Hi guys (and gals), I hope all you Hams won=92t laugh at me too much.So as a final piece to my current project we want to make an AM radioreceiver. (This is mostly to give a =91real world=92 mixingapplication.) Now the only AM radio I=92ve played with was a longpiece of wire as antenna to coil, variable cap, Ge diode to ear bud.The current idea is to NOT do the frequency selection with an LCcircuit but with a mixer.So, antenna to HF amp to mixer to audio amp.I banged this together yesterday and it worked OK, but far from great.I was able to see some nice ~10kHz wide side bands around a centralcarrier. (on a spectrum analyzer) But when I tuned the LO to becentered on the carrier and listened to the sound it wasdisappointing. The sound level was bouncing around at about a 1 Hertzrate. I could see the same thing on the =91scope, the HF signal fromthe antenna was bouncing around.My antenna was just a piece of wire maybe three meters long stuffedinto the BNC jack of an amp with a 1 kohm input impedance. My firstthought was that I need some sort of balun transformer on the input.(I=92m guessing the bouncing around of the input was because the otherhalf of my antenna was ground?)This then raises several questions.What=92s the impedance of my piece of wire. (I=92ve got to keep thetransformer impedance well above this.)OK I=92ve got several more questions, like should I use some other sortof antenna. But I=92ll wait...perhaps my first assumption is wrong!Thanks for any advice.(discrete chuckling from Hams is OK.)George H.

It has to be phase locked, not 'tuned the LO to be centered on thecarrier' if you want a clean and stable output. Any offset will causethe problem you have. Find a copy of the ARRL handbook, or their design guides.-- You can't have a sense of humor, if you have no sense.

AM radio

Nov 16, 2011 62 Replies

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Ralph Barone 14 years ago

Perhaps he graduated twice.

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Fred Abse 14 years ago

I shall have to take a look at that. I confess to using published curves (Radiotron Designer's Handbook), back in the time I needed such things. One source of information might be the coupling circuits for AM automobile antennas. IIRC, they looked like several hundred pF in series with a couple of ohms.

A coil antenna looks more or less like a pure inductance with loss resistance. It is only sensitive to the H-field, and gets rid of a lot of E-field crap from around the place. You could probably treat it as just a voltage source and damn any power match.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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George Herold 14 years ago

No 'we' both graduated together.

George H.

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