[Namaste-dev] Re: Dishes - roll call for experimentation

Tom Clark, K3IO tom.k3io at gmail.com
Thu Jun 19 22:36:09 PDT 2008


Michelle wrote:
> Thanks Grayg, 
> First step is to find a dish to experiment with. Either one, if you want to try a dual-feed experiment, or two, like Stan mentions in his email, so you can separate the transmit and receive parts. A dual feed is sort of like a sports sedan. It's not a sports car, nor is it a sedan, but it still might get you to work. 
> Next step, is to identify some feeds to experiment with, and that is where we are at the moment. I'd like to investigate whatever Kerry N6IZW can come up with. He's working on this question. Also, I have some feed designs from Kent Britain WA5VJB, who was kind enough to donate one at Dayton. You can see these sorts of feeds at http://www.wa5vjb.com/  The feed that he suggested looks like the bottom picture with the two symmetrical curves. I think they're called Vivaldis, and there are some gain results here:  http://www.csvhfs.org/ant/CSANT07.HTML
>   
One caution on Kent's PCB antennas -- he has designs that work over a 
fairly wide frequency ranges, but since they are implemented on 
fiberglass dielectric stock, they tend to be more lossy than you might 
desire. I have seen one of his dual-band feeds (see 
http://www.wa5vjb.com/images/logperio400.jpg) burned on TX when it was 
used as a dual 23/13 cm feed. And remember that every dB of loss in the 
antenna/feed is a dB added onto an LNA's noise figure (since it not only 
increases the noise but also decreases the signal, the S/N ratio takes 
an even bigger hit!).

Personally, I lean towards the idea of using two separate, side-by-side 
dishes (like figure 3 of 
http://www.haystack.mit.edu/edu/undergrad/VSRT/VSRT_Memos/009.pdf) with 
each dish optimized for a single frequency of interest. I also note that 
the 60 cm dish that's being described is only 6.8 wavelengths in 
diameter at 3.4 GHz. In general, a dish doesn't perform very well if 
it's smaller than ~10 wavelengths. My personal recommendation is the 90 
cm dishes that don't cost much more than the 60's, and will perform 
significantly better (especially at the 8.8cm "downlink"). Such a dish 
can be found at 
http://www.sadoun.com/Sat/Order/Dishes/Free-to-air-satellite-dish.htm 
for $75 each. Sadoun has a package with the same dish, plus a 60°K 
Ku-band LNB, plus a DiSEQc rotor (see my previous post) for $145 
(http://www.sadoun.com/Sat/Order/Dishes/Motorized-combo-satellite-dish.htm).

For information on the design of feeds and other microwave widgets, see 
Paul Wade's (http://w1ghz.org/) web site, and be certain to fetch his 
on-line book at http://www.w1ghz.org/antbook/preface.htm paying special 
attention to Chapters 5 & 6. For dual-band feeds, look at chapter 6.9. I 
also commend to you the ARRL UHF/Microwave Projects book, especially 
Volume 2, section 1. The ARRL book is no longer available in paper, but 
they have the CD available at 
http://www.arrl.org/catalog/?category=&words=8853 
<http://www.arrl.org/catalog/?category=&words=8853>.

73, Tom


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