Ferrite antenna com system

The propagation mode here is pure magnetic coupling, not a proper electromagnetic "radio" wave, because the antennas are so small compared to a wavelength. Dipole magnetic fields fall off with the cube of distance.

A lot more turns on the rods, and resonating with a cap, will help some. Longer rods would help some, too, but 1/d^3 is a cruel function.

How far do you need to go?

John

Hi!

I remember 1/d^4 for a full EM-field here.

The receiver is a WORKING time-code receiver. Working in distance at least

2000km from the time-code transmitter with an EIRP of 30KW. The time-code transmitter have of course a VERY BIG antenna (120m height). So I'm a little confused of your capacitor idea. That is true?: The transmitter is NOT sending an electromagnetic wave but the same antenna system at the receiving end reads it as an full established EM field?

How far: Hm, several km's if possible. Maybe I should go higher in frequency? What other small effective antennas work here?

I think the problem is not the minimum turns because I tested it with an original ferrite rod - the same as in the original time-code receiver. It have a lot of turns, probably 100 or more. The same behaviour with 100 turns AND with 10 turns. No difference!

What I understand of ferrrite antenna theory is: That the coil is simply an impedance transformer and bandpass (with a parallel capacitor for narrow-band reception) to couple the preamplifier to the antenna system (= ferrite rod).

But I miss something. Maybe something with differences between transmitting and receiving with a ferrite antenna. The antenna is not pure reciprocal - because the ferrite (or iron powder) can be nonlinear!

- Henry

Yes, a ferrite stick antenna works quite well for receivers, but not for transmitters. Try winding a few dozen turns around the whole room - i.e., up the wall, across the ceiling, down the other wall, across the floor, and so on. Or, you could wrap a piece of 50-conductor ribbon cable, and make loops by soldering the ends together offset by 1. ;-)

I don't know very much about antenna theory, but I know that the bigger the better. ;-)

Something's telling me that it's theoretically possible to transmit with a ferrite stick, but from the kind of power you'd have to run through it, it would probably blow up. =:-O

Good Luck! Rich

1. Ferrite and powdered iron are entirely different materials, with different physical and magnetic characteristics. Powdered iron isn't a good choice for this application.
2. You're not likely to drive either one into a nonlinear region when they're in the form of a rod because of the large air gap in the magnetic path.

Roy Lewallen, W7EL

In addition to what others have said, the most field you can generate with the ferrite rod antenna will occur when it is almost reaching saturation, and that takes a lot of ampere turns. You can deliver more ampere turns to the rod than your transmitter output can deliver if you resonate the coil with a capacitor. That way, you have the current bouncing back and forth through the capacitor added to the current from the amplifier. If the coil-capacitor Q is, say, 100, there will be 100 times more current through the coil than the transmitter is delivering. This will probably take a coil with a considerable mass of copper in it.

With this approach, you might reach 10 meters.

So there is a resonant circuit at the transmitter and not just a coil?

With such low number of turns (and hence low inductance), the capacitor would have to be huge to resonate it at 77.5 kHz. Where do you get high Q capacitors with such capacitances ?

The resonant circuit impedance levels are quite low in this configuration (small L/large C), how do you effectively couple power from the transmitter to this low impedance level at the resonant circuit ?

The skin depth at this frequency is about 0.25 mm, so any wire thicker than 0.5 mm will not utilise the full copper wire, so some kind of Litz wire with separately insulated strands could be used to keep the coil resistance low.

The inductance of some ferrites varies if there is some DC field present. This inductance change could detune the resonant circuit and drop the radiated power. Are you sure that the transmitter coil is not carrying any DC components or some even harmonic distortion, which would cause an unbalanced magnetic field in the ferrite rod ?

The band noise is the dominant (compared to "white" amplifier) noise when listening to the band with your transmitter switched off, the receiver noise performance should be adequate.

