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 Post subject: Is an untuned RF stage????????????
PostPosted: Mar Mon 26, 2018 4:19 pm 
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Would an untuned RF stage in a Hallicrafters S-19R Sky Buddy be worth the effort? I have read that adding an untuned RF stage does little to reject mirror images, such as a tuned RF stage would. I do not know, and would like a few scholoroly replies on the subject, please?........Boyd


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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Mar Mon 26, 2018 5:18 pm 
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Once a signal is above the minimum required for basic operation, adding more gain stages does nothing except maybe increase noise. If you need an increase in sensitivity, why not add a TUNED RF stage?
If you need the performance---and you don't want to upgrade---you can learn to live with having an extra knob to turn when tuning.

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Last edited by pixellany on Mar Wed 28, 2018 4:22 pm, edited 1 time in total.

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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Mar Mon 26, 2018 5:30 pm 
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Atwater Kent incorporated an untuned first stage in some of its receivers as a way of taming the wild impedance swings of a wire antenna over the broadcast band.

It was an impedance-matching system, not a gain stage.

- Leigh

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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Mar Wed 28, 2018 1:40 am 
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Location: Tokyo
The higher the frequency, the harder it is to get gain from an RF amplifier. Tube, socket, wiring, etc capacitance shunts the signal to ground. Not a problem at MW or lower SW frequencies, but it is overwhelming at 15MHz and higher, just where your S-19 needs help. That's why RF amps for SW are always tuned: to get the maximum possible gain to make up for the inevitable shunting losses. An untuned RF amp might even attenuate the signal rather than boost it.

An untuned RF amp could improve MW reception, assuming you're just using a 30 or 40 foot wire for an antenna. It won't help with SW, and of course it will do nothing to help with image rejection.

As Pix noted, if you're interesting in improving your receiver's SW reception, you need a tuned RF amp, usually referred to as a preselector.

Rob


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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Mar Wed 28, 2018 2:19 am 
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As Pix noted, if you're interesting in improving your receiver's SW reception, you need a tuned RF amp, usually referred to as a preselector.

And generally in the form of an external box that connects between the antenna and the receiver. They were available commercially and there are articles on how to build your own. As previously mentioned, they add another knob that must be tuned along with the receiver.

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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Apr Wed 18, 2018 4:34 pm 
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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Apr Wed 18, 2018 5:14 pm 
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Hi Boyd,

I wonder if a simple antenna tuner would not help?

This would require a parallel coil and varicap in line with your antenna lead. The coil and cap need to resonate in the range of your desired band.

I've found that this approach can give a spike in response at the tuned frequency, or can be slightly off tuned to give a deep valley.

Tim


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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Apr Fri 20, 2018 12:00 am 
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I'm going to amend my earlier comments. I was thinking only of simple, untuned vacuum tube RF amps: it is very difficult to get gain from such an amplifier at frequencies that will actually improve performance.

However, there are wide band, solid state preamps that claim 6~9dB gain. That could help the S19 where it really needs it, say above 10MHz. Should be used with an RF attenuator. Not as good as a preselector: untuned RF amps can create other problems. But if it could be switched in and out, used only when needed, I think it would help.

Rob


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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: Apr Mon 30, 2018 6:38 pm 
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Location: Champaign IL 61822
I'm talking broadband, 100kHz to 30 MHz.

20 dB gain and excellent noise performance is trivial for op-amps.

Its doable with tubes, especially frame grid triodes, but the 12AU7 will in fact do OK.
You need three triode sections, two for a cascode, one for an emitter follower
to match the existing radio input. To do well you do need to get the tube current
well up near the max spec, and careful layout.


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 Post subject: Re: Is an untuned RF stage????????????
PostPosted: May Tue 08, 2018 10:46 am 
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Location: Powell River BC Canada
As to why, the untuned RF stage might prevent leakage of the oscillator
frequency from 6K8, back out the aerial.

That signal might be triangulated to reveal your position.

See par. 3 & 4 p.992 Radiotron Designer's Handbook, re. 6K8.







7

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