On Sat, 26 Aug 2006 22:35:23 +0100, "Andrew Holme"
wrote:


"Andrew Holme" wrote in message news:...
I've erected a delta loop in my back garden for general shortwave
listening, with a total wire length of 75 feet, fed with 75-ohm coax via a
balun. The balun is a 3:1 turns ratio transformer wound on a small red
toroid from my junk box. The winding on the coax side is about 20uH, and
the winding on the loop side is about 200uH. I've no idea how close to,
or far from optimal this is. I mostly listen to hams on 40 and 80-metres.
I've no idea what the impedance of my loop is on these bands. It would be
useless for transmitting, because it's less than one wavelength, but it
seems to work OK on receive. The noise level is certainly nice and low.
My questions is: what is the impedance likely to be on 40 and 80-metres?

TIA
Andrew.


More info: the feedpoint is at the centre of the base of the delta, the apex
is supported from a chimney, and the other two corners are about 8 feet
above ground, guyed off my back fence. The plane of the delta slopes.


Hi Andrew,

Your antenna's Z in the 40M band is astronomical:
Impedance = 12740 + J 8376 ohms
in the 80M band
Impedance = 98.29 + J 1069 ohms

Skip the lore about baluns (which you couldn't possibly hope to
find/build/buy), what you need is a tuner and second smaller loop
(think half the size) in parallel with the big one (to pull down that
astronomical mismatch).

73's
Richard Clark, KB7QHC