Paul OH3LWR

I can't really help you with ferrite antennas for transmitting, but can tell you that if you google around for "lowfer" and the Longwave Club of America

you will find a lot about antenna designs that are suitable for this band. They will also might have recommendations for frequencies of operation that are legal for transmission in your home country (I don't even know what that is!)

LNA isn't really applicable here because there is so so so much man-made and natural noise in this band.

I'm a little surprised that your achieved range was so small from a ferrite rod antenna, actually. Did you really tune both antennas, in place and in circuit, for resonance? The resonance is so so super narrow that strays between design and circuit make a big difference. I mean, CRT screens with flybacks, and faulty flourescent lamp ballasts, and incadescent dimmers radiate all sorts of crap around the LF spectrum for blocks, and they aren't even trying to be intentional transmitters! And don't get me started about induction heaters and welding machines, those can be heard across several states!

Tim.

transmitting

reciprocal -

You made my day :) BTW: Your idea with the ribbon cable gives you a very easy made transformator if using clamping connectors. This works very good. I practiced it 10 years ago.

I heart it several times that a ferrite stick antenna cannot work as a useful transmitter antenna. But why?????

Sure, for reasonable antennas. But if the antenna is very VERY big in relation to wavelength it even cannot work! Read somewhere.

I found no saturation state but I have not enough power at the moment to drive it very powerful. Something I try later ...

Regards - Henry

Powdered iron should work better because of the higher permeability even under heavy load in comparision to ferrite. I think so in theory - not tested.

Can you explain this more detailed Ron? What will happen with the air gap? The losses in the air gap radiates and that is the antenna function?

- Henry

John, that is what I have seen! I resonated the antenna coil and driven it with it's resonance frequency. Seems that the achievable distance was a little more than the circuit without resonating capacitor.

You say, that driving the ferrite rod into saturation will force it to leave more power into air? Why?

- Henry

Efficient antennas at that frequency are effectively very long bits of wire. The ferrite rod is small compared to the wavelength and very inefficient at generating a far field.

This is the antenna of the DCF77 transmitter (same frequency):

Kind regards,

Iwo

I tested it as resonating circuit using the original time-code receiver antenna AND a second time without the capacitor. Maybe I got a little more power in the air with the resonating circuit, but it was not very distingiuable.

I don't know the exact manufacturer of the time-code receiver ferrite antenna but I comparable model reads: L=900uH bandwidth=700Hz n=94 see original data

It is not the same antenna but very similar.

The original foil-capacitor is 682 labeled. I don't measured it but I think it should be 6800pF reading.

For my second experiment I used no capacitor and turns=10.

If I would find a PSPICE model for an ferrite antenna ...

Hm. I thought here: just trying different turns value to achieve this. The coil is the impedance transformer for the ferrite rod (=antenna). I'm wrong here?

The original coil is thinner than 0.3mm. If I compare it to my 0.3mm wire maybe it is 0.18mm. The second experiment with the 10 turns coil is 0.3mm enamelled copper wire. I will give Litz wire a try if the system as such works...

Good question. I series blocked DC with a WIMA MKS4 1.0uF 100VDC high-quality capacitor. As measured the "big" capacitor is outside the bandwidth of the antenna.

I don't think there is any DC component left. And yes, there is no magnet on my desk laying around :-) Is there any internal rectifiation phanomen in the ferrite possible?

How much band noise should I expect?

- Henry

Thanks. I will look there.

Maybe the time-code receiving IC is a bad design. I don't know. It's operating current is 500uA only. That is very small. It can receive the time-code over 2000km with such an antenna with an transmitter EIRP of 30KW. The receiving antenna is 700Hz bandwidth. I don't think this is super narrow. Even if we look at the time-code receiver quartz filter with a bandwidth of about 10Hz I can met it with my stable wave generator. It is a PLL-design with a clock quartz. Should be typical 10ppm. I don't have a very good frequency meter to verify it.

In my second transmitter experiment I used a not-resonated driver design. So there are no problems with detuning the transmit antenna expected. It is just driven by the 77.5KHz power signal.

