LF: Transmitting Loops
Wed, 14 Jul 1999 08:55:37 -0400
Dave Sergeant wrote:
> >From Dave G3YMC
> The 200Hz bandwidth refers specifically to my transmit loop, although I
> believe many stations do indeed have tuned antennas of this sort of
> bandwidth. The bandwidth is determined primarily by the DC loss resistance
> of the loop wire (around 0.1ohms, I use heavy duty loudspeaker cable to
> minimisde the resistance). Note that for loops earth resistance really
> doesn't come into it. Matching is via an impedance matching network of two
> capacitors at the antenna feedpoint, downstairs outside the house, the
> antenna being fed via coax from the transmitter. It is not possible to
> have the matching network in the shack as then the resistance of the coax
> or other feeder would form part of the loop resistance. To retune the
> antenna needs a trip outside, that is until I sort out remote tuning.
> At present I adjust the resonance with several switched fixed capacitors.
> Since the capacitors are large (22nF tuning capacitance) normal air spaced
> variables can't really be used.
> The narrow bandwidth of the loop is a good sign that the DC resistance is
> low. If the bandwidth is wide then it is unlikely to work well as a
> transmit antenna.
> Loops are not effected much by rain and ground resistance effects, and the
> main effect I see is temperature coefficient change in the values of the
> matching capacitors. This results in a change of several hundred Hertz
> between winter and summer temperatures, the heating of the sun on the
> matching box, and even 100Hz or so due to heating of the capacitors by the
> transmitter current. The resonance however never changes by as much as
> several kHz!
> For more information on loops see my web pages:
> 73s Dave