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Antenna Ground Plane
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mike(at)vision499.com
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PostPosted: Thu May 30, 2019 5:40 pm    Post subject: Antenna Ground Plane Reply with quote

Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com antenna, why would this be?

Thanks

Mike



Mike Pienaar
mike(at)vision499.com (ike(at)vision499.com)
Home: +1 250-999-8121
Mike cell: +1 250-885-0554
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Kellym



Joined: 10 Jan 2006
Posts: 1700
Location: Sun Lakes AZ

PostPosted: Thu May 30, 2019 6:03 pm    Post subject: Antenna Ground Plane Reply with quote

I will just address your question of antenna separation. ELTs have
oscillator circuits to generate the multiple frequencies they transmit.
Those circuits can be activated by strong VHF signals, such as TV, FM
radio, etc. When that occurs the ELT re-radiates signals that are
received by your com radio. I had an aircraft where the ELT antenna was
18" behind the Loran antenna (a modified com antenna), and in front of
the Loran antenna 18 " was the com antenna. Whenever I flew within 10
miles of the local TV/FM antenna farm, I would get squelch break so bad
I couldn't hear the local tower or approach control frequencies, on both
com radios. Disconnected ELT antenna and problem went away. Reconnect
and it was back. I would suggest adhering to the antenna separation, and
if you have a second com, put the antenna for it on the belly.

On 5/30/2019 6:39 PM, mike(at)vision499.com wrote:
Quote:
Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an
aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the
adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com
antenna, why would this be?

Thanks

Mike

Mike Pienaar

_mike(at)vision499.com <mailto:ike(at)vision499.com>___

*Home: +1 250-999-8121*

Mike cell: +1 250-885-0554

1-1100 Tulip Ave, Victoria, BC, V8Z 0A2



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PostPosted: Thu May 30, 2019 6:42 pm    Post subject: Antenna Ground Plane Reply with quote

I don't have any easy answers for aluminum foil, but copper foil is
easily soldered. Another option for only a slight weight penalty is to
cut thin aluminum strips, with the 'root' end cut big enough to accept
the antenna mounting bolts. Flashing material from your local building
supply is an easy, inexpensive source.

Charlie

On 5/30/2019 9:03 PM, Kelly McMullen wrote:
Quote:

<kellym(at)aviating.com>

I will just address your question of antenna separation. ELTs have
oscillator circuits to generate the multiple frequencies they
transmit. Those circuits can be activated by strong VHF signals, such
as TV, FM radio, etc. When that occurs the ELT re-radiates signals
that are received by your com radio. I had an aircraft where the ELT
antenna was 18" behind the Loran antenna (a modified com antenna), and
in front of the Loran antenna 18 " was the com antenna. Whenever I
flew within 10 miles of the local TV/FM antenna farm, I would get
squelch break so bad I couldn't hear the local tower or approach
control frequencies, on both com radios. Disconnected ELT antenna and
problem went away. Reconnect and it was back. I would suggest adhering
to the antenna separation, and if you have a second com, put the
antenna for it on the belly.

On 5/30/2019 6:39 PM, mike(at)vision499.com wrote:
> Hello,
>
> I’m installing an ACK ELT in a composite aircraft and they recommend
> an aluminum foil ground plane for the antenna.
>
> My question is how do you ensure conductivity between the layers of
> the adhesive aluminum foil and how do you attach the antenna to the
> foil?
>
> They also want the antenna to be more than 3 ft away from the com
> antenna, why would this be?
>
> Thanks
>
> Mike
>
> Mike Pienaar
>
> _mike(at)vision499.com <mailto:ike(at)vision499.com>___
>
> *Home: +1 250-999-8121*
>
> Mike cell: +1 250-885-0554
>
> 1-1100 Tulip Ave, Victoria, BC, V8Z 0A2


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Robert Reed



Joined: 22 Oct 2009
Posts: 331
Location: Dallas/Ft.Worth

PostPosted: Thu May 30, 2019 7:14 pm    Post subject: Antenna Ground Plane Reply with quote

To see my installation go to www.kissbuild.onfinal18.comSelect MY PROGRESS
Under Instrument Panel - Select Antenna Installation.

