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Old 3rd March 2009, 01:14 AM   #1
Mark245 is offline Mark245  United States
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Default AM Radio Antenna

Hey everyone. I was not sure where this question should go. It is loosely related to diy audio. I have built an AM radio receiver from scratch. It is a simple design that includes a RF amp at the front end followed by a detector which feeds into a two stage audio amp. When I feed an AM signal from my generator into the receiver it works perfectly. The problem is when I connect the antenna in place of the generator. I do not hear anything. However, when I touch the antenna with my hand, I can hear the radio station pretty well. Does anyone have an idea of why this is happening?

The antenna is a loop type. Basically it is a piece of wire wrapped around a box. A trim cap is used to tune it to a resonance frequency of the desired radio station. I can give more detail if necessary. Thanks.
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Old 3rd March 2009, 02:55 AM   #2
infinia is offline infinia  United States
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Because you're well grounded i.e. you have no potential.



kidding aside.
or are you de-Qing the tank possibly (wider BW). Access to a network analyzer?

1) best >long wire ant
2) better> ferrite bar
3) air loop

If you use a long wire antenna and connect the low side of the LC tank to a good earth ground this should cure that effect for good.
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Old 3rd March 2009, 11:13 AM   #3
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Loops cancel most noise that could be induced by your hand, because they are inherently balanced devices.

I'd stick to the loop variety of antennas for AM. The noise on those bands is horrendous and loops for the most part, are immune.

Cheers!
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Old 3rd March 2009, 12:08 PM   #4
infinia is offline infinia  United States
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Can you post an image of the physical build? thanks
OT Not really a noise issue at the moment. Long wire is best for Dx.
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Old 3rd March 2009, 10:43 PM   #5
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THIS is a DX loop for those lower frequencies

http://geek.scorpiorising.ca/GeeK_Zo...p?topic=3929.0
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Old 4th March 2009, 12:08 AM   #6
Mark245 is offline Mark245  United States
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The design I am using came from http://www.mindspring.com/~loop_antenna/

I am trying to tune in the AM radio broadcast band. Specifically I am trying to get 1230kHz. I will try to get some pictures so you can see what I am dealing with.

Thanks for all the responses so far.
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Old 4th March 2009, 06:24 PM   #7
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There are guys in the UK who listen to a.m. radio stations in the US with loop antennas.
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Old 4th March 2009, 09:25 PM   #8
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Mark245
Please post a picture of the radio and more importantly a schematic. If in fact you have only the loop antena and a tuning cap you don't have enough selectivity in that design. When you say that you are going from an RF amplifier right into a detector that is leaving out the mixer, and some other things. Its long been accepted pratice to mix down to an intermeadate frequency and incorporate still more filtering. In short you have whats called a TRF "tuned RF" radio and they are finiky at best. This might mean more parts, but it will me much better. Look up the "Superhetrodyne Principal" Oh,- and this topic is a little unusal for this forum, I hope I'm not out of line.
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Old 4th March 2009, 09:26 PM   #9
infinia is offline infinia  United States
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Hi
Must be using some super tuner. Wonder... who do they want to hear so badly in the US?
http://www.universal-radio.com/catalog/mwant/0445.html
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Old 4th March 2009, 10:55 PM   #10
infinia is offline infinia  United States
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Default Its 12 inches long @ 2 inches diameter

If'n it were my project I'd use a really big ferrite rod similar to what these folks use/sell. Probably use several different litz windings for peaking and impedance matching. Forget transitors and big air coils. Preamp would be a LN current feedback op-amp with a balanced ferrite LPF.

AGC is must for any kind of AM tuner! also a good IF filter for selectivity couldn't hurt. But then you need a good local oscillator. All been done before us.

oops...almost forgot the link http://www.stormwise.com/11AM-530K-1...-datasheet.htm
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