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Post by Papa Alpha on Jan 24, 2008 9:40:24 GMT -5
What is the general opinion on the Avanti Sigma IV (AV 174) omni directional antenna? Still after 25 years I have a brand new one in a box at the attic.
73's Harry.
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Sandbagger
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Post by Sandbagger on Jan 24, 2008 10:18:48 GMT -5
What is the general opinion on the Avanti Sigma IV (AV 174) omni directional antenna? Still after 25 years I have a brand new one in a box at the attic. 73's Harry. Well, I've had one for over 25 years and it's the best omni I ever owned. Not that I've owned every antenna made, but where I used to live (in a relative hole), the Sigma 4 put out a better signal than a 1/2 wave ground plane, an Astro Plane, and a 5/8th wave GP, and was just under a 3 element beam. If you have one new in the box, you'd get a mint for it on E-Bay, assuming you didn't want to use it yourself.
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Post by masterchief on Jan 26, 2008 12:47:24 GMT -5
The Sigma IV is a pretty antenna. While being physically 3/4 wave long, it is only a 1/2 wave antenna. It functions much like a J-Pole (physically 3/4 wave long, bottom 1/4 being part of the matching network).
This has been argued time and time again on the Internet. My information comes from my own testing and speaking directly to one of the engineers from Avanti.
Avanti built this antenna in order to get away from the 18' footprint of the ground radials.
As we (should) know, the longest monopole that produces gain is the .64 wave or 5/8 (.625) wave antenna. A 3/4 (.75) wave antenna exhibits pattern breakup with very high angle radiation lobes. In other words, it works, but lacks the gain of a 5/8 wave antenna. They figured this out way back in the 1930s. This is why you don't see antennas longer than .64 wave by ANY manufacturer. Even the broadcast industry stays below 5/8 wave monopoles. If a 3/4 wave antenna was better, everyone would have one.
I have argued that the Sigma IV has a higher angle of radiation caused by the cone, but have yet to actually prove it. This would explain why it works better for Sandbagger, who lives "in a relative hole". A 5/8 wave antenna has a lower angle of radiation which would be blocked by higher surrounding objects and land masses.
So why did Avanti call it a 3/4 wave antenna? MARKETING......and the fact that it IS "physically" 3/4 wave tall. Bigger is better, right?! People went out in droves to buy this antenna......including me!
I replaced my Avanti Sigma II (Sigma 5/8) and saw a reduction in performance, even at various heights.
The Sigma IV also uses a gamma match which has more loss than the Sigma 5/8. I'm not a fan of the gamma match as there are way better matching systems. The gamma is CHEAP to build and EASY to tune so the manufacturers love them. They work......everything works......but some things work better.
I'd love to have a new-in-box Sigma IV to add to my collection, but use an I-10K as my base antenna.
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Sandbagger
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Post by Sandbagger on Jan 26, 2008 17:11:52 GMT -5
The Sigma IV is a pretty antenna. While being physically 3/4 wave long, it is only a 1/2 wave antenna. It functions much like a J-Pole (physically 3/4 wave long, bottom 1/4 being part of the matching network). This has been argued time and time again on the Internet. My information comes from my own testing and speaking directly to one of the engineers from Avanti. Avanti built this antenna in order to get away from the 18' footprint of the ground radials. As we (should) know, the longest monopole that produces gain is the .64 wave or 5/8 (.625) wave antenna. A 3/4 (.75) wave antenna exhibits pattern breakup with very high angle radiation lobes. In other words, it works, but lacks the gain of a 5/8 wave antenna. They figured this out way back in the 1930s. This is why you don't see antennas longer than .64 wave by ANY manufacturer. Even the broadcast industry stays below 5/8 wave monopoles. If a 3/4 wave antenna was better, everyone would have one. I have argued that the Sigma IV has a higher angle of radiation caused by the cone, but have yet to actually prove it. This would explain why it works better for Sandbagger, who lives "in a relative hole". A 5/8 wave antenna has a lower angle of radiation which would be blocked by higher surrounding objects and land masses. So why did Avanti call it a 3/4 wave antenna? MARKETING......and the fact that it IS "physically" 3/4 wave tall. Bigger is better, right?! People went out in droves to buy this antenna......including me! I replaced my Avanti Sigma II (Sigma 5/8) and saw a reduction in performance, even at various heights. The Sigma IV also uses a gamma match which has more loss than the Sigma 5/8. I'm not a fan of the gamma match as there are way better matching systems. The gamma is CHEAP to build and EASY to tune so the manufacturers love them. They work......everything works......but some things work better. I'd love to have a new-in-box Sigma IV to add to my collection, but use an I-10K as my base antenna. I