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Re: BBQ Dish Sanity???



Hi Tony,

Nice to see some well constructed measurements being made.  From you're
description there one part of the set up that is unclear to me. When you
said, "Both antennas are configured with an N-male connector so the receive
system is exactly the same for both antennas. No switch or coax was used in
front of the preamp."  I take it that means the preamp is connected directly
to the horn as the reference antenna and at the end of the "3' pigtail" for
the BBQ dish.

> The dish is spec'd at 24 DB gain and the reference antenna
> was designed for 22 dB gain so the dish should provide 2 dB
> more signal power than the reference antenna.
>
> However, the reference antenna consistently provided about
> 1.5 dB more signal strength than the dish. Hmmmmm????

It looks like you got the results I would expect if you account for a few
additional factors.

1.  The 3' LMR-400 pigtail adds 0.2 db of loss in the critical early part of
the chain.  As a result you are losing a full 1 db of G/t.

2.  The noise picked up by the BBQ antenna is probably higher than the horn.
Scott, NX7U, did an excellent analysis of this which you can read at
http://members.cox.net/nx7u/ao40/bbqdish/BBQGridNec.htm

In Scott's comparison the BBQ is expected to pick up 2db more noise than a
round dish.

If we accept for the moment that your reference horn has about the same
noise pickup as a solid round dish, then we've accounted for almost the
entire 3.5 db difference in the expected performance.  I wouldn't be
surprised if the other half db is because the horn at a high elevation is
probably quieter than a center fed dish.

> So, the only conclusion I can come to is that while the
> dish is usable, it does not perform as specified and does
> not really have 24 dB gain. If it did, it would be a very good
> antenna for AO-40. Hmmmm????

As you said it is usable, but once ALL the factors are added, polarization
loss, pigtail loss, and antenna noise, you're really about 6 dB below the
optimum performance that looking at just the gain figures would have you
expect.  That's the beauty of making Sun and Ground Noise measurements of
your system G/t.  Measured G/t really defines the performance of the whole
system.  In this case overall it is on par with an 18" offset dish with the
preamp directly at the feed.  It works. I made a lot of contacts with my 18"
dish.

As for the alligator argument, if you've noted the MB signal strength on
your receiving system and keep your own uplink 2-3 S-units below MB
everything will be fine.  Of course this applies to anyone, regardless of
whether they've got an 18" dish, BBQ grill, or 12' BUD.

Lee-KU4OS

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