Tuned Loop AM Antenna
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Modify that AM antenna by Bruce Carter
Nice article, with # loops vs. area table.
Tune your antenna for better DX - Construction of random length wire AM antenna
by Roy A. Walton (adapted by Bruce Carter)
If you use a “Pi-Section Coupler”, your antenna can be as short as 30 feet or as long as 100 feet. The combination can be made to work efficiently at all frequencies between 500 and 6800 kHz.
A MAGNETIC LOOP ANTENNA FOR SHORTWAVE LISTENING (SWL)
The PVC Loop Overall Article
How to build a tuned loop antenna
How can you vastly improve your Medium Wave reception? its quite simple really, all you need is 120 foot of wire, a few lengths of timber and an old tuning capacitor with which you can build the answer to every DX’ers prayers, a tuned loop antenna.
Our AM RADIO DX LOOP ANTENNA converts your portable transistor radio into a long distance receiver
The coupling from the antenna to the radio is through the ferrite loop antenna inside most portable AM radios. We have received several inquries about the possibility of conneting the antenna to a regular component receiver. Although every situation is different, it is generally easy to do.
The MTM Scientific AM Loop Antenna is quite a different animal from all of those however for a few reasons. First it is a large square wood frame loop measuring slightly over 17” per side which makes it at least twice the size of the other passive AM loops I mentioned. This seemed like an exciting prospect as it seemed likely it would be able to capture more signal than the smaller loops.
A loop antenna is a small multi turn loop of less than 1/10th wavelength in length. The loop is wound on a form, which may be either box (solenoid), or spiral (pancake) wound. The core material can either be air, or a powdered iron compound (Ferrite). The gain of a loop is much less than a longwire, but it has much less noise pickup. A properly designed Loop primarily responds to the magnetic component of the radio wave. Note that noise resides primarily in the electrical component. A vertical antenna responds mainly to the electrical component.
A loop antenna is an antenna primarily for the AM broadcast and the Longwave bands.
I think the generalization about ‘how many feet’ is more of a ballpark consideration. If you’re using a typical BCB range tuning cap like 365-400pf you’re going to fall into the 50-75 ft range almost inherently for most practical sizes and thats never been a bad thing.
Now if you want to use something like 200 feet of wire (a la some of these wacky broomstick or liter-coke bottle ideas) and find a cap to tune it with you’ll immediately see the uh-oh at 1700 kcs. And the other extreme…20 feet of wire at 550kc.
That said, you’ll run into the unavailability of standard parts and having to multi-bandswitch the thing to cover BCB probably before you will encounter the differences in actual peformance.
If you are building a 2-3’ size loop with a typical BCB cap you can tune the entire range without tapping it. The difference in wire length should not be noticeable.
I am proud to introduce you to my first loop tube radio. I had this in mind for a while and it took me a long time to get started. The radio design was simple, but how was I to make this into another show piece radio? Do I make it into a 1920’s look, or a rock n’ roll 50’s radio? This one is both. The design, and parts are something that you might have seen in a 1950’s Poptronics magazine. Then there are some hints that have that definite roaring 20’s look. The loop itself is an example as well as the art deco Garolite® front panel. The wood base gives this radio a breadboard look. I sure did it all. This puppy kept me up nights thinking how I was going to build her. I like that curve that decorates the front panel. I used a large pizza pan as a guide.
The radio is a simple regenerative type using a 6418 subminiature tube. This is a pentode tube that has a 30 volt plate maximum rating. The filament runs on 1.25 volts at 10ma. With 18 volts on the plate and a low current filament, the batteries will last a long time.