too, have heard the "Sigma 4 is really not a 3/4 wave antenna" arguments as well. Since I'm not an antenna engineer, I don't really have enough knowlege on this specific design to mount an effective argument. However I do know that the 3/4 wave WILL produce as much or more gain than a 5/8 wave, it's just that the radiation angle is so high that it would be all but useless unless you were intending to talk to orbiting space vehicles or talking sporatic- E skip (Which the Sigma IV truly shines at). The .64 wave is the longest antenna which produces the most effective gain at a fairly low angle of radiation. But as you accurately observed, in the real world, a low angle of radiation is not always preferred. In my own (admittedly not scientific) tests between the common antenna designs of the time (late 70's), the Sigma IV was the best performing for my situation. Had I been on the top of a hill overlooking valleys all around, my results would likely have been different. Which underscores the fact that there is no "one size fits all" antenna which will work the best in all situations. When choosing an antenna, one must consider how and where it will be mounted, and how high up in HAAT your station is.
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Post by masterchief on Jan 28, 2008 12:19:42 GMT -5
However I do know that the 3/4 wave WILL produce as much or more gain than a 5/8 wave No it doesn't. The single lobe breaks up into many smaller lobes, all of which have LESS GAIN than the .64 antenna.
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Sandbagger
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Post by Sandbagger on Jan 28, 2008 13:21:05 GMT -5
However I do know that the 3/4 wave WILL produce as much or more gain than a 5/8 wave No it doesn't. The single lobe breaks up into many smaller lobes, all of which have LESS GAIN than the .64 antenna. I'll admit it's been a few years since I took "antenna 101", but I seem to recall seeing the E-Plane radiation patterns for each of the major antenna configurations. Whereas a dipole resembled a donut shaped pattern, the 5/8th wave resembled a more "flattened" donut with the major lobe relatively horizontal, with minor lobes at the 30 and 45 degree up angle. The 3/4 wave was basically the reverse of that with the major lobes at a 30 and 45 degree angle with minor lobes in the horizontal plane and in other points. The gain on those lobes was still relatively high, but they were deemed not very useful because of the very high radiation angles. So in terms of "useful gain", the 3/4 wave falls fall short, but it can still achieve respectable gain where the major lobes are present. Anyway, I'll have to look into it again and see if my memory is still serving me, or whether I need to start looking at the early onset Alzheimer's literature As to what actual configuration the Sigma 4 really is, I guess that's for the antenna gurus to decide. Avanti must have employed a bunch of imaginative engineers with warped senses of humor. There is just as much discussion/debate/argument about the design of their Astro Plane and whether it's a 1/2 or 5/8th wave design. Whatever you want to call the S-IV, it worked darned good for me.
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Post by Night Ranger on Jan 28, 2008 16:21:12 GMT -5
I would guess the Avanti Sigma IV is basically just a J-Pole which makes it a radiating vertical half wave over a non-radiation 1/4 wave. The bottom 1/4 wave section is for matching purposes only. I think the Sirio Vector 4000 is basically the same antenna. The only way to get more gain than a .64 wave in a vertical omni-directional antenna is to start stacking vertical half wave elements or make a full vertical extended double zepp. Both the full extended double zepp and the two element vertical collinear array would be a problem to maintain from a mechanical point of view at 27 MHz due to their size. The two element collinear array would be about 36 feet tall and the extended double zepp would be about 46 feet tall. Night Ranger
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Sandbagger
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Post by Sandbagger on Jan 28, 2008 19:52:07 GMT -5
I would guess the Avanti Sigma IV is basically just a J-Pole which makes it a radiating vertical half wave over a non-radiation 1/4 wave. The bottom 1/4 wave section is for matching purposes only. I think the Sirio Vector 4000 is basically the same antenna. The only way to get more gain than a .64 wave in a vertical omni-directional antenna is to start stacking vertical half wave elements or make a full vertical extended double zepp. Both the full extended double zepp and the two element vertical collinear array would be a problem to maintain from a mechanical point of view at 27 MHz due to their size. The two element collinear array would be about 36 feet tall and the extended double zepp would be about 46 feet tall. Night Ranger That seems to be one of the more popular explanations for what type of antenna the Sigma 4 was. But if it was truly only a 1/2 wave radiator, it sure blew the socks off of the rat shack 1/2 wave ground plane, Hustler 5/8th wave antenna, and the Astro Plane antenna that I had.
