened to me a few times and it could set off fire.

Sure thing! There are hundreds to thousands A of RF current circulating in that tank circuit at several Kilovolts, and the cabling to get there is lossy and will get warm.

And it's not surprising that you'll see flashovers beween bare adjacent turns of that flat spiral coil if you have a discontinuity in the mechanical connection under these conditions - just as if you were hard-switching, only by accident.

What I'm struggling with is the link between all this and your OP. I sense a wonderful focus on direct observation of things in real life vs. piles of maths, but I'm not sure what this is really all about, underneath the surface.

I'm sensing there is some depth of fascination and wonder, but can you help me link everything more clearly back to your original query?
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Old 25th Feb 2021, 8:51 am   #19
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Default Re: Radiation resistance and gain of ferrite antenna

A ferrite rod antenna winding looks inductive, and is brought to resonance with a section of a variable capacitor. In many radios this is used as the RF selectivity ahead of the self-oscillating mixer. The oscillator coil gets the other section of a 2-gang tuning capacitor.

So ferrite rods are usually configured as part of a resonator, and they also act as an antenna, a transducer of energy between electrical current form and EM field form.

So the answer is yes, both of the above.

There is an ironic compromise that for the best Q for the best brownie points as an RF filter, it would have to be shielded from the electromagnetic environment to contain losses. This of course ruins its functionality as an antenna. For best antenna function, it should couple as strongly as possible to the universe around it.

Fortunately this can be compromised to yield a functional radio, as testified by the countless millions that have been made.

David
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Old 25th Feb 2021, 10:42 am   #20
regenfreak
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
I'm sensing there is some depth of fascination and wonder, but can you help me link everything more clearly back to your original query?
David has summarised it.

Below is the summary of my understanding so far in plain language (correct me if I am wrong as i find it confusing as well):

It seems "the new thinking" is the ferrite rod is like an aerial or a magnetic dipole proposed by Payne ( http://g3rbj.co.uk/) that should be studied from a far field and not near field. He published a few articles on the subject but they are not peer-reviewed research. So you make your own judgment if you agree with him or not. The interesting thing is that Coil 32 uses the equations in his articles. But if you follow the trails of his references. He referred to work of Lo and Lee et al. The equations he used may be from other research sources.

Payne dismissed the classical thinking that the ferrite antenna acts more like an magnetic coil. This was my confusion. I was pre-occupied with Q and the traditional thinking of a tuned LC circuit struggle to find "tangible" or understandable significance of "radiation resistance" in a receiver.

The reciprocity theorem is a maths principle but there are differences in transmitters and receivers in term of the direction of energy transfer and some design considerations to minimises heat losses. What we want to transfer maximum energy from the ferrite antenna to the RF front end. We cannot understand an antenna in the same way as a simple circuit. The impedance is a vector at any point along the transmission line which has direction and magnitude varying with time and space. To visualise the variations of voltage, current, impedance vectors along a transmission is fun thing to make your brain turning into mashed potataos. I will stay away from Maxwells; thank you.

Here is an article about the confusions of radiation resistance:
https://www.w8ji.com/radiation_resistance.htm

You know a dipole is a weird thing but I can understand it better by relating it to a tesla coil. A tesla coil behaves like a quarter wave antenna. At the top of the Toroid the voltage is 200-300kV, the current is very small; at the bottom of the coil the voltage is zero but the ground current is over 1000 ampheres.
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