[Elecraft] MFJ-1782 Loop Antenna?

[Ron D'Eau Claire]

Fred, N3CSY asked:

Has anyone tried, or using, a MFJ-1782 HF LOOP
Antenna - or homebrew of this?  

Some sites claim good HF success with this
small circular loop HF antenna.  I'm not a
fan of MFJ's quality, by a long shot - but owners
claim - with care, it works pretty well.

What do you think?  Is this a small stealth antenna
that would work?

-------------------------------------------------------

They do "work" Fred, but they are very, very lossy antennas. That's not
MFJ's fault, its inherent in the design. The problem is that the RF currents
go up as the impedance goes down. Small loops are very, very low impedance
antennas, so the currents are very, very high (wore out the "very" keys on
my machine writing that...). 

That's why MFJ makes a big deal about no "mechanical" (meaning screwed or
riveted) connections. Every milliohm of resistance counts in a big way. 

When we finally get room-temperature superconductors, small transmitting
loops will take their place alongside many other high-efficiency design. In
the meantime, they're about as good as most antennas their size. 

One of the big issues with a small loop is matching it to a 50 ohm output of
a rig. It requires an impedance matching network capable of doing that, and
almost no "normal" antenna tuner can do it. A big part of what you're buying
with that loop is the tuner that can handle that very low impedance. That's
why MFJ doesn't try to go below 10 MHz with their design.  

Like any antenna that is smaller than 1/2 wave long, the bigger you can get
it the more efficient it'll be. Wayne's suggestion to run a wire around the
top of the walls of a room will do much better for just that reason and be a
whole lot cheaper. 

Of course, using any antenna indoors subjects the RF to losses in the walls,
wiring, etc. of the building. 

AA5TB has a nice on-line article about his work with small transmitting
loops and a whole bunch of links to web sites of other Hams who are working
with them. It's at:

http://www.aa5tb.com/loop.html

No one is going to rewrite Maxwell's equations any time soon, but these guys
are all doing what they can to squeeze the most out of what they've got to
work with. Some of their work is very interesting. But, like almost all
Amateur antenna work, results are based on on-air checks are less than
meangful when you consider that a couple of watts of radiated RF can, under
the right conditions, be copied around the world. 

Ron AC7AC