Bob Reed
Sent from my iPhone

On May 30, 2019, at 8:39 PM, <mike(at)vision499.com (mike(at)vision499.com)> <mike(at)vision499.com (mike(at)vision499.com)> wrote:
Quote:
<![endif]--> <![endif]-->
Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com antenna, why would this be?

Thanks

Mike
 


Mike Pienaar
mike(at)vision499.com (ike(at)vision499.com)
Home: +1 250-999-8121
Mike cell: +1 250-885-0554
1-1100 Tulip Ave, Victoria, BC, V8Z 0A2




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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jun 02, 2019 9:18 am    Post subject: Antenna Ground Plane Reply with quote

At 08:39 PM 5/30/2019, you wrote:
Quote:
Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com antenna, why would this be?

Might be easier to ditch the whip antenna
and go with the RST design semi-dipoles
discussed here on the List about 7 years
ago:


https://tinyurl.com/yyz6anjs

This antenna is a dual frequency DIPOLE
that does not require a ground plane.
It mounts no the side of your fuselage
inside surface.

What is the material used in your
airplane's construction?



Bob . . .


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ceengland7(at)gmail.com
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PostPosted: Sun Jun 02, 2019 9:55 am    Post subject: Antenna Ground Plane Reply with quote

On 6/2/2019 12:17 PM, Robert L. Nuckolls, III wrote:

Quote:
At 08:39 PM 5/30/2019, you wrote:
Quote:
Hello,
 
I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.
 
My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?
 
They also want the antenna to be more than 3 ft away from the com antenna, why would this be?

  Might be easier to ditch the whip antenna
  and go with the RST design semi-dipoles
  discussed here on the List about 7 years
  ago:


https://tinyurl.com/yyz6anjs

  This antenna is a dual frequency DIPOLE
  that does not require a ground plane.
  It mounts no the side of your fuselage
  inside surface.

  What is the material used in your
  airplane's construction?



  Bob . . .
Don't know if this is reality, but I've seen some discussion in the past about ELTs' supplied-antennas being part of the certification 'package'. I know we're experimentals, but some things we don't get a pass on, like IFR GPS vs non-TSO nav/ILS radios.

ELTs in general don't give me a lot of warm & fuzzies, but I would like to be confident that my insurance company doesn't have an 'out' if they're asked to pay on an accident that I've survived.

Just a thought...

Charlie
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alec(at)alecmyers.com
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PostPosted: Sun Jun 02, 2019 10:07 am    Post subject: Antenna Ground Plane Reply with quote

Bob,
If you change the antenna, does the ELT still meet TSO-C126b, is it still “approved”, and does it still meet 14 CFR § 91.207?

On Jun 2, 2019, at 13:17, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
At 08:39 PM 5/30/2019, you wrote:
Quote:
Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com antenna, why would this be?

Might be easier to ditch the whip antenna
and go with the RST design semi-dipoles
discussed here on the List about 7 years
ago:


https://tinyurl.com/yyz6anjs

This antenna is a dual frequency DIPOLE
that does not require a ground plane.
It mounts no the side of your fuselage
inside surface.

What is the material used in your
airplane's construction?



Bob . . .


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PostPosted: Sun Jun 02, 2019 1:42 pm    Post subject: Antenna Ground Plane Reply with quote

At 12:58 PM 6/2/2019, you wrote:
Quote:
Bob,

If you change the antenna, does the ELT still meet TSO-C126b, is it still “approved”, and does it still meet 14 CFR § 91.207?

probably not according to the guy that
walks around with a rule book under his
arm . . . but it does meet the spirit
and intent. Jim sold a lot of kits.


Bob . . .


