From Dave G3YMC
Toni HB9ASB writes:
<Regarding the B.W. of the antennas: It's interesting that some stations
<have antennas with only 200Hz B.W. How do you get this high Q-value? My
<antenna covers the whole band and I'm adjusting the SWR with a
<"LF-matchbox" in the shack. This is helpful, when soil condx (dry, wet,
<frozen) and nearby trees (summer, winter) are changing the resonance by
<several kHz!
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:
http://ourworld.compuserve.com/homepages/sergeantd/loops.htm
73s Dave
[email protected]
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