What is the best way to feed a mag loop? I used a 1/5 feed with the shield
and center shorted at the base and a 3/8" gap at the top of the shield
(wa0vsl). Should I solder the shields at the bottom and the center to the
shield at the top? Also, what about the shape of the feed? Mine is an
eliptical shape for bandwidth. Is a circle better? I based my loop on the
octagon from the elecraft website. 73, Damon

"KC5CQW" wrote -
What is the best way to feed a mag loop? I used a 1/5 feed with the shield
and center shorted at the base and a 3/8" gap at the top of the shield
(wa0vsl). Should I solder the shields at the bottom and the center to the
shield at the top? Also, what about the shape of the feed? Mine is an
eliptical shape for bandwidth. Is a circle better? I based my loop on the
octagon from the elecraft website. 73, Damon

============================

Samon, and others,

For every impedance matching circuit there must be at least two controls.
One is related to the resistive impedance component and the other to the
reactive component.


In the case of a magloop one component is the tuning CAPACITOR and the other
is COUPLING-LOOP DIAMETER. It so happens that for a 50-ohms system a
circular coupling loop needs a diameter about 1/5 of the main loop. A nice
convenient feature of a magloop is that the 1/5th ratio remains relatively
unchanged over several bands. So only the capacitor need be re-adjusted
when changing frequency from one band to another.


Actually, it is the enclosed AREA of the two loops which matters. For a
50-ohm system the enclosed areas should be in the ratio about 1/25. So the
actual SHAPE of the coupling loop is not of great consequence. Another
feature is that the exact location of the coupling loop within the main loop
is not of great consequence. It is mechanically convenient to mount the
coupling loop near to the circumference of the main loop. For the area
ratios to hold it matters only that the two loops lie in the same plane.


In fact, the effective (diameter) area coupling ratios can be be 'pruned'
more conveniently just by rotating the coupling loop inside the main loop
without changing its size or shape. (But nitpickers may not approve of the
antenna's cosmetic properties. ;o)


The practice of extending the coax line to form the coupling loop has no
electrical/radio advantages. A bare, self-supporting wire may be better
because it is more conveniently adjusted for length/diameter/enclosed area.
Or for rotating in the plane of the main loop.


The main loop must NOT be electrically connected to the coax braid. Allow
it to find its own electrical potential within the space it has been
allocated. Ideally the main loop should be isolated from everything except
via its magnetic coupling to the small loop. Choke all connections to a
remotely-controlled tuning capacitor.


The shape of magloops is of no electrical consequence except that a circle
has the greatest enclosed area for a given perimeter. This gives the
greatest possible radiation resistance for a given conductor loss resistance
and hence also the greatest radiating efficiency. But as program MAGLOOP4
will show, it is not very critical insofar as S-units are concerned.


For a full analysis of magloop performance, download program MAGLOOP4 in a
few seconds from website below and run immediately.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg, G4FGQ wrote:
"The shape of magloops is of no electrical consequence except that a
circle has the greatest enclosed area for a given perimeter. This gives
the greatest possible radiation resistance for a given loss resistance
and hence the greatest radiating efficiency."

Great logic. What about the fractal idea that you can zig-zag the
perimeter for greater wire length and thus lower the resonant frequency
for a given area? Isn`t that inherently inefficient?

Best regards, Richard Harrison, KB5WZI

--
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........
"Richard Harrison" wrote in message
...
Reg, G4FGQ wrote:
"The shape of magloops is of no electrical consequence except that a
circle has the greatest enclosed area for a given perimeter. This gives
the greatest possible radiation resistance for a given loss resistance
and hence the greatest radiating efficiency."

Great logic. What about the fractal idea that you can zig-zag the
perimeter for greater wire length and thus lower the resonant frequency
for a given area? Isn`t that inherently inefficient?

Best regards, Richard Harrison, KB5WZI

====================================

Richard, after all these years I still refuse to be dragged into that
madhouse. Cheerio. Perhaps see you later.
----
Reg.