Using ferrite rods and loops to transmit

[Panrock]

Take a tuned ferrite rod operating without saturating, or an equivalent multi-turn loop aerial.

How efficient would such aerials be at transmitting, especially at LF and MF, compared to wire aerials? I understand these tend to be on the short side and require base inductors or capacity hats and unfeasibly low-impedance earths...

To need to ask such a question reveals that in no way am I a radio amateur. Never mind. Just curious.

Steve

[trickie_dickie]

I built a Practical Wireless Topband transmitter back in the 60s and slugged ALL the coils to get it working on the top end of the medium waveband. Worked very well. The ferrite in the tank coil was 5/8" diameter. But it fed a standard long wire aerial. Valves were 6C4, EF91 & 807. Audio was fed in via the key socket, Cathode modulation. Similar to this one.

[Herald1360]

Physically small aerials tend to have low value resistive components to their radiation impedance along with large reactive ones. This demands impractically high Q from matching circuits needed to drive them. Net result is more power lost in the matching circuit than actually radiated.

Have a read of the lengths the old 16kHz Rugby station had to go to to get a reasonable amount actually up the spout- it's fascinating if nothing else.

So they work, but not very well, as transmitting aerials.

Ferrites to increase inductance of coils as described is a different matter, it works well enough for low powers.,

[russell_w_b]

Originally Posted by Herald1360

Net result is more power lost in the matching circuit than actually radiated.

Bandwidth resistors in series with the aerial?

Originally Posted by Herald1360

Ferrites to increase inductance of coils as described is a different matter, it works well enough for low powers.

Ferrite in the tuning inductance can be deliberately saturated by applying voltage derived from the mark / space telegraph frequencies to a separate winding, so shifting the resonant frequency accordingly, and optimising the bandwidth.

[Argus25]

The answer to your questions are contained in equation 12 & 13 in the attached image. Although it looks involved, if you look at 12 you will see that the antenna radiation resistance (that you want as high as possible) is proportional to (He/wavelength) squared. What this means is that the higher the effective height He of a linear antenna and the shorter the wavelength, the higher the radiation resistance.

When the antenna is folded up into a coil or in a core with some relative permeability, you will see from equation 13 that the radiation resistance is proportional to the number of turns, the permeability and the cross sectional area (all squared) and the fourth power of the frequency. What this means in practical terms is that the small cross sectional area of a ferrite rod, won't likely be made up for by the increase in permeability of the ferrite. Also, at low frequencies like medium wave band, the radiation resistance of a coil or loop is substantially reduced by the dreaded 4th power relationship for the frequency.

So in practice, to get a medium wave loop working well as a radiator it needs a good number of turns, at least 5 or 10 (not one) and it needs to be a reasonable cross sectional area of a meter or more because all of the factors are conspiring against it being a good radiator at that relatively low frequency.

[Radio Wrangler]

All the factors above force you to do everything you can to reduce loss resistance in a small loop. The best you can do is to achieve very low efficiency, and that is with a loop operating at such a high Q that it doesn't have the bandwidth for audio modulation sidebands. The more you succeed in fighting losses, the narrower it gets.

Some time ago there were academic articles on tiny loop antennae made of superconductor. They showed the intended increase in efficiency through dramatic reduction in loss resistance, leaving only the radiation resistance to set the Q of the resonance. The Q is too high to carry any useful modulation.

Broadcasters would jump at any way of scaling down their antennae. Even in a country with a bit of empty space going spare. A lecturer in Scotland had an idea which he thought might scale down the size of antennae. A student working on the project happened to be from Egypt and got an Egyptian broadcaster interested in building a full size one. The lecturer had a small business going selling another novel antenna he'd invented, so he patented the new thing and got working on publicity to commercialise it. Time has passed, the student got his degree and the lecturer is now a 'silent key' The ex-student, now doctor seems to be carrying on trying with the idea. One got built in the Yorkshire dales, and was dismantled when its temporary planning permission required. Another is now being built on the Isle of Man. These things are rather contentious and have been the subject of a lot of argument. I've not seen any hard figures published, and if they worked as well as suggested, they'd be all over the place by now. Look uo 'Crossed-Field Antenna' but be prepared to have to sort out what you believe from all the arguments.

