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CAP Talk  |  Operations  |  Tools of the trade  |  Topic: ELT Antenna
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Author Topic: ELT Antenna  (Read 2735 times)
♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,837
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« on: November 14, 2007, 04:34:48 AM »

I know that if you transmit on a radio without an antenna connected, it can cause serious damage to the radio.  If an ELT's antenna is broken/snapped off on impact, would it cause damage to the beacon?  Would that damage completely destroy the beacon?
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           Capt. Dan Turkal
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                WI-204/CC
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SarDragon
Global Moderator

Posts: 10,464
Unit: NAVAIRPAC

« Reply #1 on: November 14, 2007, 04:46:42 AM »

I know that if you transmit on a radio without an antenna connected, it can cause serious damage to the radio. [Q1] If an ELT's antenna is broken/snapped off on impact, would it cause damage to the beacon?  [Q2] Would that damage completely destroy the beacon?

A1. Possibly. Some ELTs have a second antenna mounted directly to the unit, to enable its use outside the plane. As long as the transmitter has the correct load on it, it will not be damaged.

A2. Probably not. See A1.
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Dave Bowles
Maj, CAP
AT1, USN Retired
Mitchell Award (unnumbered)
C/WO, CAP, Ret
lordmonar
Too Much Free Time Award

Posts: 10,654

« Reply #2 on: November 14, 2007, 05:02:51 AM »

With the power levels that an ELT can put out.....probably not.

The ELT puts out 5-10 Watts or much less.  Today's electronics can usually withstand that sort of SWR (Standing Wave Radio) that you get from and "open" antenna.
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
sardak
Salty & Seasoned Contributor

Posts: 1,212

« Reply #3 on: November 14, 2007, 07:26:12 AM »

121.5 MHz ELTs transmit at a nominal 1 watt.

The 1/2 second 406 MHz burst on "new" beacons is transmitted at a nominal 5 watts.  The 121.5 MHz homing signal is a nominal 25 mW.

The technical specs for 406 beacons require them to meet all transmitter requirements at a VSWR up to 3:1.  Beyond that, the transmitter "shall not be damaged by any load from open circuit to short circuit."

The radiation pattern without an antenna, or a damaged antenna, is going to be screwed up, though.  If the transmitter itself is damaged on impact, who knows.

Mike
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Al Sayre
Salty & Seasoned Contributor

Posts: 2,510
Unit: SER-MS-001

Mississippi Wing
« Reply #4 on: November 14, 2007, 08:49:38 AM »

If you have a crash and the ELT antenna gets snapped off, your chance of getting a signal out drop to miniscule, that's why a lot of them use a rather flexible wire antenna.  On the other hand burning out the final amp on your ELT is probably not going to be your biggest worry...
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Lt Col Al Sayre
MS Wing Staff Dude
Admiral, Great Navy of the State of Nebraska
GRW #2787
capchiro
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Posts: 577

« Reply #5 on: November 14, 2007, 09:07:24 AM »

lordmonar probably meant standing wave ratio for SWR in the above post.
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Lt. Col. Harry E. Siegrist III, CAP
Commander
Sweetwater Comp. Sqdn.
GA154
Eclipse
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Posts: 29,238

« Reply #6 on: November 14, 2007, 09:10:08 AM »

If you have a crash and the ELT antenna gets snapped off, your chance of getting a signal out drop to miniscule, that's why a lot of them use a rather flexible wire antenna.  On the other hand burning out the final amp on your ELT is probably not going to be your biggest worry...

Right - most ELT's I have seen do not have the rigid, telescoping antenna found on practice beacons.

With no antenna, your transmit range is in feet - that's why removing the antenna on L-Per or scanner is a close-quarters DF technique.
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BillB
Salty & Seasoned Contributor

Posts: 1,987

« Reply #7 on: November 14, 2007, 09:32:05 AM »

I disagree. I've been to a crash where the antenna cable was torn loose from the ELT and the search aircraft still was able to vector in on the crash site from the ELT signal.
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Gil Robb Wilson # 19
Gil Robb Wilson # 104
♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,837
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« Reply #8 on: November 14, 2007, 02:20:07 PM »

Okay, I didn't think the SWR would be that important.  I ask because I was thinking about a crash scenario

Situation - A pilot crashes in the wilderness, setting off his beacon.  He survives the crash, but the antenna on his ELT doesn't.  He then tries to reconnect or fix the antenna.  Here's where my question comes into context: would the ELT have already been damaged by transmitting with no antenna, to the point that it would become inoperable?  I see now that it is unlikely that it would be.
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           Capt. Dan Turkal
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                WI-204/CC
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desert rat
Member

Posts: 81

« Reply #9 on: November 14, 2007, 05:50:06 PM »

Now if the antena splits won't it give out a distorted signal and leaving the search in wrong directions.
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SarDragon
Global Moderator

Posts: 10,464
Unit: NAVAIRPAC

« Reply #10 on: November 15, 2007, 03:54:10 AM »

Probably not. The one I got when I went Dumpster diving had the flexible antenna wrapped around it. The signal was detectable about 3 miles away, with the unit inside a large Dumpster with a bunch of metal scrap.
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Dave Bowles
Maj, CAP
AT1, USN Retired
Mitchell Award (unnumbered)
C/WO, CAP, Ret
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CAP Talk  |  Operations  |  Tools of the trade  |  Topic: ELT Antenna
 


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