Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #41   Report Post  
Old September 18th 05, 05:45 AM
David
 
Posts: n/a
Default

Cecil, Using a 1/4 wave open stub.

Cecil Moore wrote:
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.

  #42   Report Post  
Old September 18th 05, 05:53 AM
David
 
Posts: n/a
Default

Roy,

I am using your EZNEC V4.019. Would you happen to have the ez
description file for this antenna so I can see how it is modeled ?

I had a "play" and set up 2 vertical have wave lengths and set the
source at the centre I then placed a 1/4 wave wire near the lower
1/4 wave and connect them at the centre point with a horizontal wire but
the program kept complaining that the wires were overlapping etc.

I also tried modeling an open stub J-Pole and seemed to be able to get
some plots that look ok but SWR was 100 across the band. Not sure what
I did wrong here but I assume as I get more familiar with modeling
techniques, I will get more meaningful results.

Thanks

Regards

David

Roy Lewallen wrote:
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

  #43   Report Post  
Old September 18th 05, 07:07 AM
Roy Lewallen
 
Posts: n/a
Default

David wrote:
Roy,

I am using your EZNEC V4.019. Would you happen to have the ez
description file for this antenna so I can see how it is modeled ?


Sorry, I haven't had time to model it. But I dusted off a very old,
originally ELNEC, model of a 3 element collinear, and uploaded it to my
web site. It should give you the general idea. It's Collinear3.EZ at
http://eznec.com/misc/.

. . .


Roy Lewallen, W7EL
  #44   Report Post  
Old September 18th 05, 01:14 PM
Cecil Moore
 
Posts: n/a
Default

David wrote:
I also tried modeling an open stub J-Pole and seemed to be able to get
some plots that look ok but SWR was 100 across the band. Not sure what
I did wrong here but I assume as I get more familiar with modeling
techniques, I will get more meaningful results.


The open stub J-Pole (Zepp) is modeled as a shorted stub
J-Pole with the source in the middle of the shorted segment.
Mine is for 53.2 MHz. On edge it looks like this:

13.5 ft.
+---------------------------------------------------
|
(S)
|
+-----------------
4.5 ft.

The sections are 0.1 ft. apart. The resonant frequency is
53.2 MHz and the feedpoint impedance is 25 ohms for an SWR
of 2:1. The feedpoint is 30 ft. high over high-accuracy
ground. The gain over ground is 6.2 dBi, omnidirectional.
Free space gain is 3.4 dBi.

Anyone who wants a copy of this 6m J-Pole EZNEC file, please
send me an email to .
--
73, Cecil
http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
  #45   Report Post  
Old September 18th 05, 01:42 PM
Dan Richardson
 
Posts: n/a
Default

On Sun, 18 Sep 2005 07:14:39 -0500, Cecil Moore
wrote:


The open stub J-Pole (Zepp) is modeled as a shorted stub
J-Pole with the source in the middle of the shorted segment.
Mine is for 53.2 MHz. On edge it looks like this:

13.5 ft.
+---------------------------------------------------
|
(S)
|
+-----------------
4.5 ft.

The sections are 0.1 ft. apart. The resonant frequency is
53.2 MHz and the feedpoint impedance is 25 ohms for an SWR
of 2:1. The feedpoint is 30 ft. high over high-accuracy
ground. The gain over ground is 6.2 dBi, omnidirectional.
Free space gain is 3.4 dBi.


You can vary the feedpoint impedance by changing (S).

73,
Danny, K6MHE

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


  #46   Report Post  
Old September 18th 05, 02:55 PM
Cecil Moore
 
Posts: n/a
Default

Dan Richardson wrote:
You can vary the feedpoint impedance by changing (S).


Moving the elements to one foot apart increases the
feedpoint resistance to about 50 ohms at resonance
but it decreases the gain and distorts the radiation
pattern since one foot is an appreciable distance
on 6 meters.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
  #47   Report Post  
Old September 18th 05, 11:51 PM
David
 
Posts: n/a
Default

Thanks for the model, I'll try it.

What is the diameter of the elements. I understand you can also vary the
feedpoint impedance by changing element diameter ? (Zo = 276 log(2S/d)

Regards
David

Dan Richardson wrote:
On Sun, 18 Sep 2005 07:14:39 -0500, Cecil Moore
wrote:



The open stub J-Pole (Zepp) is modeled as a shorted stub
J-Pole with the source in the middle of the shorted segment.
Mine is for 53.2 MHz. On edge it looks like this:

13.5 ft.
+---------------------------------------------------
|
(S)
|
+-----------------
4.5 ft.

