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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Feb 2021, 12:18 pm   #21
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Default Re: Radiation resistance and gain of ferrite antenna

You can view a ferrite rod as a discontinuity in the magnetic continuum. Its presence distorts the fields present.

A nice copper bar similarly distorts electric fields around it.

In the case of the ferrite rod, it looks like a better 'magnetic conductor' than the free space around it, so it acts to concentrate fields around and through it.

These things work for DC field components as well as for AC ones.

Sticking a coil around the ferrite rod takes advantage of the concentrated fields.


For a bit of fun with ferrites and fields, look up how a fluxgate magnetometer works. Note the frequency doubling effect. Most people don't expect that the first time they meet the things.

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Old 25th Feb 2021, 1:19 pm   #22
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
You can view a ferrite rod as a discontinuity in the magnetic continuum. Its presence distorts the fields present.
This reminds me of the potential flow theory of aerofoils and vortex lines etc....the magnetic field are like streamlines. An aerofoil distorts the flow field surrounding it.

If you can tolerate the Manga animations, this explain Maxwell's four laws of electromagnetism; Maxwell was a genius. He could predict something on a piece of paper before they proved its existence:

https://www.youtube.com/watch?v=9Tm2c6NJH4Y

Quote:
For a bit of fun with ferrites and fields, look up how a fluxgate magnetometer works. Note the frequency doubling effect. Most people don't expect that the first time they meet the things.
Sweet. Hypothetically speaking, if I were a greedy career criminal and my long-life ambition is to rob the house of a billionaire or the gold vaults of the Bank of England. Can i use the fluxgate to locate the position of the hidden safe or the bomb proof vault?

https://www.metrolab.com/technology/...%20the%20other.

Last edited by regenfreak; 25th Feb 2021 at 1:26 pm.
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Old 25th Feb 2021, 4:24 pm   #23
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Default Re: Radiation resistance and gain of ferrite antenna

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Originally Posted by regenfreak View Post

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.
Yes, I'm familiar with the 1/4λmodel for a TC. (I've built conventional spark-gap TC's, VTTC's and a recent DRSSTC).

In practice, what happens with currents is more complex and certainly governed by the properties of materials. In the photo of your rig, it looks like the secondary is rather tall and narrow, and I'd guess is wound with 33 SWG or about that?

The skin depth at 100KHz for this sort of gauge is only about 200 μM. It seems vanishingly probable that such thin wire can carry more than a few tens of A without heavy losses, whatever the theory predicts.

The opposite end is also interesting. Streamers as they break out and vary in length also vary in self-capacitance and impedance. One researcher stuck a scope probe (connected to a 1,000 worth of 'scope via a 5GHz wireless link) and current inside a Faraday cage and modelled currents at the toroid with and without ground strikes. Currents aren't trivial - 10A or so for a large rig.
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Old 25th Feb 2021, 4:33 pm   #24
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Default Re: Radiation resistance and gain of ferrite antenna

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Originally Posted by regenfreak View Post
Maxwell was a genius. He could predict something on a piece of paper before they proved its existence:

https://www.youtube.com/watch?v=9Tm2c6NJH4Y

No doubt that he was a genius, but predicting something on a piece of paper before its existence is proven is an established quirk of the human imagination. Of course, we've rather moved away from the model of one particularly gifted being illuminating things for the masses, probably about 100 years ago. Collaboration is the only path to progress.


And particle physics wouldn't be the well-funded thing that it is if proof had to follow theory within a given period of time.
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Old 25th Feb 2021, 5:06 pm   #25
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
The reciprocity theorem is a maths principle but there are differences in transmitters and receivers
I am prepared to be shot down by this, anyway.
Antennas are reciprocal, I wouldn't expect a ferrite rod to be a good transmitter because it only (well nearly) uses the H (magnetic) field and used as a transmitting aerial will give the same amount of H field but very little E (the electrostatic one). You can easily receive either from the electromagnetic wave, or even both with, say a dipole. You need both H and E to transmit past the near field (a few wavelengths).

