[Moon-Net] 2.4M PRIME FOCUS / 2.4M OFFSET/ ALSO SEPTUM FOR 1296
peter.hansen at dk.saabgroup.com
Tue Nov 22 08:36:40 CET 2016
Hi All I am an extensive user of a Offset dishes.
I started out with a 1.8meter offset dish (6foot) with 35W in the feed. I worked easily 35initials on CW and it would have been going on. But 1½ year ago I changed to a 2.4meter Offset dish and this really opened the world. I use the dish on 3 and 6cm currently but will do 24Ghz also. My dish is channel master/Andrew dish. The mesh is small enough for use on 24Ghz.
I can confirm most of the things KL7UW states. The dish is easy to feed linear on 3cm as there are waveguide feeds used originally that works great. On 6cm where I am circular I designed my own feed type as I find W2IMU feed little off peak being used for my dish. I had 0.2dB too little Moonnoise on 6cm. I know I could have tried with larger W2IMU feeds but I went another way.
Efficiency is much better on the Offset dish due to low blockage and higher efficiency in the Feed to illuminate a higher F/d.
What can be worked on a 2.4meter ??
I worked a 90cm that was using 12W in JT4F EME
I worked a 90cm that was using 25W in CW
I worked a 1meter and 10W in CW
Many qso is possible just try it out.
All forgot one VERY NICE feature on Offset dish. When I work on my Feeds, Change system etc I simply sit on the ground and have the feed just in front of me. Or I can tilt the dish and work on it standing up.
73 Peter OZ1LPR
From: moon-net-bounces at mailman.pe1itr.com [mailto:moon-net-bounces at mailman.pe1itr.com] On Behalf Of Edward R Cole
Sent: 22. november 2016 00:55
To: serge; moon-net at mailman.pe1itr.com
Subject: Re: [Moon-Net] 2.4M PRIME FOCUS / 2.4M OFFSET/ ALSO SEPTUM FOR 1296
Just my opinion but if you can acquire an offset feed dish I suggest its slightly better:
1. Feed does not block part of the dish surface (no apparent shadow)
2. It requires a narrower beamwidth feed that may improve rejection of ground noise pickup (lower sidelobes; better G/T)
3. You can mount all the electronics at the feed eliminating feedline losses (max bang for the buck)
4. Often these dish surface are rated to 10-12 GHz so multi-band use is greater.
The feed support would need to be robust to handle the higher weight at the feed.
Dish may be fiberglass vs mesh so heavier weight.
Cost may be higher
Az-El requirements are the same.
Offset dish needs a feed designed for a F/d of 0.6-0.7 (W2IMU would work). Ordinary septum would not be good.
I believe VE4MA is using one on 24/47 GHz so ask him about feedhorns.
I have a idle 1.8m offset feed dish I am considering for 10-GHz use. It used to be our satellite-TV dish until recently switching to Direct-TV (which included free dish and HD receiver). Recently occurred to me that 6-foot (1.8m) makes it a candidate for portable use with our RV trailer.
73, Ed - KL7UW
At 12:00 PM 11/21/2016, serge wrote:
Which is best a 2.4M prime focus dish or a 2.4M offset dish? & WHY? What kind of septum would you use on both dishes for 1296???
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73, Ed - KL7UW
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