Fergus Kyle wrote:
>The only way to increase the radiation is to turn the flat disc
> of alu into a cone at up to 45deg from the axis of the 'spike'
> this might tune the antenna more closely to the 50ohm characteristic
> of cable and spike. You should not have to place the spike on the
> exterior to improve it.
> ...
> PS: - maybe the metal nearby 'blanked' your reply - try changing
> direction if you get a complaint.
Ferg, I agree with your analysis technically, but FAA requires only
125 watts to pass the test, which means their equipment must be able
to accept some lower output, may be even 50W. The SL-70 puts out
250W, not "nominal" but minimum. These things have to 100% reliably
work in the enroute IFR environment, where in the U.S., at least, they
have paint you with two radar sites, and one of which can be a
backstop "super site" in each sector, capable of receiving your
interrogation at MEAs of only 'bout 3,000' AGL a couple hundred miles
away, a rather enormous path loss.
Tweaking impedance or radiation pattern I think is unlikely to fix
anything, especially if ATC can receive your interrogations, under
that antenna "law of reciprocity" thing!
Reg,
Fred F.
|