[Antennas] Do you really need that duplexer?

2002年1月24日 13:07:52 -0500

In my experience, duplexers are used to isolate transmitters from each 
other, not antennas (I know they could be used this to isolate antennas, 
but I have never done so, and agree that not all antennas actually need 
isolation).
The duplexer will isolate the "A" transmit port from the "B" receive port 
and keep from frying B's input rx circuitry when you tx on A, and vice versa.
If we are talking about 2 M + 70 CM radios, ones that use different coax 
outputs for the different bands (like my FT-847) would die if a single 
antenna feedline with paralled antennas were connected to both radio 
outputs without a duplexer. Ones that use the same coax output (like my 
706MK2g) will live.
It would be very risky to "parallel" those radio outputs without 
considering the radio. Hope this saves someone some heartache.
Jerry W4UK
At 07:24 PM 1/24/2002 +1100, Kevin W Forbes VK3UKF wrote:
>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
>