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1.57542 GHz Filter

 
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speedy11



Joined: 29 Jun 2015
Posts: 61
Location: Port Orange, FL

PostPosted: Sat Jul 22, 2017 10:10 am    Post subject: 1.57542 GHz Filter Reply with quote

In the Garmin installation manual, it says the following:

"The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."

Does anyone know of anyone who had to do this?

Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.

Thanks,

Stan Sutterfield


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nuckolls.bob(at)aeroelect
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PostPosted: Sat Jul 22, 2017 11:44 am    Post subject: 1.57542 GHz Filter Reply with quote

At 01:10 PM 7/22/2017, you wrote:
Quote:
In the Garmin installation manual, it says the following:

"The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."

Does anyone know of anyone who had to do this?

Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.

Thanks,

Stan Sutterfield

These were found useful on some makes of comm transceivers that
had unhappy amounts of harmonic radiation on or near
GPS frequencies. I don't now recall the brands/models of
radio involved but as I do recall, it was just a select
few.

Try it without the filter. It's easy to add later if
indeed you do need it. It's simply inserted into
the transceiver coax feedline


I doubt you will find it necessary.





Bob . . .


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alec(at)alecmyers.com
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PostPosted: Sat Jul 22, 2017 6:29 pm    Post subject: 1.57542 GHz Filter Reply with quote

I've tried the filters and they didn't make any difference.

On Jul 22, 2017, at 14:10, speedy11(at)aol.com (speedy11(at)aol.com) wrote:
In the Garmin installation manual, it says the following:

"The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."

Does anyone know of anyone who had to do this?

Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.

Thanks,

Stan Sutterfield


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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jul 23, 2017 7:54 am    Post subject: 1.57542 GHz Filter Reply with quote

At 09:28 PM 7/22/2017, you wrote:
Quote:
I've tried the filters and they didn't make any difference.

What symptoms suggested that the filter
was, perhaps, necessary?


Bob . . .


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alec(at)alecmyers.com
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PostPosted: Sun Jul 23, 2017 8:37 am    Post subject: 1.57542 GHz Filter Reply with quote

I'm sorry - I should have given more detail.
The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. It’s an old GPS and not a new problem. I’ve put a fair amount of resource into trying to fix it some while ago, but not found a resolution.

The inline filters didn’t make any difference.
Quote:
On Jul 23, 2017, at 11:53 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
At 09:28 PM 7/22/2017, you wrote:
Quote:
I've tried the filters and they didn't make any difference.
What symptoms suggested that the filter was, perhaps, necessary?
Bob . . .



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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jul 23, 2017 12:23 pm    Post subject: 1.57542 GHz Filter Reply with quote

At 11:35 AM 7/23/2017, you wrote:
Quote:
I'm sorry - I should have given more detail.

The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. It’s an old GPS and not a new problem. I’ve put a fair amount of resource into trying to fix it some while ago, but not found a resolution.

Understand.

Some background on cross-system interference issues
may be helpful . . .

When one runs the DO160/MIL-STD 461/462 gauntlets,
it is incumbent on the manufacturer of a new product
to measure and document extraneous emissions to
be sure they are below tolerable limits for other
systems. We know that its difficult if not impossible
to drive all potential interference emission to zero
but the design goals call for reducing them to insignificance.
I.e. the victim system's performance is not degraded.


[img]cid:.0[/img]

A fairly recent edition of DO160 offers these testing
limits for radiated emissions. Note those 'notches' in
the allowable emissions plot. They are centered over specific
ranges of spectrum where RECEIVERS of itty-bitty signals
would like to have a clear shot at detecting the information
on those frequencies.

Note the notches cover vhf nav/comm, uhf nav/comm, transponder/
tcas and gps bands. Not sure what service is being protected
up round 5 ghz . . .

The energy levels allowed are the sum total of energies
radiated from all components of a device and associated
cables when laid out on a copper-top table like so.

[img]cid:.1[/img]
All this is fine and dandy for potential antagonists
that are NOT designed to radiate buckets of energy on purpose
. . . i.e. transmitters. Tests for bkackbox/harness radiation are
generally no big deal . . . I've never failed one.

Then there are transmitters. By nature, they're intended to
radiate signal levels on the order of 130 dBuv/m at 3 meters
from an antenna. Given that 'extraneous' emission limits
at GPS frequencies are on the order of 85 dBuv/m LOWER, this
means that the 13th harmonic (121 mHz x13 = GPS) power has to
be on the order of 1 times 10^-8 lower than the fundamental.

