ONAIR
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« Reply #30 on: November 16, 2011, 02:46:37 PM » |
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Wow, I haven't seen one of those in years! Isn't that the old lampshade CB antenna from the 1960s? |
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ONAIR
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« Reply #32 on: November 16, 2011, 08:01:01 PM » |
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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. |
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BOOTYMONSTER
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« Reply #33 on: November 16, 2011, 08:18:38 PM » |
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W4OP
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« Reply #34 on: November 17, 2011, 05:49:42 AM » |
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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
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ONAIR
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Posts: 1631
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« Reply #35 on: November 17, 2011, 02:29:21 PM » |
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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? |
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BOOTYMONSTER
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« Reply #36 on: November 17, 2011, 05:20:01 PM » |
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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  |
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W4OP
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« Reply #37 on: November 18, 2011, 06:21:00 AM » |
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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
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W8JI
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« Reply #38 on: November 18, 2011, 08:00:17 PM » |
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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. :-) |
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BOOTYMONSTER
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« Reply #39 on: November 19, 2011, 10:20:03 AM » |
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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 . |
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W8JI
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« Reply #40 on: November 19, 2011, 12:54:22 PM » |
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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. |
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W4OP
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« Reply #41 on: November 21, 2011, 09:00:31 AM » |
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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
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K1ZJH
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« Reply #42 on: November 21, 2011, 09:26:18 AM » |
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I'd like to see an 11-meter Isopole |
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KE1IZ
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« Reply #43 on: January 30, 2013, 09:58:14 AM » |
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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.
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AC2EU
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« Reply #44 on: January 30, 2013, 03:07:34 PM » |
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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".  |
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