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nuckollsr(at)cox.net
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 Posted: Fri Jun 15, 2007 5:24 am Post subject: MB antennnas |
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I poked around in the catalogs for some of my suppliers
and found some pictures to illustrate the text below.
The "sled runner" antenna is still sold as a spare for about
$250. You can see a picture of it at:
http://www.aeroelectric.com/Pictures/Antenna/AV-533.jpg
There's a low profile device offered by the same source
http://www.aeroelectric.com/Pictures/Antenna/DM_N27-2.jpg
this puppy is about $550 dealer net. If anyone is out at
an airport and sees a sled runner installation that they
can measure. We can publish the dimensions for folks to
exploit in building their own. Need OVERALL length of
antenna element from skin surface to trailing tip. Need
height of antenna off the skin. Need position of feedline
tap measured along element length (skin to tap location).
I could eyeball it and probably produce a perfectly good
working antenna but it would be handy to have some dimensions
off a production article.
Bob . . .
| Quote: | >Would that 40" piece of wire work outside a metal airplane if the conductor
>was insulated from the skin?
>
>I'm thinking of a wire or tape laminated to the belly between layers
>fiberglass.
>
>For that matter, could you laminate an Archer antenna to the bottom of a
>metal plane and expect it to work?
>
>If no, why not?
There needs to be some separation. A conductor laying right
next to a ground plane looks more like a transmission line
with high SWR than an antenna. The optimal belly mounted
marker beacon antenna for metal airplanes can be seen on
the 60-80's Cessnas. It's been affectionately dubbed "the
sled runner".
The more elegant antennas for metal airplanes are like
these from A/S.
http://www.aircraftspruce.com/catalog/avpages/comt_markerantennas2.php
One of my cohorts at Cessna was playing with a flush antenna
design. It had a slot in the belly skin about 2" wide and
extending between tail cone formers. The opening was closed
off outside with fiberglass and inside with a hat-section.
A tuned radiator was supported centrally within the hat-sections
profile.
I seem to recall it worked well but was pretty expensive
to build compared to the "sled runner" so was never incorporated
into our products.
Bob . . .
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john(at)morgensen.com
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| Posted: Fri Jun 15, 2007 11:23 am Post subject: MB antennnas |
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I have a "sled runner" on the bottom of my "RV Trainer" aka Grumman AA1B
that is no longer connected to anything. I am about to remove it. Does
anyone want it?
John Morgensen
RV-9A
Robert L. Nuckolls, III wrote:
| Quote: |
<nuckollsr(at)cox.net>
I poked around in the catalogs for some of my suppliers
and found some pictures to illustrate the text below.
The "sled runner" antenna is still sold as a spare for about
$250. You can see a picture of it at:
http://www.aeroelectric.com/Pictures/Antenna/AV-533.jpg
There's a low profile device offered by the same source
http://www.aeroelectric.com/Pictures/Antenna/DM_N27-2.jpg
this puppy is about $550 dealer net. If anyone is out at
an airport and sees a sled runner installation that they
can measure. We can publish the dimensions for folks to
exploit in building their own. Need OVERALL length of
antenna element from skin surface to trailing tip. Need
height of antenna off the skin. Need position of feedline
tap measured along element length (skin to tap location).
I could eyeball it and probably produce a perfectly good
working antenna but it would be handy to have some dimensions
off a production article.
Bob . . .
>> Would that 40" piece of wire work outside a metal airplane if the
>> conductor
>> was insulated from the skin?
>>
>> I'm thinking of a wire or tape laminated to the belly between layers
>> fiberglass.
>>
>> For that matter, could you laminate an Archer antenna to the bottom of a
>> metal plane and expect it to work?
>>
>> If no, why not?
>
> There needs to be some separation. A conductor laying right
> next to a ground plane looks more like a transmission line
> with high SWR than an antenna. The optimal belly mounted
> marker beacon antenna for metal airplanes can be seen on
> the 60-80's Cessnas. It's been affectionately dubbed "the
> sled runner".
>
> The more elegant antennas for metal airplanes are like
> these from A/S.
>
> http://www.aircraftspruce.com/catalog/avpages/comt_markerantennas2.php
>
> One of my cohorts at Cessna was playing with a flush antenna
> design. It had a slot in the belly skin about 2" wide and
> extending between tail cone formers. The opening was closed
> off outside with fiberglass and inside with a hat-section.
