Antenna ferrite loopsticks verses air core?

snipped-for-privacy@downunder.com wrote: Phil Allison

** You have not understood the question, by trying too hard to be a smartarse. ** Try actually answering the question sometime - d*****ad. ** FFS - you have the attention span of a gnat.

** No more than the vast majority of people living in Sydney.

( snip more smartase s**te)

You are so full of bollocks it smells.

.... Phil

Right, It's a JVC "ultra compact component system" FS-1000

I made a wooden box with handles, so to me it's semi-mobile. :^)

It's got seven turns of insulated cable, in a 1 cm width. The coil is not a square, ~10 cm X

12 cm. I could stick it in my back pack and take it to work (if it mattered) and measure inductance and Q/R on our SRS box, 100 kHz and lower.

Knowing almost nothing about AM, (and assuming I don't care about price.) I figure I want the highest Q possible on the front end.. times some area... And I have no idea how to calculate the "area" of a ferrite. Measurements would be in order. (Hopefully someone has already done them.)

George H.

Larkin is great, I like you too.... :^)

George H.

The most power? That implies a conjugate load, which will kill selectivity.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

There are many variables you're not considering, but maybe this will spark some ideas.

AIUI, in a uniform magnetic field (such as far from a radio transmitting source), a loop antenna produces a voltage proportional to the area the loop encloses. A ferrite rod will increase a loop's effective area by a factor equal to the effective permeability of the ferrite rod.

So, a 8mm^2 loop wound on a core with an effective permeability of 50 produces the same output voltage as an 400mm^2 loop with no core.

The effective permeability of a ferrite rod, in turn, is mostly a property of its length-to-diameter ratio.

e.g.,

Coil geometry also affects a rod's effective permeability. Highest effective mu is attained by short coils, in the center of the rod.

e.g. Fig. 1, makearadio.com/tech/files/Ferrite_Rod_Inductance.pdf

Cheers, James Arthur

Rush Limbaugh, Mark Levin, Dennis Prager, Michael Medved, George Noory, Bill Handel, John and Ken, Tim Conway Jr. and other political hacks. Also there are financial shows like Ric Edelman, George Chamberlain, Ken Moraif, and others. Keeps me busy 24/7 .

The Antenna Ferrite Loopstick Verses (an exercise in mixed metaphors) --------------------------------

I raised an aerial into the air, Pumped in some watts; they landed, but where? For, so swiftly they flew, the sight Could not follow the watts in their flight.

But now we all know (thanks to Edward J. Snowden) They were scooped from the ether By one William ('Bill') Bowden.

Whose giant loop, all loaded with ferrite Had grabbed all my flux (that ain't fair, right?) To warm up his home, and light LEDS, And listen to Rush whenever he pleased.

Actually, when I was overseas in the south pacific working at a telegraph station, we put up a long wire antenna tuned to the local AM broadcast station and managed to light a neon bulb (NE-51) or whatever. I probabaly could have charged a battery with the thing by just stealing the RF power from the air. I think it was a 10KW station about a mile away.

.

I used to have a Yamaha CT-7000 FM tuner, perhaps one of the best FM tuners from the 1970s.

From

:

"The CT-7000 has a 7-gang tuning capacitor (two gangs in front of the first MOSFET RF amp, two in front of the second MOSFET RF amp, two in front of the MOSFET mixer and one for the bipolar local oscillator)."

Alignment must have been fun.

Allan

And second harmonic distortion. I'm seeing the same issue in a photodiode now. The customer wants my amp to have -80 dB THD, but his photodiode has -40 from varicap effect.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

So the next topic is to compare and contrast the merits of an H field sensing loop antenna vs an E Field sensing whip antenna for use on the AM broadcast band. In practice, most car radios use an E Field whip and most other radios use an H field loop. How do they compare? M

Wild guess: indoors, there is a lot of e-field noise. Outdoors, not so much.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

An H field antenna is directional - the signal would fade whenever you went round a corner. The H field antenna has to be horizontal to pickup the field.

kevin

In the far field (radio station), both the E and H fields are inversely proportional to the distance (and hence power density proportional to the inverse square of distance). In this area the free space impedance (relation between E and H) is 120 pi or 377 ohms.

In the near field (local noise), the H-field drops faster than the E-field with distance, thus a better SNR can be obtained with H-antenna in the near field of a noise source.

One can argue where is the border line between near and far field, but at least 0.1 wavelenghts is definitively near field, i.e for 1 MHz at

30 m. In a residential area, there are a lot of noise sources within that range. In a car on a highway, there are only a few noise sources constantly within that distance, as long as you have suppressed any noise sources from your own car.

