Re: R: [amsat-bb] 18" RCA Dish Question

["Frederick M. Spinner" <fspinner@xxxxxxxxxxx>]

Yeah, I8CVS is absolutely right in the description.

The only thing I have to add is that 5.25 turns is just about right for the 
real f/D ratio of the RCA dish. (If I recall right about 0.7)  So you might 
get a little more signal out if you optimize, but you are most likely in the 
ballpark.

Fred W0FMS


>From: "i8cvs" <domenico.i8cvs@tin.it>
>To: "Joel Black" <jbblack1@earthlink.net>, "AMSAT-BB" <amsat-bb@amsat.org>
>Subject: R: [amsat-bb] 18" RCA Dish Question
>Date: Thu, 11 Jul 2002 15:02:55 +0200
>
>
>----- 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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>
>
>
>
>
>
>
>
>
>
>
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