> A tuned radiator was supported centrally within the hat-sections
> profile.
>
> I seem to recall it worked well but was pretty expensive
> to build compared to the "sled runner" so was never incorporated
> into our products.
>
> Bob . . .
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longg(at)pjm.com
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| Posted: Fri Jun 15, 2007 5:36 pm Post subject: MB antennnas |
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Thanks Bob,
Not only is that $$$, it looks like it will cost you about 3 knots. I'll
take it upon myself to write down the dimensions and post next time I
see one. Looks darn simple. With the Lancair Legacy I am trying to keep
all of the antennas indoors.
Glenn
--
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nuckollsr(at)cox.net
Guest
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| Posted: Sun Jun 17, 2007 5:41 am Post subject: MB antennnas |
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At 09:35 PM 6/15/2007 -0400, you wrote:
| Quote: |
Thanks Bob,
Not only is that $$$, it looks like it will cost you about 3 knots. I'll
take it upon myself to write down the dimensions and post next time I
see one. Looks darn simple. With the Lancair Legacy I am trying to keep
all of the antennas indoors.
Glenn
|
Hmmmm . . . during the course of this thread I've
been remiss in not pointing out that the inside antennas
work only in fiberglas composite structures . . . the carbon
fiber airplanes offer too much attenuation.
Bob . . .
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nuckollsr(at)cox.net
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| Posted: Sun Jun 17, 2007 5:55 am Post subject: MB antennnas |
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At 12:21 PM 6/15/2007 -0700, you wrote:
| Quote: |
I have a "sled runner" on the bottom of my "RV Trainer" aka Grumman AA1B
that is no longer connected to anything. I am about to remove it. Does
anyone want it?
|
I don't need the antenna . . . but if you could
get me the dimensions I asked about earlier, it
would be helpful.
Bob . . .
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Sun Jun 17, 2007 7:13 am Post subject: MB antennnas |
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Hi Bob;
Not trying to be a "smart ass" but in the light of accuracy, not ONLY fibreglass, but also wood structures such as the F8L Falco where internal antennas are used to good advantage. (and possibly other non-conductive constructions as well).
Bob McC
[b]---
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_________________
Bob McC
Falco #908
(just starting) |
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nuckollsr(at)cox.net
Guest
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| Posted: Sun Jun 17, 2007 8:40 am Post subject: MB antennnas |
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At 11:08 AM 6/17/2007 -0400, you wrote:
| Quote: | Hi Bob;
Not trying to be a "smart ass" but in the light of accuracy, not ONLY
fibreglass, but also wood structures such as the F8L Falco where internal
antennas are used to good advantage. (and possibly other non-conductive
constructions as well).
Bob McC
|
Point well taken. Let's refine the idea as limited to
non-conductive composites. Some folks have even reported
"satisfactory" performance from antennas contained within
a tube and fabric tailcone. I'm a bit skeptical as to the
quantification of "satisfactory" . . .
Bottom line is try it an see if it meets your needs knowing
that carbon fiber composites are have yielded unsatisfactory
results in a number of repeated experiments.
Bob . . .
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john(at)morgensen.com
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| Posted: Wed Jun 20, 2007 11:44 am Post subject: MB antennnas |
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Robert L. Nuckolls, III wrote:
| Quote: | I don't need the antenna . . . but if you could
get me the dimensions I asked about earlier, it
would be helpful.
Bob . . .
|
Down 6" from the belly and then 88" long. In addition, there is a copper
wire coil about 6" long in the middle of the antenna. Hope this helps.
John Morgensen
RV-9A
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nuckollsr(at)cox.net
Guest
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| Posted: Thu Jun 21, 2007 7:44 am Post subject: MB antennnas |
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At 12:42 PM 6/20/2007 -0700, you wrote:
| Quote: |
Robert L. Nuckolls, III wrote:
> I don't need the antenna . . . but if you could
> get me the dimensions I asked about earlier, it
> would be helpful.
> Bob . . .
Down 6" from the belly and then 88" long. In addition, there is a copper
wire coil about 6" long in the middle of the antenna. Hope this helps.
|
Interesting!!! Let's dissect these figures in the light
of what we know about antenna design and performance.
A full wavelength at 75 Mhz is 300/75 = 4 meters.