Small transmitting loops do work a bit and are used primarily in the shortwave (SW) region (3 to 30MHz) the vagaries of SW mean that upon occasion even a few mW of radiated power can get you "round the world", no one remembers the "contact" that didn't work. Hence the reports of good results from such loops.

Note, not a single SW broadcaster uses a loop, some tried the "crossed field" antenna, they worked due to the co-ax radiating.
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Old 25th Feb 2021, 6:44 pm   #26
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
Yes, I'm familiar with the 1/4λmodel for a TC. (I've built conventional spark-gap TC's, VTTC's and a recent DRSSTC).

In practice, what happens with currents is more complex and certainly governed by the properties of materials. In the photo of your rig, it looks like the secondary is rather tall and narrow, and I'd guess is wound with 33 SWG or about that?

The skin depth at 100KHz for this sort of gauge is only about 200 μM. It seems vanishingly probable that such thin wire can carry more than a few tens of A without heavy losses, whatever the theory predicts.

The opposite end is also interesting. Streamers as they break out and vary in length also vary in self-capacitance and impedance. One researcher stuck a scope probe (connected to a 1,000 worth of 'scope via a 5GHz wireless link) and current inside a Faraday cage and modelled currents at the toroid with and without ground strikes. Currents aren't trivial - 10A or so for a large rig.
I think the secondary uses AWG30 which is roughly near 33 SWG. I built a few different versions of the primary and secondary coils with software iterations and modelling; different resonance caps, power factor and sizes of toroids The photo shows the last 1700W version with a 50cm diameter toroid stacked on top of a smaller toroid to modify the electric field (reduce the risk of arcing down the secondary if i remember correctly.) In the software modelling, I included the distributed capacitance of the streamers because it have big effect on the resonance frequencies. If i omitted the capacitance of the streamers, I can get the resonance frequencies wrong. Running a detuned tesla coils with high power is a dangerous business; the energy will transfer to the spark gaps and no amount of air blow quenching can interrupt the sparks. The coil would behave like a wild bear escaping from the zoo. I used Richard Quick's multi-copper pipes spark gaps and an elaborate Terry filter, ferrite chokes everywhere ..etc Everything is over-engineering for high voltage, RF filtering and low losses


In the photo on the left, it is the half bridge SSTC CW 700W kit with mosfets based on Steve Ward's topology. On the right, it is One Tesla DRSSTC kit using IGBTs designed by a bunch of MIT students.

I was going to built a vaccum tube tesla coil with two 800W microwave transformers (one as a current limiting choke ). Photo shows the pair of new 811A transmitter valves that i was going to use. That was 2018 and then I got interested in radios and abandoned the project. I still have numerous HT capacitors, neon sign transformers, TV flybacks lying around. My flat has turned into an electric junkyard full of clutters and vintage radios yard

My neighbours have been happy since the day i stopped testing tesla coils
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Last edited by regenfreak; 25th Feb 2021 at 7:06 pm.
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Old 25th Feb 2021, 7:17 pm   #27
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Default Re: Radiation resistance and gain of ferrite antenna

Any burglars ought to feel very discouraged.

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Old 25th Feb 2021, 9:10 pm   #28
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Default Re: Radiation resistance and gain of ferrite antenna

Quote:
Note, not a single SW broadcaster uses a loop, some tried the "crossed field" antenna, they worked due to the co-ax radiating.
From wiki

The radiation resistance of a small loop is generally much smaller than the loss resistance due to the conductors composing the loop, leading to a poor antenna efficiency. Consequently, most of the power delivered to a small loop antenna will be converted to heat by the loss resistance, rather than doing useful work.

