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two airband antenna

PostPosted: Fri Jun 08, 2018 2:25 pm
by ben2311
I have got a DIAMOND D-777 120-300MHZ and a halfwave Dipole 118 - 137 MHz Bandwidth antenna, can i put them both into one receiver.

Re: two airband antenna

PostPosted: Sat Jun 09, 2018 8:37 am
by barrylewis1234
You cannot connect them both at the same time due to impedance. You could however buy an antenna switch to flick between the two.

Re: two airband antenna

PostPosted: Sat Jun 09, 2018 7:10 pm
by G4RMT
The impedance mismatch is not that critical, so you could just experiment and try them out. Proper combiners will introduce a little loss and maintain impedance - but as most scanners, especially portable never get a proper impedance from their compromise antennas anyway - the mathematical loss is a bit less important. The real snag is simply what happens. As soon as you combine antennas, the results are difficult to predict. As an example, in comms systems it's quite common to combine two identical antennas to alter how they perform, and of course using a properly designed matching system for the transmitters. Two identical dipoles that were individually pretty much omni directional, can be combined to offer 3db (double) the gain, by stacking vertically, but with a definite directional advantage, or they could be stacked on opposite sides of the mast to produce a more two directional pattern. With four of them, space 90 degrees around the mast you'd be close to 6dB gain and roughly omnidirectional.

Once you combine dissimilar antennas, the adding and subtracting of their performance peaks and troughs can give rise to quite bizarre performance, so it's quite rare to combine things unless you have a real requirement to meet. You could stick up an omni for general coverage but add a long yagi to grab that important source, that the omni can't get. The trouble is in practice, you just need to have the time and finance to experiment.

Me? Like Barry - get a switch!