I've always used the definition of near/far as whether the source can be resolved or whether it's a point source. IOW, if the source has a "visible" area, it's near field.

OK, I did the experiment. EDIT; I reread the OPs post, he didn't say 6" dia he said 6" length,

so, my experiment was a bit of a waste and doesn't answer his question.

But here's the info from my experiment.

First, I don't know if the OP was tuning or not, I could not measure any signal without tuning. So, I tuned.

Two 6.75" diameter coils using 660/46 litz, both approximately close wound on a styrene pipe coupler. Coils tuned with a high quality cap and a 20pf fine tune trim cap. The air core coil has 33 turns and measures 236uh The core with ferrite has 30 turns and 232uh. without the rod, the coil measures 195uh. The rod is 3/8" diameter x 8" long. Not a large inductance change to put a small ferrite rod in a large air core coil. To center the rod in the coil, I cut two, 1/2' styrofoam circles to fit the inside of the coupler and put a hole in the center for the rod. To measure the signal I have high input impedance amp with a gain of 1. I used the amp to drive a scope (ch 2) set at 50mV/div. I took the channel 2 output from the back of the scope to drive a Boonton 92BD RF millivolt meter. I used the scope to compare the visual to audio from a portable radio to know where I was tuned. I have three local stations 590kHz, 1290kHz, and 1430kHz. I did two separate tests, but they were so close it didn't matter. Modulation made the signal vary, but the Boonton has an analog meter to help peak the signal. I did try a second larger diameter poorer quality rod 1/2" diameter x 7.25 long. It may have out did the air core on the low end with a few more turns to bring the inductance up to match the others, but it failed on the upper end.

Best Ferrite Poorer ferrite 236uh 232uh 216uh 33 turn 30 turn 30 turn Air core coil Ferrite Coil Ferrite Coil

590kHz 59 mv 43 mv 59 mv

1290kHz 15.5 mv 6.7 mv 7.1 mv

1430kHz 10.5 mv 4.2mv 3.2 mv

Mikek

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I'll run the experiment.

Do you want it tuned?

If not, I have no way to measure the signals of my local stations. I need the resonance peaking to see the signal.

What diameter PVC?

I have 1/2" OD polystyrene that will allow a little closer coupling between the ferrite and the wire. 400 turns #28 = 203uh air core.

I have 1/2" CPVC. actual OD. 0.615"

290 turns #28 = 200uh air core

I have 1/2 PVC, actual OD. 0.832. 175 turns #28 = 205uh air core.

Pick one.

I'll also wind one with less turns and use my best Q rod that is 8" long x 0.375" diameter.

I will check three frequencies, 590Khz, 1290kHz and 1430Khz.

I made a post last night of the wrong experiment (6"dia not 6" long) It has not shown up this morning, so I'll repeat my measurement method.

To measure the signal I have a very high input impedance amp with a gain of 1. I use the amp to drive a scope (ch 2) set at 50mV/div. I took the channel 2 output from the back of the scope to drive a Boonton 92BD RF millivolt meter. I use the scope to compare the visual to audio from a portable radio to know where I am tuned. Modulation causes a bit of amplitude bounce, but I do a visual average.

Let me know what you want. Mikek

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My monitor does both. It has switched capacitors (S-correction, the amount of "S" required depending on sweep rate), and Litz in the deflection coil, so there. :-p

(Obviously, the "large signal" (100V?) regime makes it quite effective to use individual power MOSFETs to switch capacitors into the circuit. It's still an okay method in the small signal < 0.3V range too, but in that inbetweeny range, err... :-) )

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Wait, "dynamic impedance" == mechanical resonance? Oh...

In other words, he made an ear. ;) Or, a log periodic, or...

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Actually, I'm just interested in comparing the response of two identical loopsticks, one using an air core and the other using a ferrite core. I could do the experiment since there is a 50KW station about 7 miles away and I can see the signal from the antenna loop directly on a scope. I can get about 1 volt peak using a loop antenna of about 15 inches square. I just thought someone would know the answer without a lot of experimenting. I have a portable car radio with a air core loop antenna mounted on the chassis that measures 6.5 inch by 3/4 diameter and about 300 turns of small wire. Works fine and gets stations 130 miles away. But it's a power hog and draws

100mA from a 12 volt battery. I suppose a good test would be to use a shorter ferrite rod and fewer turns to compare the results. But I'm lazy and just want to know which idea is better.