Ferrite aerials for reception in cars

[Panrock]

In another thread, it has been mentioned that there was a BBC study in the 1970s examining the use of ferrite rods for car radio aerials, in particular for VHF radio reception.

I wonder what the conclusions were?

The dayroom at my 1960s boarding school was situated in a modern steel framed building and had fluorescent lighting. Trying to operate an old valve 'steam radio' on medium or long waves with a wire aerial always resulted in terrible, interference-ridden reception. But using a transistor radio with a ferrite rod, reception was much better.

I understood this was due to a wire being sensitive to the electric field and the ferrite to the magnetic. The interference was contained mostly in the electric field.

Could this property of ferrites have been exploited in another situation where interference was an issue, namely the motor car?

For LF/MF use maybe you'd need two crossed ferrites, to provide omnidirectional reception.

For VHF/FM use, I guess a single vertical ferrite rod would have given omni-directional coverage with the horizontal polarisation used at the time. More compact than a whip too. Sounds ideal.

I remember the problems I had obtaining reception free of ignition interference on my VHF car radio in the early '70s. If I had used a ferrite, perhaps this problem would have been much less.

Also (very much guessing here), perhaps the field-concentrating property of the ferrite, by 'pulling in the field' from over a wider area, would have meant the reception suffered less from 'plopping' sounds. I got these when the car moved through local nulls caused by screening and reflection of the signal.

All-in-all a promising idea. I wonder why it never took off?

Steve

[kalee20]

I'm not sure that two crossed ferrites would provide omnidirectional response! wouldn't they produce the same as a single rod at 45°? (With a null at -45° and 135°?)

The only way to get the omnidirectional response would be for each of the two crossed ferrites have its own front-end, right up to the detector - and be combined at AF.

Feel free to disagree, I'm not an experienced aerial analyst!

[Panrock]

An interesting point. I'm not an experienced aerial analyst either!

Two crossed dipoles (picking up the electric field) could be used as omnidirectional arrays. I had such an array on my chimney for TV-DX'ing.

Could there be a difference when picking up the magnetic field though? Something to do with the inability to suitably phase the two incoming signals perhaps?

Steve

[Nuvistor]

I found this BBC report, snippet attached.
http://downloads.bbc.co.uk/rd/pubs/reports/1969-51.pdf

[Hartley118]

Originally Posted by Panrock

Trying to operate an old valve 'steam radio' on medium or long waves with a wire aerial always resulted in terrible, interference-ridden reception. But using a transistor radio with a ferrite rod, reception was much better........

......... The interference was contained mostly in the electric field.

Steve

But would Maxwell have agreed that electromagnetic waves can possess an electric component without the corresponding magnetic?

I know that practical experience suggests that a magnetic loop aerial can vastly reduce interference pickup - my new Wellbrook Loop aerial gives testimony, with a startling improvement in S/N ratio on AM stations.

But I don't understand it. Is an interference signal typically not a radiated wave at all?

Martin

[Junk Box Nick]

Back in the days when built-in car radios required a separate licence (separate from the radio licence you had for use at home) my parents would take a battery portable for days out where it rested on the shelf below the rear window. You could bet that no matter which direction we were driving the ferrite aerial was always in a null position...

[Panrock]

Originally Posted by Hartley118

But I don't understand it. Is an interference signal typically not a radiated wave at all?
Martin

Perhaps we're back here to the difference between 'induction' and 'radiation' as discussed in another thread. The interference may only be conveyed by induction.

But doesn't induction also necessarily have a magnetic component?

Steve

[G8HQP Dave]

Radiated fields have a fixed ratio between electric and magnetic fields, called the impedance of free space. Local induction fields do not have a fixed ratio so it is quite possible for local interference to be mainly electric or mainly magnetic - depending on exactly what combination of potentials and currents is responsible.

There is also the issue that the induction fields may be longitudinal (radiation fields are purely transverse). A simple 'long wire' (for MW reception this is often actually a short wire!) may be sensitive to longitudinal fields, while a broadside dipole cancels them out and is more sensitive to transverse electric fields (although it can still pick up longitudinal fields from its ends - antenna polar patterns can be different for radiation and induction fields). Hence the common finding that a dipole is 'quieter' than a long wire.