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Post by Night Ranger on Jan 28, 2008 20:38:06 GMT -5
Well the only other explanation I can see is that the upside down cone acts a 1/4 wave phase reversal sleeve, and that would make the Sigma 4 a radiating 1/2 wave over a radiating 1/4 wave. You would have to reverse the phase of the bottom section from the top half wave section or else the out of phase signal from the top 1/2 wave and the bottom 1/4 wave would cancel each other out when radiating perpendicular to the plane of the vertical element. The out of phase signals in a straight 3/4 wave or longer end fed wire antenna are the reason nulls open up perpendicular to the plane of the wire and lobes form at 45 degree angles from the plane of the antenna. The null is where the out of phase signals are cancelling each other out, and the lobes are where they are combining. Click the image below to see an example of a phase reversal sleeve or "skirt" as it is sometimes called. My first guess is still that the Sigma 4 is probably just a J-Pole. -Night Ranger
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Sandbagger
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Post by Sandbagger on Jan 29, 2008 7:56:12 GMT -5
Well the only other explanation I can see is that the upside down cone acts a 1/4 wave phase reversal sleeve, and that would make the Sigma 4 a radiating 1/2 wave over a radiating 1/4 wave. You would have to reverse the phase of the bottom section from the top half wave section or else the out of phase signal from the top 1/2 wave and the bottom 1/4 wave would cancel each other out when radiating perpendicular to the plane of the vertical element. The out of phase signals in a straight 3/4 wave or longer end fed wire antenna are the reason nulls open up perpendicular to the plane of the wire and lobes form at 45 degree angles from the plane of the antenna. The null is where the out of phase signals are cancelling each other out, and the lobes are where they are combining. Click the image below to see an example of a phase reversal sleeve or "skirt" as it is sometimes called. My first guess is still that the Sigma 4 is probably just a J-Pole. My gut "theory" was along those same lines. The hoop and "umbrella" radials acted as a phase angle "correction" which allowed a 3/4 wave to have a lower radiation angle. It would be interesting to see something like this properly modeled on EZNEC or something similar. But whatever the Sigma 4 ends up actually being, it worked really well for me. Whether this had to do with my lower than average terrain location, or a credit to the overall performance of the antenna itself, I really don't know for sure. But I can't argue with the results. I had increased signals from all of my locals. One guy in particular was my "signal standard". He would turn his RF gain all the way down and look at my signal. On the 1/2 wave antenna, my signal was anywhere between 3/4 and 1-3/4 S-units depending on conditions and weather. The Astro Plane was 1/2 - 1-1/2 S units. The Hustler Trumpet 5/8th wave was about 2-1/2 - 3 S units. The Sigma was over S3 and sometimes was as high as S4. Later on, my Astro Beam on the top of my 40' tower would best S5 on his meter (which was also the signal I gave him with the 5/8th wave antenna while running a 2 tube amp). Now I'll admit that over the years, I used some different radios, but they all put out within a watt of each other, so I don't think that differences in radios would account for the consistent signal differences. The guy who was my receive standard never changed anything. So as his antenna aged and weathered (he had a Hustler Jam Ram 5/8th wave), his performance, if anything, should have decreased somewhat over the years. I've always wanted to do a real live antenna test using base antennas. We've already done it in the past with mobile antennas, where someone will drive to a high open spot and swap between various antennas and see which one works best. But I'd love to do it with base antennas, It's a little awkward to do, and most people aren't into it enough to take down their base antenna to donate to the cause. But I'm sure it would be a real eye opener, especially in my area now, where there are hills and valleys all over the place which might favor one antenna design over another depending on where each receive station is located and what they see.
