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 Post subject: Antenna Amplifier/Filter Design
PostPosted: Mar Thu 09, 2017 4:16 am 
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Not sure if this is a completely rediculous idea or not, but I would like to try building a small attachment that goes from antenna to ground of any vintage AM radio to both amplify the received RF signal and block noise outside the operable frequency range. Here's a frequency sweep of the proposed filter:

Attachment:
FLTR1.JPG
FLTR1.JPG [ 51.1 KiB | Viewed 467 times ]


And the circuit:

Attachment:
FLTR2.JPG
FLTR2.JPG [ 19.95 KiB | Viewed 467 times ]


I haven't got around to adding much gain, but this is good enough for starters and discussion. My ultimite goal here is to pick up stations on an AK20 battery set from a longer distance and/or block noise from fluorescents and dimming bulbs. Now for my question. Since the tuning condensers in a radio already tune in a specific frequency, is the filter redundant? I figure if enough gain is added, it should be strong enough to swamp out most of the unwanted noise.


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Thu 09, 2017 4:26 am 
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isn't each one of those op-amps a noise source?

My gut reaction is twofold:

1. Increased SNR in the front end is typically done with a tunable narrow-band RF stage and a low-noise gain element. Wide-band means large noise bandwidth also.

2. At some level of "completeness" in a design, it is usually not possible to improve it by adding parts.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Thu 09, 2017 4:39 am 
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What would be a good example of a "low-noise gain element" besides a tube? How about a simple NPN? Maybe a wide band amplifier can be used and let the radio filter out any noise. It wouldn't be practical to add another tuning stage.


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Thu 09, 2017 2:15 pm 
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By " simple NPN", I assume you mean a transistor, which is also a gain element. All amplifying devices have noise. I'm not at all current on noise performance of various devices, but Google will find it.

My hunch is that you can't improve the performance of an RF front end by simply adding wide-band gain. It would be instructive to see if you could find any examples.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Thu 09, 2017 8:31 pm 
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Tuned loop will accomplish everything you have designed - greater signal pickup and rejection of out-of-band signals.
You could add some gain between the loop and the radio if you really wanted it.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 4:36 am 
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I have no knowledge of loop antennas, and googling for schematics results in so many different designs. Any recommendations? Would the tuned cap remain fixed, or will it have to vary with each station?


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 4:50 am 
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The typical loop is a fixed inductance, and is tuned with a variable capacitor. Look at the circuits used in various "AA-5" and "AA-6" radios.

Most radios use a fixed inductance in the front end...plus a variable capacitor. When using a loop, it is the inductor, and nothing else is needed. For a set using a "long-wire" there is a coupling coil, which creates a transformer.

You would not normally take a set with a tuned front end and then simply connect a separately tuned loop. But I think it can be done with the right matching transformers. (Search here for "loop antennas")

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 4:32 pm 
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I would like to avoid building a tuned loop if possible, just for the sake of being user friendly. Below is the RF section of the AK20.

Attachment:
AK20.JPG
AK20.JPG [ 50.6 KiB | Viewed 242 times ]


I would have to see schematics of what you recommend to really understand. When you say "it is an inductor and nothing else", that implies that I can just take a long wire, wrap it around a loop, and connect from antenna to ground. Then there's the question of what gauge to use, what length to use, what should be the overall inductance value, etc.

Would it still be common practice to ground a radio when using a loop antenna?


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 4:55 pm 
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I think we need to go back to the question: What problem are you trying to solve?

Compared to using a long-wire antenna, as the set is designed for, you are not likely to improve performance with a loop, unless it is tuned. If you keep the existing antenna transformer, then I think the typical setup has the loop connected to a matching transformer so that the loop has an impedance similar to the long wire ( nominally 377 ohms ). I am not certain about this, so do the search that I recommended.

Also, are you comfortable with the principle of operation of the current front end?

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 5:03 pm 
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More random answers:
Any length of wire is an inductor. If it's wound in a coil, the inductance is higher. Do a google search for one of the many online inductance calculators.

Grounding:
With a long-wire, the return for the received signal is the local ground plane. One way or the other, the radio must connect to the ground plane...often just through the AC wiring.

With a loop, the return is at the end opposite the signal, so a ground is not required.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 10:20 pm 
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You cannot filter noise that is within the passband of the signal you are listening to. But,a directional loop will allow you to null the direction of the noise source.

Pete

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sat 11, 2017 10:56 pm 
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The tongue-in-cheek definition of the 3 laws of thermodynamics somehow fits a lot of things in life.

1st law: You can't win.
2nd law: You can't break even.
3rd law: Don't even try.

Further simplified: There's no free lunch.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sun 12, 2017 4:28 pm 
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pixellany wrote:
My hunch is that you can't improve the performance of an RF front end by simply adding wide-band gain.

In some circumstances that can degrade performance, such as when there are strong signal sources such as nearby radio stations.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


Last edited by Dale H. Cook on Mar Sun 12, 2017 4:41 pm, edited 1 time in total.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Sun 12, 2017 4:40 pm 
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pixellany wrote:
The typical loop is a fixed inductance, and is tuned with a variable capacitor.

A typical commercial example of a tuned loop is the Radio Shack 15-1853 (long out of production) which I have used for many years.

A loop can sometimes be beneficial when a local station is causing interference to the desired station or the local station is desensitizing the receiver. If the interfering station is around 90 degrees from the desired station the loop can be oriented to null out the interfering station while providing some gain to the desired station.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 12:06 am 
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pixellany wrote:
I think we need to go back to the question: What problem are you trying to solve?



1) I want to be able to use this radio in a semi-rural location. Currently, I barely get a signal from the strongest station.

2) If I am able to solve problem 1, I would like to try reducing interference from fluorescents and dimmable lights. Currently about three or four lights have to be turned off to get rid of this interference, which makes it somewhat impractical for demonstration.

Regarding the antenna, I figure a ball park inductance value must be achieved in order to resonate with a practical variable capacitor value. Then, it must be capable of being tuned from approximately 550 to 1700KHz. Does this idea hold merit?


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 2:01 am 
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I wonder if there is a problem with your radio. I looked up your location and this list shows a ton of stations that you should be able to hear even with some noise sources in your house.
http://radio-locator.com/cgi-bin/locate ... =freq&sid=


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 2:10 am 
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The radio works great where I live. Taking it up to Michigan, not so much.


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 3:51 am 
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There are 2 design issues: the loop, and the coupling to the front end. I've never built a custom loop, so take that into account.

I would think the easiest would be to emulate what is done with the ubiquitous AA5s and AA6s, to wit: use the loop as the single tuned circuit in the front end.

Your picture of the design of the tuned circuit is correct....Here are the basic steps:
Select a variable capacitor.
Do the math to see what the tuning range will be with a simple parallel LC circuit.
( f = 0.159 / sqrt (L*C) )
If necessary, calculate the value of a "padder" capacitor, which allows you to adjust the tuning range.
The inductance value will fall out of the math.
Use the L in an inductance calculator to design the coil.

But first, do the search here on how other people have done this.

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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 4:06 pm 
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Is the loop antenna design usually an LC series or parallel connection? I am finding contradicting schematics.


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 Post subject: Re: Antenna Amplifier/Filter Design
PostPosted: Mar Mon 13, 2017 4:41 pm 
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A front end tuned circuit is normally parallel resonance. The impedance goes to a maximum at resonance, which equates to more signal passing through.

If you post the schematics (or links) that you're looking at, we can be more specific.

Padders are in series

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