Today's Message Index:
----------------------
1. 03:09 AM - Re: Re: Dipole antenna fabrication (Jan de Jong)
2. 03:13 AM - Re: Re: Dipole antenna fabrication (Jan de Jong)
3. 07:55 AM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III)
4. 09:38 AM - Re: Re: Dipole antenna fabrication (Jan de Jong)
5. 02:26 PM - Re: Re: Dipole antenna fabrication (Noel Loveys)
6. 02:28 PM - Re: Re: Dipole antenna fabrication (Noel Loveys)
7. 03:17 PM - Re: Re: Dipole antenna fabrication (Noel Loveys)
8. 08:39 PM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III)
Message 1
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Subject: | Re: Dipole antenna fabrication |
Bob,
When you're at comparing I would be interested in the effectiveness of a
Pawsey stub balun as it is so easy to make from scrap coax.
The following antenna gets good results using such, apparently:
http://chrusion.com/BJ7/InvVeeAntenna4ULs.pdf
I also wonder about the effectiveness of a coax air coil in the feedline
(people use 10 turns of 2 or 3 " diameter or so). Also easy to make.
As I understand it the Pawsey stub works by compensating, the air coil
(and ferrites) by choking.
Jan de Jong
Robert L. Nuckolls, III wrote:
> <nuckolls.bob@aeroelectric.com>
>
> At 11:41 AM 2/26/2012, you wrote:
> <emjones@charter.net>
>
> The Wiki site shows a simple dipole and the caption says:
>
> "A schematic of a half-wave dipole antenna connected to an unbalanced
> coaxial cable. Better practice is to connect the balanced dipole to
> the unbalanced line with a balun."
>
> . . . and I published instructions for adding this
> feature to a coax fed dipole at
>
> http://aeroelectric.com/articles/BALUN/Balun_Fabrication.html
>
> . . . the same technique could be applied to a comm
> dipole.
>
> I've been thinking of putting aluminum foil on the TV rabbit ears....
>
> Which will lower the resonant frequency of the antenna
> and perhaps lower the Q . . . wider bandwidth. But
> no benefit for relative efficiency compared to an "ideal"
> dipole at the same frequency.
>
>
> Proper termination is the key to antenna efficiency. If Cessna didn't
> do it, that doesn't mean the homebuilder shouldn't do it right.
>
> I can assure you that Dr. Wood read the same books
> we've all read about antennas with professional
> attendance to "better practice." The point I endeavored to
> make was that after he came to understand all the
> deleterious effects over which we had no control on
> a C-172, adding baluns on the VOR antennas yielded very
> small gains compared the sum of other losses.
>
> Next time I can get into the test lab, I'll do some
> measurements on the difference between a balun-fed
> dipole and a bare foot, coax-fed dipole.
>
> Bob et al. having flown extensively in the Western states and Mexico,
> I have plently of experience with being a zillion miles from a
> station. At those times I start to think that having a good radio and
> good antennas is a wise and cost effective thing to do. But do what
> you want.
>
> But you paint a picture suggesting that a balun would
> make the difference between communicating with some distant
> station . . . and not. Given the light of sight character
> of VHF+ frequencies, you're more likely not to have a useable
> path because you can't see the other station than because
> your effective radiated signal is down by a few db due
> to violations of feed point protocols. Further, it's
> insufficient to be able to see the other station
> peeking over the horizon (or mountaintop). The effects
> of intervening "roughness" in the "Fresnel zone" can
> have some profound effects on what might otherwise be
> considered a free-space, line of sight situation.
>
> http://web.arundale.co.uk/docs/ais/PathlossCalculationsforAmateurs.pdf
>
> Mountain peaks and valleys intruding into the Fresnel
> zone can kick your free space performance predictions
> in the teeth.
>
> Currents flowing on the coax shield are but part of
> the 'violations' . . . the center impedance of a perfect
> dipole is on the order of 73 ohms. This means that the
> BEST SWR to be achieved without application of some
> impedance matching is on the order of 73/50 = 1.46:1
> So we've tossed off some efficiencies right out of the
> box. When we plot radiation patterns on VHF antennas
> on airplanes, one generally expects 10 db or greater
> 'lumpiness' in the pattern due to geometry of the
> conductive elements around the antenna. So again,
> if one is at 'extreme range' for useful communications,
> it's more likely that you'll improve the path by turning
> the airplane a few degrees as opposed to optimizing
> feed point design.
