Thanks to all who commented and joined in the discussion.
For some there is a real distinction between mechanically tuning an antenna
versus electrically matching the complex impedance at the station end of the
transmission line to a transceiver or RF amp. For some the distinction, if
it exists at all, is trivial, just a matter of semantics, or an argument
over how many angels can dance on the head of a pin.
Somewhere along the way during the discussion of antenna tuners / match
boxes / couplers / coax flatteners / adjustable impedance matching networks
another parallel discussion got mixed in. That parallel discussion was about
if it were best to insert the antenna tuner into the system at
antenna-to-feedline connection or at the station end of the feedline.
Someone commented that it if low-loss coax were used the difference would be
only a few "silly" tenths of a dB. That is not correct. If you feed an 80M
dipole up 50 feet with 100 feet of low-loss coax, something like TMS LMR400,
and then use a antenna tuner at the station to match it, for example, on 40M
the losses can be significant - more than 6dB, with 1500 watts into the
antenna tuner and somewhere around 300 watts to the antenna (per TLW
software that is included with the ARRL Antenna Book). For that application
450 ohm ladder line or 600 ohm open-wire line would have much lower loss.
My antenna tuner, and I know it is an antenna tuner because it says so on
the face plate, has a variable capacitor and a roller inductor. The
capacitor can be adjusted from 40 to 500 uF and is rated at 3500 volts, and
the inductor can be adjusted from 0.2 to 18 uH. There is a finite range of
impedances it can match. The antenna tuner can match the impedance of some
antenna / transmission line systems, it can re-reflect back to the antenna
power reflected to it by the impedance mismatch at the transmission
line-to-antenna connection (see Reflections by Maxwell), and it has some
loss. But the antenna tuner does not change the feedpoint impedance of the
antenna which in my view would be actually "tuning" the antenna.
Marsh, KA5M
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|