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Post by Sniper..Unit 305 on Jan 29, 2008 12:59:31 GMT -5
Sandbagger I remember years ago, (now I'm talking in the early 70's) whether this is true or not I can't say for sure because I never owned one and at that time lived on very high ground. People that used to live in low areas surrounded by hills allways swore that the old Astroplane was the way to go to get out, because of that hoop (I guess) at the bootom let it radiate better for them. Any ideas as to the truth of this and could this be why the Sigma 4 worked out so well for you ? Just had to ask, because it became a such a common occurance back then that the folks who did live in low areas ended up buying the Astroplanes like they were going out of style. At least that was the way it was in our area around Pittsburgh, Greensburg, and McKeesport, there were so darn many of those things. Just wonder if it really did have an advantage that much or if it was just wishfull thinking on their part ? Sniper OT2194
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Sandbagger
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Post by Sandbagger on Jan 29, 2008 13:12:59 GMT -5
Sandbagger I remember years ago, (now I'm talking in the early 70's) whether this is true or not I can't say for sure because I never owned one and at that time lived on very high ground. People that used to live in low areas surrounded by hills allways swore that the old Astroplane was the way to go to get out, because of that hoop (I guess) at the bootom let it radiate better for them. Any ideas as to the truth of this and could this be why the Sigma 4 worked out so well for you ? Just had to ask, because it became a such a common occurance back then that the folks who did live in low areas ended up buying the Astroplanes like they were going out of style. At least that was the way it was in our area around Pittsburgh, Greensburg, and McKeesport, there were so darn many of those things. Just wonder if it really did have an advantage that much or if it was just wishfull thinking on their part ? Sniper OT2194 You know, we found just the opposite to be true for the Astro Plane (and to a lesser extent, the Starduster as well) . The people who had them raised up real high and on clear unobstructed terrain seemed to do better than those with 1/2 or 5/8th wave GP's, while those in the valleys and "holes" seemed to do better with standard ground planes and later on, the Sigma 4 (I don't think the S4 came out until the very late 70's or early 80's). My own comparison with my brief ownership of an Astro Plane would support this as well. Although, to be fair, I mounted the AP on the same mast that I had my original Radio Shack Super Maxim 1/2 wave GP on. As you know most of the AP hangs down below the mast, so I was actually losing 5' to 8' of height. It would have been a fairer test to have added another 5' of mast, but at that time, I had parental restrictions.......
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Post by Sniper..Unit 305 on Jan 29, 2008 14:42:55 GMT -5
Sandbagger, Thats what I allways wondered, if it really did help them. I had a friend that lived in McKeesport along the river but about midway up a hill overlooking the river. He was surrounded by the rest of the hill to his back ..open to the front for about 500 yds accross the river to another hillside and then open up and down the river of course. He had both an Astroplane and an Astro beam and he swore it was the astroplane that really helped. Again, this was another case of him ever only trying and having an astroplane as his only omni. Thanks for your input on what you actuallly tested and found to be more fact than what I had become to believe. Sniper OT2194
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Post by masterchief on Feb 17, 2008 14:56:34 GMT -5
That seems to be one of the more popular explanations for what type of antenna the Sigma 4 was. But if it was truly only a 1/2 wave radiator, it sure blew the socks off of the rat shack 1/2 wave ground plane, Hustler 5/8th wave antenna, and the Astro Plane antenna that I had. Sure it did. The Sigma IV (not a ground plane antenna) has a higher angle of radiation than the 1/2 wave ground plane antenna. Don't confuse gain with the angle of radiation. A 1/2 wave antenna will exhibit the same amount of gain, but the design of the antenna will determine where that gain goes. As you pointed out, the Sigma IV works better in the hole you live in.