>
> Yes, a BALUN can be "better" but experience on small
> aircraft has shown that the difference is so small
> compared to all other effects combined that the balun
> was not 'cost effective'. Further, it won't fix the
> fundamental mis-match so 1.46:1 is still the best
> we can expect. The textbooks are full of examples of
> data taken from the idealized antenna and feed line
> examined in the anechoic chamber with test equipment
> that can split a DB into small pieces.
>
> As soon as you put that same antenna is a real world
> environment, lots of things with potential for
> altering performance happen. This is why
> the guy flying 'around with a radio capable of
> talking to another one just like it 1000 miles
> away finds that the practical limits are much
> shorter for reasons mostly beyond control of
> the antenna designer.
>
> But just for grins, I've got some DIY antenna's I've
> used in my weekend seminars and articles. I'll
> fit one with a balun and leave the other barefoot.
> Then see if my buddy Don P can tell the
> difference without looking to see which one is
> hooked up. Hmmmm . . . I think I know how to
> do the experiment with equipment I have. Need
> warmer weather though . . .
>
> Bob . . .
>
>
Message 2
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Subject: | Re: Dipole antenna fabrication |
Bob,
Sorry about this. The balun you describe making IS a Pawsey stub -
should have taken a look before responding.
In any case - I'm interested in the comparison.
Jan
Message 3
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Subject: | Re: Dipole antenna fabrication |
At 05:12 AM 2/27/2012, you wrote:
>
>Bob,
>
>Sorry about this. The balun you describe making IS a Pawsey stub -
>should have taken a look before responding.
>In any case - I'm interested in the comparison.
You are correct. If one compares the Pawsey stub with a
1/4 wave, de-coupling sleeve or "sleeve balun" as shown in Wikipedia:
Emacs!
. . . we see that the top end of the sleeve is the open end
of a shorted 1/4-wave transmission line. I.e., the
impedance of the top rim of the sleeve looking at the
shorted end is very high. Hence, connecting it to the
center conductor adds no 'load' to purposeful energy
but it would have the effect of collecting differential
currents between coax shield and the center conductor
and force them toward equivalency.
The Pawsey stub functions in the same manner except that
its behavior is more like a short piece of balanced
transmission line as opposed to a piece of coax. This
brings to light an error in my balun article cited
earlier. Since the stub IS NOT operating as a free-space
1/4-wave but instead as a transmission line, dielectric
effects on velocity come into play. The velocity factor
of this piece of transmission line is on the order of
.95 to .97 which means that my 26" dimension for the
stub would be closer to optimum were it shortened by
an inch or so.
The 'coiled coax ploy" has good foundations
in physics. Consider this image from the Wikipedia
article:
Emacs!
The current balun is a common mode choke which
may be implemented by wires on a high-efficiency
core (as in many TV couplers), a coil of coax
immediately adjacent to the attach point on the
dipole or consider this image also from Wikipedia
article on baluns:
Emacs!
As you can see here, the common mode choke effects
are greatly enhanced by winding the coax onto
the core. Inductance in these windings is proportional
to the permeability of the magnetic path material.
Air is 1.0, the ferrite may be 5-20 . . . thus
multiplying the beneficial effects by that factor.
The neat thing about this balun is that it is not
resonant or tuned; thus effective over a wider range
of frequencies. The stubs in coax or balanced transmission
lines are tuned and optimum at one frequency only.
The 'hard' thing about the coax on core ploy is finding
coax that will wind tightly through a relatively
small core WITHOUT dropping below the recommended
bend radius for that coax. Coaxes with solid center
conductors are at risk for center conductor migration
through the insulation over time and temperature
cycles. RG-141 wound through a core like that has
potential for becoming a shorted feed line years from
now. The commercial antenna I saw with this technique
used a small diameter, very flexible coax and offered
a BNC connector on the stub for extending the feed line
to the appliance with more conventional coax.
