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From: jimmie68 on 15 Jul 2008 00:16
On Jul 13, 11:06 pm, Jeff Liebermann <je...(a)cruzio.com> wrote:
> On Sun, 13 Jul 2008 13:29:18 -0500, "amdx" <a...(a)knology.net> wrote:
> >I have what I call my reference antenna an MFJ-1800 spec says 15db gain (hmm
> >?)
> >best signal is -72dbm to -70dbm.
>
> >My test signal is in a building about 100 yards away.
>
> >I built the biquad with 30cm lips my best signal strength is -74dbm .
> >when I mount the biquad on a 18"x36" offset feed dish (looks like this)
> >http://www.2000networks.com/accessories/dishes_lnbs/dish_directv_inte...
> >the best signal I get is -64dbm, a 10 db gain over biquad alone.
>
> >Is that the gain you would expect?
>
> >I found the original feed position wasn't optimum, I had to raise the biquad
> >about 2"
> >and also back it away from the dish about 2" to peak the signal.
>
> >What is are your thoughts about that?
>
> Ok, let's play with the numbers.  First, with a reflector type
> antenna, the gain of the biquad feed is not important.  It's sole
> purpose is to "illuminate" the reflector.  If the beamwidth of the
> feed is too wide, you lose gain in transmit.  If the beamwidth of the
> feed is too narrow, you lose gain in both receive and xmit because
> you're not using the full area of the reflector.  See:
> <http://www.w1ghz.org/antbook/contents.htm>
> <http://www.w1ghz.org/antbook/chap4.pdf>
>
> When dealing with elliptical reflectors, that are designed to handle
> multiple feeds (for multiple satellites), you don't get the benefits
> of the entire dish.  The offset feed also means the effective cross
> sectional area is somewhat less than the physical area.  I'm too lazy
> to work out the exact number, but my guess(tm) is that the DirecTV
> dish has an effective circular diameter of about 60 cm.  In other
> words, the bulk of the reflector is not used.
>
> The maximum gain of a dish antenna can be calculated.
> Assuming you did everything perfectly, and the feed was properly
> matched to the dish f/D ratio, then:
> Maximum gain for a 60 cm meter diameter dish:
>    gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency)  
>    gain = 9.87 * 600mm^2 / 125mm^2 * 0.4
>    gain = 91
>    dBi = 10 log(91) = 19.5dBi
> The 40% efficiency (that's the 0.4) is probably optimistic for your
> home made feed.  Again, the numbers assume that everything is
> lossless, perfectly matched, built correctly, and properly designed.  
>
> Your 10dB difference is gain is very close to what I would expect
> (i.e. calculate).  The typical biquad has 8-11dBi gain.  See:
> <http://802.11junk.com/jeffl/antennas/Biquad/>
> So if I give the biquad a 9dBi gain and the dish perhaps 18dBi of
> theoretical gain, then an 9dB difference is about what you would see.
> Close enough to your 10dB difference.
>
> Some stuff on dishes and feeds from my bookmark pile:
> <http://www.findmorecollectibles.com/wifi.html>
> <http://www.weijand.nl/wifi/>
> <http://www.qsl.net/ki7cx/wgfeed.htm>
> <http://www.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-an...>
> <http://www.nodomainname.co.uk/parabolic/parabolic.htm>
>
> >I question the the MFJs gain because the modified (110mm and lips) biquad
> >is only 2 to 4 db less than the MFJ.
>
> MFJ means "Might Fine Junk".  They also no longer sell or support the
> MFJ-1800 antenna.  The link:
> <http://www.mfjenterprises.com/products.php?prodid=MFJ-1800>
> is dead.  My guess(tm) is about 13dBi gain at best.
>
> >If I did good, and got 8db gain from
> >the biquad then that looks like 10 to 11db gain for the MFJ. I do understand
> >antenna measurements can be difficult.
>
> Ummmm..... yeah, that's an understatement.  Actually, it's not
> difficult to make the measurement.  The problem is finding a place to
> do the testing that isn't full of reflections and interference.  I've
> found the deep dark forest to be fairly good as the trees absorb all
> the reflections.  The only problem is that I have to elevate the test
> antennas well above the roadway to prevent ground bounces.
>
> >That said, I have used the MFJ-1800
> >for almost a year now and think it is
> >a good buy at $30.00 plus shipping.
>
> I have a bad attitude about yagis.  They have their place but are a
> PITA to build, maintain, install, and design.  I much prefer a panel
> antenna which is much simpler, easier to mount, waterproof, wider
> bandwidth, and only slightly more dollars.  How about 13dBi gain for
> $36?
> <http://www.fab-corp.com/product.php?productid=893&cat=0&page=1>
>
> --
> Jeff Liebermann     je...(a)cruzio.com
> 150 Felker St #D    http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann     AE6KS    831-336-2558- Hide quoted text -
>
> - Show quoted text -

I have some friends who tinkered around a lot trying to get a used sat
TV dish to work. They had simliar results as the OP. I think they
finally opted for a QUagi antenna consisting of quad driven element
reflector and one director and several yagi type directors. They also
experimented with some of the large TVRO antennas and the gain on
these was awsome.


Jimmie
From: Jeff Liebermann on 15 Jul 2008 00:36
On Mon, 14 Jul 2008 21:16:08 -0700 (PDT), jimmie68(a)gmail.com wrote:

>I have some friends who tinkered around a lot trying to get a used sat
>TV dish to work. They had simliar results as the OP.

I've done it successfully. This is an early version:
<http://802.11junk.com/jeffl/antennas/Misc/slides/primestar.html>
The only tricks are to make sure the feed matches the f/D ratio of the
dish, and to *NOT* use dishes made from multiple LNB's.

>I think they
>finally opted for a QUagi antenna consisting of quad driven element
>reflector and one director and several yagi type directors. They also
>experimented with some of the large TVRO antennas and the gain on
>these was awsome.

Reminder.... there is a legal limit to EIRP.

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: amdx on 24 Jul 2008 17:48
..
>
> I have a bad attitude about yagis. They have their place but are a
> PITA to build, maintain, install, and design. I much prefer a panel
> antenna which is much simpler, easier to mount, waterproof, wider
> bandwidth, and only slightly more dollars. How about 13dBi gain for
> $36?
> <http://www.fab-corp.com/product.php?productid=893&cat=0&page=1>
>
I think I found the same antenna a little cheaper, but to late for me.
http://www.sparcotech.com/cgi-bin/commerce.cgi?preadd=action&key=PA24-13
$31.00


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