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Post by masterchief on Feb 17, 2008 15:02:31 GMT -5
Well the only other explanation I can see is that the upside down cone acts a 1/4 wave phase reversal sleeve, and that would make the Sigma 4 a radiating 1/2 wave over a radiating 1/4 wave. My first guess is still that the Sigma 4 is probably just a J-Pole. With the Sigma IV, this is not the case. Here is what one of the Avanti Engineers sent to me: If that doesn't dispell all the myths, I don't know what will.
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Sandbagger
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Post by Sandbagger on Feb 18, 2008 8:33:04 GMT -5
Well the only other explanation I can see is that the upside down cone acts a 1/4 wave phase reversal sleeve, and that would make the Sigma 4 a radiating 1/2 wave over a radiating 1/4 wave. My first guess is still that the Sigma 4 is probably just a J-Pole. With the Sigma IV, this is not the case. Here is what one of the Avanti Engineers sent to me: If that doesn't dispell all the myths, I don't know what will. Well, that's about as "official" as you can get. But whatever you call it, all I know is that it worked well. Even now, where I live, I'm no longer in a hole and I'm actually at a fairly good elevation. I put the S-IV on a ground-mounted 8' pole and it was only about 1 "S" unit below my Maco 3 element beam on 30' of mast. I've recently given the antenna to a friend of mine who lives 20 miles from me who is currently running a PDL-2 on a 10' pole. I'm waiting to see the difference on his end.
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Post by zman on Sept 29, 2008 20:52:53 GMT -5
I regularly talk to an old timer on CH 38 LSB that lives about 25 west of me. He runs a Sigma 4 that is up about 20' off the ground mounted on the end of a garage running about 25 watts or so. On any given night he gives me about an S7 to an s9 to me on my GP. Of course a tad more on my beam if i aim it a him. But anyways, his antenna seems to work great! Our terain here is pretty flat with some small hills but overall is petty level here. There is another old timer that lives te same distance over in Ohio that has a Sigma 4 up on a 40' tower. He used to give me about an s6 to a 9 on my omni, but he hardley ever gets on the air anymore. But pretty much everyone that i know that have the Sigma 4 (including me) like them so much they refuse to sell them!!! I know mine is not going anywere!
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Sandbagger
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Post by Sandbagger on Sept 30, 2008 7:29:51 GMT -5
I regularly talk to an old timer on CH 38 LSB that lives about 25 west of me. He runs a Sigma 4 that is up about 20' off the ground mounted on the end of a garage running about 25 watts or so. On any given night he gives me about an S7 to an s9 to me on my GP. Of course a tad more on my beam if i aim it a him. But anyways, his antenna seems to work great! Our terain here is pretty flat with some small hills but overall is petty level here. There is another old timer that lives te same distance over in Ohio that has a Sigma 4 up on a 40' tower. He used to give me about an s6 to a 9 on my omni, but he hardley ever gets on the air anymore. But pretty much everyone that i know that have the Sigma 4 (including me) like them so much they refuse to sell them!!! I know mine is not going anywere! I know from my own personal experience with the S-IV, that it was my best performing out of all the omni antennas I owned back in the 70's and 80's. It outperformed a dipole, 1/2 wave Groundplane, an Astro Plane, and a 5/8th wave GP. All were mounted to the same mast so it was pretty much a fair comparison. There have been all sorts of discussions on the subject of the Sigma 4 and how some feel that it's electrically no more than a 1/2 wave antenna, and how the radiation angle is high and what not. Now I'm not an antenna engineer (My expertise is at the other end of the feedline), so I can't comment on the validity of these claims. But I do know how it worked for me in the real world. Only a beam does better.....
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Post by zman on Sept 30, 2008 20:29:09 GMT -5
Im the same way. Put at the buisness end of the coax and not at the paper and book end and im happy! :-)
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Post by viper926dave on Oct 4, 2008 7:30:54 GMT -5
i just got a sigma IV and its missing the ring and the spreader for the radials so i need to make a ring and spreader but i need the size of the ring and the spacing of the radials on the ring thanks for any info
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Post by Sniper..Unit 305 on Oct 4, 2008 16:51:54 GMT -5
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Post by viper926dave on Oct 4, 2008 18:28:11 GMT -5
thanks alot sniper i completely over looked the measurements in the diagram dave
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Post by Sniper..Unit 305 on Oct 4, 2008 20:54:57 GMT -5
Not a problem and its easy to do !! Sniper
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Post by marconi390 on Oct 31, 2008 6:50:54 GMT -5
dave, my S4 has only three radials and I will be glad to measure the items you need if you let me know you have only three radials also.
