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Antenna Ground Plane
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ceengland7(at)gmail.com
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PostPosted: Mon Jun 03, 2019 1:08 pm    Post subject: Antenna Ground Plane Reply with quote

On 6/3/2019 3:25 PM, Joe Keenan wrote:

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


Great book, right?
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PostPosted: Mon Jun 03, 2019 1:39 pm    Post subject: Antenna Ground Plane Reply with quote

Quote:
>
Great book, right?

He was my go-to author growing up. And I’ve named characters in many computer games after his characters. My two original World of Warcraft characters are Mycroft and Wyoming.


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PostPosted: Mon Jun 03, 2019 3:08 pm    Post subject: Antenna Ground Plane Reply with quote

At 03:25 PM 6/3/2019, you wrote:
Quote:
--> AeroElectric-List message posted by: Kelly McMullen <kellym(at)aviating.com>

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.

I'm certain that this is a classic demonstration
of inter-modulation of multiple strong
signals that mix and remix in a cacophony of
of new signals all of which carry vestiges of
the original modulation. Got some real hard
lessons in this phenomenon back in my two-way
radio days when EVERYBODY wanted to put their
company's repeater on top of the tallest buildings
in the city.

Some building owners would hold roof-leases
in their hip-pocket and dole them out to all
comers irrespective of proposed operating
frequencies and equipment. I was anointed with several calls
from a new client who complained that a
brand new $kilo$ repeater we had just installed
became unusable at certain times of the day.

Of course, it was our fault . . . in fact it
was the uncoordinated exploitation of premium
rooftop real estate. That rooftop radio farm
could have as many as ten, 50-100 watt uhf
and vhf transmitters talking simultaneously
into antennas with 6 to 9 db of gain.

In one case, an intermod problem had nothing
to do with the local electronics . . . was
traced to corroded joints in a very old
Decibel Products antenna that wasn't even
in service! It was sitting up there with nobody
knowing that the associated radio had been off
the roof for years . . . hence the antenna's
mechanical condition deteriorated to the point
of becoming a nuisance neighbor.

Those problems were materially eliminated
by leasing such juicy spots to a single,
talented re-leasing company that coordinated
all suitable tenants by conducting a potential
intermod products study . . . by hand . . .
no whippy desktop computers back then.

Of course, flying past an antenna farm of
television and FM broadcast stations
can wreak havoc in a lowly vhf comm radio,
a condition you can fly out of in a matter
of seconds.









Bob . . .


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PostPosted: Mon Jun 03, 2019 3:09 pm    Post subject: Antenna Ground Plane Reply with quote

Quote:
Quote:
Great book, right?

Arguably his best



Bob . . .


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PostPosted: Mon Jun 03, 2019 7:32 pm    Post subject: Antenna Ground Plane Reply with quote

Quote:
Of course, it was our fault . . . in fact it
was the uncoordinated exploitation of premium
rooftop real estate. That rooftop radio farm
could have as many as ten, 50-100 watt uhf
and vhf transmitters talking simultaneously
into antennas with 6 to 9 db of gain.

Just for grins, snatched a recent photo of
that building

[img]cid:7.1.0.9.0.20190603204853.05ddeb88(at)aeroelectric.com.0[/img]

It's still got an aluminum whisker-farm
growing out the top . . . brings back
memories. Last time I was up there it was
over 100F and no breeze . . .



Bob . . .


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PostPosted: Mon Jun 03, 2019 7:32 pm    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?

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

Thanks again

The antenna is two dipoles . . . one centered on
121.5 MHz, the other on 402 MHz.



Bob . . .z


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mike(at)vision499.com
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PostPosted: Mon Jun 03, 2019 9:31 pm    Post subject: Antenna Ground Plane Reply with quote

The antenna is two dipoles . . . one centered on
121.5 MHz, the other on 402 MHz.


You have lost me here, I do not understand two dipoles concept. Please explain in more detail if you don’t mind

Thanks

Mike
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PostPosted: Tue Jun 04, 2019 4:58 am    Post subject: Antenna Ground Plane Reply with quote

Think I just figured it out, it is the one with 2 V shapes. One leg 7.3 inches and one 22.5 as per

www.matronics.com/forums/viewtopic.php?t=95222&highlight=&sid=23bc0da12eeb24ba261ca5ac45ec04b3


Thanks again for the help

Mike


The antenna is two dipoles . . . one centered on
121.5 MHz, the other on 402 MHz.