Modern designs have no problem achieving this goal but
there's a goodly number of legacy comm equipment that
was designed and qualified before GPS became the backbone
of aerial navigation.

GPS signals are exceedingly weak . . . generally at or below
atmospheric noise. The predictable nature of the incoming
signal makes if possible to digitally sift useful data
out of what would otherwise be noise.

If adding the notch filter didn't fix the symptoms, then
perhaps the interference isn't being radiated from the
antenna . . . there have been cases of incompatible
appliances were the interference was getting out of the
enclosure (i.e. wouldn't pass the bench test as described above).

Will your gps interference symptom repeat on the ground?

You might try dummy loading the transceiver and see
if the interference goes away. Oops, just noticed that it
repeats on two separate transmitters (same brand/model?)
How far apart are your comm and GPS antennas?









Bob . . .


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Eric Page



Joined: 15 Feb 2017
Posts: 241

PostPosted: Sun Jul 23, 2017 2:04 pm    Post subject: 1.57542 GHz Filter Reply with quote

On Jul 23, 2017, at 1:22 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:
Quote:
Not sure what service is being protected up round 5 ghz . . .

Probably 802.11a WiFi systems. In the U.S., four channels between 5.180 and 5.240 GHz, and five channels between 5.745 and 5.825 GHz. Might be other stuff as well; that's in an Industrial, Scientific and Medical (ISM) band.

Eric


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alec(at)alecmyers.com
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PostPosted: Sat Aug 05, 2017 4:34 pm    Post subject: 1.57542 GHz Filter Reply with quote

Hi Bob
I’m *very* happy to have your expert attention consider this, even though it’s not a homebuilt.

VHF comm equipment is (are?) a King KX165 (com1) and a KX155 (com2), each connected to its own roof-mount antenna.

The GPS is an Apollo GX50, with the “patch” antenna on the tailcone, so there’s about 6 feet between the antenna systems. The coaxes are routed diversely, the GPS under the floor and the COM via the pillars to the cabin roof.

The interference is generated from both com1 and com2.

A
Quote:
On Jul 23, 2017, at 4:22 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
At 11:35 AM 7/23/2017, you wrote:
Quote:
I'm sorry - I should have given more detail. The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. It’s an old GPS and not a new problem. I’ve put a fair amount of resource into trying to fix it some while ago, but not found a resolution.
Understand. Some background on cross-system interference issues may be helpful . . . When one runs the DO160/MIL-STD 461/462 gauntlets, it is incumbent on the manufacturer of a new product to measure and document extraneous emissions to be sure they are below tolerable limits for other systems. We know that its difficult if not impossible to drive all potential interference emission to zero but the design goals call for reducing them to insignificance. I.e. the victim system's performance is not degraded. <b12a4d0.jpg> A fairly recent edition of DO160 offers these testing limits for radiated emissions. Note those 'notches' in the allowable emissions plot. They are centered over specific ranges of spectrum where RECEIVERS of itty-bitty signals would like to have a clear shot at detecting the information on those frequencies. Note the notches cover vhf nav/comm, uhf nav/comm, transponder/ tcas and gps bands. Not sure what service is being protected up round 5 ghz . . . The energy levels allowed are the sum total of energies radiated from all components of a device and associated cables when laid out on a copper-top table like so. <b12a51e.jpg> All this is fine and dandy for potential antagonists that are NOT designed to radiate buckets of energy on purpose . . . i.e. transmitters. Tests for bkackbox/harness radiation are generally no big deal . . . I've never failed one. Then there are transmitters. By nature, they're intended to radiate signal levels on the order of 130 dBuv/m at 3 meters from an antenna. Given that 'extraneous' emission limits at GPS frequencies are on the order of 85 dBuv/m LOWER, this means that the 13th harmonic (121 mHz x13 = GPS) power has to be on the order of 1 times 10^-8 lower than the fundamental. Modern designs have no problem achieving this goal but there's a goodly number of legacy comm equipment that was designed and qualified before GPS became the backbone of aerial navigation. GPS signals are exceedingly weak . . . generally at or below atmospheric noise. The predictable nature of the incoming signal makes if possible to digitally sift useful data out of what would otherwise be noise. If adding the notch filter didn't fix the symptoms, then perhaps the interference isn't being radiated from the antenna . . . there have been cases of incompatible appliances were the interference was getting out of the enclosure (i.e. wouldn't pass the bench test as described above). Will your gps interference symptom repeat on the ground? You might try dummy loading the transceiver and see if the interference goes away. Oops, just noticed that it repeats on two separate transmitters (same brand/model?) How far apart are your comm and GPS antennas?
Bob . . .



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