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Help with Sleeve Dipole



 
 
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  #1  
Old October 2nd 05, 08:16 AM
David
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Posts: n/a
Default Help with Sleeve Dipole

I am a little confused with calculation of appropriate length of metal
tubing used in a sleeve antenna design (the 1/4 wave section).

I understand that as RF travels through various mediums, Velocity of the
wave slows down relative to Er of the material. So for coax the
wavelength would be say 0.66 Lambda.

With the tubing over a coaxial cable though it seems there is Vp related
to the dielectric formed between the sleeve and the outer sheath of the
coax cable but also the Metal has an Er value.

1. Do you need to count for both these when calculate how the tube
should be.
2. Is there relationship that needs to be considered between sleeve
length and distance between sleeve and coax (ie. If the sleeve is made
larger diameter does it effect the length required for the tube or is
this still defined strictly by Er) also, Does the diameter of the tube
effect the impedance ?

The specific example is RG174 Coax cable vertical dipole. The top
radiating element is 1/4 long. At the bottom of this element is a length
of RG174 Coax. 1/4 wave length of this coax has a 5/32 Brass sleeve. The
sleeve is soldered to the RG174 coax at the point where the coax meets
the top 1/4 wave element.
I need to understand what needs to be used to calculate the length of
Brass tubing sleeve given the tubing diameter is 5/32 and besides air,
the outer sheath of the RG174 is also between the sleeve and Earth
braid. Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).

The final point would be if I place some ferrite rings across the coax
below the sleeve, will this improve SWR ?

Thanks

Regards

David
  #2  
Old October 2nd 05, 08:50 AM
Richard Clark
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Posts: n/a
Default

On Sun, 02 Oct 2005 07:16:01 GMT, David wrote:

I need to understand what needs to be used to calculate the length of
Brass tubing sleeve given the tubing diameter is 5/32 and besides air,
the outer sheath of the RG174 is also between the sleeve and Earth
braid. Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).


Hi David,

The Er of the metal is not something that comes quickly to mind
(especially this late at night). Besides, it sounds like what you are
describing is the radial component which would be fantastically small
(many decimal places, not simply tens of percent).

It also seems like your application is for UHF (you need to be more
specific). This makes such "calculations" little better than SWAGs.
It doesn't take much to be way off. What is even more confounding is
your problem contains a hidden trap.

The sleeve of brass that drops below the feed point is embracing the
jacket of the RG174. THIS constitutes the dielectric constant of
interest and is a material that is different from the insulator of the
inner line. Even more, and as you note above, there is some air mixed
in to really gum up the back of the napkin calculations. This is the
G of the SWAG.

Worst yet, some jacket material is actually quite lossy, or so it has
been reported - another G of the SWAG.

All-in-all, what you need to do is build one and measure it. No doubt
that will lead to another trail of tears. On the other hand, there is
nothing so stimulating to learning than pain. Most of my best
subjects centered on projects I thoroughly screwed up.

As the saying goes:
"If you haven't failed, you aren't trying hard enough."

73's
Richard Clark, KB7QHC
  #3  
Old October 2nd 05, 09:18 AM
David
Guest
 
Posts: n/a
Default

Richard,

Thanks for the feedback. No problem with the failing issue, plenty of
that going on.

The way I see it though is that if I can add some knowledge to my
experimentation then at least I can "play" with some hope of moving to a
solution in the right direction.



Richard Clark wrote:
On Sun, 02 Oct 2005 07:16:01 GMT, David wrote:


I need to understand what needs to be used to calculate the length of
Brass tubing sleeve given the tubing diameter is 5/32 and besides air,
the outer sheath of the RG174 is also between the sleeve and Earth
braid. Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).



Hi David,

The Er of the metal is not something that comes quickly to mind
(especially this late at night). Besides, it sounds like what you are
describing is the radial component which would be fantastically small
(many decimal places, not simply tens of percent).

