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.

Hello Andrew.

About your 75ft (22.86m) delta loop (vertical above ground).

For 80m, the loop is relatively small, so you can omit transmission
line effects (to get an approximate value). The impedance consists of
the wire resistance, radiation resistance and the inductance.

I expect the inductance to be about 28uH and the radiation resistance
about 0.8 Ohm (series equivalent circuit). So your loop behaves at 80m
as an inductance with impedance of about j660 Ohms. When you connect
this to a 75 ohm line (via a 1:1balun). Most of the induced voltage
falls across the inductance.

Using a 3:1 balun will give a significant improvement (in order of
6dB). Now the load for the loop is about 700 Ohms. In fact, your balun
is good for 80m.

You may reduce the turns, since a reduction of the primary inductance
to about 100uH is no problem. Reducing the turns may reduce capacitance
between the windings improving balance. If the noise that you are
receiving is above that of your receiver, everything is OK. You do not
need an antenna tuner. Don't worry about mismatch. The only need for a
tunable filter or antenna tuner is to reject strong signals that may
overload your receiver.

For 40m the situation is a somewhat different. Now the wire length is
about half a wavelength and the current will not be uniform anymore, so
general inductance and radiation resistance formulas do not apply. It
is possible that the capacitance between the wire ends (also via
ground) may reduce the self resonant frequency of the loop. Your
antenna may be capacitive.

I think the best way is to find out whether the received noise exceeds
the receiver's noise floor. If at 40m still the received noise is
dominant, your antenna is OK. Don't be afraid of mismatch. If
performance is worse at 40, you may increase the turns ratio. Make
sure that your balun is constructed well to avoid that interference
coming from domestic equipment is guided to your loop via common mode
transmission along your 75 Ohms cable.

If the signals and noise are far above the receiver's noise floor
(check with attenuator), you may decrease the size of the loop. This
will give better performance on the higher frequencies. Since
inductance reduces more then the received output voltage.


With respect to transmitting, you can convert such a loop into a narrow
band transmitting loop. In that case you need a tuner to correct the
mismatch. The mismatch is that worse, that if you would correct the
mismatch just behind the transmitter, most power will be dissipated
inside the balun and or the 75 Ohms cable. In that case tuning the
loop in combination with a inductive coupling will give good results
(even for this loop). The disadvantage of this setup is that your
tuning capacitor must be located at the loop.

Andrew, I hope this information will help you somewhat.

Best Regards and Good SW listening.


Wim Telkamp
PA3DJS

On Sat, 26 Aug 2006 22:25:40 +0100, Andrew Holme wrote:

My questions is:
what is the impedance likely to be on 40 and 80-metres?

Different.