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Old 5th Oct 2019, 7:41 pm   #1
Christoffrad
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Default Ferrite rod antenna and RF amp for Valve radios

I am building an antenna system for my valve radios ussing an old ferrite rod which has been recovered from a transistor radio.
It seems to work fine on Long Wave. A good resonant peak and clear directionality when turning the rod. But I'm having some problems on Medium Wave. The ferrite rod antenna consists of two coil sets. The Long Wave coil primary is 4.85mH and secondary is about 10uH. The Medium Wave coil set has a 1mH primary (approx) and 5uH secondary.
If the MW secondary is connected one way round the antenna doesn't seem to resonate with the tuning capacitor and if the secondary is reversed it becomes unstable.
The circuit is shown below.

Any suggestions?
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Old 5th Oct 2019, 11:45 pm   #2
TrevorG3VLF
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Default Re: Ferrite rod antenna and RF amp for Valve radios

You need to find the feedback on MW.
Move the rod well away from the radio.
Perhaps the 465 kHz fom the I.F. is saturating the rod.
The LW coil may be resonating around MW with no load, try shorting it.
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Old 6th Oct 2019, 7:38 am   #3
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Try a 'stopper' in series with the connection to the transistor base. With the emitter fully decoupled like that, its input impedance can go negative resistance at HF. This can cause oscillation with no obvious feedback path.

Otherwise try the circuit in a screened box.... feedback could be coming from radiation from the subsequent radio as well.

David
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Old 6th Oct 2019, 8:26 am   #4
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Default Re: Ferrite rod antenna and RF amp for Valve radios

By a stopper David means place a 47R to 100R resistor in series with the 10nF input coupling capacitor. Another place you can put it is in series with the transistor's emitter which will lower the gain a little and increase the stability.In that location a 27R would be OK probably. It will set the AC gain at about roughly 470/27 ignoring the the transistor's little re. Otherwise the gain is about 470/re , re is about 25/the collector current in milliamperes.

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Old 6th Oct 2019, 8:31 am   #5
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Are the earthy ends of the two windings on the ferrite rod actually connected to the negative line of the circuit? They should be for it to function correctly.
Also 470 ohms seems a bit on the low side for the collector load. Perhaps 1 or 2k may improve things here.
Radio Wrangler's suggestion of a stopper in the base circuit is a good one, as the impedace in that base circuit may be quite low which can cause various unexpected feedback paths to occur. Maybe a different type of transistor may help. A type used in an A.M. I.F. would be probably the best choice.
I made a similar device in the '60s using a 717A pentode valve. A truly startling increase in sensitivity! Tony.

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Old 6th Oct 2019, 10:04 am   #6
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Thank you all for your advice. I'll make some changes.

Chris
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Old 6th Oct 2019, 10:30 am   #7
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Default Re: Ferrite rod antenna and RF amp for Valve radios

I built something similar years ago with an even simpler design than yours. It was prone to oscillate at the HF end of MW unless I limited the supply voltage to 3V and was careful with the layout. Hand selecting a relatively low gain transistor also helped.
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Old 6th Oct 2019, 1:25 pm   #8
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Default Re: Ferrite rod antenna and RF amp for Valve radios

The 300pF variable cap is a bit too much for that coil! It will resonate around 290kHz.
If the coil is indeed 1mH on MW then you need only about 97pF to resonate at 520kHz, to tune in the low end of the MW band.
I suspect the coil inductance is less, it is about 0.5mH or so.
Polivaricon caps in transistor radios radios usually had about 230pF max capacitance or so for the antenna circuit.
You can take the matching cap from the donor radio and use it, or you can add a padding cap in series withe 300pF variable cap to bring down the capacitance.
For a 1mH coil you need to add about 150pF in series to get 97pF, to achieve 230pF (with a 0.5mH coil) you need to add about 600pF.

Not sure you need the extra gain coming from a transistorized RF-stage.

In the early '60s tube radios started to appear with ferrite loopsticks added.
Here is a snippet from the Orion AR-612 radio schematics, there they just switched out the "regular" MW-coil with the one on the loopstick.
https://www.radiomuseum.org/r/orion_ar612ar_61.html
The loopstick could also be rotated around in the radio with a knob to get the best reception.