A quarter wave antenna would therefore be 1 meter
or 39.36 inches. The sled runner I illustrated at
http://www.aeroelectric.com/Pictures/Antenna/AV-533.jpg
the far end bolts to the airframe and is grounded to the
skin at that point. The antenna is supported off the
skin by some distance set by the design of the bend
at the the "ground" end and duplicated by a non-conducting
support post . The feedline for routing antenna energy
to the receiver is tapped onto the antenna some distance
away from the ground end. In antenna parlance, this is
called a "gamma match". The idea is that we know the
impedance of the system at the ground end of the antenna is
zero ohm. On the free end, it will approach mucho-ohms.
Someplace between mucho and zero, we'll find a point that
is very close to 50 ohms.
Hence, you see the feedline tap fastened to the antenna
rod at some distance nearer the ground end but optimized
during design and testing to maximize the energy
transfer between antenna and feedline.
Now, what about this 88" figure? This is longer than
1/2 wavelength. Further, he does not speak to a feedpoint
attachment.
John, do you know that you have ALL of the antenna as-
removed? Is there any evidence that a feedpoint tap was
ever installed some distance from the forward end of
the antenna? Is the mounting hardware still present
for the forward end? Is it all metal . . . or does the
collection of components offer an insulated penetration
of the belly skin?
Pending John's future findings and based on what we
know so far, it's my best guess that this antenna feeds
from what is usually the "ground end" of the sled-runner
illustrated above. Further, we know that antennas offer
a useable, relatively low impedance feed-point at the ground
end that repeats at odd quarter-wave intervals for overall
length. For example, a ham's 1/4-wave vertical antenna
optimized for operation at 7.3 MHz offers some opportunity
for useful performance on the 21 MHz band where
the antenna is nominally 3x 1/4-wave.
The antenna John is describing to us may well be a
nominally "too long" antenna for 1/4-wave performance
and "too short" for 3/4-wave service. However, by
inserting a small inductance at some point along the
antenna's length, the system can be electrically
"loaded" to appear as if it were really 3/4-wave
in length when in fact it is much shorter.
Amateur radio mobile antennas operate over a range
of 3.0 to 30 MHz which would require physical lengths
of 25 to 2.5 meters for 1/4 wave performance. Typically
these antennas are 3 meters or so in length and fitted
in an adjustable "loading coil" for the purpose of optimizing
the antenna's performance at any frequency of interest.
My guess is that the designer intended this antenna
to appear as if it were a 3/4-wave antenna with what
I would suppose was an intent to improve performance
as a receptor of energy from the ground based marker
transmitter. This is a bit mystifying for modern systems
because the markers used during approach are quite strong
and your distance above them is in hundreds of feet.
Now, it may be that this antenna was designed for
installation on a larger airplane and used for IFR
navigation back in the 30s and 40s when the airways
were electronically marked laterally with low frequency
A-N radio range stations and your location along any given
airway was made known by the occasional electronic
milestone called an airway marker.
I've not found a description of the US versions of
the visual/aural range system that used low frequency
ground stations (150-500 Khz) however, in the 50's
when we were phasing out our LF VAR systems for
VHF VOR systems, the Australians were adapting
VHF Localizer and ILS technologies to their own
version of the 4-course VARS but with the advantage
of being able to fly a needle on the panel instead
of having to listen to the aural signature of the
AN-range station signal through static crashes of
hopefully not to near thunderstorms.
Both the US and AUS 4-course range systems used 75
MHz marker transmitters for locating one's position
along a route. See page 6 of . .
http://www.aeroelectric.com/Reference_Docs/RadioNavigation/VAR_Marker-Beacon_Notes.pdf
I believer the US airways marker used to operated the white
indicator on the suite of marker lamps and the morse
code tone was unique to the marker so you knew which
transmitter was talking to your receiver. This functionality
was replaced by "inner marker" and "fan markers"
on the ILS to some approaches. See:
http://en.wikipedia.org/wiki/Marker_beacon
Back then receiver performance limitations could be
enhanced by an extraordinarily large antenna so that
the crew would get reliable indication of having
passed a marker that was thousands of feet below
and perhaps a mile or two off to one side or the
other.
The US 4-course, AN Ranges stations and the Australian
VAR systems were replaced by early cousins to the
VHF Visual Omni Range stations that are still in
use today. See
http://en.wikipedia.org/wiki/VHF_omnidirectional_range
The need for receiving airway markers faded away
with the advent of VOR and DME. We may never know
for sure but it seems likely that John is in
possession of a relic of our radio-navigation yesteryear.
Bob . . .
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