You can plug different numbers into this octo loop design calculator to get a sense the order of magnitude of transmitting efficiency and the radiation resistance value:

https://www.66pacific.com/calculator...alculator.aspx

For highest efficiency, the conductor length for a small transmitting loop antenna should be greater than 1/8 wavelength

Last edited by regenfreak; 25th Feb 2021 at 9:20 pm.
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Old 25th Feb 2021, 9:27 pm   #29
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Default Re: Radiation resistance and gain of ferrite antenna

For 21MHz, 2.1 diameter octagonal loop with copper tube of 1cm, 10W; it is very impressive:

Antenna efficiency: 98% (-0.1 dB below 100%)
Antenna bandwidth: 673 kHz
Tuning Capacitance: 11 pF

Capacitor voltage: 456 volts RMS
Resonant circulating current: 0.684 A
Radiation resistance: 10.4 ohms
Loss Resistance: 0.266 ohms

Inductance: 5.05 microhenrys
Inductive Reactance: 666 ohms
Quality Factor (Q): 31.2
Distributed capacity: 19 pF

Antenna "circumference": 7 meters
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Old 25th Feb 2021, 9:33 pm   #30
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Default Re: Radiation resistance and gain of ferrite antenna

Example of a mismatch with 8MHz: note the Q and radiation resistance. Q goes up!

Antenna efficiency: 33% (-4.9 dB below 100%)
Antenna bandwidth: 17.0 kHz
Tuning Capacitance: 121 pF

Capacitor voltage: 880 volts RMS
Resonant circulating current: 5.35 A
Radiation resistance: 0.057 ohms
Loss Resistance: 0.117 ohms

Inductance: 3.27 microhenrys
Inductive Reactance: 164 ohms


Quality Factor (Q): 471
Distributed capacity: 13 pF

Antenna "circumference": 5 meters

Suppose reciprocity applies to this calculator for receivers, i shall be tempted to make one for SW radios

Last edited by regenfreak; 25th Feb 2021 at 9:43 pm.
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Old 26th Feb 2021, 2:51 am   #31
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Default Re: Radiation resistance and gain of ferrite antenna

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Originally Posted by regenfreak View Post
For 21MHz,

Antenna "circumference": 7 meters
THat's no longer a 'small' loop the circumference is almost lambda/2

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Old 26th Feb 2021, 11:18 am   #32
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Default Re: Radiation resistance and gain of ferrite antenna

You are right. The equations for small loop only applies for circumference lambda/10. In the radiation resistance model they assume the current is constant around the loop so the above calculations are inaccurate.

IEEE paper; the estimated efficiency is below 25% in this paper:

https://www.mpoweruk.com/papers/loop_antennas.pdf
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Old 26th Feb 2021, 11:27 am   #33
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Default Re: Radiation resistance and gain of ferrite antenna

In above IEEE paper, the point about tuning capacitor contact resistance is very interesting as i have encountered the same problem with measuring the Qs of high Q inductor (Q>1000). The quality of contact/solder joints and solder flux can impair the measured Q massively.
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Old 26th Feb 2021, 11:59 am   #34
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Default Re: Radiation resistance and gain of ferrite antenna

You cannot measure high Q using off-the-shell aluminium tuning capacitor. Thats what i found with the tuning capacitor of my broken Heathkit Q-meter. Ideally it should be silver or gold plated like the HP Q meter 4342.

Belrose IEEE paper:

https://citeseerx.ist.psu.edu/viewdo...=rep1&type=pdf
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Old 26th Feb 2021, 6:23 pm   #35
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Default Re: Radiation resistance and gain of ferrite antenna

There is no shortage of disagreement in the world of HAM radio; different experts come up with "new theories" that one accuses others of being "wrong". Underhill claimed that the mini magnetic loop has efficiency of 80 to 90% based on his thermo measurements. He attacked Belrose's work and classical theory:

https://www.nonstopsystems.com/radio...efficiency.pdf

So much politics and egos
Sometimes one has to take it with a pinch of salt when they come up with a "new theory". Well i guess it adds to the confusion I find it very weary
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