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R: using a 2mtr dish




----- Original Message -----
From: Keith Bainbridge <vk6xh@arach.net.au>
To: amsat <amsat-bb@AMSAT.Org>
Sent: Monday, September 29, 2003 8:03 AM
Subject: [amsat-bb] using a 2mtr dish


>
> Subject: using a 2mtr dish
>
>
>  G'day from Western Australia .
>  I have a W0LMD dual patch feed 70 cms/ 13 cms on my 2 mtr dish.
>  I dont believe I have the most efficient system possible and was
wondering
>  if I ran a few things past all who read these bulletins and study the
> comments you make, I might get some positive suggestions..
>  The dish is 2mtrs dia, mesh construction, with a f/d ratio of 0.656 the
>  focal point is 1.312 mtrs from the face.
>
Hi Keith VK6XH,

Your F/D of 0.656 is too big for a pach feed.

The radiation pattern of a patch at -10 dB points is about 120 degrees wide
but the angle subtended from the focal point and the edges of a dish
with FD= 0.656 is about 84 degrees only.

This means that a patch overilluminate such dish and all the energy radiated
by the feed between 120 and 84 degrees toward the dish is lost.
For the same reason,because of the overillumination on receiving,the feed
gets more unwanted noise from the warm ground.

  My 70 cms uplink is an FT 736r with mirage amp and an output of approx 85
>  watts. The feed line is belden 9913 and is approx 16 mtrs long.
>  The RX is an aidc 3731aa from K6GNA. noise fig 1.0 db
>  I have a mode L set up on the way, approx 1 month off completion, it will
> be
>  a HTX 100 tx on 28 mhz to a Parabolic transverter with 1 watt out  and a
>  SSB electronics 0.5 to 10 watt amplifier.

The Parabolic transverter uses an IF of 28 MHz and so to transmit at 1270
MHz for example you need a local oscillator of 1270-28= 1242 MHz
This represent a problem because the transverter selectivity at this high
frequency is low  and is not sufficient to prevent that your local
oscillator arrives at the power amplifier and be amplified and radiated
togheter with the 1270 MHz wanted signal.

In addition,using a 28 MHz IF the image frequency will be at
1270-(28x2)=1214 MHz with the same problem as for the local
oscillator.

For this reason,to stop the local oscillator and the image  frequency
you need  a 1270 MHz interdigital filter connected between the Parabolic
transverter and your power amplifier

>  I have just completed an 18 turn helical for the uplink.
> I sat back and looked at all this and I'm not sure that I'm doing things
the
>  best way.
>  Would a dual helical feed for 23 and 13 cms on the dish and a separate 9
>  turn helical on 70 cms be a better prospect?

In your case with a F/D= 0,656 a dual helical feed made each by 3 and half
turns will be better  because the edge of your dish with an angle of 84
degrees from the focal point will be illuminated at  the -12 dB points of
the helix radiation pattern.

A 2 meter dish at 435 MHz has a gain of  only 16 dBi and so it is easier to
get about the same gain with crossed yagis connected for RHCP or with a
helix antenna with about 15 turns

>  Or can anyone  suggest another way .
>  I would appreciate all your comments as I seem to reach mental overload
>  everytime I run the options through my brain !!
> Construction of the antennas is not a problem, I'm a toolmaker by
>  apprenticeship ( many years ago ) so thats the part I find easiest.
>  I look forward to hearing from you and meeting you all on AO-40 if poss,
I
>  reached country # 50 this morning on AO-40 , after just 10 weeks on the
> satellite, so todays a good day !!
>  73 for now, and many thanks
> > Keith Bainbridge  VK6XH
> > vk6xh@arach.net.au
> >

73" de

i8CVS Domenico
>
> ----
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