Bob . . .
Message 4
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Subject: | Re: Dipole antenna fabrication |
Bob, thank you for the explanation.
There was some discussion on the web whether the velocity factor to be
used for finding the length of the Pawsey stub was that for the coax
insides (NO) or the coax outsides (YES). The latter would be the .97 to
.99 then.
Jan
Message 5
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Subject: | Re: Dipole antenna fabrication |
Dipole antenna erected vertically is similar to the 1/4 wave whip that we
now use. The tricks are to make sure you have a large enough groundplane
(section of the aircraft skin). When I first saw the talk of dipole
antennae I assumed incorrectly you wanted to be able to remove a radio and
take it with you and still have good range in case of a crash.
Aircraft radios are restricted to power. Basically the powers to be don't
want you to be transmitting thousands of miles otherwise they would up the
power of transceivers and drop the frequency closer to six meters than to
two meters. Main problem with that is you could end up with a lot of
crosstalk like they used to have on the CB band (11 meters).
While flying in mountainous areas for safety I would not be without the new
400 mHz ELT. There are lots of things about those ELTs I don't like but we
are stuck with it.
Noel
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Eric M.
Jones
Sent: February 26, 2012 2:12 PM
Subject: AeroElectric-List: Re: Dipole antenna fabrication
<emjones@charter.net>
The Wiki site shows a simple dipole and the caption says:
"A schematic of a half-wave dipole antenna connected to an unbalanced
coaxial cable. Better practice is to connect the balanced dipole to the
unbalanced line with a balun."
I've been thinking of putting aluminum foil on the TV rabbit ears....
Proper termination is the key to antenna efficiency. If Cessna didn't do it,
that doesn't mean the homebuilder shouldn't do it right.
Bob et al. having flown extensively in the Western states and Mexico, I have
plently of experience with being a zillion miles from a station. At those
times I start to think that having a good radio and good antennas is a wise
and cost effective thing to do. But do what you want.
--------
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=367293#367293
Message 6
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Subject: | Re: Dipole antenna fabrication |
"Good enough" is everyone's enemy when it comes to ceasing to operate at
3000' !
Noel
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Richard
Dudley
Sent: February 26, 2012 3:06 PM
Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication
<rhdudley1@bellsouth.net>
A wise (and practical) person once said:
"Perfection is the enemy of good enough."
Anymouse
RHDudley
On 2/26/2012 12:41 PM, Eric M. Jones wrote:
> --> AeroElectric-List message posted by: "Eric M.
Jones"<emjones@charter.net>
>
> The Wiki site shows a simple dipole and the caption says:
>
> "A schematic of a half-wave dipole antenna connected to an unbalanced
coaxial cable. Better practice is to connect the balanced dipole to the
unbalanced line with a balun."
>
> I've been thinking of putting aluminum foil on the TV rabbit ears....
>
> Proper termination is the key to antenna efficiency. If Cessna didn't do
it, that doesn't mean the homebuilder shouldn't do it right.
>
> Bob et al. having flown extensively in the Western states and Mexico, I
have plently of experience with being a zillion miles from a station. At
those times I start to think that having a good radio and good antennas is a
wise and cost effective thing to do. But do what you want.
>
> --------
> 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=367293#367293
>
>
Message 7
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Subject: | Re: Dipole antenna fabrication |
Bob:
Have you ever used a broadband windom antenna?
Noel
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Robert L.
Nuckolls, III
Sent: February 27, 2012 1:06 AM
Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication
<nuckolls.bob@aeroelectric.com>
At 11:41 AM 2/26/2012, you wrote:
<emjones@charter.net>
The Wiki site shows a simple dipole and the caption says:
"A schematic of a half-wave dipole antenna connected to an unbalanced
coaxial cable. Better practice is to connect the balanced dipole to
the unbalanced line with a balun."
. . . and I published instructions for adding this
feature to a coax fed dipole at
http://aeroelectric.com/articles/BALUN/Balun_Fabrication.html
. . . the same technique could be applied to a comm
dipole.
