[Elecraft] Magnetic Loop antenna...off Elecraft topic

[Ron D'Eau Claire]

I'll certainly second David's, G3UNA, comments about using a wire for
transmitting. 

Loops can be very effective for receiving though, as you noted. Any decent
receiver (like the K2) has plenty of excess gain to make up for the losses
in the loop itself, and they do tend to pick up less noise. With a receiving
antenna, signal-to-noise ratio is everything while in a transmitting antenna
efficiency becomes very important. 

You'll likely see more articles about small transmitting loops during the
next sunspot peak when extreme low power will "work the world" and the very
poor efficiency of a small loop is not so apparent. 

If you have access to the attic space in your building you might consider a
single wire or doublet directly under the roofing material if it's not a
metal roof. You can fabricate open wire line with some nominal sized wire
and makeshift spacers. The spacing isn't important nor does it have to be
entirely consistent. Such feeders can pass through tiny holes in most
ceilings no larger than a small nail and which are easily patched when you
leave. A bit of "spackle" or even the apartment dweller's friend (tooth
paste) will plug the little holes when you're done.  

Depending upon the composition of those bricks (some clay has much more
metal ore in it than others), you may not see as much attenuation as you
expect if you're limited to a wire inside your unit. 

You wrote: 

"A big plus is being able to match the antenna directly bypassing the
KAT2 for higher efficiency. My built-in K2 tuner is more efficient than my
MFJ tuner even though the MFJ has some usefulness for use with balanced
lines and built-in dummy load."

I wouldn't assume that is true unless you are talking about transmission
line losses between the loop and the K2. You don't mention how far apart
they are, but indoors it's usually a very short distance. In either case you
are resonating the system with lumped values of inductance or capacitance.
Whether they are at the antenna or at the rig in the KAT2 should make no
difference except, as you noted, it's much easier to tune a high-Q antenna
at the rig. 

If your MFJ tuner is one of their most common 300 watt (or lower) units,
it's a T-network. While they can be very good matching networks, a T-network
is notoriously inefficient when matching to a very low impedance load like a
small loop, so I wouldn't expect it to do as well as the L-network in your
KAT2. Looking at some scenarios in an on-line T-network simulator
(http://www.ve3sqb.com/hamaerials/w9cf/), an antenna presenting a
non-reactive feed point impedance to the T-network of 100 ohms at 7 MHz will
see 0.1 dB loss while an antenna presenting a non-reactive impedance to the
tuner of 0.5 ohms (not unusual for a small loop) at 7 MHz will show a loss
of over 5 dB. Like the small transmitting loop, those loses are resistive
losses in the inductor in the tuner and go up as the inductance required
goes up at lower frequencies. The losses just about double, for example, on
80 meters.

The bottom line is to get as much wire out there as possible to raise the
impedance at the feed point. That reduces circulating currents which are the
greatest source of loss, whether they are in the antenna as in a small loop,
in the a transmission line with high SWR, or in the matching network, either
at the antenna or at the rig.  

Ron AC7AC