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mike(at)vision499.com
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PostPosted: Sun Jun 02, 2019 3:47 pm    Post subject: Antenna Ground Plane Reply with quote

Thank you very much for the help.

The aircraft is a KIS made from honeycomb and fiberglass

On the dipole antenna what length should the folded balun be and do I make the toroid balun with 3 toroids as in the instruction?

Just out of interest why should the ELT antenna be separated from the com antenna

Thanks again

Mike

From: owner-aeroelectric-list-server(at)matronics.com <owner-aeroelectric-list-server(at)matronics.com> On Behalf Of Robert L. Nuckolls, III
Sent: June 2, 2019 10:18 AM
To: aeroelectric-list(at)matronics.com
Subject: Re: Antenna Ground Plane

At 08:39 PM 5/30/2019, you wrote:

Quote:

Hello,

I’m installing an ACK ELT in a composite aircraft and they recommend an aluminum foil ground plane for the antenna.

My question is how do you ensure conductivity between the layers of the adhesive aluminum foil and how do you attach the antenna to the foil?

They also want the antenna to be more than 3 ft away from the com antenna, why would this be?


Might be easier to ditch the whip antenna
and go with the RST design semi-dipoles
discussed here on the List about 7 years
ago:
https://tinyurl.com/yyz6anjs

This antenna is a dual frequency DIPOLE
that does not require a ground plane.
It mounts no the side of your fuselage
inside surface.

What is the material used in your
airplane's construction?


Bob . . .
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echristley(at)att.net
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PostPosted: Mon Jun 03, 2019 6:20 am    Post subject: Antenna Ground Plane Reply with quote

I can answer this with some real world experience. I mounted my COM antennae just behind the canopy on my 601XL, and then I put the ELT about halfway between the COM and the rudder. There was about 18 inches or so separation between each.
In there air, I could not reliably communicate with KRDU once I got out of their airspace.
Once I moved the ELT to the bottom of the fuselage (and away from the COM antennae), performance has been like every other plane I've ever flown in.


On Sunday, June 2, 2019, 7:48:30 PM EDT, <mike(at)vision499.com> wrote:

Just out of interest why should the ELT antenna be separated from the com antenna




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PostPosted: Mon Jun 03, 2019 7:58 am    Post subject: Antenna Ground Plane Reply with quote

Isn't the ELT turned off (not transmitting) all the time during flight?  If this is the case please explain why it would affect the Com performance.
Roger
Sent from my Verizon Wireless 4G LTE smartphone

-------- Original message --------
From: Ernest Christley <echristley(at)att.net>
Date: 06/03/2019 10:20 (GMT-05:00)
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Antenna Ground Plane
I can answer this with some real world experience.  I mounted my COM antennae just behind the canopy on my 601XL, and then I put the ELT about halfway between the COM and the rudder.  There was about 18 inches or so separation between each.
In there air, I could not reliably communicate with KRDU once I got out of their airspace.
Once I moved the ELT to the bottom of the fuselage (and away from the COM antennae), performance has been like every other plane I've ever flown in.


On Sunday, June 2, 2019, 7:48:30 PM EDT, <mike(at)vision499.com> wrote:

Just out of interest why should the ELT antenna be separated from the com antenna
 

 

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PostPosted: Mon Jun 03, 2019 8:16 am    Post subject: Antenna Ground Plane Reply with quote

If you ring a tuning fork of a certain pitch on one side of the room, a tuning fork of the same pitch will vibrate across the room. Both are as turned off as can be. I had a case where transmissions on 121.7 were bleeding onto 121.5. Moving the ELT antenna farther away fixed it. The story we told ourselves was that it must have been a resonance through the inactive ELT's circuit.

