Hi Andy,
1) When you state that the Radiation Resistance of a loop antenna is
proportional to the square of its Area, you are of course quoting the
accepted formula, but you will remember that Prof. Mike Underhill challenged
this formula in his talk at Windsor. Unfortunately I have not yet managed to
find this talk published anywhere.
2) Surely using a coupling loop is not a matter of matching the loop
impedance to 50 ohms, but rather providing input coupling which does not
unduly dampen the Q of the primary loop (which should be tuned with a
capacitor). While this is a popular method it does not provide maximum
signal to the receiver and should not be coupled by a long line.
In the case of the G3LNP loop, the measured Q was 65, and the inductance
(calculated using G4GFQ's RJELOOP3 program) is 6.3 mH, so the dynamic
resistance of the tuned loop is 84 ohms. Using a Z-match to couple to the
receiver would halve the Q and would require a 29 turn secondary, obviously
excessive. The loss of signal due to using a single turn coupling loop is
not adequately made up by the receiver in my experience, however there may
be some bright ideas out there to learn from.
Best 73, John, G4CNN
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