I've been thinking of putting aluminum foil on the TV rabbit ears....
Which will lower the resonant frequency of the antenna
and perhaps lower the Q . . . wider bandwidth. But
no benefit for relative efficiency compared to an "ideal"
dipole at the same frequency.
Proper termination is the key to antenna efficiency. If Cessna didn't
do it, that doesn't mean the homebuilder shouldn't do it right.
I can assure you that Dr. Wood read the same books
we've all read about antennas with professional
attendance to "better practice." The point I endeavored to
make was that after he came to understand all the
deleterious effects over which we had no control on
a C-172, adding baluns on the VOR antennas yielded very
small gains compared the sum of other losses.
Next time I can get into the test lab, I'll do some
measurements on the difference between a balun-fed
dipole and a bare foot, coax-fed dipole.
Bob et al. having flown extensively in the Western states and Mexico,
I have plently of experience with being a zillion miles from a
station. At those times I start to think that having a good radio and
good antennas is a wise and cost effective thing to do. But do what you
want.
But you paint a picture suggesting that a balun would
make the difference between communicating with some distant
station . . . and not. Given the light of sight character
of VHF+ frequencies, you're more likely not to have a useable
path because you can't see the other station than because
your effective radiated signal is down by a few db due
to violations of feed point protocols. Further, it's
insufficient to be able to see the other station
peeking over the horizon (or mountaintop). The effects
of intervening "roughness" in the "Fresnel zone" can
have some profound effects on what might otherwise be
considered a free-space, line of sight situation.
http://web.arundale.co.uk/docs/ais/PathlossCalculationsforAmateurs.pdf
Mountain peaks and valleys intruding into the Fresnel
zone can kick your free space performance predictions
in the teeth.
Currents flowing on the coax shield are but part of
the 'violations' . . . the center impedance of a perfect
dipole is on the order of 73 ohms. This means that the
BEST SWR to be achieved without application of some
impedance matching is on the order of 73/50 = 1.46:1
So we've tossed off some efficiencies right out of the
box. When we plot radiation patterns on VHF antennas
on airplanes, one generally expects 10 db or greater
'lumpiness' in the pattern due to geometry of the
conductive elements around the antenna. So again,
if one is at 'extreme range' for useful communications,
it's more likely that you'll improve the path by turning
the airplane a few degrees as opposed to optimizing
feed point design.
Yes, a BALUN can be "better" but experience on small
aircraft has shown that the difference is so small
compared to all other effects combined that the balun
was not 'cost effective'. Further, it won't fix the
fundamental mis-match so 1.46:1 is still the best
we can expect. The textbooks are full of examples of
data taken from the idealized antenna and feed line
examined in the anechoic chamber with test equipment
that can split a DB into small pieces.
As soon as you put that same antenna is a real world
environment, lots of things with potential for
altering performance happen. This is why
the guy flying 'around with a radio capable of
talking to another one just like it 1000 miles
away finds that the practical limits are much
shorter for reasons mostly beyond control of
the antenna designer.
But just for grins, I've got some DIY antenna's I've
used in my weekend seminars and articles. I'll
fit one with a balun and leave the other barefoot.
Then see if my buddy Don P can tell the
difference without looking to see which one is
hooked up. Hmmmm . . . I think I know how to
do the experiment with equipment I have. Need
warmer weather though . . .
Bob . . .
Message 8
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Subject: | Re: Dipole antenna fabrication |
At 05:12 PM 2/27/2012, you wrote:
>
>Bob:
>
> Have you ever used a broadband windom antenna?
No, I've known hams that did. Most of my activity
for the past 40 years has been on 2M repeaters.
In the picture at . . .
http://aeroelectric.com/Pictures/Misc/KTVH.jpg
you can see the toes of my boots while peering
down the center of KTVH tower east of Hutchinson
KS. I was standing on the 1200' platform were
the 22/82 repeater was quartered back then.
Antennas to die for on that system were things
like DB-228 arrays of 8 dipoles on a mast about
40' long and weighing in at 75 pounds. Got to
hang two of those off the leg of this tower
about 100' apart.
Bob . . .
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