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  #31   Report Post  
Old September 16th 05, 08:25 PM
Steve Nosko
 
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"Cecil Moore" wrote in message
...
...
But I've given you all the information in Kraus' book
about that particular antenna. Laid on its side:

| + - +
|----------/////--------------------
| 1/4WL coil 1/2WL

The coil is a 180 deg. phase reversing coil. [snip]
73, Cecil,


Yep and this makes it a "phasing" coil and the antenna a colinear. The
phasing coil causes the current in the two sections of the antenna to be in
phase. Note my addition of polarity indicators to the above diagram. The
right end of both elements are the same = in phase currents. Now you have a
half wave and a quarter wave in phase, yielding gain by a flatter pattern
(whicn is vertical in this drawing).
You can stack half waves up like this and this is done in common cellular
gain antennas - probably WiMax as well. There are several ways to do the
phasing... Coils like this, quarter-wave shorted stubs (parallel-line),
reverse connecting alternate sections of coaxial t-line as elements, phasing
harnesses on the feed line... Probably more.

73, Steve, K;9'D.C,I



  #32   Report Post  
Old September 17th 05, 12:15 AM
David
 
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I would like to express my appreciation for all those who have taken the
time to respond to my post. Being new to antenna design and construction
I understand some of my questions and comments may seem a bit "silly"
and it would be easy for people to suggest that I go learn more first or
simply use trial and error.

I have a practical requirement for a flexible whip antenna that results
in "good" gain that works but at the same time I want to learn what I
can because it is my intention to later experiment with other style of
antenna (Yagi at 434MHz is next on my list).

Now, back to the 1/4 wave, phasing coil, 1/2 wave design.

Does this means for a practical antenna I could have...

SMA plug centre pin - 1/4 length of wire - several turns of wire
around the Radome - 1/2 length of wire ? (ie. coax earth braid stops at
SMA and rest of antenna is made up of inner conductor and dielectric only).

Is it that "simple" ? If so, that would be easy to construct in a
flexible whip (which is my aim) compared to many of the other suggestions.

Is the gain the same as a dipole ?

Also, I have EZNEC 4. but have not had a chance to learn it yet. Can you
model the coil in the program or can you only enter segments ?

Thanks.

Steve Nosko wrote:
"Cecil Moore" wrote in message
...

...
But I've given you all the information in Kraus' book
about that particular antenna. Laid on its side:

| + - +
|----------/////--------------------
| 1/4WL coil 1/2WL

The coil is a 180 deg. phase reversing coil. [snip]
73, Cecil,



Yep and this makes it a "phasing" coil and the antenna a colinear. The
phasing coil causes the current in the two sections of the antenna to be in
phase. Note my addition of polarity indicators to the above diagram. The
right end of both elements are the same = in phase currents. Now you have a
half wave and a quarter wave in phase, yielding gain by a flatter pattern
(whicn is vertical in this drawing).
You can stack half waves up like this and this is done in common cellular
gain antennas - probably WiMax as well. There are several ways to do the
phasing... Coils like this, quarter-wave shorted stubs (parallel-line),
reverse connecting alternate sections of coaxial t-line as elements, phasing
harnesses on the feed line... Probably more.

73, Steve, K;9'D.C,I



  #33   Report Post  
Old September 17th 05, 05:07 AM
Cecil Moore
 
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David wrote:
Is it that "simple" ? If so, that would be easy to construct in a
flexible whip (which is my aim) compared to many of the other suggestions.


The non-simple part is designing the 180 deg. phase
shifting coil. I think I have one of those on my
Diamond mobile antenna.

Is the gain the same as a dipole ?


Higher than a dipole - close to 6 dBi compared to the
1/4WL ground plane at *about* 0 dBi.
--
73, Cecil http://www.qsl.net/w5dxp

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  #34   Report Post  
Old September 17th 05, 06:40 AM
Jerry Martes
 
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"Richard Harrison" wrote in message
...
Cecil, W5DXP wrote:
"The 180 deg. phase reversing coil is the tricky part."

For UHF, you might prefer to use a 1/4-wave short-circuited stub in
place of a coil to reverse the phase. My 19th edition of the ARRL
Antenna Book shows such an antenna, "the super J-pole on page 16-25. At
other frequencies, this might be called a "Franklin Antenna". It`s a
1/2-wave in-phase with another 1/2-wave, one mounted directly over the
other.

The super J-pole is designed for 144 MHz, but can be scaled for any
frequency with proper mechanical allowances. Gain is about 6 dB over a
1/4-wave whip.

Best regards, Richard Harrison, KB5WZZI


Richard

I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave
stub over a ground. is it easy to show where i've missed something?

Jerry



  #35   Report Post  
Old September 17th 05, 08:55 AM
David
 
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I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto
the end of a SMA plug based on help from you guys. I have some pics of
the construction if anyone is interested in taking a look.
Where is the appropriate place to post these jpg files ?

I am accessing this newsgroup from my email client presently.

I am not sure how I can optimize this yet. My antenna Analyzer is the
MJF unit that only goes to 440MHz.

I have a 1GHz spec analyzer, 1GHz Sig Gen and a Telonic VSWR kit that
goes to 2.5GHz but I think the drive required to the VSWR "Rho-Tector"
needs to be higher than the 10dBm from the sig gen.

Could I optimize the design with this equipment, or maybe would I be
better to use RSSI levels from a receiver placed say 3m away from
Antenna under test ?

