[Antennas] Do you really need that duplexer?

2002年1月24日 19:24:43 +1100

If you want to operate 2 antennas that are resonant on different
frequencies, I believe you may not always need a duplexer. Consider the
following.
Antennas can be likened to a resistance. They are mostly 50 Ohms. Now think
of parallel resistors. Two 50 ohm resistor in parallel = 25 Ohms. No good
for you as you need it to be 50 Ohms. But, an antenna resonant at 2m is
going to be NOT resonant (hopefully) at say 6m, and vica versa. If you are
using the 2 m antenna to transmit at 2 m the impedance should be very close
to 50 Ohms. A 6m antenna may be found to be 1000 ohms at 2m. Back to
parallel resistors. 50 Ohms in parallel with 1000 Ohms is 49 point something
ohms. In fact the more the other antenna ( the one you are not using at that
moment ) is not resonant at the current frequency, the better this will
work.
If your antennas are getting close to each other in impedance, this theory
will not work.
I have been using this method of attatching VHF and UHF antennas with no
problems. As long as the impedances are different enough at your operating
frequency to allow the parallel resistor theory to become effective and
leave the resulting impedance so close to what you want. it makes buting a
duplexer a waste of time.
I would like some feedback on this simply wonderful idea, first put to me by
Noel, VK3JOO.
If someone can shoot me/us down in flames, Please do so, as I really hate
being incorrect or wrong.
Hope it saves someone a headache.
Kevin. VK3UKF