I flew with a unit recently and placed it on the coaming next a F***M box. I have two antennas at the moment, one for the ADS-B and another for 868MHz. By comparison the unit and antennas were both large, cumbersome and awkward. There wasn't really enough space to have the 868MHz antenna vertical as it should be.
1) Do I need both antennas? The ADS-B dongle on the hardware manual doesn't have one attached, so I wondered if I could dispense with that one.
2) There was another thread about using a smaller stubby one. Does anyone have any experience/results from those and how much do they affect range/performance.
3) How important is it to have the antenna vertically oriented?
Thanks in advance
hi,
1) Yes you need to be able to receive with both systems. If you don't have an antenna for ADS-B, then you won't be able to pick up these transmissions.
2) The stubby antennas have slightly reduced performance over the longer ones (on paper!). In practise however, you may find that a stubby works fine, especially as the ADS-B antenna.. ModeS transponders have a much higher transmit power output that the 868MHz ARF module, so you can pick ADS-B up from literally 10's or 100's of miles away - which is a bit unnecessary for what we are trying to do here.
unfortunately, since a lot of these come from the Far East where the testing is a bit shoddy; one stubby antenna is not as good as another stubby antenna, so it can be a bit hit and miss. Frankly, some of them are not even tuned to the right band. I've had some claiming to be 868MHz antennas, only to measure them on a network anaylser and find actually they'd work better for WiFi at 2.45GHz!
What you could try is a stubby antenna for both. I had an idea of making up a wire dipole with suction pads that suck onto the windscreen and plug into the pilotaware (for the 868MHz). It would be a half wave dipole (a little bit like the FM antennas you can get with some HiFi units that people 'blu tack' to the wall - but use suction cups instead), it's flexible so you could form it to the shape of your screen. There's no reason why this shouldn't work.
3) Re vertical polarisation: Antennas emit most efficiently perpendicular to the antenna element. So you can imagine that with a vertical antenna you will be most efficient around you in the horizontal sense (i.e. you'll be looking out to the horizon). Think of it as a 'doughnut shape' around the antenna (although this is rather theoretical).
For a diagram, see
http://www.cisco.com/c/dam/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.doc/_jcr_content/renditions/0900aecd806a1a3e_null_null_null_08_07_07-04.jpgImagine, then, mounting your antenna horizontally. Your maximum efficiency will be pointing down to the ground or straight up into the air, so you will not be looking out to the horizon, which is what you really need to be doing. in fact, you will have what is refereed to as a 'null' in the horizontal direction (i.e. out to the horizon) pointing exactly out of the 'end' of the antenna - this is where the antenna really does not work well (think of it like a bar magnet and how the fields flow from N to S pole)
Of course, there are practical constraints and compromises as you've noted..in addition to this there is the screening effect of any metal near the antenna, especially a metal fuselage if you have one. The idea really is to mount as vertically as possible.
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