Today's Message Index:
----------------------
1. 07:41 AM - Re: Servo problem (Eric M. Jones)
2. 09:38 AM - Dipole antenna fabrication (Robert L. Nuckolls, III)
3. 10:46 AM - Re: Dipole antenna fabrication (Eric M. Jones)
4. 10:57 AM - Re: Re: Dipole antenna fabrication (Dj Merrill)
5. 07:14 PM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III)
Message 1
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Subject: | Re: Servo problem |
Etienne drew a T-antenna on his discussion of the servo problem. This incluced
stripping back the insulation on a coax, separating the shield and core, and stretching
out each to form a Tee.
Clever. Actually too-clever-by-half. Would that it could be so easy! The problem
lies in the termination, since the core and shield do not share the signal.
In fact neither the core nor the shield even CARRY the signal.
See: periheliondesign.com/downloads/Dabbling%20with%20electricity.pdf
Then grab your SWR meter and Google any of the ham radio sites and see how to design
a T-antenna.
Might it work the way Etienne shows? A little probably, but let's do it right.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones@charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=367126#367126
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Subject: | Dipole antenna fabrication |
At 09:39 AM 2/24/2012, you wrote:
Etienne drew a T-antenna on his discussion of the servo problem. This
incluced stripping back the insulation on a coax, separating the
shield and core, and stretching out each to form a Tee.
Clever. Actually too-clever-by-half. Would that it could be so easy!
The problem lies in the termination, since the core and shield do not
share the signal. In fact neither the core nor the shield even CARRY
the signal.
See: periheliondesign.com/downloads/Dabbling%20with%20electricity.pdf
Then grab your SWR meter and Google any of the ham radio sites and
see how to design a T-antenna.
Might it work the way Etienne shows? A little probably, but let's do it right.
Can you cite a website specific to the
fabrication of VHF dipoles fed with
coax?
Bob . . .
Message 3
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Subject: | Re: Dipole antenna fabrication |
Bob,
I see that there are thousands of sites when you Google them. But here a general
purpose one that will get one started and discusses the issues (sorry for all
the equations...):
http://en.wikipedia.org/wiki/Dipole_antenna
But Bob, in your vast experience, have you seen many dipoles per Etienne's drawing?
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones@charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=367147#367147
Message 4
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Subject: | Re: Dipole antenna fabrication |
On 2/24/2012 1:44 PM, Eric M. Jones wrote:
> http://en.wikipedia.org/wiki/Dipole_antenna
>
> But Bob, in your vast experience, have you seen many dipoles per Etienne's drawing?
>
>
I'm not Bob, but his diagram looks like the one at the top right of
the page on that link you sent, and looks normal to me. I've made many
of these types of antennas for ham equipment when a quick-n-dirty setup
was needed. In fact, I just made one a couple of weeks ago for an APRS
setup. They tend to work okay. Not great, just okay. Much better than
the flexible antennas that come on a handheld radio.
What specifically are you seeing wrong with it?
-Dj
--
Dj Merrill - N1JOV
Sportsman 2+2 Builder #7118 N421DJ - http://deej.net/sportsman/
Glastar Flyer N866RH - http://deej.net/glastar/
Message 5
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Subject: | Re: Dipole antenna fabrication |
At 12:44 PM 2/24/2012, you wrote:
Bob,
I see that there are thousands of sites when you Google them. But
here a general purpose one that will get one started and discusses
the issues (sorry for all the equations...):
http://en.wikipedia.org/wiki/Dipole_antenna
But Bob, in your vast experience, have you seen many dipoles per
Etienne's drawing?
Oh yeah . . . tens of thousands. For many years during
the vacuum tube days of aviation radio, tens of thousands
of single engine Cessna's went out the door with VOR whiskers
in the vertical fin cap that were nothing more sophisticated
than Etienne's coax fed dipole.
Granted, the whiskers were stuck out in the breeze and
assembled to the end of the coax but electrically, no
different. I think we had a brief romance with adding
BALUNS to these antennas. But without spending money
to do really scientific tests on the antenna range,
we generally relied on the anecdotal observations of
production and flight test pilots. Cessna was putting
10,000 airplanes a year out the door on 5 production
lines.
Some years later, Jim Weir suggested adding ferrite
toroidal cores the coax right at the junction with
the antenna. The idea was to reduce coax radiation
depicted in the Wikipedia article
Emacs!
. . . radiation caused by less than ideal termination
of the feedline. I tested this work-around in the EMC
lab at HBC about 15 years ago and found that an array
of 10 cores had no appreciable effect on these stray
shield currents.
Years later I suffered this "hot-d@#n" moment. I saw
an antenna product wherein the string of ferrites was
replace with a single ferrite core. The coax was
wound through it several times.
Emacs!
But of course! Inductance in a single magnetic circuit
varies with the SQUARE of turns. The picture seen here
has 6 passes of the coax through the core. It is the
electrical equivalent of 36 single cores. Now, having
'discovered' that, it would still be interesting to
go to the lab and see how effective it is.
Had we possessed the knowledge and materials to
duplicate this technique on a 1967 Cessna 172, I'll
bet the pilots would have continued to report "no
observable difference" in hearing the VORs.
Of course, transmit is a different story. Energy
levels are perhaps 130db higher (10 trillion
stronger). The effects of poorly terminated coax
don't have much to do with transmit/receive
performance but MIGHT have an impact on how
much energy is coupled into unwanted systems
radiated from coax feed lines bundled in with
other wires.
But then, the potential for interfering signals
radiated DIRECTLY from the antenna is already
very high on a small airplane. If it's a plastic
airplane, it's worse yet. And those 'el cheeso
vertical dipoles tend to be really popular for
installation in vertical fins of plastic airplanes.
Admittedly, these antennas are not the best we
know how to do . . . but they are easy to implement.
They pose some risks for EMC issues. But the
relative success with these installations suggests
that the ship's 'other' appliances are reasonably
designed immunity to strong external fields.
The genesis thread discovered that making the servo
more robust to the strong RFI environment was
a good solution. But given that it's a plastic
airplane, it's not clear that refining the
antenna's feed point design would have made any
difference at all.
Bob . . .
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