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roughleg(at)gmail.com
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 Posted: Sun Feb 02, 2020 5:02 pm Post subject: GPS notch filter? |
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Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.
Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it?
Pat
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 02, 2020 6:23 pm Post subject: GPS notch filter? |
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At 06:59 PM 2/2/2020, you wrote:
| Quote: | Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.
Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it? |
It would not hurt anything to
install it in the antenna feedline
for vhf comm. The 13th harmonic
of vhf comm energy falls in close
proximity to the gps signals at 1575
or so MHz. GPS signals are very weak . . .
in fact you cannot generaly tune them
in on a normal receiver. Their signals
are literally below the atmospheric
noise floor.
The predictable, encoded nature of
received gps signals allows digital
processing to sniff out these tiny
voices and make sense of them.
Up until the dawn of gps implementation
on civil aircraft, there was no special
attention given to DO160 qualification
of vhf comm transceivers to offer special
protection to gps spectrum . . . after
all it WAS the 13th harmonic we're fussing
about.
But it turns out that significant numbers
of vhf comm systems already in place
generated 13th harmonic energy bad enough
to degrade if not snafu a gps signal.
Hence, the 'gps notch' filter band-aid
allowed these slightly-less-than-civilized
transceivers to coexist with gps equipment
on board an aircraft.
In the interim, gps receiver performance has
taken some quantum leaps . . . you got the
things in cell phones, cameras and wrist-watches.
At the same time, DO160 qualification requirements
added a band of additional protection to gps
spectrum. You can buy surface-mount gps notch
filters to be included in your vhf-comm
offering . . . and getting your DO160
blue ribbon requires that these frequencies
be protected.
The short answer is, you probably don't
need it. But do some flight testing to see
if gps signal strength values are depressed
while transmitting . . . you'd want to do
testing at 25 Khz steps from 120 MHz to
122 MHz . . .
-OR-
Check with the manufacturer of your transceiver
as to compliance with CURRENT gps protection
requirements . . .
-OR-
Stick the thing in anyhow . . . won't hurt a thing.
Bob . . .
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roughleg(at)gmail.com
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| Posted: Sun Feb 02, 2020 8:25 pm Post subject: GPS notch filter? |
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Thanks, Bob. As always, a very helpful and complete answer.
On Sun, Feb 2, 2020 at 7:29 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 06:59 PM 2/2/2020, you wrote:
| Quote: | Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.
Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it? |
 It would not hurt anything to
 install it in the antenna feedline
 for vhf comm. The 13th harmonic
 of vhf comm energy falls in close
 proximity to the gps signals at 1575
 or so MHz. GPS signals are very weak . . .
 in fact you cannot generaly tune them
 in on a normal receiver. Their signals
 are literally below the atmospheric
 noise floor.
 The predictable, encoded nature of
 received gps signals allows digital
 processing to sniff out these tiny
 voices and make sense of them.
 Up until the dawn of gps implementation
 on civil aircraft, there was no special
 attention given to DO160 qualification
 of vhf comm transceivers to offer special
 protection to gps spectrum . . . after
 all it WAS the 13th harmonic we're fussing
 about.
 But it turns out that significant numbers
 of vhf comm systems already in place
 generated 13th harmonic energy bad enough
 to degrade if not snafu a gps signal.
 Hence, the 'gps notch' filter band-aid
 allowed these slightly-less-than-civilized
 transceivers to coexist with gps equipment
 on board an aircraft.
 In the interim, gps receiver performance has
 taken some quantum leaps . . . you got the
 things in cell phones, cameras and wrist-watches.
 At the same time, DO160 qualification requirements
 added a band of additional protection to gps
 spectrum. You can buy surface-mount gps notch
 filters to be included in your vhf-comm
 offering . . . and getting your DO160
 blue ribbon requires that these frequencies
 be protected.
 The short answer is, you probably don't
 need it. But do some flight testing to see
 if gps signal strength values are depressed
 while transmitting . . . you'd want to do
 testing at 25 Khz steps from 120 MHz to
 122 MHz . . .
 -OR-
 Check with the manufacturer of your transceiver
 as to compliance with CURRENT gps protection
 requirements . . .
 -OR-
 Stick the thing in anyhow . . . won't hurt a thing.
 Bob . . .
|
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 02, 2020 9:15 pm Post subject: GPS notch filter? |
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At 06:59 PM 2/2/2020, you wrote:
| Quote: | Rooting around in boxes at the hangar the other day I came across a "thingy" (technical term) with part number TED 4-70-54. Apparently it is a notch filter to stop radio harmonics interfering with GPS reception (see SkyGeek listing). I'm using an MGL V16 radio, and GPS in the form of a Garmin G5 (internal antenna) and a SkyFYX for the ADS-B setup, plus possibly iPad on the panel and/or handhelds. The V16 will be under the passenger's seat, the SkyFYX above the passenger's head, and the G5 on the pilot's side of the panel. Airplane is a Zenith STOL CH 750.
Is there any good reason to incorporate the notch filter in my design? And if so, where best to put it? |
It would not hurt anything to
install it in the antenna feedline
for vhf comm. The 13th harmonic
of vhf comm energy falls in close
proximity to the gps signals at 1575
or so MHz. GPS signals are very weak . . .
in fact you cannot generaly tune them
in on a normal receiver. Their signals
are literally below the atmospheric
noise floor.
The predictable, encoded nature of
received gps signals allows digital
processing to sniff out these tiny
voices and make sense of them.
Up until the dawn of gps implementation
on civil aircraft, there was no special
attention given to DO160 qualification
of vhf comm transceivers to offer special
protection to gps spectrum . . . after
all it WAS the 13th harmonic we're fussing
about.
But it turns out that significant numbers
of vhf comm systems already in place
generated 13th harmonic energy bad enough
to degrade if not snafu a gps signal.
Hence, the 'gps notch' filter band-aid
allowed these slightly-less-than-civilized
transceivers to coexist with gps equipment
on board an aircraft.
In the interim, gps receiver performance has
taken some quantum leaps . . . you got the
things in cell phones, cameras and wrist-watches.
At the same time, DO160 qualification requirements
added a band of additional protection to gps
spectrum. You can buy surface-mount gps notch
filters to be included in your vhf-comm
offering . . . and getting your DO160
blue ribbon requires that these frequencies
be protected.
The short answer is, you probably don't
need it. But do some flight testing to see
if gps signal strength values are depressed
while transmitting . . . you'd want to do
testing at 25 Khz steps from 120 MHz to
122 MHz . . .
-OR-
Check with the manufacturer of your transceiver
as to compliance with CURRENT gps protection
requirements . . .
-OR-
Stick the thing in anyhow . . . won't hurt a thing.
Bob . . .
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http://www.matronics.com/Navigator?AeroElectric-List |
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