Today's Message Index:
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1. 08:54 AM - Re: 1.57542 GHz Filter (Robert L. Nuckolls, III)
2. 09:37 AM - Re: 1.57542 GHz Filter (Alec Myers)
3. 10:14 AM - Re: AeroElectric-List Digest: 3 Msgs - 07/22/17 (speedy11@aol.com)
4. 01:23 PM - Re: 1.57542 GHz Filter (Robert L. Nuckolls, III)
5. 02:48 PM - ADS-B Out Antenna (Charles Brame)
6. 03:04 PM - Re: 1.57542 GHz Filter (Eric Page)
Message 1
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Subject: | Re: 1.57542 GHz Filter |
At 09:28 PM 7/22/2017, you wrote:
>I've tried the filters and they didn't make any difference.
What symptoms suggested that the filter
was, perhaps, necessary?
Bob . . .
Message 2
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Subject: | Re: 1.57542 GHz Filter |
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=99s an old GPS and not a new problem. I=99ve put a fair
amount of resource into trying to fix it some while ago, but not found a
resolution.
The inline filters didn=99t make any difference.
> On Jul 23, 2017, at 11:53 AM, Robert L. Nuckolls, III
<nuckolls.bob@aeroelectric.com> wrote:
>
> At 09:28 PM 7/22/2017, you wrote:
>> I've tried the filters and they didn't make any difference.
>
> What symptoms suggested that the filter
> was, perhaps, necessary?
>
> Bob . . .
>
Message 3
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Subject: | Re: AeroElectric-List Digest: 3 Msgs - 07/22/17 |
Thanks to all for your advice.
I will install without and add later if needed.
Thank you,
Stan
Do not archive
Time: 12:44:12 PM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
Subject: Re: AeroElectric-List: 1.57542 GHz Filter
At 01:10 PM 7/22/2017, you 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
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
4-70-54-Inline-Gps-Notch-Filter-Bnc
I doubt you will find it necessary.
Bob . . .
Message 4
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Subject: | Re: 1.57542 GHz Filter |
At 11:35 AM 7/23/2017, you wrote:
>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=99s an old
>GPS and not a new problem. I=99ve 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.
Emacs!
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.
Emacs!
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 . . .
Message 5
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Subject: | ADS-B Out Antenna |
I recently purchased a uAvionix Echo ADS-B Out device for my RV-6A. The
instructions say to position the ADS-B Out antenna at least 3 feet from
my existing transponder antenna. I will be forced to locate the new
antenna in the aft fuselage with at least a 15-20 foot cable - not an
ideal situation for me.
Is there really a significant reason for the distance requirement for
the ADS-B Out antenna? How about proximity to a VHF antenna?
Charlie Brame
RV-6A N11CB
San Atonio
Message 6
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Subject: | Re: 1.57542 GHz Filter |
On Jul 23, 2017, at 1:22 PM, Robert L. Nuckolls, III <nuckolls.bob@aeroelectric.com>
wrote:
> 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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