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jrevens
Joined: 21 Nov 2013
Posts: 34
Location: Arvada, CO
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 Posted: Thu Dec 10, 2015 4:17 pm Post subject: ADS-B antenna location |
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I stupidly did a very nice ADS-B UAT antenna installation on my new project before doing all of my homework. It is 45 inches from my transponder antenna. I just received my ADS600-EXP unit. NavWorx says that there must be a minimum of 5 feet of separation or the ADS-B unit will fail (their words). Has anyone broken this rule and had successful operation with less than 5 feet of separation? Can a closer distance actually cause damage to the unit? In this particular instance, moving the ADS-B antenna will be a difficult and messy change to have to make. Can't believe I did that.
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John Evens
Thorp T-18 N71JE (sold)
Kitfox SS7 N27JE (built & flying) |
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skywagon
Joined: 11 Feb 2006
Posts: 184
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| Posted: Thu Dec 10, 2015 6:57 pm Post subject: ADS-B antenna location |
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John,
Others will answer with greater detail, but here is what you are up against.
. .
The transponder puts out high wattage pulse returns in answer to a radar
inquiry. That pulse of power is going to hit your ADS antenna pretty hard.
I don't have a clue what modern receiver solid state circuits can deal with
these days but, that pulse is made up of a huge bandwidth of harmonics, etc.
that typically make up/define a "pulse". My thinking is that 60" and more
is needed to not cause problems. . .
David
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matronics(at)rtist.nl
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| Posted: Thu Dec 10, 2015 11:17 pm Post subject: ADS-B antenna location |
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The transponder transmits a certain amount of energy. As you move away
from the antenna, the energy spreads out. Only some of the transmitted
energy will hit the receiving antenna and the further you move away, the
less energy will be 'caught'. When too close, the amount of energy
induced in the receiver antenna can overwhelm or even damage the
sensitive ADS-B receiver.
The attenuation between transmitter antenna and receiver antenna is
called 'free space path loss'. There is a formula to calculate that
loss, and here's a link that will let you play with it:
https://www.pasternack.com/t-calculator-fspl.aspx
Using that calculator, the difference between 45 inches and 5ft at 1GHz
is less than 3dB. That is not a whole lot. A poor quality antenna cable
will already give you that. That means for your situation that you can
simulate the extra distance by adding a 3dB attenuator in your receiver
antenna cable. The drawback is that you'll have slightly worse coverage
(the signals that you want are also attenuated a bit), but should be safe.
You may want to call NavWorx and ask them about this. Things also depend
on the rest of your installation, such as type and output of your
transponder, antenna cable type and lengths for both devices, etc.
Rob
On 12/11/2015 3:55 AM, David Lloyd wrote:
[quote]
<skywagon(at)charter.net>
John,
Others will answer with greater detail, but here is what you are up
against. . .
The transponder puts out high wattage pulse returns in answer to a
radar inquiry. That pulse of power is going to hit your ADS antenna
pretty hard. I don't have a clue what modern receiver solid state
circuits can deal with these days but, that pulse is made up of a huge
bandwidth of harmonics, etc. that typically make up/define a "pulse".
My thinking is that 60" and more is needed to not cause problems. . .
David
---
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