Why pattern testing caused TV interference?

[waveboy]

Dear Engineers,

Whilst doing antenna patterns outdoors, we interfered with a nearby television set's reception, quite unexpectedly, -does anybody know why? (don't worry, we are remotely located and it was just the boss's TV and he laughed about it)

We were testing at 100MHz to 1GHz. -The interesting thing was that the transmit antenna was a horn and was pointing directly AWAY from the TV's aerial!!...the antenna_under_test was a cavity backed planar spiral antenna. -For their to have been such interference, the antenna_under_test must(?) have been re-radiating the RF it received -and thus interfering with the television's reception.

So my question is.. "is such re-radiation possible"?
-This brings a related question.....antennas are "reciprocal" devices, -as good in receive as in transmit. So when an RF wave hits an antenna and instigates RF currents in it...then what's to stop this antenna simply re-radiating this RF back out again?.....so what is it that determines whether the RF in the receive antenna goes to the receiver circuitry, or just gets radiated back out into space?

I suppose this is infringing on radar theory...which relies on re-radiation from the target.

 

[VEBill]

Remote location = weak TV signal?

You can do the math to compare the approximate relative strengths from the illuminated target antenna versus sidelobes directly from the xmit antenna. I'll put a dollar on sidelobes.

Another reason for chambers with doors. You might want to think about passing aircraft even in your remote location.

 

[Higgler]

100 Mhz horns have a big backlobe, It probably had high gain towards his TV. Plus RF bounces (re-radiates) off structures. If you sweep fast it may not bother the boss's TV.
That's a good job your boss has.

kch