CRT screen is off if I experiment. Otherwise I seen a very big CRT signal at the receiver...

If the two ferrite rods will detune because of the close proximity I cannot control it. I don't think so.

If you can hear induction heaters or something this is surely with a very big antenna and a resonable good receiver design.

- Henry

Henry Kiefer wrote: and that takes a lot of ampere

You misunderstood what I said. It was, " the most field you can generate with the ferrite rod antenna will occur when it is almost reaching saturation,"

If you saturate the rod, the field you generate will have lotsof 3rd harmonic components in it, but little more of the fundamental. I was trying to emphasize that you will need as strong a magnitic field as possible aat the transmitting antenna, and just below saturation is that limit, when a ferrite core is involved.

If the rod has a large lenght to diameter ratio (say , above 10) then I think the uptimum coil arrangement on the rod also doffers considerably for the transmitting and receiving cases, since the receiving case does not deal with saturation.

In the receiving case, the end sections of the rod act as flux collectors, and only the middle thirs or so has almost all the collected flux passing through it, so this third is the optimum place for the coil. /in the transmitting case, the rod has a tendency to saturate at the center, first, with this arrangement, and you want essentially the whole rod to approach satuation at the same ampere turns. This will produce a field that acts as if it has been produced by the full length of the rod. You can achieve something close ot this by spreading the turns out, all over the rod, with an extra concentration (a second or third layer layer, perhaps) at the ends. Something like this (shown in cross section. View with fixed width font i.e. Courier, so charcters are on grid pattern):

• = wire in cross section # = rod

*** *** ****** ****** ************************ ########################## ************************ ****** ****** *** ***

I understand that. I added a second coil on the ferrite rod to measure the antenna current and set it just below the point where I saw harmonics (or say non-sinusial) waveform on the scope.

That is a very interesting configuration. Never seen such a design. I read about a old-fashion remote controller system having a ferrite antenna transmitter. There someone wrote, the transmitter antenna was a mignon battery-shaped ferrite rod. e.g. much shorter but wider than mine. So an optimum ferrite transmitter antenna is maybe more like a fat battery shaped.

- Henry

The problem with loop antennas made of ribbon cable (or other multiconductor cable connected this way) is the stray capacitance between turns. The self resonance frequency (without external capacitor) may be below the band of interest, so you can not resonate such antenna with an external capacitor.

Paul OH3LWR

I found it very useful for design of hard-driven Power MOSFET driver transformator till 400KHz without problems.

- Henry

Ob-flippant remark: Sure you can, you just need a negative capacitor! ;-)

I suppose if one actually built a negative capacitor out of, e.g., a gyrator, the noise performance would be pretty much shot? Might be OK for a transmitting antenna, though?

The effective permeability of a rod is dictated largely by the air gap in the magnetic path, which is a function of the length/diameter ratio of the rod. Powdered iron in general has very low permeability compared to ferrite. If you really wanted to apply a huge amount of power to a rod antenna, powdered iron might be a better choice because of its high saturation flux density. But I doubt you could get the Q of a ferrite rod antenna at the frequency in question, so it would be considerably less efficient. You'd probably end up with less power radiated than if you ran less power to a ferrite rod antenna, and a less efficient antenna would impact your received signal. You'd have to crunch some numbers or make measurements to find out for sure.

The presence of even a small air gap has the effect of reducing the effective permeability of the core and therefore the inductance of the winding. It also dramatically reduces the core flux density for a given number of winding amp-turns. This makes it very hard to saturate. Inductors used for power applications commonly have a small core gap for this reason. A rod has a very large air gap in the path -- from one end of the rod, curving around outside the rod, to the other. And for many ferrites used at radio frequency, the material loss is high enough that the core would be hot enough to explode well before you reach a flux level anywhere close to saturation. This isn't true of all materials at all frequencies, of course.

The radiation takes place from the field outside the core, i.e., in the air gap. If you didn't have a gap, you wouldn't have any significant radiation.

And it's Roy, not Ron.

Roy Lewallen, W7EL