|
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. |
|
Thread Tools |
25th Feb 2021, 1:18 pm | #21 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,901
|
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. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
25th Feb 2021, 2:19 pm | #22 | ||
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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:
https://www.metrolab.com/technology/...%20the%20other. Last edited by regenfreak; 25th Feb 2021 at 2:26 pm. |
||
25th Feb 2021, 5:24 pm | #23 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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.
__________________
Al |
|
25th Feb 2021, 5:33 pm | #24 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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.
__________________
Al |
|
25th Feb 2021, 6:06 pm | #25 | |
Guest
Posts: n/a
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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. |
|
25th Feb 2021, 7:44 pm | #26 | |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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 Last edited by regenfreak; 25th Feb 2021 at 8:06 pm. |
|
25th Feb 2021, 8:17 pm | #27 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,901
|
Re: Radiation resistance and gain of ferrite antenna
Any burglars ought to feel very discouraged.
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
25th Feb 2021, 10:10 pm | #28 | |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Radiation resistance and gain of ferrite antenna
Quote:
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 10:20 pm. |
|
25th Feb 2021, 10:27 pm | #29 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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 |
25th Feb 2021, 10:33 pm | #30 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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 10:43 pm. |
26th Feb 2021, 3:51 am | #31 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,901
|
Re: Radiation resistance and gain of ferrite antenna
THat's no longer a 'small' loop the circumference is almost lambda/2
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
26th Feb 2021, 12:18 pm | #32 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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 |
26th Feb 2021, 12:27 pm | #33 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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.
|
26th Feb 2021, 12:59 pm | #34 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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 |
26th Feb 2021, 7:23 pm | #35 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
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 |