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

[K5OE@xxxxxxx]

Hi Joel.
For the size, these small DSS dishes work extremely well.  I think this is due to two factors:  no feed blockage due to the offset design and good surface regularity.  Both of these factors affect the dish efficiency in a positive way.  

The helix feed does not add gain to the dish, as gain is determined solely by the surface area of the dish and the "efficiency" of the dish (see above).  Correct matching of the feed's radiation pattern to the shape of the dish is important to getting good "illumination efficiency."  If you can, access the "MMDS" article on my web page for the math:  http://members.aol.com/k5oe

Also of importance in using a helix is aigning the "phase center" of the helix, near the end of the first turn, with the "focus" of the antenna.  Since you have the W1GHZ software, it will also give you the focus for your antenna.

If you also select the "Patch Feeds" menu item on my page you will see an easy-to-build example of the K3TZ design (with my addition of a ring collar for the offset dishes) for your dish.  My testing has shown this design to be superior to any helix I have tried on the DSS dish.  Ron, W6ZQ, reports exactly the opposite, so your mileage may vary :-)

The problems at the AOL site seem to be server-location related (where you access it from) and not the site itself.  I get reports from people they cannot access the site from time to time and can't seem to figure out a pattern.  Sorry.
73,
Jerry, K5OE

> 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)?
> 
> 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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