You have lost me here, I do not understand two dipoles concept. Please explain in more detail if you don’t mind

Thanks

Mike


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PostPosted: Tue Jun 04, 2019 8:00 am    Post subject: Antenna Ground Plane Reply with quote

At 12:57 AM 6/4/2019, you wrote:
Quote:
I am working on an antenna for a homebuilt and I read from the AEROLECTRIC column in KITPLANES about using ferrite toroid’s for a balun arrangement. However, I seem to remember that the material used in the toroids must be of a certain mixture to get the desired effect for RF signals. Can anyone tell me which toroids to use and which ones not to use. Apparently some work for RF and some work better for audio frequencies.


I included Jim Weir's string-of-beads concept in the 'Connection
many moons ago . . . the idea was intuitively logical but physically flawed.
That fact was not discovered until a years later when I was
working some EMC issues at RAC/Beech and gained access to the
EMC lab.

I did some testing of Jim's string-of-beads design and
found that the differences to be barely perceptible
for beads-on versus beads-off. As mentioned in an
earlier post, a single toroid with multiple passes
through the core is many times more effective . . .
however . . .

It is true that the material from which the toroid is molded
should be optimized for the frequency of interest . . . in
the case of the glideslope antenna illustrated in the
'Connection, the toroids should demonstrate good performance
at 300+ Mhz. Looking through the various manufacturer's
catalogs for toroids, we find very few recommended for
service above 100Mhz with most at 30Mhz and below.

The antenna chapter needs some work and the string-of-
beads idea is going to be removed.

What kind of antenna are you contemplating a
need for a balun?



Bob . . .


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PostPosted: Tue Jun 04, 2019 8:01 am    Post subject: Antenna Ground Plane Reply with quote

At 07:56 AM 6/4/2019, you wrote:
Quote:


Think I just figured it out, it is the one
with 2 V shapes. One leg 7.3 inches and one 22.5 as per . . .

You got it . . .



Bob . . .


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PostPosted: Tue Jun 04, 2019 9:04 am    Post subject: Antenna Ground Plane Reply with quote

I did some testing of Jim's string-of-beads design and
found that the differences to be barely perceptible
for beads-on versus beads-off. As mentioned in an
earlier post, a single toroid with multiple passes
through the core is many times more effective . . .
however . . .

Did some digging in the archives and I think
these (or very similar) cores were used in Jim's
kit:

https://tinyurl.com/y2laaf78

This is Type 52 material which offers pretty
good performance up to 300Mhz . . . but
it still takes a LOT of cores for an effective
implementation of string-of-beads.

A multi-pass common mode choke could be
implemented with a small diameter
feedline (like RG174) through the
center of this one

https://tinyurl.com/y58fqdlh

This might be suited for de-coupling
the feedline on the dual-dipole ELT
antenna.




Bob . . .


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PostPosted: Wed Jun 05, 2019 7:42 am    Post subject: Antenna Ground Plane Reply with quote

At 12:10 AM 6/4/2019, you wrote:
Quote:
Bob

Thank you very much for the detailed explanation, I’m not sure that I understand it all, took a Ham radio course a while back so will go back to my notes.

If I understand you correctly I must make Jim’s dipole antenna as per instruction and attach the core of the coax to one “leg” and the braid to the other “leg” without a balun or toroid.

I found the remains of the copper tape that I got (I think) from Jim in 1998, will sen a photo before I install.

I believe that will do the job and you
don't need to put any holes in the airplane.

The 'inside' antenna does not negate concerns
for potential intermod situations discussed in
this thread. Those problems are not common.
They are transient and circumstance based.
The risks are low.



Bob . . .


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PostPosted: Wed Jun 05, 2019 8:38 am    Post subject: Antenna Ground Plane Reply with quote

Quote:
I wrote: 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.

Not necessarily true. I've ordered some
-52 cores and will do some measurements
on the bench. I may have a multi-pass
toroidal balun to offer to your task.



Bob . . .


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mike(at)vision499.com
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PostPosted: Thu Jun 06, 2019 7:28 am    Post subject: Antenna Ground Plane Reply with quote

Thank you very much, I look forward to seeing your test results

Keep well

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 5, 2019 8:45 AM
To: aeroelectric-list(at)matronics.com
Subject: RE: Antenna Ground Plane
Quote:

I wrote: 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.


Not necessarily true. I've ordered some
-52 cores and will do some measurements
on the bench. I may have a multi-pass
  toroidal balun to offer to your task.


Bob . . .
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Demixl
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PostPosted: Wed Jun 26, 2019 10:01 pm    Post subject: Re: Antenna Ground Plane Reply with quote

Thank you for sharing guys it really helps me

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