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In line filter

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davelammers(at)mchsi.com
Guest

Posted: Mon Mar 08, 2010 7:05 am Post subject: In line filter

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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douglist(at)macnauchtan.c
Guest

Posted: Mon Mar 08, 2010 9:14 am Post subject: In line filter

At 08:42 -0600 3/8/10, Dave Lammers wrote:

 	  Quote: 	
		  



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  <--

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Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Mon Mar 08, 2010 2:25 pm Post subject: 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:

 	  Quote: 	
		   

 


	


 	  Quote: 	
		   

 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.

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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Mon Mar 08, 2010 5:49 pm Post subject: 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:

 	  Quote: 	
		   

 

 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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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

Bill & Sue

Joined: 10 Jan 2006
Posts: 48
Location: Malvern, Worcs. UK (Defford, Croft Farm)

Posted: Tue Mar 09, 2010 2:43 am Post subject: Re: In line filter

You could try a simple (and cheap) 1/4 wave stub:

Just connect a short open ended length of coax cable to a BNC tee-piece in line with the ELT antenna. The coax stub just needs to be cut to the right length - about 3.5 cm should do it  but it slightly depends on what coax you use. cut it to say 5 cm and snip tiny bits off till the problem disappears!



Hope that helps!



Bill

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jadecuir(at)comcast.net
Guest

Posted: Wed Mar 10, 2010 10:44 am Post subject: In line filter

I had problems like yours, and I tried notch filters in the comm ant lead to no avail. I replaced comm and GPS coax with RG400 and eliminated the problem.John DeCuir

RV4

[quote]

Time: 07:05:22 AM PST US

From: Dave Lammers <davelammers(at)mchsi.com (davelammers(at)mchsi.com)>

Subject: In line filter

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



[b]

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