Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 2 [3] 4 5 6 7 8 Next   Go Down
  Print  
Author Topic: home-brew sigma 4 11 meter antenna  (Read 69194 times)
ONAIR
Member

Posts: 1738




Ignore
« Reply #30 on: November 16, 2011, 02:46:37 PM »

its up in the air . no coax on it , im beat , my back is aching , ill get it tuned and on the air tomorrow .
i did make a few changes to the antenna , ill post pics of those tomorrow also .



http://i867.photobucket.com/albums/ab234/BOOTY-MONSTER/IMG_0158-1.jpg
     Wow, I haven't seen one of those in years!  Isn't that the old lampshade CB antenna from the 1960s?
Logged
BOOTYMONSTER
Member

Posts: 73




Ignore
« Reply #31 on: November 16, 2011, 07:53:12 PM »

yep . the ones currently made by sirio have a tendency to break in high winds faster than other antennas ..... so i made a stronger one .

http://www.cbtricks.com/ant_manuals/avanti/av174/graphics/sigma4_av174_om.pdf

mine is different though . it has 108 inch basket elements , a 34 1/2 inch diameter loop and its vertical is 31 1/2 ft tall .
Logged
ONAIR
Member

Posts: 1738




Ignore
« Reply #32 on: November 16, 2011, 08:01:01 PM »

yep . the ones currently made by sirio have a tendency to break in high winds faster than other antennas ..... so i made a stronger one .

http://www.cbtricks.com/ant_manuals/avanti/av174/graphics/sigma4_av174_om.pdf

mine is different though . it has 108 inch basket elements , a 34 1/2 inch diameter loop and its vertical is 31 1/2 ft tall .
   Yeah, wasn't it called the Avanti Astro Plane?  We used to laugh our butts off when we first saw those things going up on top of homes!  I remember that even though they looked odd, they were good performers.   
Logged
BOOTYMONSTER
Member

Posts: 73




Ignore
« Reply #33 on: November 16, 2011, 08:18:38 PM »

its based on the sigma 4 design . heres a link to the Astro Plane

http://www.cbtricks.com/ant_manuals/avanti/av101/graphics/astro_plane_av101_om.pdf
Logged
W4OP
Member

Posts: 413


WWW

Ignore
« Reply #34 on: November 17, 2011, 05:49:42 AM »

I was an engineer at Avanti when the Sigma IV design was being built. It's a J pole- nothing more and nothing less. No magic,

Dale W4OP
Logged
ONAIR
Member

Posts: 1738




Ignore
« Reply #35 on: November 17, 2011, 02:29:21 PM »

I was an engineer at Avanti when the Sigma IV design was being built. It's a J pole- nothing more and nothing less. No magic,

Dale W4OP
    Interesting.  Did it have any measurable gain or performance advantage over the host of colinears, ground planes, and other 11M antennas that were being offered at that time?
Logged
BOOTYMONSTER
Member

Posts: 73




Ignore
« Reply #36 on: November 17, 2011, 05:20:01 PM »

i don't believe in magic .
i do believe when i hear distant stations most other locals dont .

W4OPdo you think there is or was a better ground plane made for 11 meters , keeping a equal feed-point ?
i've herd the ham international big mac was a really good antenna ..... its way to difficult for me to try to make though . and i really like the narrow horizontal area the sigma 4 needs . thanks for designing such a fine antenna Smiley


Logged
W4OP
Member

Posts: 413


WWW

Ignore
« Reply #37 on: November 18, 2011, 06:21:00 AM »

It's an end fed half wave- so same gain as a dipole IF you can keep radiation currents off the mast and the outside of the coax- this is the downfall of all of the groundplanes, J poles etc.

W4OP
Logged
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #38 on: November 18, 2011, 08:00:17 PM »

It's an end fed half wave- so same gain as a dipole IF you can keep radiation currents off the mast and the outside of the coax- this is the downfall of all of the groundplanes, J poles etc.

W4OP

I see a smart antenna person on verticals and groundplanes who knows how they work and what the problems are has arrived on scene.

:-)

Logged
BOOTYMONSTER
Member

Posts: 73




Ignore
« Reply #39 on: November 19, 2011, 10:20:03 AM »

same gain as a dipole
W4OP

well , i can say it kicks major ass over any dipole or 1/4 wave ground plane i ever used and does better on TX and RX than the 5/8wgp's i've used also . i'm just going by my real world results here ........ but i do appreciate all comments .
Logged
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #40 on: November 19, 2011, 12:54:22 PM »

same gain as a dipole
W4OP

well , i can say it kicks major ass over any dipole or 1/4 wave ground plane i ever used and does better on TX and RX than the 5/8wgp's i've used also . i'm just going by my real world results here ........ but i do appreciate all comments .

