[amsat-bb] Re: Avoiding statics on helix antennas

[i8cvs]

----- Original Message -----
From: "Lars Petersen" <lars.petersen at pobox.com>
To: <AMSAT-BB at amsat.org>
Sent: Thursday, August 06, 2009 12:56 PM
Subject: [amsat-bb] Re: Avoiding statics on helix antennas

> Hello list,
>
> What measure would you recommend to avoid statics building up on helix
> antennas?
>
> Several suggestions have been given, both here and as PM. Thank you to you
> all! Here is a summary of the mails received:
>
> Answer 1: Try a grounded radome to keep rain and air movements off the
> antenna. (G8IFF/W8IFF)
>
Hi Lars, OZ1BXM

A grounded radom is still transparent for the static electricity and
obviously for RF as well so that a radome is unusefull to avoid statics.

> Answer 2:  The shorted quarter-wave coax line suggested in the ARRL
> Handbook 2008 and 2009 pages 23.20-21 will work great because it
> is a short circuit  for the static electricity and a theoretically
> infinite impedance at the resonant frequency so that for the RF input
> signal it is completely ignored by the input stage of a converter or
> preamplifier.
> If you enlarge a shorted quarter-wave by any even or odd number of  1/2
> wavelenght of coax line thing do not changes because a 1/2 wavelenght
> of transmission line represents a full turn of the Smith Chart. Do it as
> suggested by the Handbook! (I8CVS)
>

This one is a good method because the input stage do not see the very high
impedance of the shorted quarter-wave coax line so that the original Noise
Figure of the downconverter is not degraded while the static voltages are
shorted out.

> Answer 3: Can I suggest: a) A 47k resistor across the input of your
> converter. The static will have a very high impedance and I've found 47k
> will remove it without increasing the noise figure of your converter. b)
> For really severe cases: An inductor to ground. An 0805 or smaller device.
> Something that presents a couple of Kohms at the frequency of interest
> would be OK. Or perhaps a high pass filter designed for 50 ohms with
> an inductor to ground. Cut off frequency at 30% lower than 23 or
> 13cms (G0MRF).
>
A 47 k resistor or an inductor to ground of your converter implies that you
have to open the converter box and modify it.In addition a resistor shoves
inductance as well and an inductor do shoves capacitance as well so that
at 23 cm or 13 cm they will modify the input mached impedance for the
lovest Noise Figure of your downconverter.
More over a high pass filter designed for 50 ohm with an inductor to ground
has an insertion loss that in dB adds to the actual Noise Figure of your
downconverter.
>
> Answer 4: An earlier version of the ARRL handbook has a multi turn helical
> for 23cms with the usual quarter wave strip of brass soldered to the first
> quarter  turn of the helix. For static protection it has about 2cm of wire
> wound around a former to make about half of a turn. This is soldered to
> the pin of  the N connector and grounded. I can send you a picture later
> today. Done.(G7HIA)
>
For static protection about 2 cm of wire wound around a former to make
about half a turn and then soldered from the pin of the N connector and
ground do short out the stastic voltages but it modifies the impedance of
the helix antenna so that you have to compensate for it twiking with the
usual quarter wave strip of brass soldered to the first quarter turn of the
helix until you get again a VSWR very close to 1

> 73 de OZ1BXM Lars Petersen, Holstebro, Denmark
>
Best 73" de

i8CVS Domenico

>   ----- Original Message -----
> From: "Lars Petersen" <lars.petersen at pobox.com>
> To: <AMSAT-BB at amsat.org>
> Sent: Tuesday, August 04, 2009 6:01 PM
> Subject: [amsat-bb] Avoiding statics on helix antennas
>
>
> > Hello AMSAT list,
> >
> > I have a problem caused by static electricity building up on my two
helix
> > antennas (23 cm and 13 cm). Two converters have already been zapped, so
I
> > have to do something about it.
> >
> > What measure would you recommend to avaoid statics building up on helix
> > antennas?
> >
> > Does anyone have experience using the shorted quarter-wave-stub
mentioned
> > in
> > "The ARRL Handbook 2008" pages 23.20-21?
> >
> > Vy 73, Lars Petersen, OZ1BXM,
> > AMSAT-Member 36222,
> > oz1bxm at pobox.com
> > www.oz1bxm.dk
> >
> > _______________________________________________
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> _______________________________________________
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