When is 1090 not actually 1090 ?

[GregHudson]

I've recently downloaded HDSDR (High Def Software Defined Radio) and connected it to my external 1090 base antenna.
To my surprise, HDSDR is showing the adsb signals are actually being transmitted NOT on 1090 Mhz, but 1089.5Mhz !

This may not sound like much of a difference (just 500 Khz) but for those building their own antennae, that minor difference may make a slight improvement in reception.

Just a thought, but could be worth recalculating the length of directors / reflectors / collinear elements (slightly longer than before).

Regards, Greg.

[Chris-G0WTZ]

They definitely transmit on 1090mHz so I suspect you need to look at radio settings

[G4UMW]

Transponder tx spec is 1090 +/- 3 MHz or 1090+/- 1 MHz, depending on the transponder class and/or optional features. See FAR Part 43 Appendix F.

[viking9]

For reception, as opposed to transmission, the antenna SWR is not that critical.

[Truetrack]

Hi all
deviation of 500 kHz or more the 3 MHz at 1090 is nice to know about
but for practicable purposes ?
e.g.
1/4 wave element length is at
1089.5 is 68.838 mm
1090.5    68.775
even at a tolerance of 3MHz to each side
1087 68.9972
1093 68.6184 mm
in any case that is less than 1/2 mm
Now whether one is constructing the antenna of wire(rod) material or soldering coax sections
with their even more important velocity factor it will be hard to stay w/i this target.

my 2€cents