It also seems like your application is for UHF (you need to be more
specific). This makes such "calculations" little better than SWAGs.
It doesn't take much to be way off. What is even more confounding is
your problem contains a hidden trap.

The sleeve of brass that drops below the feed point is embracing the
jacket of the RG174. THIS constitutes the dielectric constant of
interest and is a material that is different from the insulator of the
inner line. Even more, and as you note above, there is some air mixed
in to really gum up the back of the napkin calculations. This is the
G of the SWAG.

Worst yet, some jacket material is actually quite lossy, or so it has
been reported - another G of the SWAG.

All-in-all, what you need to do is build one and measure it. No doubt
that will lead to another trail of tears. On the other hand, there is
nothing so stimulating to learning than pain. Most of my best
subjects centered on projects I thoroughly screwed up.

As the saying goes:
"If you haven't failed, you aren't trying hard enough."

73's
Richard Clark, KB7QHC

  #4  
Old October 2nd 05, 06:03 PM
David G. Nagel
Guest
 
Posts: n/a
Default

David wrote:
I am a little confused with calculation of appropriate length of metal
tubing used in a sleeve antenna design (the 1/4 wave section).

I understand that as RF travels through various mediums, Velocity of the
wave slows down relative to Er of the material. So for coax the
wavelength would be say 0.66 Lambda.

With the tubing over a coaxial cable though it seems there is Vp related
to the dielectric formed between the sleeve and the outer sheath of the
coax cable but also the Metal has an Er value.

1. Do you need to count for both these when calculate how the tube
should be.
2. Is there relationship that needs to be considered between sleeve
length and distance between sleeve and coax (ie. If the sleeve is made
larger diameter does it effect the length required for the tube or is
this still defined strictly by Er) also, Does the diameter of the tube
effect the impedance ?

The specific example is RG174 Coax cable vertical dipole. The top
radiating element is 1/4 long. At the bottom of this element is a length
of RG174 Coax. 1/4 wave length of this coax has a 5/32 Brass sleeve. The
sleeve is soldered to the RG174 coax at the point where the coax meets
the top 1/4 wave element.
I need to understand what needs to be used to calculate the length of
Brass tubing sleeve given the tubing diameter is 5/32 and besides air,
the outer sheath of the RG174 is also between the sleeve and Earth
braid. Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).

The final point would be if I place some ferrite rings across the coax
below the sleeve, will this improve SWR ?

Thanks

Regards

David

David;

That's why they call it the ART of antenna design. First you make the
best mathematical calculations you can then you start cutting
wire/tubing. Make your lengths slightly longer than the math says then
prune the lengths until the antenna works the way you think it should.

There are so many variables in antennas that this is the only practical
way to make a new one. Once you find your optimal lengths you can make
more to the same specs. However remember that just like in housing
antenna's depend on three things: location, location, location and have
to be custom adjusted for each site.

Dave WD9BDZ
  #5  
Old October 2nd 05, 06:26 PM
Dave VanHorn
Guest
 
Posts: n/a
Default


With the tubing over a coaxial cable though it seems there is Vp related
to the dielectric formed between the sleeve and the outer sheath of the
coax cable but also the Metal has an Er value.


From what I understand, the RF will be traveling on the outside of the
sleeve, and won't "see" the internals at all.
So, the sleeve ends up as a 1/4 wavelength stub.

Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).


It shouldn't be

The final point would be if I place some ferrite rings across the coax
below the sleeve, will this improve SWR ?


Not if the sleeve is doing it's job.
The sleeve's open end should look like a short at the operating frequency,
and keep RF off the feedline.


Thanks

Regards

David

David;

That's why they call it the ART of antenna design. First you make the
best mathematical calculations you can then you start cutting wire/tubing.
Make your lengths slightly longer than the math says then prune the
lengths until the antenna works the way you think it should.

There are so many variables in antennas that this is the only practical
way to make a new one. Once you find your optimal lengths you can make
more to the same specs. However remember that just like in housing
antenna's depend on three things: location, location, location and have to
be custom adjusted for each site.

