[amsat-bb] Re: AO-51 L band requirements

i8cvs domenico.i8cvs at tin.it
Tue Apr 27 13:24:50 PDT 2010

----- Original Message -----
From: "Bob- W7LRD" <w7lrd at comcast.net>
To: <amsat-bb at amsat.org>
Sent: Tuesday, April 27, 2010 8:40 PM
Subject: [amsat-bb] AO-51 L band requirements
> Reading the AO-51 book by Gould where he discuses the L band requirements.
> For occasional contact 150 W EIRP is needed. For superior performance 1 Kw
> EIRP needed. Assuming I start out with 10W to a 16 turn helix 40 feet
> away.Guesstimating- 5-8 W at the antenna, helix gain 8-12db. Guestimated
> EIRP about 40W (worst case). Am I figuring this correct? If so then AO-51
> is performing better than what the book says. I am trying to "fine tune"
> what I have.
> Thanks 73 Bob W7LRD
> Seattle

Hi Bob ,W7LRD

The gain of a 16 turn helix is much more than 8 - 10 dBi .Supposing that the
circunference of a turn = Lambda or one wavelenght = 23 cm and the pich
angle between turns is Alpha = 12.5 ° then the half-power beamwidth can be
calculated using  the following formula from Kraus book "ANTENNAS"
Half-power beamwidth  = 52° / SQR ( n x tan alpha ) where n= numbar of
turns and so for 16 turns helix we have:

Half power beamwidth  = 52° / SQR ( 16 x tan 12.5° ) = 27.6°

An antenna having a lobe large as 27.6° at the -3dB points in 1268 MHz is
corresponding to a parabolic dish with 70 cm in diameter and has a gain of
16 dBi corresponding to 40 time in power ratio.

If the power reaching your 16 turns helix antenna is only 5 watt then your
EIRP=5 x 40=200 watt and if the power is 8 watt your EIRP= 8x40=320 watt

It seems that the book by Gould is correct.

73" de

i8CVS Domenico

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