On June 3, 2019 12:04:40 Roger Curtis <rnjcurtis(at)charter.net> wrote:
Quote:
Isn't the ELT turned off (not transmitting) all the time during flight? If this is the case please explain why it would affect the Com performance.
Roger
Sent from my Verizon Wireless 4G LTE smartphone

-------- Original message --------
From: Ernest Christley <echristley(at)att.net>
Date: 06/03/2019 10:20 (GMT-05:00)
To: aeroelectric-list(at)matronics.com
Subject: Re: Antenna Ground Plane
I can answer this with some real world experience. I mounted my COM antennae just behind the canopy on my 601XL, and then I put the ELT about halfway between the COM and the rudder. There was about 18 inches or so separation between each.
In there air, I could not reliably communicate with KRDU once I got out of their airspace.
Once I moved the ELT to the bottom of the fuselage (and away from the COM antennae), performance has been like every other plane I've ever flown in.


On Sunday, June 2, 2019, 7:48:30 PM EDT, <mike(at)vision499.com> wrote:

Just out of interest why should the ELT antenna be separated from the com antenna




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PostPosted: Mon Jun 03, 2019 9:56 am    Post subject: Antenna Ground Plane Reply with quote

At 06:46 PM 6/2/2019, you wrote:
Quote:
Thank you very much for the help.

The aircraft is a KIS made from honeycomb and fiberglass

On the dipole antenna what length should the folded balun be and do I make the toroid balun with 3 toroids as in the instruction?

This antenna can't use a coax balun . . . it's a dual frequency
configuration that precludes using a resonant piece of coax
cable to improve feedline performance. Unfortunately
the highest frequency of operation (402Mhz) precludes
the use of broad-band magnetics.

Jim was fond of the string-of-beads approach to
reducing effects of mismatch on the feedline
but I researched that in the EMC lab at Beech
about 30 years ago . . . minimally effective.

The most effective decoupling technique using
ferrite beads or toroids was utilized in this
product a few years ago . . .

https://tinyurl.com/y4qpkqoc

Talked with this fellow by email and phone.
Made some questionable claims including
the assertion that the thing was 'patented'.
Never could find that patent . . . nonetheless,
here's what was inside the 'miracle box':

http://www.aeroelectric.com/articles/Antennas/AirWhip/AirWhip_Inside.jpg

This de-coupling technique wound several
turns on a common core. Inductance goes
up as the square of turns. In the photo
we count 7 turns or 49 times the inductance
of a same core with one turn. Jim's string-
of-beads gives 1x potential inductance/per/core
or perhaps 8-12x depending on how many cores
are strung onto the coax.

Played with this in the EMC lap at RAC/Beech
and determined that the string of beads wasn't
worth the trouble. So . . . how about the
airwhip technique for multiple turns on a
single core? Not a bad choice. I've seen
it done with antennas over a broad spectum
of frequencies . . . within limits.

Seems that ferrite inductive qualities
versus attenuation qualities swap duties
at about 200 Mhz or so . . . that's why
they are effective for EMC management
at vhf/uhf frequencies, not so much
as transformers at 402 Mhz . . . but still worth
investigating with the right test equipment.

I've got a uhf vector analyzer sitting
on the shelf above my desk (sigh) . . . someday
maybe . . .

Given that this antenna is intended to
operated on two, unchanging discrete
frequencies, one COULD consider fabricating
a passive components matching network to
transfer feedline energy to the two antennas
effectively . . . but I'd bet that the
performance differences between uncle Jim's
shade-tree engineering approach an one by
Dorne-Margolin would be observable only
in the lab and of little advantage in
practice.

In the mean time, fabricating uncle Jim's
paralleled dipoles arrangement is easy.
Attaching directly to this antenna with
coax is also easy.

[img]cid:.0[/img]


It seems that your airplane (epoxy/glass)
is a likely candidate for an all internal
ELT antenna which would, I suspect, be
less vulnerable to damage by unintended
arrivals with the earth. Best yet, no
ground plane . . .

Quote:

Just out of interest why should the ELT antenna be separated from the com antenna


Kelly offered the strongest rationale for
separation . . . ELT's are particularly
susceptible to cross-modulation products
because unlike receivers, there are
'high power', non-linear components continuously
connected to the antenna. I suspect that modern
ELTs with FET output stages might be
better in this regard but the risks are
not zero.

