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Re: Re: Question "nesting Helix antennas"



You are very correct. I have a homebrew L band helix for INMARSAT reception and the
DEM LNA amp (simple input match only) gets bombed with cellular images all the time.
I think this is what Kraus called the "normal mode" of the helix.

Douglas Braun & Nadia Papakonstantinou wrote:

> Here is one thing to watch out for when using a S-band helix:
>
> If you look at a helix, it bears a curious resemblance to a helically
> loaded ground-plane dipole antenna.  In fact, it will probably
> resonate somewhere in the VHF range.  If you are unlucky, there may be
> some strong signal source at this frequency that might cause an overload problem
> with your S-band preamp.  Most, but not all, preamps will have an input
> network that acts as a high-pass filter and reduces the problem.
> But if you hear voices on your S-band setup, you might suspect
> VHF overload...
>
> One advantage of an antenna like a loop Yagi, or a dish with a horn or
> "coffee can" style feed, is that it will have very little response to
> frequencies below the design frequency.
>
> Doug
> NA1DB
>
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