Dave WD9BDZ




  #6  
Old October 2nd 05, 08:21 PM
Dan Richardson
Guest
 
Posts: n/a
Default

On Sun, 2 Oct 2005 12:26:00 -0500, "Dave VanHorn"
wrote:

Not if the sleeve is doing it's job.
The sleeve's open end should look like a short at the operating frequency,
and keep RF off the feedline.


If you wish to keep common mode off the feedline you will need to add
decoupling.

That's the old AEA ISO-Loop had that extra sleeve.

Danny, K6MHE

email: k6mheatarrldotnet
http://users.adelphia.net/~k6mhe/

  #7  
Old October 3rd 05, 03:02 AM
David
Guest
 
Posts: n/a
Default

I am still confused on this one.
I see a 1/4 wave radiating element joined to another 1/4 wave radiating
element that just happens to also act as a choke to reduce RF on the
return braid of the RG174 coax.

The point where they meet is supposed to look like the centre of a 1/2
wave dipole so the impedance must be around the 72 Ohms

If the Sleeve is acting as a choke(Stub) then does it not need a second
element as the return (ie is it not acting like a 1/4 length of coax
where the tube is the outer braid and the RG174 cable running up the
centre outer sheath is the dielectric and the outer braid of the RG174
becomes the inner conductor of the stub )?

If this is correct then the electrical length of the sleeve must be
modified by the presence of the RG174 coax outer sheath. The sleeve is
sort of performing a dual function.

As it is difficult to cut the tube once soldered in place, I will need
to make heaps of these to get the length right. At least if I had a
reasonable starting point it would minimize the number of iterations.

The other issue is that I read about cutting lengths for elements and
not that even an element in free space is trimmed down by about 3% to
account for some "end effect" ? Would this also apply to my sleeve ? If
so, then the length may be difficult to get right because if I make
allowance for the end effect then the choking effect of the sleeve will
not work as well.

I do not want to reinvent the wheel here as these antennae are sold
already being sold commercially and I am sure there are numerous people
who have successfully made these. Therefore if I can learn from other
people's mistakes and success, the starting point is much closer to the
solution.

I have a practical application but want to understand also. The
requirement is that I have a small antenna that can be built into the
end of a RF connector and plug directly onto a radio modem operating in
the 915MHz band. The power is 1 Watt max. The design needs to be
repeatable and not require trimming at every location (being portable I
have no control over where the unit is located), as there is no decent
ground plane, I need a unit that does not require radials or large
ground plane.
The low radiation angle and no requirement for large ground plane made
the J-Pole look like a good contender but it was physically difficult to
mount in this application.

After looking around and opening up some commercial units, I noted many
used the Choke Sleeve, 1/2 wave dipole antenna.

This is why I am now wanting to understand how these work and have a
good starting point for making them. The commercial units used molded
construction that was difficult for me to emulate. I have started with
RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4"
nylon tubing as a radome. It goes together well and looks good but does
not perform very well.

Thanks for any further info.


Dave VanHorn wrote:
With the tubing over a coaxial cable though it seems there is Vp related
to the dielectric formed between the sleeve and the outer sheath of the
coax cable but also the Metal has an Er value.



From what I understand, the RF will be traveling on the outside of the
sleeve, and won't "see" the internals at all.
So, the sleeve ends up as a 1/4 wavelength stub.

Is the diameter of the tubing critical (will it impact adversely

on SWR looking into this antenna).



It shouldn't be


The final point would be if I place some ferrite rings across the coax
below the sleeve, will this improve SWR ?



Not if the sleeve is doing it's job.
The sleeve's open end should look like a short at the operating frequency,
and keep RF off the feedline.



Thanks

Regards

David


David;

That's why they call it the ART of antenna design. First you make the
best mathematical calculations you can then you start cutting wire/tubing.
Make your lengths slightly longer than the math says then prune the
lengths until the antenna works the way you think it should.