Having said that, there is zero risk for
damaging anything. I'd recommend you install
for convenience and see if you have problems.
The problems will be nothing worst than
an occasional nuisance with a high
probability that you'll not suffer
the effects at all.






Bob . . .


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PostPosted: Mon Jun 03, 2019 10:02 am    Post subject: Antenna Ground Plane Reply with quote

At 11:16 AM 6/3/2019, you wrote:

Quote:
If you ring a tuning fork of a certain pitch on one side of the room, a tuning fork of the same pitch will vibrate across the room. Both are as turned off as can be. I had a case where transmissions on 121.7 were bleeding onto 121.5. Moving the ELT antenna farther away fixed it. The story we told ourselves was that it must have been a resonance through the inactive ELT's circuit.

Close . . . big bears in the woods
are the solid state devices with matching
networks that feed their energies to the
antenna. In transceivers, a relay
disconnects that path except while
transmitting. In ELTs the pathway
is enduring.

One could just connect a diode across
the BNC connector at the base of the ELT
antenna and create a really whippy cross-mod
generator.

Take the final stage transistor out
of the ELT and the problem would go away . . .




Bob . . .


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PostPosted: Mon Jun 03, 2019 10:03 am    Post subject: Antenna Ground Plane Reply with quote

The ELT is "off", meaning that the circuits that produce and amplify a signal then apply it to the antennae are not working. It is "off" in the sense that the circuits that detect and amplify an incoming signal are not working. But, measuring from the antennae to ground, and you will notice that the resistance is not infinite. In fact, it will read a dead short if I'm not mistaken. The antennae is still "sensing" electromagnetic fluctuations and supplying them for reading. There's just nothing there to read them. The only way for the antennae to "sense" the fluctuations is for the fluctuations to do the work of producing a current in the antennae. When the ELT is off, that "work" just gets dumped as heat.

By placing the antennas very close, my COM was providing a VERY large signal to the ELT. The ELT could hear me clear as day. The ELT then proceded to convert that clear signal into heat. That heat came from my COM radio, which cut out a major part of the energy that was meant to propagate to ATC and other pilots.
It's worth noting that with the ELT antennae next to my COM, my ability to communicate with ATC was very much dependent on which direction I was flying. If I flew directly to or away from the airport, the signal was even weaker.


On Monday, June 3, 2019, 11:59:33 AM EDT, Roger Curtis <rnjcurtis(at)charter.net> wrote:




Isn't the ELT turned off (not transmitting) all the time during flight? If this is the case please explain why it would affect the Com performance.
Roger

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nuckolls.bob(at)aeroelect
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PostPosted: Mon Jun 03, 2019 10:04 am    Post subject: Antenna Ground Plane Reply with quote

At 09:20 AM 6/3/2019, you wrote:
Quote:
I can answer this with some real world experience. I mounted my COM antennae just behind the canopy on my 601XL, and then I put the ELT about halfway between the COM and the rudder. There was about 18 inches or so separation between each.

In there air, I could not reliably communicate with KRDU once I got out of their airspace.

Once I moved the ELT to the bottom of the fuselage (and away from the COM antennae), performance has been like every other plane I've ever flown in.

Interesting! That would have been
a fascinating installation to study
in the lab.


Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Jun 03, 2019 11:13 am    Post subject: Antenna Ground Plane Reply with quote

At 01:02 PM 6/3/2019, you wrote:
Quote:
The ELT is "off", meaning that the circuits that produce and amplify a signal then apply it to the antennae are not working. It is "off" in the sense that the circuits that detect and amplify an incoming signal are not working. But, measuring from the antennae to ground, and you will notice that the resistance is not infinite. In fact, it will read a dead short if I'm not mistaken. The antennae is still "sensing" electromagnetic fluctuations and supplying them for reading. There's just nothing there to read them. The only way for the antennae to "sense" the fluctuations is for the fluctuations to do the work of producing a current in the antennae. When the ELT is off, that "work" just gets dumped as heat.