[Biggles]

Yes I remember my older sister being in charge of keeping the transistor radio held in the best orientation for reception as we drove off on holiday in the Morris Minor. Five of us in it! ( the car, not the radio)
Alan.

[Jon_G4MDC]

I will chip in that I reckon the only ferrite rod aerial I have seen intended for VHF was in a Pye Pager - think it was the PG1.

The www. seems not the know the PG1, it was made for Firemans callout etc.
No pictures. Black with an orange set/reset actuator best I can recall. <edit> I found one, which is FM, I believe AM made in this series too. An earlier AM version - the Alerter also found.

I'm sure it had a Ferrite rod antenna (probably Neosid F27 grade - guesswork) wound with silver plated copper tape about 2.5 or 3mm wide. Maybe 4.5 or 5 turns?

[Biggles]

That picture has brought back some memories. I used to use one of these for callouts around about 1985. Ours worked on the VHF AM main scheme and were low band. The fire alerters used to be triggered by a separate line controlled Multitone base station, local to the Fire Station and were FM. Pye made an FM pager, I think it was the PG7FM?, a bit later on. I seem to remember coding them with wire links. I definitely remember seeing the rod type antenna with a couple of turns of quite thick looking tape. The Motorola Pageboy actually used the metal case as a sort of folded antenna.
Alan.

[Jon_G4MDC]

It could be - years scramble the memory.
Lets wait for assistance from others

Hmmm, now you mention it , I used to have a fire callout transmitter. The Main scheme was Low Band but Callout TX was highband.
I will sleep on it - see what surfaces....

[Herald1360]

Could be that the terrible interference on the steam radio was getting in via the mains rather than the aerial?

[Biggles]

At the risk of hijacking this thread Jon, it was always a bit of a gamble when flashing up an alerter base station as even when transmitting into a dummy load, and sending a separate TEST signal, if a standby fireman (or woman) just happened to work right next to the station, a few seconds after a test was done, there was often the sound of hurried footsteps and doors banging then "oh not you lot again!" Happened quite a lot actually.
Alan.

[Damo666]

I have a couple of the Pye pagers here using a Ferrite rod as the antenna. They're called the Pye Sentinel SR1, and they were normally activated by a test tone determined by a reed, and the alert tone was activated by following with a 3Khz tone.

They were initially on 147.800 Mhz, but mine has been converted to the 2M amateur band and I must say, the ferrite rod does a great job of pulling in the signals.

[Radio Wrangler]

Originally Posted by Hartley118

But would Maxwell have agreed that electromagnetic waves can possess an electric component without the corresponding magnetic?

I know that practical experience suggests that a magnetic loop aerial can vastly reduce interference pickup - my new Wellbrook Loop aerial gives testimony, with a startling improvement in S/N ratio on AM stations.

But I don't understand it. Is an interference signal typically not a radiated wave at all?

Martin

The travelling pair of magnetic and electric waves is just one of the solutions of Maxwell's equations. This solution is valid in the 'far field'

More generally, the equations describe how a travelling magnetic wave progressively induces a travelling electric wave to go with it and how a travelling electric wave progressively induces a travelling magnetic wave to go with it. The key word is 'progressively'

If we took a an idealised transducer which made one sort of wave, but not the other, we would see its sort of wave in its immediate vicinity and none of the other. Moving away from the transducer we would see the other sort of wave grow progressively from zero.

This induction of one wave from another represents a power transfer, and as there re are two directions of inductions, there is power transferring from each wave to the other. Conservation of energy says that the total energy in both waves must be constant. Superposition/linearity says that the rate of energy transfer in each direction must be proportional to the strength of the source field, and so as time and distance go by, the strengths of the two fields will settle into an equilibrium, with the two energy transfer rates balanced, and a predictable ratio of the strengths of their inducing fields. This condition is what we call the 'far field'. It's an asymptotic approach to equilibrium, but for practical work, it's usually considered to be settled about 20 wavelengths from the source.

