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Re: G3RUH vs K5GNA

At 01:45 PM 10/21/03 -0700, Emily Clarke wrote:
>I started to work AO-40 about 2 months ago with a K5GNA complete package 
>that I had modified by adding a screen covering the grill.  I was generally 
>pleased with the results although the QSB was a bit annoying.
>Over the weekend I purchased a G3RUH from a friend who upgraded to a 4' 
>dish.  It offers quite a few features I was planning to homebrew into the 
>K5GNA.  These were to modify the BBQ into a round dish, and adding a 
>circular feed of some sort.  Since I live in a townhouse apartment I was 
>still trying to workout the logistics of how to build the mods (space, 
>tools, materials, etc) it was attractive to buy something that was 
>virtually plug and play.  The G3RUH also offered a smaller size and 
>circular patch feed. Here are my impressions:
>G3RUH =60cm round (2827 cm2)
>K5GNA = 62 X 90 cm rectangle (5580 cm2)
>G3RUH=Right Hand Circular patch feed
>K5GNA=Horizontal Dipole
>I had to build an adapter plate to match the 2x2" pattern of the G3RUH to 
>the 3.5" width of the K5GNA brackets.  This was important it needed to fit 
>over a 1" PVC "T" to allow me to easily tilt the dish up and down.  (see 
>http://www.emilyshouse.com/W0EEC/AO-40.php for details)  Since I was going 
>to use the same downconverter (AIDC 3731) I also had to remove the dipole 
>and replace it with an N connector.  I have to hand it to Bob K5GNA - he 
>thought of everything.  It took 3 minutes to remove 4 screws, swap the 
>connection and replace the 4 screws.
>While no means scientific, the noise level of both antennas is comparable. 
>I believe this is because the vast majority of noise is coming from the D/C.
>It was very quickly apparent that the signal strength of the G3RUH dish was 
>greater.  Once I found the middle beacon it was about 6db stronger than 
>before using the K5GNA.  In terms of relative signal strength (with a noise 
>level attenuated to S0) the middle beacon is now S8/S9 over the noise 
>floor, and QSOs are S6+. The K5GNA levels under similar conditions produced 
>signals of S5-6 for the middle beacon and S2-3 for QSOs. The patch feed did 
>not appear to decrease noise from 2.4GHz appliances but I'm they weren't 
>any stronger either.  This lead me to conclude the S/N ratio is better.
>One thing I didn't expect was the gain of the G3RUH to be flat over a wider 
>beam than the K5GNA.  Since I live in an apartment and manually position 
>the dish this is a real benefit.  The K5GNA needed to be repositioned about 
>every 15-20 minutes.  Today there was virtually no change in signal 
>strength for almost 40 minutes.  I've enjoyed rain free weather since May 
>but they are predicting a very wet winter.  So I'm going to really 
>appreciate not having to go outside so often the rains come.
>Lastly, the G3RUH is about 30 percent lighter than the G3RUH. Since I move 
>my dish around a lot, this is important to me.

Interesting comparison!

1)  From the ratio of aperture area:  5580 vs 2827 cm^2,  you would expect
the larger antenna to produce 1.974 more gain (that is 2.95 dB more).
2)  From the fact the square grill antenna is linear and the G3RUH is
circular one would expect the latter to exhibit 3 dB better performance
3)  Combining the effects of points 1 and 2 there should be a no net
difference in performance.

Since you see approx 6 dB better performance, then what can explain this?
I would offer that the G3RUH dish with patch is a better matched feed/dish
arrangement.  It is optimized for low feed "spill-over" and thus lower
"ground-noise" pickup!

The rectangular grill antenna is designed for terrestrial TV reception of
strong wideband signals where one looks at the horizon so there is less
advantage for a design reducing "ground-noise" pickup.  That is my guess!

I would suggest making some sun-noise vs cold-sky comparisons.  This may
really open you eyes!  Sun noise will have a very small rise over cold-sky
with a 60cm dish but I imagine you may see about 1/2 s-unit (or 2 dB if you
can use an attenuator before the IF receiver).

It does sound as the G3RUH performs quite well for its size.  I would like
to hear of side-by-side comparison with a 3-foot dish, though!

This does demonstrate that not all antennas are "equal" even though size
may imply an advantage!!!

73, Ed - AL7EB  

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