That can only be because your OTHER antennas are less than good. Most CB omni antennas I have seen have terrible common mode.

When I designed a commercial four radial groundplane, I had to insulate the radials from the mast and use a common mode isolator between the feedpoint and the radial-antenna junction.

Prior to doing that, the four radial groundplane had terrible common mode.
Logged
W4OP
Member

Posts: 413


WWW

Ignore
« Reply #41 on: November 21, 2011, 09:00:31 AM »

I think the common mode issue is why so many hams today believe they can adjust VSWR by trimming the coaxial cable length.
If trimming your coax changes the VSWR:
1. You have common mode issues
2. Your coax is other than 50 Ohms- a real possibility these days I suppose
3. Your measuring instrument is not 50 Ohms

There's probably other reasons  that I overlooked.

A walk down a Smith Chart should convince you that changing coax length only rotates around the chart on a constant VSWR circle. This from meory- haven't done it in years.

Dale W4OP
Logged
K1ZJH
Member

Posts: 949




Ignore
« Reply #42 on: November 21, 2011, 09:26:18 AM »

I'd like to see an 11-meter Isopole Smiley
Logged
KE1IZ
Member

Posts: 30




Ignore
« Reply #43 on: January 30, 2013, 09:58:14 AM »

It's not my goal to offend anyone here but it sure would be nice if those who were commenting on the Sigma IV design actually had any first hand experience experimenting with it prior to offering their opinions. Years ago before he passed L.B. Cibik analyzed the design and claimed it was a "non apparent collinear". He obviously understood the function of the basket at the base as a means of containing the out of phase radiation in the lower portion of the vertical it shields while the basket itself radiates in phase with the upper 1/2 section of the vertical. Does anyone here have more antenna experience than Cebik? Has anyone here that thinks it behaves like a 1/2 wave compared gain on the horizon between the Sigma design and a 1/2 wave before commenting?

I respect Tom as a knowledgeable person due to his background but I have to respectfully disagree with his thoughts that the Sigma needs horizontal radials or that its pattern falls apart when modeled with coax or mast. I've experimented with both of those ideas extensively. When a second set of horizontal radials are added to a Sigma gain on the horizon drops! I experimented with every position possible on the second set of radials and the only time gain went up was when these radials were swept upwards close to the angle of the original radials. What everyone overlooks is those radials are in phase with the currents on the upper vertical. If they are not bent upwards towards the angle of the vertical, their radiation currents do not reinforce the vertical radiation pattern on the horizon.

Furthermore, mast and coax radiation at the base of this antenna are actually in phase with the currents radiating from the antenna. Obviously the entire coax and mast would not be in phase if they were long but that begining length is where the current is strongest and modeling shows adding mast and coax adds to the gain. One other problem that has led to a misunderstanding of this antenna is the advent of EZNEC software. For some reason this program agrees with the myth the antenna is only a half wave. I'm not looking to open a debate regarding EZNEC on this antenna but I will tell you that the program does not identify the radiation current and phase correctly and that becomes apparent when you try to design a collinear version of the antenna where the program has to identify the current correctly to show gain.

If you doubt what I'm saying, buy EZNEC+ and a more advanced program like CST Microwave Studio. Model the antenna in both programs and watch how much closer CST results match what happens in the field. EZNEC+ will show the currents to be inaccurate by 90 degrees ( the exact amount of radiation taking place in the basket) with almost no gain because it somehow misses the in phase basket radiation while CST clearly shows constructive radiation in the basket with two current nodes that compress the pattern down on the horizon.
Logged
AC2EU
Member

Posts: 361


WWW

Ignore
« Reply #44 on: January 30, 2013, 03:07:34 PM »

Yup a lot of us started out on CB. if it hadnt been for CB i wouldn't have had any interest in ham radio. I enjoy ham radio but i dont like the atitudes a lot of hams have.  

I'm sure you know why you "upgraded".  Sure, everyone whether they admit it or not, has noodled with CB in some form, but I wouldn't go so far as to defend  "the band".  Grin
Logged

Pages: Prev 1 2 [3] 4 5 6 7 8 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!