So if you're 20 lambda or more from the transmitting antenna, it doesn't matter what sort of wave the antenna radiated, the permeability and permittivity of free space have done the balancing act for you.

But if you're closer, then you could see a dominant magnetic component, or a dominant electric component.

Short probe-y rods or small capacitive plates act as E-field transducers, small loops do for mag fields. So most of our antennae are biased towards one sort of field or the other, as are most things radiating electromagnetic energy.

So your radio in the school common-room gets hit with a nicely balanced pair of electric and magnetic waves from the distant transmitter. It doesn't matter whether its antenna is better at E or H components.

But it is nowhere near 20 wavelengths from the fluorescent lights and the building wiring. It does matter which field is dominant in the near field radiation of the interference, and which field the radio antenna is most sensitive to.

On top of all this, the ferrite rod can be rotated and might just give a position where the broadcast is peaked a bit and the worst offender is nulled a bit.


Maxwell's equations were a remarkable feat of understanding. They've survived the onslaught of special and general relativity and of quantum mechanics and still come up smiling. Few other things have.

David

[ms660]

VHF ferrite rod antenna:

http://downloads.bbc.co.uk/rd/pubs/reports/1977-11.pdf

Can't see it working inside a vehicle very well unless it's glued to a window or somehow on a remote head unit externally unless it's a Trotter's Independent Trading Co type vehicle or an open top jobie, Radio waves don't like big sheets of metal in the way.

Lawrence.

[Radio Wrangler]

With the benefit of hindsight, we can look back and ask why, if it was any good, didn't it catch on?

There are some disadvantages:

1) It needs an extra gang on the tuning toresonate it - Either another varactor or else an extra section on a mechanical variable capacitor.

2) it needs an extra RF stage to get the performance up to 'adequate' which costs money as well as several mA more from a battery. There had been articles by Mullard etc on how bipolar front-ends could be run at lower current consumption than FET ones of like performance.

3) He makes the statement that you need 20uV/m to break even with the noise floor, and then goes on to relate this to the antenna Q. Hmmm does the series-R or shunt-R that you could use to model the Q-defining losses also model the noise figure? I'd suspect multiple mechanisms, and not all working in the same direction, also ferrites can have excess noise mechanisms.

It seems to have sunk without trace

David

[Hartley118]

Originally Posted by Radio Wrangler

The travelling pair of magnetic and electric waves is just one of the solutions of Maxwell's equations. This solution is valid in the 'far field'

More generally, the equations describe how a travelling magnetic wave progressively induces a travelling electric wave to go with it and how a travelling electric wave progressively induces a travelling magnetic wave to go with it. The key word is 'progressively'

If we took an idealised transducer which made one sort of wave, but not the other, we would see its sort of wave in its immediate vicinity and none of the other. Moving away from the transducer we would see the other sort of wave grow progressively from zero...............

........Maxwell's equations were a remarkable feat of understanding. They've survived the onslaught of special and general relativity and of quantum mechanics and still come up smiling. Few other things have.

David

Thanks David for a very lucid explanation of the improvement in S/N ratio obtained by concentrating on receiving only the magnetic portion of a radio wave. As I'd suspected, the key is the difference between the uniformity of the far field and the eccentricities of the near field less than a few wavelengths from the source. It helps to explain that the most dramatic improvement I get from a magnetic loop is in the long-wave band where, in my Cambridge location, RTE1 on 252kHz is barely usable on any long-wire 'electric' aerial but is received with ample S/N ratio for good entertainment value on a magnetic loop.

The phenomenon illustrates that what we think of today as a frustratingly general impenetrable 'fog' of interference on the AM bands is typically a relatively locally generated electric interfering field which can be rejected by a magnetic loop aerial.

I agree that Maxwell's prescience in creating his electromagnetic wave equations was remarkable. I do find it disturbing when a mathematician forecasts a phenomenon that we practical types haven't yet noticed.

Maxwell's ghost clearly still haunts these Cambridge streets!

Martin

[ms660]

From what I read up on yonks ago the interference is mainly vertically polarized...same orientation as most MW/LW wire antennas and whips, sod's law.

Lawrence.