David

[Hartley118]

I guess that there's an acoustic analogy here.

Will a microphone work as a loudspeaker? Answer: Well......yes it will, but not very well, because:

1. Its impedance match to the medium is poor.
2. It can't handle the power involved.

Martin

[Argus25]

Originally Posted by Radio Wrangler

Broadcasters would jump at any way of scaling down their antennae. Even in a country with a bit of empty space going spare. A lecturer in Scotland had an idea which he thought might scale down the size of antennae.

Well , assuming the laws of Physics cannot be violated (at least in our reality) the equations demonstrate that for a linear geometry antenna ideally the effective height is a good proportion of a wavelength. We are all familiar with 1/4 wavelength antennas. Sure, you can go shorter with loading coils etc, but they are then less efficient radiators.

Once the structure is made into a coil with some cross sectional area, it is a pretty poor radiator unless it is a large area and with a good number of turns, unless the frequency is high (>30MHz) and single turn loops become practical. So a coil sitting on your bench with a few square centimetres cross sectional area cannot rightly be called an "antenna" for transmitting purposes, whether it has a ferrite core or not.

The ferrite rod in the AM radio though is a bit deceptive because, for its size and cross sectional area, it works astonishingly well for receiving (albeit very directional) due to its sensitivity to the magnetic component of the field, but it is hopeless for a transmitting antenna due to its small cross sectional area. This seems to contravene the general rule that if an antenna is good at receiving it will also be good at transmitting.

When I read stories about supposed miniaturized antennas that work as well as full sized ones, I struggle to believe it. That won't stop such projects attracting funding though.

Of course at UHF frequencies all kinds of interesting tiny antenna designs can work well, but it doesn't mean they will have any use at a few MHz.

[G8HQP Dave]

Ferrite rods work well as receive antennas because the signal strength is fairly strong, and reception is limited by external noise. Hence the fairly weak coupling between the antenna and the external field can still extract enough signal to be useful. The law of reciprocity still works for ferrite rods (it radiates as well as it receives) but it just happens that on MW external noise means that you need good radiators but can cope with a poor receiver. Hence the appearance of reciprocity failure.

[Argus25]

That is a good explanation of how the ferrite rod seems to break the reciprocity rule. Probably ferrite would improve a transmitting antenna like a large MW loop with a reasonable cross sectional area, but it would have to be a pretty big slab of it.

[kalee20]

Well, I'm not sure of that! Reciprocity works fine as long as things are linear.

So... A ferrite rod would be effective as a transmitting aerial rather than a receiving, provided that your receiver was connected to a gigantic mast array as a receiving aerial rather than a transmitting one.

[Argus25]

The ferrite rod antenna doesn't break the reciprocity rule it only "seems to" due to the factors pointed out. But as a transmitting antenna, the fundamental reason why it is poor, despite the permeability increase of the ferrite beyond that of the air cored coil, is the small cross sectional area of the coil wound on it. That is why a larger slab of ferrite in a large coil would be better, but its impractical.

[julie_m]

Woukd it make a difference if the ends of the ferrite core were specially shaped for the most efficient transfer of magnetic flux to/from the air?

[Argus25]

Originally Posted by julie_m

Woukd it make a difference if the ends of the ferrite core were specially shaped for the most efficient transfer of magnetic flux to/from the air?

That is a good idea, that say the ferrite could be flared out at the ends of the rod to increase the surface area. I guess it could be tried if some large ferrite washers could be added to the end of the rod. It would make for a very interesting experiment. I wonder if it were flared out to part of a section of the surface of a sphere that would correspond to the imaginary lines of magnetic flux cutting the surface perpendicularly.......it would have an amazing look to it.