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Post by bob85 on Oct 31, 2008 14:38:02 GMT -5
the sigma4 loop is made from 4x 2ft sections diameter is 30",
double skin the lower section/improve the way the gamma fastens to the so239 and you have one of the best 11mtr verticals ever made, much misunderstood, its not a jpole, i believe it is a 1/2wave over 1/4wave none aparent colinear array, when tuned for maximum signal/correct phasing it will outperform any 11mtr groundplane antenna apart from the 5/8 over 1/4 ham big-mac, the 4 leg 32ft copies work even better but they are far too weak in bad weather.
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Post by dgtr43 on Nov 1, 2008 13:53:06 GMT -5
Can anyone tell me if the Tram 1498 GP or the Antenna Specialist M-417 POLECAT is a good antenna?
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Post by viper926dave on Nov 4, 2008 9:58:58 GMT -5
thanks marconi i think i have it
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Post by mark4 on Jan 18, 2009 9:16:59 GMT -5
As for the Astroplane this was one of the worst performing antennas I ever owned. It was good at one thing TVI! And I highly doubt this is truly a 5/8 wave.
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Sandbagger
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Post by Sandbagger on Jan 18, 2009 9:50:48 GMT -5
As for the Astroplane this was one of the worst performing antennas I ever owned. It was good at one thing TVI! And I highly doubt this is truly a 5/8 wave. I would have to agree. In my situation (YMMV), the Astro Plane barely kept up with the 1/2 wave Radio Shack ground plane I started out with. The AP got blown away by a 5/8 wave ground plane, and the Sigma 4 was even better than that. But I lived in a creek valley, with gradually sloping (up) hills on 2 sides of my location, and a gradual incease in altitude upstream. I only had one "good" direction, which was only in the clear for about 2 miles where, once again, the altitude increased sharply. Someone running an AP on top of a hill or otherwise in the clear would probably see better performance.
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Post by ic751a on Mar 2, 2009 20:10:53 GMT -5
the sigma4 loop is made from 4x 2ft sections diameter is 30", double skin the lower section/improve the way the gamma fastens to the so239 and you have one of the best 11mtr verticals ever made, much misunderstood, its not a jpole, i believe it is a 1/2wave over 1/4wave none aparent colinear array, when tuned for maximum signal/correct phasing it will outperform any 11mtr groundplane antenna apart from the 5/8 over 1/4 ham big-mac, the 4 leg 32ft copies work even better but they are far too weak in bad weather. I beg to differ. I owned (for a short time) the 3/4wave gold anodized Larry's LW-150 version which was incredibly well-built and gave a whole new meaning to the term 'heavy-duty'. I bet it was at least as heavy as TWO I-10Ks, and used a 7" long 4-allen bolt machined cylinder of forged Aluminum, necked/stepped down in the center for joining each section to the next one up. Incredibly well made. A machinists dream. I was certain that I had the best antenna in the world, and probably did when it came to construction quality. Removed my Penetrator from it's 67' perch, atop the 5 section mast above the 2-story roof and replaced it with the LW-150 3/4 wave. Lost performance. The further out I tried to talk the more my signal loss was evident. Mastercheif is correct, there is no antenna with as low or lower a take-off angle of radiation than a .64, period, end of story. Funny thing was, and even though it was taller by ~5 feet, I actually LOST receive signal strength by about the same amount as I lost transmit signal strength; about 1.5 S-units at 45 miles. Sold it. Hated to, as it was truly a work of machining art. Last I heard, it now lives in Boise, Idaho behind my old Yaesu FT-107M. I still have & use the Hy-Gain Penetrator, but I sure do miss that radio. . . . . .
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