Today's Message Index:
----------------------
1. 07:05 AM - In line filter (Dave Lammers)
2. 09:14 AM - Re: In line filter (Doug McNutt)
3. 02:25 PM - Re: In line filter (Kelly McMullen)
4. 05:49 PM - GA7 RPM Gauge Urgently Required (Shaun Wilkinson)
5. 05:49 PM - Re: In line filter (Kelly McMullen)
Message 1
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Hi, does anyone on the list know where to purchase an in-line (BNC/BNC)
low-pass filter to pass 243 mhz, and reject the GPS carrier of 1.5 Ghz
and above?
It turns out that my comm radio causes the ring diodes in the output of
my ELT to resonate and retransmit at frequencies that saturate and shut
down the GPS reciever front end.
I don't want to move antennas (already 4 feet apart) and think that a
simple low pass filter in the ELT antenna line will do. I can easily
make one but would rather just purchase one already packaged.
Thanks
Dave Lammers
Message 2
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| Subject: | Re: In line filter |
At 08:42 -0600 3/8/10, Dave Lammers wrote:
>
>Hi, does anyone on the list know where to purchase an in-line (BNC/BNC) low-pass
filter to pass 243 mhz, and reject the GPS carrier of 1.5 Ghz and above?
>It turns out that my comm radio causes the ring diodes in the output of my ELT
to resonate and retransmit at frequencies that saturate and shut down the GPS
reciever front end.
>I don't want to move antennas (already 4 feet apart) and think that a simple low
pass filter in the ELT antenna line will do. I can easily make one but would
rather just purchase one already packaged.
>Thanks
Huh?? Some things you say just are not clear.
Your 121.5/243 MHz ELT had better not be transmitting when you poke a mic key on
a comm transmitter.
Ring diodes? Are you talking about the diodes in a double-balanced mixer? I suppose
something like that could be used in an ELT to create the harmonic of 121.5
that it needs but I haven't seen one that does that and it would more likely
be done before a non-linear final amplifier.
The GPS carrier is generated only by the satellites. The receivers are only receivers
and radiate nothing at 1.5 GHz. Some might have a local oscillator for
heterodyning but the level would be quite low.
121.5/243 is dying and is already not being monitored by satellites. Are you planning
to replace it with a 409 MHz version.
As for a filter about 20 feet of RG-58 will attenuate pretty well at 1.5 GHz and
still pass 121.5 MHz.
--
--> Halloween == Oct 31 == Dec 25 == Christmas <--
Message 3
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| Subject: | Re: In line filter |
ELT transmit sections are not isolated in any way and do resonate
harmonically when energized by strong RF signal in same spectrum.
Especially the early ones. It takes very little harmonics to overwhelm
the front end of a GPS. While a com receiver needs something better than
-40db signal for you to perceive it, GPS receivers operate with -150 db
signal, so a tenth of a watt output of a 5th or higher order harmonic
from an ELT will easily overwhelm the GPS receiver.
I had ELT harmonics(excited by a dozen TV broadcast antennas within 5
miles) overwhelming both com receivers to break squelch with loud static
when all 3 antennas were top-side on the airframe. Moved com 2 antenna
to the belly and problem went away. I knew it was the ELT because the
same flight with ELT antenna disconnected had no problems. Reconnecting
ELT antenna and they came right back.
Doug McNutt wrote:
>
>
> Huh?? Some things you say just are not clear.
>
> Your 121.5/243 MHz ELT had better not be transmitting when you poke a mic key
on a comm transmitter.
>
> Ring diodes? Are you talking about the diodes in a double-balanced mixer? I
suppose something like that could be used in an ELT to create the harmonic of
121.5 that it needs but I haven't seen one that does that and it would more likely
be done before a non-linear final amplifier.
>
> The GPS carrier is generated only by the satellites. The receivers are only receivers
and radiate nothing at 1.5 GHz. Some might have a local oscillator for
heterodyning but the level would be quite low.
>
> 121.5/243 is dying and is already not being monitored by satellites. Are you
planning to replace it with a 409 MHz version.
>
> As for a filter about 20 feet of RG-58 will attenuate pretty well at 1.5 GHz
and still pass 121.5 MHz.
Message 4
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| Subject: | GA7 RPM Gauge Urgently Required |
Dear All,
Ive been given the avionics-list info in the hope that you/somoene might
be able to help me.
I have a Grumman Cougar 1979 serial number GA7-0105 in the UK.
It went for its annual and someone has stolen the rpm guage along with a
few other items.
I have replaced everything but the rpm guage.
I cant get the annual finished till i have one !!!
I will pay for your services along with the part and mailing
Can anyone help in any way at all.
Kind regards
Shaun Wilkinson
Shaun Wilkinson
Owner
Cloud 9 Aviation (UK)
Cloud 9 Aviation (CZ)
Leeds Flying School
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Message 5
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| Subject: | Re: In line filter |
I believe you can get them from avionics shops. 1st generation ELTs that
came out in the '70s are worst for this, and will even interfere with
your com radios, often breaking squelch with static if near strong FM/TV
transmitters. 2nd generation ELTs from the 90s have better suppression,
and I believe the 406 units are supposedly even better.
With your GPS, be sure you aren't getting interference directly from
your com or nav radios. Most, including King and Narco have specific
frequencies that cause GPS interference when tuned to those
frequencies..does not require transmit, just being tuned to receive the
frequencies will shut down GPS reception. Avionics shops that install
IFR certified units are supposed to specifically test all of those
frequencies and install notch filters if any of them cause any interference.
Dave Lammers wrote:
>
> Hi, does anyone on the list know where to purchase an in-line (BNC/BNC)
> low-pass filter to pass 243 mhz, and reject the GPS carrier of 1.5 Ghz
> and above?
> It turns out that my comm radio causes the ring diodes in the output of
> my ELT to resonate and retransmit at frequencies that saturate and shut
> down the GPS reciever front end.
> I don't want to move antennas (already 4 feet apart) and think that a
> simple low pass filter in the ELT antenna line will do. I can easily
> make one but would rather just purchase one already packaged.
> Thanks
> Dave Lammers
>
>
>
>
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