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Converting C-Band Dish to 900 Mhz

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MacHeadDover

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Does any one have suggestions about attempting to convert an old C-Band Dish
to a 900 MHz antenna?

Seems like there must be some sort of feed design that's been tried.
Much googling has yet to produce any hints.

Seems like a 10dbi panel located at the focal point might work,
but I'm guessing there's a better answer than that.

Any thoughts would be welcomed.

Dave

Dave AT RTWB dot NET
 

zz0468

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175 DME, HEC 358° Radial
The first question to come to mind would be... why? A feed COULD be made, but depending on the reflector could be difficult to get it optimized. A simple dipole and splash plate would be easy enough to fabricate, but if a directional antenna is what's needed, there are better choices than a converted c band dish.

Your 10 db panel would probably be a very poor choice.
 

ind224

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Oct 30, 2004
Messages
406
Location
Indianapolis
what they said

The elements for a 900 would be so small it only makes sense to build a yagi or quad with as many elements as you think you need.

That way, the dish can be given to me for its real purpose, HD feeds.:wink:
 

prcguy

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A 10dB gain antenna may under illuminate the dish and a dipole with reflector may be a better choice. It depends on the dish diameter and focal length. You want the feed antenna to be about 10dB down at the edge of the dish compared to the center, so if you have to know the approximate beamwidth of the feed antenna and how many degrees it takes to span the dish from the focal point. If you Google "parabolic dish illumination" or anything similar, it should point you in the right direction.
prcguy
 

MacHeadDover

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Why Big Dish?

As a follow up.

My issue is this:

I'm attempting to serve a wireless internet customer thru an awful lot of old growth
Oak Woods. At present I'm using a 17 dbi yagi. The average link quality is pretty marginal.
(-82 to -85 db) with a noise floor of -95 to -100.

While the link holds with perfect weather conditions, wind or rain (or just wet leaves)
kicks the link into the "flap mode" (up and down, up and down).

I cannot convince the client to clear any of the trees, and in one other case
the tree cutting option is not possible since the client doesn't own the wooded area.

I'm hoping that managing a Higher Focused gain might help me Punch thru the
trees and provide me with a link margin sufficient to hold when the wind blows.

Since there are many abandoned C-Band Dishes in my area, the cost to acquire
these would probably be no more than the muscle to move them.

IF this is a wokable solution, it might permit me to serve some cleints
that otherwise simply are not workable.

Any assistance will be cheerfully accepted

Dave

Dave AT RTWB dot NET
 

prcguy

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It should be fairly easy to put a 900MHz feed on a C-band dish and the gain of a 9ft dish at 900MHz will be around 25 to 27dBi or 8 to 10dB more than the yagi your using. That would put the signal strength in the mid -70s under the same conditions. Are you using some of the spread spectrum 900 stuff or is it 802.11B/G, etc? At 2.4GHz a 9ft dish will have a lot more gain, somewhere around 34 or 35dBi and at 5.8GHz maybe 42dBi. I doubt if you can legally use an antenna like were describing and if I were an Inernet service provider I would want to do everything by the book to keep my compitition from turning me in to the feds.
prcguy
MacHeadDover said:
As a follow up.

My issue is this:

I'm attempting to serve a wireless internet customer thru an awful lot of old growth
Oak Woods. At present I'm using a 17 dbi yagi. The average link quality is pretty marginal.
(-82 to -85 db) with a noise floor of -95 to -100.

While the link holds with perfect weather conditions, wind or rain (or just wet leaves)
kicks the link into the "flap mode" (up and down, up and down).

I cannot convince the client to clear any of the trees, and in one other case
the tree cutting option is not possible since the client doesn't own the wooded area.

I'm hoping that managing a Higher Focused gain might help me Punch thru the
trees and provide me with a link margin sufficient to hold when the wind blows.

Since there are many abandoned C-Band Dishes in my area, the cost to acquire
these would probably be no more than the muscle to move them.

IF this is a wokable solution, it might permit me to serve some cleints
that otherwise simply are not workable.

Any assistance will be cheerfully accepted

Dave

Dave AT RTWB dot NET
 

zz0468

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Joined
Feb 6, 2007
Messages
5,958
Location
175 DME, HEC 358° Radial
It's certainly helpful to know what the intended application is. In this particular case, it might be easier to distrubute the required antenna gain among both ends of the path. Shooting through trees is a dicey proposition, and depending on the depth of the fades that occur when it's raining or windy, you could find that no amount of practical antenna gain is sufficient to hold the link.

A good on-line resource for the information you're looking for is the W1GHz on line microwave antenna book... http://www.qsl.net/n1bwt/contents.htm.

One thing to keep in mind is that there are ERP limitations, the effect of which is, the higher the antenna gain, the lower the allowable transmitter power. That can limit the legality of such a large dish.
 

MacHeadDover

Member
Joined
May 18, 2008
Messages
3
Wireless Internet Info

Guys - I had thought this thread was dead.

More follow up info.

My use is 900 Mhz Internet using Motorola Canopy Gear.
These systems us an 8Mhz Wide Carrier at one of the following
Freqs: 906 915 or 924 Mhz.
I'm sure these carriers are Spread Spectrum and use BPSK or QPSK
modulation depending on adjustable settings. The troubled links are locked down to
BPSK requiring only 3db SNR to maintain the link.

I have 3 120 degree sector antennae mounted on a tower at 200 feet (768 AGL).

these MTI 12.5 dbi panels are down tilted to about 3 degrees with a Vertical
beam width of about 14 degrees. These are Horizontally polarized antennae.

The target systems (two are having troubles) are 1.6 miles and 2.6miles away
respectively.

In both cases the elevation is around 500 to 550 ft with wads of Old Oaks
and other assorted trees in or near the freznel zone.

So far as the "legal" concerns go, my sense of the intent is to limit field strength
as not to cause interference with other part 15 devices. Since my issue is the tree cover
eating most of the energy in my link path before emerging into the free space between us.
My thinking is that the trees in the path are the only things being "interfered with"
and I doubt they'll complain <wink>. When the leaves drop, come autumn, we can easily
turn down the transmit power on the user end, and If need be, add attenuation on
the subscribers end however these units seem to work fine with pretty High input power.

At least one of my subscribers is seeing my tower at about -50dbm and works fine.

I Truly appreciate the thoughts you guys have shared and I'm going to look at the link provided.

Any further thoughts can be directed to me directly if you choose at
Dave (at) RTWB (dot) net

Thanks again

Dave
 

k9rzz

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Joined
Dec 12, 2005
Messages
3,162
Location
Milwaukee, WI
Well, have fun hauling a C band dish up 200 ft, but in any case, I was playing around with using my C band dish to pick up Inmarsat last fall. I had the best luck with a simple pigtail sticking out of a piece of coax and just experimentally finding the best focal point of the old 7 ft mesh dish. I tried a simple 5 element yagi feed, but the 1/4 wave stub worked better. I used a TV type pre-amp and that worked quite well, not that there was much to hear anyway, but I just wanted to see if I could do it. I got good sigs out of the telemetry up there. Many hams put those dishes to use for various uses, so there's no reason it shouldn't work for you. Just be prepared to put a little time in to experiment with it.
 
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commstar

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Premium Subscriber
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Apr 22, 2001
Messages
349
Location
Spokane, WA
Kudos

Well, I wish to offer Kudos on your commitment to customer service. Bravo, Sir, Bravo.

Most companies would tell these two customers to cut down some trees or get stuffed.

I reviewed your webpage and I wish you swift luck on the buildout and success of
your company. Growing a business from scratch is a huge undertaking- Godspeed to you and your dreams.

Mike
 
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