[G8HQP Dave]

Originally Posted by kalee20

So... A ferrite rod would be effective as a transmitting aerial rather than a receiving, provided that your receiver was connected to a gigantic mast array as a receiving aerial rather than a transmitting one.

As I said, external noise is the problem. Your big receive antenna would pick up noise very effectively. Reciprocity is not the issue. It is true, but almost irrelevant.

A ferrite rod is already shaped for efficient magnetic coupling to the environment. You could make a lower loss (higher Q) tuned circuit by using a ferrite toroid, but that would not work very well as an antenna. The problem with a ferrite rod antenna is that the conductor resistance is a significant fraction of the radiation resistance. To make a good radiator unavoidably involves size, because radiation requires retardation. Maybe a really long ferrite rod would help?

[G6Tanuki]

As far as I'm concerned the 2 issues are saturation and the Curie-point of the magnetic core.

Saturation means that the classic B-H hysteresis-curve is no longer applicable. The resultant non-linearity will do horrible things to the quality of the transmitted signal! [think of it like a gruesomely-overloaded output transformer in an audio amp]

Curie-point: once a certain temperature is reached [and it will be if it's lossy and you're whacking significant Wattage into the thing] the magnetic properties suddenly go AWOL.

In the past I unfortunately pushed rather too many Watts of RF through a ferrite-cored transformer. The results were not-good for a while, then suddenly became very-bad when the poor thing's Curie-point was reached!

[Panrock]

Maybe a ferrite based transmitter would be ideal where very low power and short range were needed in a compact package... one of those 1.7 MHz analogue phones mentioned in another thread comes to mind.

A related matter with regards to short range applications. How do we know when 'induction' is happening rather than 'radiation'? Indeed, what is the difference?

Steve

[Tim]

A ferrite rod picks up from the ends. Does that mean if one does use it to transmit then it would then radiate from the ends too.

[Argus25]

Originally Posted by G6Tanuki

As far as I'm concerned the 2 issues are saturation and the Curie-point of the magnetic core.

Of course it is relatively easy to saturate the core in a magnetically closed structure like a toroid where the leakage flux is very low. But with a rod or a bar, its a lot more difficult because of the reluctance of what amounts to an enormous air gap in the magnetic circuit. Obviously a toroid can't be successfully used as a good transmitting antenna.

Looking at an iron cored example, it was interesting when "transformer style" ignition coils replaced the conventional ones. What the manufacturers didn't tell everyone is that they folded the core around to look like a transformer core, but left an enormous air gap if about 1/4 inch or more. It is not possible to see it because of the plastic molding. If you apply a current for the primary, say an amp or so, and run a screwdriver blade around the perimeter of the coil body, its very easy to find where this large air gap is.

[G8HQP Dave]

Originally Posted by Panrock

Maybe a ferrite based transmitter would be ideal where very low power and short range were needed in a compact package... one of those 1.7 MHz analogue phones mentioned in another thread comes to mind.

They used a piece of wire as a transmit antenna on the base station and a ferrite rod for reception in the handset. If they could have used a ferrite rod at both ends they probably would have done.

A related matter with regards to short range applications. How do we know when 'induction' is happening rather than 'radiation'? Indeed, what is the difference?

At sufficiently short distances the induction field dominates. At sufficiently long distances the radiation field dominates. The induction field falls off with distance like 1/(r^2) so power falls as 1/(r^4); the radiation field falls like 1/r so power falls like 1/(r^2). The two fields may have quite different patterns, and also different ratios of electric vs. magnetic fields.

Originally Posted by Tim

A ferrite rod picks up from the ends.

Does it? Maybe for local induction, but radiated fields are picked up broadside by a ferrite rod. The two fields may have different patterns.

Does that mean if one does use it to transmit then it would then radiate from the ends too.

No, it will radiate broadside. The transmission radiation pattern will be the same as the reception pattern.