There are so many variables in antennas that this is the only practical
way to make a new one. Once you find your optimal lengths you can make
more to the same specs. However remember that just like in housing
antenna's depend on three things: location, location, location and have to
be custom adjusted for each site.

Dave WD9BDZ





  #8  
Old October 3rd 05, 03:40 AM
Richard Clark
Guest
 
Posts: n/a
Default

On Mon, 03 Oct 2005 02:02:13 GMT, David wrote:

If this is correct then the electrical length of the sleeve must be
modified by the presence of the RG174 coax outer sheath. The sleeve is
sort of performing a dual function.


Hi David,

I think I pointed that out already. The effects of this jacket are
variable across many suggestions offered here over the years.

As it is difficult to cut the tube once soldered in place, I will need
to make heaps of these to get the length right. At least if I had a
reasonable starting point it would minimize the number of iterations.


Ah yes, the pain of it all. I mentioned that too. You may as well
grit your teeth and set to it, there is no shortcut to this all.

The other issue is that I read about cutting lengths for elements and
not that even an element in free space is trimmed down by about 3% to
account for some "end effect" ? Would this also apply to my sleeve ? If
so, then the length may be difficult to get right because if I make
allowance for the end effect then the choking effect of the sleeve will
not work as well.


You might want to work this backwards. That is start off with a
successful choke section of the sleeve. Let that dictate what follows
because its isolation will mediate what I describe. If you have any
experience with what is called an Offset Center Fed Dipole, you would
find that it offers a closer match to 50 Ohms. You are using the
dipole as a variable match by finding the 50 Ohm portion along its
full length. This means that the portion above the sleeve's
attachment to the drive point may not be the classic length (and
certainly not accounted by the 3% of the "end effect" - not even
close).

That is neither here nor there because at that point you will have
simultaneously achieved resonance, a match, and isolation; and yet by
no fixed formula pulled out of a hat, nor accredited by an institution
of higher learning.

And, by the way, it isn't going to pull together in the first pass. I
hope you have a stack of tubing, so put on a happy face and get down
to it.

This is why I am now wanting to understand how these work and have a
good starting point for making them. The commercial units used molded
construction that was difficult for me to emulate. I have started with
RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4"
nylon tubing as a radome. It goes together well and looks good but does
not perform very well.


How do you know? It might be doing the best job you could ask of any
design. Your statement requires FAR MORE qualification than a rather
subjective toss-off.

In other words:
1. To what parameters?
2. Compared to what?
3. By what measure?

73's
Richard Clark, KB7QHC
  #9  
Old October 3rd 05, 04:14 AM
David
Guest
 
Posts: n/a
Default

Richard,

With regard to performance.

Doing an A/B comparison between the commercial antenna and my version.
The range achieved by the commercial antenna was approximately 4 times
the range of my version.
The range was determined by loss of using one end as a beacon and moving
the test unit further away until it could no longer receive valid
packets, then move in and out of range several times to confirm
consistency. The same was done with my version.

Before the test, I monitored the return port on the directional coupler
on a Spectrum analyzer while seeping frequency from the sig. gen and
noted the SWR on the commercial unit was lower than my version. (Though
the measurement was not calibrated I could see the SWR was better but
did not determine how much).

Previously I designed a Microwave movement detector that operated at 1.2
GHz using Doppler principle. It used a microstrip tank circuit connected
to a negative resistance oscillator. The design was published (RF Design
magazine Dec 1986).
Though there were many technical issues regarding the oscillator (how to
adjust impedance in the base for optimal negative resistance looking
into emitter, effect of moving the antenna, ground plane under the
antenna etc etc), others were able to make the design just as per my
prototype and it worked. They could then proceed to"play" with the
design to change its performance.

I really would have expected that a similar scenario would exist with
antenna design (Someone has made one just like what I am wanting to make
and is able to share with me the details on what they did that worked,
then I would play with the design).

I am grateful for the many responses I have received from my postings to
this group and am confident there is much good information in what I
have received but mixed with people's actual experience is a bunch of
theoretical information that makes it hard to pick out what is useful.
(ie. some of the information has conflicted and left me not
understanding what is actually happening here).

