JUST A CONCEPT
A 1/4 wave monopole whip with horizontal groundplane has a curve like a bandpass filter. See below a curve for such a monopole for 400 Mhz.
If two such monopoles are used, enclosed in metallic container, and one is connected to antenna and other to receiver, the signal picked by antenna will be transmitted by first monopole, and received by second monopole. There will be double the filtering effect of one monopole.
Can this be a workable filter?
How much is insertion loss?
How much is bandwidth of the pass-band?
A VNA is required to determine all these, and I dont have a VNA
That starts to look like a cavity filter… The resonance within the cavity is pretty important, remember that the return loss etc usually assumes “free space” and two antennas in a metal box is anything but free space.
I was browsing eBay, and came across these two 1090 MHz filters, one from Hungary, other from China.
Both are priced around $19 + shipping.
Hungarian one is simply a TAI SAW chip on a small PCB with SMA connectors.
Chinese one does not show internals, but guessing from its size and specs, it also seems to have a TAI SAW chip inside.
Their price is similar to FA external filter, but I doubt if these can perform as good as Flightaware external filter which has superior LC Ladder circuitry.
The SAW filter has a narrower BW and 1.8-2 dB center loss. The FA filter seems to vary a bit, different measurements show 1.9-2.7 dB loss, and it is too wide, which is a real problem with the FA design.
So. I would say on most accounts the SAW design is superior.
The filter has SMA connectors and can be attached directly to the Pro Stick. See the photo on our web site (on the right side): flightaware.com/adsb/prostick/
Direct attachment is desirable to avoid losses introduced by extra connectors and cable.
Hungarian Filter
(TAI SAW chip)
I’m also intrigued. It’s cheaper for me because I’m outside US and I would like to buy his LNA too. Seller has a solid reputation on ebay
And they do work just fine.
I’m got from 200 messages to 1200 messages (depending on time of the day)
But i’m 30 meters away from several GSM (mobile phone) transmitters.
See for your self, NL3EHV - 62.251.100.171:1090