By placing the antennas very close, my COM was providing a VERY large signal to the ELT. The ELT could hear me clear as day. The ELT then proceded to convert that clear signal into heat. That heat came from my COM radio, which cut out a major part of the energy that was meant to propagate to ATC and other pilots.

It's worth noting that with the ELT antennae next to my COM, my ability to communicate with ATC was very much dependent on which direction I was flying. If I flew directly to or away from the airport, the signal was even weaker.

Not sure I can grok the heat thing. While the
comm signal is 'strong', comm power measured
at the base of the ELT antenna cannot be more
than a tiny fraction of total comm output power.
But the 'lensing' effects of parasitic radiators
is another matter . . . distortion of the
comm radiation pattern would be my first guess.

Would be fun to 'sniff' it in the lab . . .


Bob . . .


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ceengland7(at)gmail.com
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PostPosted: Mon Jun 03, 2019 12:18 pm    Post subject: Antenna Ground Plane Reply with quote

On 6/3/2019 2:12 PM, Robert L. Nuckolls, III wrote:

Quote:
At 01:02 PM 6/3/2019, you wrote:
Quote:
The ELT is "off", meaning that the circuits that produce and amplify a signal then apply it to the antennae are not working.  It is "off" in the sense that the circuits that detect and amplify an incoming signal are not working.  But, measuring from the antennae to ground, and you will notice that the resistance is not infinite.  In fact, it will read a dead short if I'm not mistaken.  The antennae is still "sensing" electromagnetic fluctuations and supplying them for reading.  There's just nothing there to read them.  The only way for the antennae to "sense" the fluctuations is for the fluctuations to do the work of producing a current in the antennae.  When the ELT is off, that "work" just gets dumped as heat.

By placing the antennas very close,  my COM was providing a VERY large signal to the ELT.  The ELT could hear me clear as day.  The ELT then proceded to convert that clear signal into heat.  That heat came from my COM radio, which cut out a major part of the energy that was meant to propagate to ATC and other pilots.

It's worth noting that with the ELT antennae next to my COM, my ability to communicate with ATC was very much dependent on which direction I was flying.  If I flew directly to or away from the airport, the signal was even weaker.

 Not sure I can grok the heat thing. While the
 comm signal is 'strong', comm power measured
 at the base of the ELT antenna cannot be more
 than a tiny fraction of total comm output power.
 But the 'lensing' effects of parasitic radiators
 is another matter . . . distortion of the
 comm radiation pattern would be my first guess.

 Would be fun to 'sniff' it in the lab . . .


  Bob . . .
OK, show of hands: How many of you had to google 'grok'? (Brag mode on; I didn't... If you had to google it, don't stop there; read the book. It's *great*.)

I wonder if a better way of talking about the interference would be in SWR terms. Was the ELT antenna causing parasitic degradation/reflection of the comm transmission?

Charlie
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FlyingDiver



Joined: 28 Mar 2019
Posts: 2

PostPosted: Mon Jun 03, 2019 12:25 pm    Post subject: Antenna Ground Plane Reply with quote

