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Old 13th Sep 2018, 10:11 pm   #1
davidgem1406
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Default Ferrite rod aerial information please.

I am building a Ferrite rod antenna for use with older valve radios that normally require a long wire antenna.

I have a rod with MW and LW coils each with a primary and secondary winding.
Each coil has 4 wires coded Red, Green, Black, and copper.

Measuring the winding resistance I have the following results:

MW Green to Red = 0.4 Ohm therefor the Primary winding.
Copper to Black = 2.8 Ohm therefore the Secondary winding.

LW Green to Red = 1.1 Ohm therefor the Primary winding.
Copper to Black = 11.5 Ohm therefor the Secondary Winding.

First I would like to verify that what I have concluded about the windings is correct and that the Primary is the front end winding to pick up the RF signal.

Next, would anybody know the color belonging to the start and finish of the windings?

I am wiring this just like any standard AM radio that uses this type of Ferrite rod, Roberts for instance, where one side of each coil is grounded. 2 of the remaining four will be switched and go to the tuning capacitor. The remaining 2 will also be switched and go to the signal amplifier stage.

Many thanks,
Regards to all,
Dave.
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Old 13th Sep 2018, 10:46 pm   #2
Herald1360
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Default Re: Ferrite rod aerial information please.

The higher resistance windings will be the ones tuned by the variable capacitor. I'd call these the primary windings. The lower resistance ones are the secondary windings used to couple the high impedance signal developed in the primaries to the low input impedance transistor circuit.

For use with valve sets, I'd try using them as you describe, but also just connecting the tuned winding directly between aerial and earth inputs- see which gives you best results.

I don't think it will matter much which way round any of the windings is connected, but connect the moving vanes to whichever end you decide to treat as earthy. If the coupling winding is at one end of the main winding, treat that end of the windings as the earthy end.

You don't say explicitly whether you will be making an active unit with its own amplifier stage or just using the ferrite rod aerial passively, but depending on whether any active stage is bipolar, FET, or valve, you may or may not need to use the coupling windings.
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Old 13th Sep 2018, 10:48 pm   #3
brightsparkey
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Default Re: Ferrite rod aerial information please.

I'm not sure you can assume all rod antennas are similar. There are many different arrangements.

You can have just a single coil for each band. In other cases the rod is also used as a transformer when using an external wire antenna.

The inductance of the windings must match the tuning capacitor in order to allow the correct frequency range. This determines the number of turns on a given rod.

When used as a transformer, the turns ratio will give the correct impedance transformation to match the input amplifier optimally for noise and selectivity. When no wire antenna is present the the windings act as the antenna themselves.

If you have a pre-made rod then you will need to measure it's inductance to check if it goes with your tuning cap.

Kevin.
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Old 14th Sep 2018, 6:10 am   #4
crackle
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Default Re: Ferrite rod aerial information please.

Isn't it simply a matter of replacing the existing M & L wave aerial windings with the corresponding primary winding on the ferrite rod.
I would guess that it would help if the original variable capacitor used with the ferrite rod in its previous life, was the same value as the radio you were adapting. I would think with out this, getting the tracking right so that the aerial winding is in tune throughout the whole range would be difficult.


Mike

Last edited by crackle; 14th Sep 2018 at 6:17 am.
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Old 14th Sep 2018, 8:56 am   #5
brightsparkey
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Default Re: Ferrite rod aerial information please.

I guess it would help if you could provide a schematic of the target radio input stage, and some picture of the antenna you have.

It sounds like a more modern antenna for transistor/IC use - I recognise the coloured wires you refer to from distant memories of 1980's radios I dismantled or repaired in my youth..

You may find that the impedances are not a good match to your radio. Its not unusual for transistor input stages to have the tuning cap at the top of the coil and the input to the amplifier taken from a lower tap, and the input impedance is low (10s-100s ohms) whereas valves have input impedances of kohms -10's of kohms.

Kevin.
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Old 14th Sep 2018, 9:44 am   #6
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Default Re: Ferrite rod aerial information please.

If you intend to use this ferrite rod aerial with any valve radio fitted with the 'A' and 'E' sockets, you have 2 main choices.

Use it as an untuned coil of wire or put a variable capacitor in parallel with the windings which will increase the sensitivity and selectivity but add an additional tuning control.
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Old 14th Sep 2018, 11:31 am   #7
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Default Re: Ferrite rod aerial information please.

