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Old 9th Oct 2019, 12:21 pm   #21
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

John
Getting a perfectly good null on a couple of other transistor sets ( same medium strength station) even one with much shorter rod.

Chris
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Old 9th Oct 2019, 12:24 pm   #22
paulsherwin
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Don't dismiss the possibility of something trivially simple, like a wiring error or a faulty coil.

There should be a very obvious peak as you adjust the capacitor.
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Old 9th Oct 2019, 12:25 pm   #23
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

And am already using screened output lead to the radio.
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Old 9th Oct 2019, 12:34 pm   #24
John_BS
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Do you have a scope?

J
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Old 9th Oct 2019, 12:36 pm   #25
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Hi Paul
I have checked continuity of all 4 windings ie the 2 LW and 2 MW.
I have measured the inductances of the LW and MW primaries 5mH and 530uH respectively and am using a air spaced capacitor that goes 30 - 350pf via a change over switch for the two sets of coils.

The hot end of the MW primary is clear to see so cold end is grounded. On MW it is not clear which is intended to be the hot end of the primary but I have made the assumption that as the LW hot is a red wire then of the two primary MW wires the red one is likely to also be the hot end. so each red goes via the c/o switch to the tuning C. On the secondary side for coupling into the transistor (via the c/o switch) it is anyones guess how the polarity goes. so one side of each coil is again grounded and the other goes via the switch to the transistor. (see my earlier posting for the circuit (which now has 47R added in the emitter)
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Old 9th Oct 2019, 12:37 pm   #26
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

yes I have a scope.
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Old 9th Oct 2019, 12:38 pm   #27
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

also a very old (Taylor 75) RF sig gen
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Old 9th Oct 2019, 12:51 pm   #28
John_BS
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Default Re: Ferrite rod antenna and RF amp for Valve radios

OK: try setting up the LW coil only and tune to 198KHz. Disconnect the output coax and monitor the voltage on the 470 ohm output of your buffer with a 10:1 probe. You should see >50mV p-p when it's working and tuned. I'm assuming your Droitwich reception is good!


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Old 9th Oct 2019, 1:20 pm   #29
paulsherwin
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Reception of 198kHz in East Anglia isn't great, though normally perfectly listenable.

I've attached the circuit of my ultra simple design from years ago (waveband and power switching omitted). You might want to lash it up and compare the performance with your own design.
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Old 9th Oct 2019, 1:35 pm   #30
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

I'm about 100mls from Droitwich. (Cambridge)
When tuned to 198kHz I don't see any signal but the radio has considerable slush noise. nevertheless I don't see any instability. Voltages around the transistor seem OK.
Must go out now - back this eve. Thanks for your help.
C
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Old 9th Oct 2019, 1:55 pm   #31
Mr 1936
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Hi Christoffrad. Ferrite rod aerials are used 99% of the time as a built-in aerial right next to the radio itself. I'm assuming that in your setup the rod is instead some distance from the radio it is feeding ? If this is the case, there is the risk that whatever cable you use to connect the two will also act as an aerial. This unwanted aerial will become more efficient as the frequency rises. A classic symptom that this is happening is the nulls in the rod's reception pattern becoming "filled in" and no longer as deep as they were. In days of yore this plagued designers of DF systems using loops, and was known as "Vertical Effect".
To solve this problem you can do one or more of the following:
Reduce the length of the interconnecting cable to reduce its pickup.
Increase the pickup of the rod, by using a longer rod or even a frame aerial.
Make the rod better electrically balanced: You will need a balanced to unbalanced conversion (BALUN) somewhere in the system. Only then will screening the interconnecting lead be an advantage.
One thing you could try for balance improvement is to make the tuned winding of the rod and its variable capacitor electrically floating, and try and minimise the length of wiring involved. Take the output to your preamp from the unbalanced coupling winding.
I'm sorry I can't be more specific, as there are so many variables involved. But I hope that understanding what is going on may help you arrive at a solution.
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Old 9th Oct 2019, 4:35 pm   #32
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Paul
Thank you for your circuit. I note that your antenna uses a tapped single coil. The antenna I have uses two discrete windings a primary tuned winding and a isolated coupling coil.

1936
In fact I have been using a screened 50ohm cable on the output to avoid the effect you describe. I have not tried floating the primary windings so I'll give that a go.

Thanks for your comments

Chris
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Old 9th Oct 2019, 5:22 pm   #33
Bazz4CQJ
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Although a slightly more complicated approach to the requirement to feed signals to vintage radios, a magnetic-loop (magloop) antenna would be a good solution.

If you search the Forum or Google for info on the Wellbrook antenna (a commercial product) or the Wellgood (its homebrew clone), you'll find lots of info on those and other ideas. Highly immune to to local electronic noise and fairly directional.

B
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Old 9th Oct 2019, 5:49 pm   #34
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Thank you Bazz. I wish I had 0.6m of space. I am aiming to assemble a display cabinet for several radios in a living room. That is why a compact ferrite antenna is appealing.

Chris
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Old 9th Oct 2019, 5:49 pm   #35
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Quote:
Originally Posted by Christoffrad View Post
Paul
Thank you for your circuit. I note that your antenna uses a tapped single coil. The antenna I have uses two discrete windings a primary tuned winding and a isolated coupling coil.
You can wire a coil with an overwinding to work as a single tapped coil. It's only for impedance matching, and one arrangement is pretty much the same as the other. I just happened to have a tapped coil when I built it (from a scrap transistor radio).

Your circuit doesn't show a cap to decouple the battery, which just might be significant. Try 0.1uF or a small electrolytic (or both).
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Old 9th Oct 2019, 10:21 pm   #36
Christoffrad
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Default Re: Ferrite rod antenna and RF amp for Valve radios

OK tried that but those C's don't make any difference. Of course the supply is effectively decoupled by the 10n that sits between the two 470Rs
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Old 10th Oct 2019, 8:55 am   #37
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Default Re: Ferrite rod antenna and RF amp for Valve radios

Yes, you're right.
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