Quote:
On Jun 3, 2019, at 4:17 PM, Charlie England <ceengland7(at)gmail.com (ceengland7(at)gmail.com)> wrote:
On 6/3/2019 2:12 PM, Robert L. Nuckolls, III wrote:
Quote:
At 01:02 PM 6/3/2019, you wrote:
Quote:
The ELT is "off", meaning that the circuits that produce and amplify a signal then apply it to the antennae are not working. It is "off" in the sense that the circuits that detect and amplify an incoming signal are not working. But, measuring from the antennae to ground, and you will notice that the resistance is not infinite. In fact, it will read a dead short if I'm not mistaken. The antennae is still "sensing" electromagnetic fluctuations and supplying them for reading. There's just nothing there to read them. The only way for the antennae to "sense" the fluctuations is for the fluctuations to do the work of producing a current in the antennae. When the ELT is off, that "work" just gets dumped as heat. By placing the antennas very close, my COM was providing a VERY large signal to the ELT. The ELT could hear me clear as day. The ELT then proceded to convert that clear signal into heat. That heat came from my COM radio, which cut out a major part of the energy that was meant to propagate to ATC and other pilots. It's worth noting that with the ELT antennae next to my COM, my ability to communicate with ATC was very much dependent on which direction I was flying. If I flew directly to or away from the airport, the signal was even weaker.
Not sure I can grok the heat thing. While the comm signal is 'strong', comm power measured at the base of the ELT antenna cannot be more than a tiny fraction of total comm output power. But the 'lensing' effects of parasitic radiators is another matter . . . distortion of the comm radiation pattern would be my first guess. Would be fun to 'sniff' it in the lab . . .
Bob . . .
OK, show of hands: How many of you had to google 'grok'? (Brag mode on; I didn't... If you had to google it, don't stop there; read the book. It's *great*.)


Nope, been in my vocabulary since my teens. Was my very first password on a multi-user computer in college.

joe


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Kellym



Joined: 10 Jan 2006
Posts: 1700
Location: Sun Lakes AZ

PostPosted: Mon Jun 03, 2019 12:27 pm    Post subject: Antenna Ground Plane Reply with quote

While I don't doubt that this configuration contributed to the problem,
a much more common situation is when the aircraft is near a high powered
facility broadcasting on VHF. For instance, it is common for TV and FM
broadcast towers to be located near each other, especially if there is
convenient high ground. They can put out thousands of watts. They will
cause the ELT output stage to oscillate and re-radiate to nearby com
antennas. I experienced this in Phoenix, where 90% of broadcast antennas
are on "South Mountain", which is 7-8 miles south of Phoenix Sky Harbor
Airport. When flying the VFR transition over the top of PHX, I would get
terrible squelch break on both com radios, making communications with
the Tracon difficult on both frequencies they used (120.7 and 123.7). I
verified this by flying under the class B on the south side of the
mountain, with and without the ELT antenna connected. Without ELT
connected, no problem. I then removed one of my 2 com antennas from the
topside of the fuselage to the belly. Problem was 95% eliminated.

On 6/3/2019 12:12 PM, Robert L. Nuckolls, III wrote:
Quote:
At 01:02 PM 6/3/2019, you wrote:
> The ELT is "off", meaning that the circuits that produce and amplify a
> signal then apply it to the antennae are not working.  It is "off" in
> the sense that the circuits that detect and amplify an incoming signal
> are not working.  But, measuring from the antennae to ground, and you
> will notice that the resistance is not infinite.  In fact, it will
> read a dead short if I'm not mistaken.  The antennae is still
> "sensing" electromagnetic fluctuations and supplying them for
> reading.  There's just nothing there to read them.  The only way for
> the antennae to "sense" the fluctuations is for the fluctuations to do
> the work of producing a current in the antennae. When the ELT is off,
> that "work" just gets dumped as heat.
>
> By placing the antennas very close,  my COM was providing a VERY large
> signal to the ELT.  The ELT could hear me clear as day. The ELT then
> proceded to convert that clear signal into heat.  That heat came from
> my COM radio, which cut out a major part of the energy that was meant
> to propagate to ATC and other pilots.
>
> It's worth noting that with the ELT antennae next to my COM, my
> ability to communicate with ATC was very much dependent on which
> direction I was flying.  If I flew directly to or away from the
> airport, the signal was even weaker.

 Not sure I can grok the heat thing. While the
 comm signal is 'strong', comm power measured
 at the base of the ELT antenna cannot be more
 than a tiny fraction of total comm output power.
 But the 'lensing' effects of parasitic radiators
 is another matter . . . distortion of the
 comm radiation pattern would be my first guess.

 Would be fun to 'sniff' it in the lab . . .

  Bob . . .



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