Regards, Peter
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Old 6th Oct 2019, 1:39 pm   #9
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Default Re: Ferrite rod antenna and RF amp for Valve radios

You certainly don't need all the gain from a high gain silicon transistor to get good results and trying to use one causes stability problems. I think the one I eventually used was hfe 120 with 3V as the supply, and it still had enough gain to receive very weak stations.
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Old 6th Oct 2019, 7:08 pm   #10
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Most valve radios have a coil connected to the tuning capacitor in parallel which then connects directly into the grid of the RF/mixer valve (secondarywinding). The primary side with fewer turns is connected to a wire aerial. To make a ferrite rod aerial from a transistor radio the MW or LW coil has two windings each. you only need to use the winding with the most turns, the other winding is used to impedance match with the relative low input impedance of the transistor radio.
Now the valve radio has a high input impedance so this winding can be directly connected in place of the secondary coil. The only problem with a transistor radio ferrite coil is it is designed for a tuning capacitor in the radio of around 350pF Most valve radios have a tuning capacitor of around 550pF The set up is usually done with the radio tuned to 550kHz, which means th capacitor is set to around 550pF. In order to get resonance with L and C at the desired frequency the value of L (the ferrite coil winding) needs to have some turns taken off.
I have successfully put ferrite aerials in a couple of valve radios. Unfortunately I only found the correct coil windings by trial and error. (take a few off at a time).
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Old 6th Oct 2019, 7:14 pm   #11
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Default Re: Ferrite rod antenna and RF amp for Valve radios

In the 60s there was a 'thing' for outboard amplified-antennas which used a coupling-coil to induce their outputs to the ferrite-rod antennas of portable radios so to get better reception of Radio Luxembourg and the pirates.

I had a thread about this a while back -

https://www.vintage-radio.net/forum/...d.php?t=144226
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Old 7th Oct 2019, 3:34 pm   #12
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Thank you for all your further comments. I can report that the MW coil primary is 530uH. I'm currently using a variable capacitor that goes from 30 - 350pf so should resonate between 370 to 1260kHz with this coil. Indeed having introduced the 47R resistor into the emitter ( I also changed to a RF screened output cable) the circuit is now stable but although it exhibits resonance arount the mid MW band when connected to my valve set, it is not possible to detect any reception null as the rod is rotated so I'm not yet happy with this and suspect it may be regeneration from the radio (although proximity between rod/amplifier and radio makes little effect.

Chris
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Old 7th Oct 2019, 3:59 pm   #13
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Default Re: Ferrite rod antenna and RF amp for Valve radios

I think part of the problem is that you have a very high impedance node (c. 300kohm) formed by the hot end of the coil & tuning cap. So all the wiring between the two, plus the switch, are susceptible to E-field pick-up. This will affect the apparent directionality of the rod, and might provide a feedback path from the radio. By introducing the 47 ohms to reduce the gain, you've also raised the input impedance of the buffer, so the rod unloaded Q will be high: at least 100 if the coils are good. At 600kHz this results in a rather narrow resonance so it might be better to dampen the tuned elements to a Q of 40 or so. Ditto on LW where you need even less loaded Q.


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Old 7th Oct 2019, 4:48 pm   #14
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Thanks John

Would you just dampen the primary with a resistor? or something less crude?
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Old 7th Oct 2019, 8:21 pm   #15
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Default Re: Ferrite rod antenna and RF amp for Valve radios

You won't dampen the primary unless you wet it!



You could damp it with a resistor quite happily though the value may need trial and error. Try somewhere around 5k for starters.
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Old 7th Oct 2019, 10:24 pm   #16
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Yes, a resistor might seem crude, but the only alternative would be to apply negative feedback to the rod, and that starts to get a bit involved. Try about 75 ohms in series with the MF coil to start with.

BTW, I recall reading somewhere that if the LW coil of a MW/LW rod is left open circuit when MW is operating, it's possible for the LW coil to interact as it heads towards self-resonance.

John
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Old 8th Oct 2019, 5:48 pm   #17
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

John
As a test I tried shorting out the LW winding. This improves the performance of the MW reception. It clearly peaks now avoer the band and the rod is somewhat directional.
Why should this be?
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Old 8th Oct 2019, 9:10 pm   #18
John_BS
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Default Re: Ferrite rod antenna and RF amp for Valve radios

If your LW coil inductance is as you stated, it only takes about 5pF self-capacitance to resonate at about 1MHz. So you have a second fixed-tuned resonator coupled to the MF coil via the rod. A kind of crude second-order filter. Is your LW coil at the other end of the rod from the MW?
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Old 9th Oct 2019, 11:42 am   #19
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

The Rod is 120mm long. MW centre is at 40mm from one end. LW centre is at 90mm from same end.

i've tried the ferrite antenna on another radio (GEC 4750) In this case it comes to resonance but has no directionality when rotated. It is about as effective as a small untuned frame antenna that I have.

So the rod is not working properly with this set either
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Old 9th Oct 2019, 12:10 pm   #20
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Default Re: Ferrite rod antenna and RF amp for Valve radios

I've designed a realtively sophisiticated ferrtite antenna with a balanced topology such that, in theory, it should only respond to the H field. Even with this arrangement I don't get a perfect null inside a room. It may be that you have a mashed-up field in the places where you're experimenting, or the E field pick-up is quite strong. Can you try a portable transistor radio to see if that responds as you'd expect?

The other thing you could try is to add an emitter-follower to the output of your amp, add a series 47 or 75 ohms from the output and feed the radio through a metre or so of coax, so giving a bit of isolation.

John
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