If the antenna works I will be very excited as it is very simple to
construct in around 10 minutes and at a cost of around $3 (no counting
labour).

Thanks for any help.

Jerry Martes wrote:
"Richard Harrison" wrote in message
...

Cecil, W5DXP wrote:
"The 180 deg. phase reversing coil is the tricky part."

For UHF, you might prefer to use a 1/4-wave short-circuited stub in
place of a coil to reverse the phase. My 19th edition of the ARRL
Antenna Book shows such an antenna, "the super J-pole on page 16-25. At
other frequencies, this might be called a "Franklin Antenna". It`s a
1/2-wave in-phase with another 1/2-wave, one mounted directly over the
other.

The super J-pole is designed for 144 MHz, but can be scaled for any
frequency with proper mechanical allowances. Gain is about 6 dB over a
1/4-wave whip.

Best regards, Richard Harrison, KB5WZZI



Richard

I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave
stub over a ground. is it easy to show where i've missed something?

Jerry





  #36   Report Post  
Old September 17th 05, 03:00 PM
Cecil Moore
 
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David wrote:

I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto
the end of a SMA plug based on help from you guys.


How are you matching the very high feedpoint impedance?

I have some pics of
the construction if anyone is interested in taking a look.
Where is the appropriate place to post these jpg files ?


Some of us have web pages from qsl.net for that purpose.
There is also a netnews group for that purpose. I think
it is alt.binaries.
--
73, Cecil http://www.qsl.net/w5dxp


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  #37   Report Post  
Old September 17th 05, 07:20 PM
Jerry Martes
 
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"Cecil Moore" wrote in message
...
David wrote:

I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto the
end of a SMA plug based on help from you guys.


How are you matching the very high feedpoint impedance?

I have some pics of the construction if anyone is interested in taking a
look.
Where is the appropriate place to post these jpg files ?


Some of us have web pages from qsl.net for that purpose.
There is also a netnews group for that purpose. I think
it is alt.binaries.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil

He is feeding the 1/2 wave antenna in the center, where the impedance will
be somewhere around 70 ohms. His coax feed line is located within one half
of the antenna and exits the 1/2 wave, center excited, dipole at the "high
impedance" end of the dipole.

David sent me some pictures. The antenna looks quite good.

The effectiveness of the "choke" that attempts to disconnect the dipole
from the feed line may be less than ideal, but it is yet unknown how good
this antenna needs to be.

Jerry


  #38   Report Post  
Old September 17th 05, 08:46 PM
Roy Lewallen
 
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Jerry Martes wrote:

I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave
stub over a ground. is it easy to show where i've missed something?


I think it should be more like 3 dB, but hadn't said anything until I
had a chance to model it. The quarter wave stub doesn't radiate
significantly, so it can be ignored. A half wavelength element should
have about 1.5 dB gain over a quarter wave. Two of them would give
another 3 dB if it weren't for mutual coupling, but the mutual coupling
of collinear elements reduces the gain to about 1.5 dB over a single
element. (See for example Fig. 39 on p. 8-35 of the ARRL Antenna Book,
20th Edition; look up Collinear, Gain and directivity in the index of
other editions; or model it with your favorite program.)

Roy Lewallen, W7EL
  #39   Report Post  
Old September 17th 05, 09:03 PM
Jerry Martes
 
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"Roy Lewallen" wrote in message
...
Jerry Martes wrote:

I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave
stub over a ground. is it easy to show where i've missed something?


I think it should be more like 3 dB, but hadn't said anything until I had
a chance to model it. The quarter wave stub doesn't radiate significantly,
so it can be ignored. A half wavelength element should have about 1.5 dB
gain over a quarter wave. Two of them would give another 3 dB if it
weren't for mutual coupling, but the mutual coupling of collinear elements
reduces the gain to about 1.5 dB over a single element. (See for example
Fig. 39 on p. 8-35 of the ARRL Antenna Book, 20th Edition; look up
Collinear, Gain and directivity in the index of other editions; or model
it with your favorite program.)

Roy Lewallen, W7EL


Thanks Roy

Your posts always add alot to my understanding. I appreciate your taking
time to evaluate this antenna and then pass the information on to the group.

Jerry


  #40   Report Post  
Old September 17th 05, 10:28 PM
Roy Lewallen
 
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Jerry Martes wrote:
"Roy Lewallen" wrote in message

I think it should be more like 3 dB, but hadn't said anything until I had
a chance to model it. The quarter wave stub doesn't radiate significantly,
so it can be ignored. A half wavelength element should have about 1.5 dB
gain over a quarter wave. Two of them would give another 3 dB if it
weren't for mutual coupling, but the mutual coupling of collinear elements
reduces the gain to about 1.5 dB over a single element. (See for example
Fig. 39 on p. 8-35 of the ARRL Antenna Book, 20th Edition; look up
Collinear, Gain and directivity in the index of other editions; or model
it with your favorite program.)

Roy Lewallen, W7EL



Thanks Roy

Your posts always add alot to my understanding. I appreciate your taking
time to evaluate this antenna and then pass the information on to the group.

Jerry


I'm sorry, I see I wasn't clear in saying that I haven't yet had a
chance to model it. There might be a flaw in my reasoning, and the
antenna might have more gain than I think. I'll try to get to it as soon
as I can -- unless someone else wants to take it on. But I'm extremely
busy just now.

Roy Lewallen, W7EL
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