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R: 18" RCA Dish Question




----- Original Message -----
From: Joel Black <jbblack1@earthlink.net>
To: AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Thursday, July 11, 2002 1:11 PM
Subject: [amsat-bb] 18" RCA Dish Question


> I have just installed an 18" RCA dish with helix feed for my 2.4GHz
> downlink.  I'm very impressed compared to the Conifer BBQ dish I was
using!
> This morning, I detected (not decoded) the MB at a squint of 55° and a
range
> of ~27,000km.  That's something I could have never done with the Conifer.
>
> I have used hdl_ant to analyze this offset dish focal point (which I have
> yet to optimize) and gain.  At 50% efficiency, the gain is ~18.3dBi and at
> 60% efficiency, the gain is ~19.1dBi.  I'm using a helix feed.  Now for
the
> question, does the helix feed add gain to the antenna system (I'm assuming
> it does) and what is that amount of gain (or how do I calculate the gain)?
>

Hi Joel  W4JBB,

The helix feed do not add directly gain to the antenna system but the gain
of the feed affect the performance of the dish and obviously its gain.

You have used hdl_ant to analyze this offset dish focal point and gain and
found that the gain is about 18.3 dBi at 50% efficiency

Now supposing that the gain of your 5.25 turn helix feed alone is 6 dBi  you
cannot add 18.3 + 6 and say that the gain of your antenna system is 24.3 dBi
because the gain of your system is 18.3 dBi provided that your 5.25 turn
helix properly illuminate your dish in such a way its efficiency is 50%

If  for example your helix feed has less than optimum turns numbar it
illuminate the edge of your dish with RF energy wich is grater than -10 dB
at the edge and so you overilluminate the dish loosing energy on the space
all around the surface of your dish loosing efficiency and system gain.

This condition of overillumination decreases the efficiency of your dish
that will be less than the computed 50% and hence the overall gain of the
antenna system will be less than the computed 18.3 dBi

This is the way by wich the helix feed affect the efficiency,directivity and
gain of a dish.

For more detailed information on this matter and fundamentals of microwave
antennas i recommend to read the following articles by Paul Wade,N1BWT

Practical Microwave Antennas Part-1 QEX September 1994

Practical Microwave Antennas Part-2 QEX October 1994

Practical Microwave Antennas Part-3 QEX November 1994

More on Parabolic Dish Antennas QEX December 1995

High-Performance Antennas for 5760 MHz QEX January 1995

Paul Wade,N1BWT is the author of the program hdl_ant that
you use.

I hope this help

Best 73" de i8CVS Domenico

> I'm using a helix design I found on Jerry's, K5OE, website (his website
> appears to be down at the moment) that doesn't use the matching section
per
> se, rather, it leaves the insulation on some 10AWG for the first 1/4 turn.
> The helix is 5.25 turns.
>
> If calculating the gain of the helix has already been discussed here, I
> apologize for the redundancy.
>
> Tnx es 73,
>
> Joel Black, W4JBB
> w4jbb@amsat.org
> jbblack1@earthlink.net
>
> "Uvajed - The overwhelming
> feeling that I've never been
> here before."
> -- Unknown
>
>
>
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