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Open Stub fed J antenna



 
 
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  #1  
Old September 14th 05, 07:21 AM
David
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Default Open Stub fed J antenna

Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.

Thanks in advance
  #2  
Old September 14th 05, 03:07 PM
Cecil Moore
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Default

David wrote:
Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.


An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.
--
73, Cecil http://www.qsl.net/w5dxp


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  #3  
Old September 14th 05, 11:15 PM
David
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Default

Cecil,

Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and having
low radiation angle.

Cecil Moore wrote:

David wrote:

Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug
and coaxial cable.



An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.

  #4  
Old September 15th 05, 12:03 AM
Jerry Martes
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Default


Dave

Have you considered making a coaxial version of the J-pole?

http://www.ansoft.com/news/articles/04.05_MWJ.pdf

Jerry





"David" wrote in message
...
Cecil,

Thanks for the info. The open stub J looks like it should be suitable for
my application being ground independent, omni directional and having low
radiation angle.

Cecil Moore wrote:

David wrote:

Does anyone know of where I could find an example of a homebrew open
stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.



An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.



  #5  
Old September 15th 05, 12:13 AM
Cecil Moore
Guest
 
Posts: n/a
Default

David wrote:
Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and having
low radiation angle.


Be sure to include a choking function at the feedpoint.
--
73, Cecil http://www.qsl.net/w5dxp

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  #6  
Old September 15th 05, 01:34 AM
David
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Default

Thanks for the feedback everyone.

The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).




Cecil Moore wrote:
David wrote:

Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and
having low radiation angle.



Be sure to include a choking function at the feedpoint.

  #7  
Old September 15th 05, 02:09 AM
Dave Platt
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Posts: n/a
Default

In article ,
David wrote:

The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).


Yes, it does. Take a look at the following:

http://download.antennex.com/hws/ws1002/sperrtof.pdf

A Sperrtof, in effect, is a J-pole whose matching section is a coaxial
tube rather than a single rod or wire. It sounds rather like what
you're describing.

As with all such (I think), the radiating section is 1/2 wavelength
long, give or take a smidge, and behaves as an end-fed 1/2-wave
dipole. The matching section isn't supposed to radiate significantly.

The overall radiation pattern would, I expect, be essentially the same
as other J-poles and other end-fed 1/2-wave radiators - similar to a
center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the
feedpoint.

You can distinguish a Sperrtof-type antenna from one of the coaxial
dipoles Jerry referred you to, by the length of the single-wire
radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial
dipole (which is really a center-fed dipole).

There's an interesting dual-band 2m/440 antenna which was written up
in QST in October 2000 - ARRL members can get the article at
http://www.arrl.org/members-only/tis...df/0010050.pdf

It's interesting because it's _called_ a J-pole, _looks_ like a
J-pole... but electrically it isn't. It's actually a center-fed
vertical, not a Zepp. The stub at the bottom acts as a
choke/decoupler, not as an impedance transformer.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
  #8  
Old September 15th 05, 02:41 AM
David
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Default

Dave,

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax instead of the copper tubing.The outer tube would be
5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I
do not need to slide the tap point up as their design did. I suppose I
should also add the choke coil at the antenna base for additional
isolation of ground currents.

The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If
the bottom part of their design is 505mm then the antenna must be used
for 2m band. If this is the case, how come the radiating element is 1480
long ?

The Coaxial antenna mentioned in the article mentioned by Jerry looks
good. The version they mention with a GAP would be simple to construct.
It is a 1/4 wave radiating element at the top (earth braid stripped
back), then 1/4 wave of full coax, then a "small" gap, then the
transmission line. They do not mention the size of the gap but I assume
any small gap has the same effect of isolating the reverse current ?

Should the actual lengths of these sections be modified by "K" depending
on diameter of conductor or are they exact 1/4 wave length cuts ?

Thanks heaps.


Dave Platt wrote:
In article ,
David wrote:


The article Jerry pointed me to starts looking like the commercial
antenna I was initially studying. Their unit though only had a 1/4 wave
above the coaxial sleeve.

From the SMA plug, the coax coiled around the radome must have been the
choke to help reduce RF currents radiating from the earth braid.
Then the 1/4 wave length coax forms the match followed by the expose
radiating section.

It looks like an open stub fed J-pole where the stub is enclosing the
inner element rather than being constructed as a rod next to it.

Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
theory and would like to understand what is happening so that I can
experiment with some kind degree of success).



Yes, it does. Take a look at the following:

http://download.antennex.com/hws/ws1002/sperrtof.pdf

A Sperrtof, in effect, is a J-pole whose matching section is a coaxial
tube rather than a single rod or wire. It sounds rather like what
you're describing.

As with all such (I think), the radiating section is 1/2 wavelength
long, give or take a smidge, and behaves as an end-fed 1/2-wave
dipole. The matching section isn't supposed to radiate significantly.

The overall radiation pattern would, I expect, be essentially the same
as other J-poles and other end-fed 1/2-wave radiators - similar to a
center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the
feedpoint.

You can distinguish a Sperrtof-type antenna from one of the coaxial
dipoles Jerry referred you to, by the length of the single-wire
radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial
dipole (which is really a center-fed dipole).

There's an interesting dual-band 2m/440 antenna which was written up
in QST in October 2000 - ARRL members can get the article at
http://www.arrl.org/members-only/tis...df/0010050.pdf

It's interesting because it's _called_ a J-pole, _looks_ like a
J-pole... but electrically it isn't. It's actually a center-fed
vertical, not a Zepp. The stub at the bottom acts as a
choke/decoupler, not as an impedance transformer.

  #9  
Old September 15th 05, 03:10 AM
Cecil Moore
Guest
 
Posts: n/a
Default

David wrote:
If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax ...


I wouldn't use RG58 for anything above HF. It has 20 dB
matched line loss per 100 ft at that frequency.
--
73, Cecil http://www.qsl.net/w5dxp

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  #10  
Old September 15th 05, 03:41 AM
David
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Posts: n/a
Default

So for the 1/4 wave (81mm) section of the antenna the loss would be
about 0.05dB ? at 915 MHz.

I typically have between 150mm (When antenna mounted on enclosure) and
2m length (when antenna external to enclosure) cable between transmitter
and Antenna. (Around 1.2dB max. loss in coax between transmitter and
antenna). Would this usually be acceptable or do you aim for much lower
loss in the transmission line ? Max. power is 1 Watt. Most of the
transceivers I play around with are around 10mW.

Thanks

Cecil Moore wrote:
David wrote:

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax ...



I wouldn't use RG58 for anything above HF. It has 20 dB
matched line loss per 100 ft at that frequency.

 




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