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Incredible difference in two AM antennas

bobbyrae

bobbyrae

The Audiophile Farmer
I can't believe the difference in two AM loop antennas that look like that they were made almost identically. Look at the photo. The smaller one on the left came with a Sangean HD radio; the one the right I purchased at a local electronics store. I have a Yamaha T-85 tuner I plan to keep but it needs an AM antenna and I want to sell the Sangean. I thought these things would all be about the same, but NO!!

The smaller Sangean version brings in every AM station I try and the T-85 meter registers 100% every time. The larger one has trouble getting the meter to register AT ALL on some stations! Rotating the antenna helps, but it is still getting a much weaker signal. What an amazing difference! Could it be the physical size of the loop? Or maybe the type of wire they used?

Do you antenna experts have any theories?
 

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It could be one has a broken connection; measure the resistance across the terminals. Of course one may have many turns and the other a few so the impedance will differ. The difference in dimensions is trivial.
 
difference in resistance

Bob91343 said:
It could be one has a broken connection; measure the resistance across the terminals. Of course one may have many turns and the other a few so the impedance will differ. The difference in dimensions is trivial.
Click to expand...

The big one measures .8 ohms; the small one .4 ohms. That is straight resistance with a Fluke. Using the BK Precision LCR meter set at 10kHz, I get 21 ohms on the big one and 1 ohm on the small one! I guess that's it.

Both appear to have 6 turns around the plastic loop.
 
I have these antennas, the smaller one is wider with more turns and i get 5 HD AM stations from a basement, vs 3 with the larger one.
 
Another question

Since it seems that probably the difference is the impedance of the wire, and these things are just plastic rectangles with some wire wrapped around them, would it make sense for me to take some new 18 guage copper stranded wire and just wrap it around? Or do they have some special wire for this purpose?

And if that would work, are more turns better?
 
Probably it would work best with Litz wire but you could use, say, 24AWG and make your own. 18 gauge is bigger than necessary.
 
bobbyrae said:
My "smaller" antenna is smaller in all dimensions.
Click to expand...



Bobbyrae, can you tell us the dimensions of that smaller loop? How wide is the antenna groove where the wire is laid? Is the wire neatly laid in rows or is it scramble wound? And are there two wires to the tuner? I'd love to replicate this at home if I could.

I've got over 50 feet of wire around the edge of the ceiling in the listening room and my T-80 sucks royally with AM and I'd like something better. An outdoor antenna isn't possible.

Tom
 
thank you!

dumptruck said:
Check out this page: http://www.hard-core-dx.com/nordicdx/antenna/loop/amloop.html
(instructions for loop antenna start about 1/2 way down)
Click to expand...

That's very interesting stuff. As usual, it ends up being more complicated than I thought and giving me more info than I can handle. Here's an interesting comment: "Sometimes the new antenna will overload the radio because it has so much gain. You might have to de-tune it slightly to receive the station you want. " This makes me wonder if that larger loop antenna just doesn't match up to the radio well.

Maybe the best question is where do I go to buy one of these loop antennas that's as GOOD as the small one? There's a Sony AM loop antenna for sale at Amazon for $10. But how do I know how good it will be?
 
bobbyrae said:
That's very interesting stuff. As usual, it ends up being more complicated than I thought and giving me more info than I can handle.
Click to expand...
You can pretty much skip all the info and just use that table to make a loop antenna. Say you want to make one that's an 11x11" square, then you find 11 in the third column, move over to the first column, and it says you should use 22 turns of wire.
 
P.S. I was just re-reading that page and a whole lot of it is related to modifying your radio and antenna for a radio that has no external AM connectors originally. If your radio already has connectors for an AM antenna, then none of that applies to you. You just have to make the loop (and any stand, rotating thing, etc. you want).
 
Sangean products seem to be very good, I have a CC radio plus that has excellent reception. I still prefer a 100 foot longwire antenna strung across my back yard.
 
a little more info

Since the larger loop was inferior and only cost $5, I decided to try some other wire on it. I unwound the original loop, which was about 20 loops, and then used my own 24 gauge wire at 15 loops (that's all the wire I had). The resulting impedance and reception were about the same as the original wire, so I decided to put the original back on. OOPS! Now the impedance has gone up to 66 ohms! There must be something in the way the wire is wound on there. I tried real hard not to cross over, but couldn't help it a couple of times.

Anyway, I guess there is no substitute for a well-engineered antenna. I don't think I am up to making my own. If taking the wire off and rewrapping it causes the impedance to triple, it's just too tricky!
 
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