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Re: To helix or Not to helix



I think a really cool idea would be a single PC board
that has etched on it all of:

The helix reflector.
The helix matching network.
The entire preamp circuit.

Then you could be guaranteed that the antenna and amplifier
are all well-matched to each other.

Doug
NA1DB

At 05:28 PM 5/30/01 , you wrote:
>Sorry to hear your disenchantment Pieter-
>Let's see...King and Wong got 16.8dBic out of a 22T helix (with "cup")
>The DEMI/Directive Systems 21ele looper is rated at 17.5dBi
>With the CP mismatch the yagi "should" be 2dB+ less effective.
>
>Why the difference?
>         0. Very rarely is 1 S-unit=6dB...to get a quanitative measure you'll need
>a calibrated receiver w/o AGC
>         1. Did you use a cup on the helix?  Your pix I've seen before don't have a
>cup.  That's allegedly worth 2dB right there.
>         2. Impedance mismatch?  I feel compelled to stress again that narrowband
>LNAs are not very tolerant of input mismatch.  You can easily burn several
>tenths of a dB NF in the 2.0:1 
>VSWR circle, depending on the exact device/design used.  And phase matters
>too...two antennas that are both 1.5:1 are not *necessarily* guaranteed to
>behave identically with a given LNA.
>
>For the UHF/microwave antenna homebrewer, it would be *really* nice if
>LNA/downconverter manufacturers could supply a Smith chart plot of NF and
>available gain vs. input impedance.  Of course that requires the homebrewer
>to have vector impedance data which I imagine is very rare.

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