[FoxHunt] Moded ELTS

[de Rick K5RIC]

Good work Dale, 

One of the major problems I have seen during actual missions is an ELT with
out any modulation.  

I was talking to an aircrew that keep saying "No Signal."  When I asked
them for a signal reading, it full scale. It was an FAA ground station with
keyed TX, and no modulation.  I could hear the carrior in my mobile unit
under the aircraft, about 3000 feet lower.  They swore they had no signal,
until I pointed out big signal no audio.  

I want to set up a practice ELT with no modulation one of these days.  


73

rick sohl
k5ric
CAP West 4



At 09:08 PM 3/22/02 -0800, you wrote:
>>-Here in Yamhill County, Oregon, we got some old ELT beacons and
>>-recrystalled them for
>>-121.775......
>>
>>
>>Great!  Go for it Dale.
>>
>>One thing to check is to see if your ELT is radiating strong RF on the
>>second harmonic. .....
>
>
>     We paid attention to that in the conversion.  One box has an outboard
>     low pass filter with BNC connectors that goes in line with the antenna.
>     In another we replaced the output circuit with a low-pass "T" network.
>
>     Fortunately, we have access to a spectrum analyzer, and made good use
>     of it!  We have also reduced the power output a bit.
>
>     The ELTs we converted are probably similar to many - the modulation is
>     applied in the collector lead one or two stages away from the final
>     (to minimize the amount of audio power required, and perhaps to
>     broaden the signal to make it easy to hear!)  An AM signal
>     amplified by one or two class-C stages is going to splatter all over
>     the band!  So one of the future projects will be to clean up the
>     modulation and keep it within a reasonable bandwidth (+/- 10 kHz or
>     so.)  Unmodified, the output at 100kHz off frequency is only down
>     40dB or so, which is a big reason why 121.6 MHz didn't work well
>     for a practice frequency.
>
>
>     (While I was working on the other units, I repaired the practice
>     beacon for the local CAP group.  Didn't look like it had ever
>     worked since someone had put in the 121.775 MHz crystal, plus one
>     transistor had blown, and the 1988 Radio Shack nicads were no
>     longer in peak condition.)
>
>          - Dale WB6BYU
>
>
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