I would have a go at calibrating the antenna by making a crystal set out of it and fiddle about until you have the correct range.
You may need to bias the diode a little bit especially if it is a silicon type.
Once done remove the diode and add it to the aerial input of the radio and use it like an ATU. A quick tweak of the aerial tuner on the chassis would help too.
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Old 14th Sep 2018, 1:34 pm   #8
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Default Re: Ferrite rod aerial information please.

Quote:
Originally Posted by brightsparkey View Post
I'm not sure you can assume all rod antennas are similar. There are many different arrangements..
No, you can't. However, in addition to the inductance matching the original and, therefore, the tuning capacitor, most MW/LW aerials have the windings in series on LW, so the phasing is important, the 'hot' end of the LW coil being connected to the earthy end of the MW coil. As has been pointed out, the position of the coupling coil should indicate the earthy end.

There is an exception to this, though. If you are lucky enough to have acquired an aerial intended for a Bush TR82, both coils are exceptionally connected in parallel on MW! (I expect the earlier valved MB60 used the same technique).
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Old 14th Sep 2018, 2:09 pm   #9
davidgem1406
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Default Re: Ferrite rod aerial information please.

Hello all,

Thanks for your input, to clear up some of the points made I will add the circuit here and a drawing of my mods to accommodate the Ferrite rod I will be using.
The circuit is one used many times before and originated from this forum.
I will get back with the drawings A.S.A.P.

Regards,
Dave
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Old 14th Sep 2018, 8:07 pm   #10
davidgem1406
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Default Re: Ferrite rod aerial information please.

Here is the information for this unit.

Images:
Circuit diagram.
The circuit of my mods to accommodate the Ferrite rod.
Picture of the Ferrite rod.

I also have the inductance figures for the 2 coils:
370uH and 4.1mH

The rod length is 127mm with a dia. of 10mm.

This is a Maplins item LB12N, the data given came with the rod.

Best regards
Dave.
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Old 17th Sep 2018, 10:17 am   #11
brightsparkey
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Default Re: Ferrite rod aerial information please.

At a rough estimate - your MW coil, for instance, is designed for a 6pF -120pF variable capacitor - fairly typical for a small Varicon type. Your 300pF vane capacitor is probably too large and won't reach the high end (1.6MHz), most likely.

Also the Q is not great at the high end with a 2n2222. Now maybe you don't want lots of selectivity as you're going to have to tune it separately, but if you want more Q then a faster device might help (higher Ft). Running it at 0.5mA collector makes the operating Ft lower.. the base looks like ~ 200pF // 100R at 1.6MHz.

Probably the LW is the same, but I haven't looked at it yet.

If you want to try a different coil, I have one from a Roberts R700, and its attendant tuning capacitor that you could have for the price of postage. These at least are a matched set. Probably a bigger antenna than you already have. I can take some pics if you're interested. Also you get a SW coil thrown in too.

Regards,

Kevin.
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Old 17th Sep 2018, 11:56 am   #12
brightsparkey
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Default Re: Ferrite rod aerial information please.

Closer inspection of the schematic (i.e. actually looking at it ) reveals the extra coil is in fact for external antenna coupling. This can be ignored or you could have provision for an external antenna socket..

Schematic attached.

L18 and L20 form the MW coil, and L19 and L21 for the LW coil, much the same as your current antenna. Trimmer caps are just there to line up the oscillator frequency with the front-end selectivity - if you hand tune then these are not necessary. L16 is just a suppression bead - a loop through a ferrite bead - these are on the tuning capacitor already.


Pink windings with 3 wires is LW. The blue winding with 2 wires next to it is the ext. antenna coupling winding. The MW winding is the grey one with 3 wires.

Kevin.
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Old 20th Sep 2018, 3:28 pm   #13
davidgem1406
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Default Re: Ferrite rod aerial information please.

Hi Kevin,

Thanks for your input and offer, also for the PM.

I built the unit just using the circuits as modified with the short rod shown in the images on 14/Sept.

It worked very well, I used a 1940's HMV for the test. This radio without an external aerial will basically produce zero.
With the Ferrite rod unit connected it would pick up faint signals, tuning the aerial unit would bring in each station loud and clear with plenty of volume at about 1/4 travel of the volume control.
That applied to both bands and across the range.

I also changed the rod for one that is 180mm, whilst the coils remained the same. That did improve things, but by how much is difficult to determine.

The tuning capacitor VC1 is shown to be 300pf, however, I have no idea as to the value of the unit I used as it is not marked, and I have no means to check the value. It is the only single gang unit I had.

I will now try a Ferrite rod from a Roberts to see if things can be improved even more.

Best regards,
Dave.
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