If someone has actually made these types of antennas and has some
practical experience with them, that would be a fantastic starting point
for me. (ie. Low power, UHF, portable).




Richard Clark wrote:
On Mon, 03 Oct 2005 02:02:13 GMT, David wrote:


If this is correct then the electrical length of the sleeve must be
modified by the presence of the RG174 coax outer sheath. The sleeve is
sort of performing a dual function.



Hi David,

I think I pointed that out already. The effects of this jacket are
variable across many suggestions offered here over the years.


As it is difficult to cut the tube once soldered in place, I will need
to make heaps of these to get the length right. At least if I had a
reasonable starting point it would minimize the number of iterations.



Ah yes, the pain of it all. I mentioned that too. You may as well
grit your teeth and set to it, there is no shortcut to this all.


The other issue is that I read about cutting lengths for elements and
not that even an element in free space is trimmed down by about 3% to
account for some "end effect" ? Would this also apply to my sleeve ? If
so, then the length may be difficult to get right because if I make
allowance for the end effect then the choking effect of the sleeve will
not work as well.



You might want to work this backwards. That is start off with a
successful choke section of the sleeve. Let that dictate what follows
because its isolation will mediate what I describe. If you have any
experience with what is called an Offset Center Fed Dipole, you would
find that it offers a closer match to 50 Ohms. You are using the
dipole as a variable match by finding the 50 Ohm portion along its
full length. This means that the portion above the sleeve's
attachment to the drive point may not be the classic length (and
certainly not accounted by the 3% of the "end effect" - not even
close).

That is neither here nor there because at that point you will have
simultaneously achieved resonance, a match, and isolation; and yet by
no fixed formula pulled out of a hat, nor accredited by an institution
of higher learning.

And, by the way, it isn't going to pull together in the first pass. I
hope you have a stack of tubing, so put on a happy face and get down
to it.


This is why I am now wanting to understand how these work and have a
good starting point for making them. The commercial units used molded
construction that was difficult for me to emulate. I have started with
RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4"
nylon tubing as a radome. It goes together well and looks good but does
not perform very well.



How do you know? It might be doing the best job you could ask of any
design. Your statement requires FAR MORE qualification than a rather
subjective toss-off.

In other words:
1. To what parameters?
2. Compared to what?
3. By what measure?

73's
Richard Clark, KB7QHC

  #10  
Old October 3rd 05, 05:21 AM
Richard Clark
Guest
 
Posts: n/a
Default

On Mon, 03 Oct 2005 03:14:52 GMT, David wrote:

Doing an A/B comparison between the commercial antenna and my version.
The range achieved by the commercial antenna was approximately 4 times
the range of my version.
The range was determined by loss of using one end as a beacon and moving
the test unit further away until it could no longer receive valid
packets, then move in and out of range several times to confirm
consistency. The same was done with my version.


Hi David,

This suggests your problems are indeed very profound. A 4 times range
difference would suggest a massive mismatch.

Before the test, I monitored the return port on the directional coupler
on a Spectrum analyzer while seeping frequency from the sig. gen and
noted the SWR on the commercial unit was lower than my version. (Though
the measurement was not calibrated I could see the SWR was better but
did not determine how much).


Then I would bet that your system topology is to blame. That is, your
line from the coupler and test equipment to the antenna is not
normalized out of the measurement as a network analyzer would do. And
when you shift from measurement to application, your line length
shifts such that your test match goes to hell. This implicates the
isolation of the sleeve - which is already in serious jeopardy by many
variables anyway. Your test scenario does not reveal the efficacy of
this isolation, it simply affirms that using the spectrum analyzer
establishes a suitable, but serendipitous arrangement.

You might add ferrite beads to the lead going to the antenna/sleeve
and move them along its length to test this hypothesis. This
prophylactic measure is also commonly employed as a necessary adjunct
to design.

73's
Richard Clark, KB7QHC
 




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