The sections are 0.1 ft. apart. The resonant frequency is
53.2 MHz and the feedpoint impedance is 25 ohms for an SWR
of 2:1. The feedpoint is 30 ft. high over high-accuracy
ground. The gain over ground is 6.2 dBi, omnidirectional.
Free space gain is 3.4 dBi.



You can vary the feedpoint impedance by changing (S).

73,
Danny, K6MHE

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

  #48   Report Post  
Old September 19th 05, 12:27 AM
David
 
Posts: n/a
Default

That model seems to work fine in EZNEC.

I tried a 918 MHz version with long length = 219mm, short length = 72mm,
24mm apart, 1.5mm solid aluminum construction.

The results show excellent SWR and feedpoint impedance close to 50
Ohms.(50.29 -j0.0148), SWR = 1.006
The unit is not so omnidirectional showing a front/back ratio of
5.79dB.(Beamwidth 214.4 Degrees).

The maximum gain was 4.74dB(ref) at 4 degrees (elevation). dB ref =
2.14dBi so I suppose this is saying gain is 4.74dBd.



Dan Richardson wrote:
On Sun, 18 Sep 2005 07:14:39 -0500, Cecil Moore
wrote:



The open stub J-Pole (Zepp) is modeled as a shorted stub
J-Pole with the source in the middle of the shorted segment.
Mine is for 53.2 MHz. On edge it looks like this:

13.5 ft.
+---------------------------------------------------
|
(S)
|
+-----------------
4.5 ft.

The sections are 0.1 ft. apart. The resonant frequency is
53.2 MHz and the feedpoint impedance is 25 ohms for an SWR
of 2:1. The feedpoint is 30 ft. high over high-accuracy
ground. The gain over ground is 6.2 dBi, omnidirectional.
Free space gain is 3.4 dBi.



You can vary the feedpoint impedance by changing (S).

73,
Danny, K6MHE

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

  #49   Report Post  
Old September 19th 05, 01:05 AM
Roy Lewallen
 
Posts: n/a
Default

David wrote:
That model seems to work fine in EZNEC.

I tried a 918 MHz version with long length = 219mm, short length = 72mm,
24mm apart, 1.5mm solid aluminum construction.

The results show excellent SWR and feedpoint impedance close to 50
Ohms.(50.29 -j0.0148), SWR = 1.006
The unit is not so omnidirectional showing a front/back ratio of
5.79dB.(Beamwidth 214.4 Degrees).


That's pretty strange. The only ready explanation is radiation from the
transmission line section. Apparently it's significant with the 0.074
wavelength conductor spacing.

The maximum gain was 4.74dB(ref) at 4 degrees (elevation). dB ref =
2.14dBi so I suppose this is saying gain is 4.74dBd.


If the antenna is in free space, the currents in the elements are
exactly in phase, and the transmission line radiation is negligible, the
gain should be maximum broadside to the antenna, not at 4 degrees
elevation. So one or more of those conditions isn't true. The horizontal
directivity indicates that the transmission line section is radiating,
but I wonder also if you might be modeling it over ground.

The gain is 4.7 dB relative to a dipole in free space. If you're
modeling it over ground, the gain isn't 4.7 dB over a dipole at the same
height above ground, which is why I dislike the term "dBd". (But antenna
manufacturers who can capitalize on the ambiguity love it.) If you're
modeling it over ground and want to know the gain over a dipole at the
same height, model a dipole at that height and compare gains.

Roy Lewallen, W7EL
  #50   Report Post  
Old September 19th 05, 01:08 AM
Roy Lewallen
 
Posts: n/a
Default

David wrote:
Thanks for the model, I'll try it.

What is the diameter of the elements. I understand you can also vary the
feedpoint impedance by changing element diameter ? (Zo = 276 log(2S/d)


Open the EZNEC file. In the main window, click the Wires line to open
the Wires Window. Find the Diameter column, where you'll see the
diameter of each conductor.

The equation you give is approximately the characteristic impedance of a
transmission line, not the feedpoint impedance of a J-pole. The
relationship between the two is very tenuous and complex.

Roy Lewallen, W7EL
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF} RHF Shortwave 23 November 3rd 04 02:38 PM
Antenna Advice Chris Shortwave 5 September 20th 04 02:04 AM
Help Please! Extremely Poor Reception In Turkey Rich Shortwave 12 December 30th 03 11:43 PM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM


All times are GMT +1. The time now is 02:41 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017