antenna specialist . Sigma IV mod AV-174

[missouri 69]

hi
I am thinking I may want to try a 3/4 wave ground plane...
maybe the antenna specialist Sigma IV mod AV-174...
anyone had experience with this antenna or know where one can be found?
thanks
sam
missouri 69

 

[Master Chief]

OK, here we go.... 8) 8) 8) :twisted:

 

[Master Chief]

I am a big fan of the Sigma IV! Visually, it is a beautiful antenna! I remember my first one and how proud I was of it! I had it on an overbuilt, homemade crank up tower and the tip was at 90'! I was disappointed that it didn't perform as well as my Sigma 5/8 though. The height above ground was certainly different which could have been part of the performance differences.

Years later the Internet came available and through a chance meeting I met one of the engineers from Avanti. I specifically asked him about the design of the Sigma IV and he stated that the design was similar to your average "J-Pole" antenna, fed with a gamma match.

You can look it up for yourself, but basically, a J-Pole is a 1/2 wave antenna that is physically 3/4 wave long. The bottom 1/4 wave section that pairs with the 1/4 wave matching stub essentially does not radiate. Only the upper 1/2 wave section radiates. This makes your every day "J-Pole" a 1/2 wave antenna and this is NOT disputed.

Looking at the design of the J-Pole and the layout of the Sigma IV, its easy to see the similarities. This also explains why my Sigma 5/8 outperformed my Sigma IV!

Freecell has convincedly argued, based on the patent information and his own extensive experience, that the loop and its three radials actually pulls the signal down towards the horizon thusly increasing the gain. While he is not convinced that it is truly a 3/4 wave antenna, he feels it certainly is not an average 1/2 wave J-Pole as I've argued.

Then you have bob85 from the other side of the pond. He LOVES the design and especially the 7/8 version. Bob's experience tells us that electrically, the 7/8 version has been his best performing antenna to date......and he has an I-10K too! Mechanically, the 7/8 version is weak and he beffed it up using original Avant parts.

The three of us have argued, with respect, over the last few years. We all agree that a true 3/4 wave antenna goes beyond the .64 magic number for wave pattern optimization and shows pattern distortion. In other words, the true 3/4 wave antenna doesn't work as good a s 5/8 wave antenna. I believe we also agree that the Sigma IV is NOT a true 3/4 wave antenna however which is why it works so well. Yes, it is 3/4 wave tall physically, but not electrically. Clear as mud?

The antenna is fed with a gamma match (I hate those!). This tells us that the "load" is less than 50 ohms as the gamma is used to increase the impedance of the load.

I like the Sigma IV! Bob loves his! Freecell makes a fantastic argument! If you have one and like how it works, use it! There is no doubt that the Sigma IV is a great antenna, but stay AWAY from the imported crap! They are cheap and unreliable.

For MY money, I'd rather have an I-10K than a Sigma IV. My experience teaches me that the 5/8 ground plane performs better than the Sigma IV. I do however, own three of the original Sigma IVs and try to pick them up whenever possible. I sure would like to find one new in the box for my collection!

Good luck with that antenna! Its a winner!

 

[bob85]

yes 3/4 and yes 7/8 or .82 just not a conventional endfed with transformer feed otherwise my 5/8 would whip it,
your right i do LOVE em but i would like to try an astroplane at this location even though the astro was not as good as my sigma when i tried them at home in an open field i feel it may do the trick here while i have to keep the antenna low relative to my surroundings,
i realise the surroundings also hurt the performance of my 5/8 even when mounted at 34+feet theres still too much crap and buildings near to the antenna to be ideal, the surroundings here are better suited to the avanti design i guess.

 

[missouri 69]

so how tall is the 7/8 version??
I know the sigma IV is 27'6"....
so the 7/8 version would be like....34'10" ????
And you could convert a 3/4 version to a 7/8 version???

so could a penetrator 500 5/8 wavew be converted to a 3/4 or 7/8 by making it longer??

thanks
sam
missouri 69

 

[bob85]

the 7/8 is about 32 foot and very weak,
i doubt the hygain could be just extended without spoiling the pattern,
lemm made a superlemm 3/4 conventional groundplane, i tried one and liked my other antennas better.

 

[Master Chief]

so could a penetrator 500 5/8 wave be converted to a 3/4 or 7/8 by making it longer??

NO! FIrst off the matching network is set up for a 5/8 wave. As I stated earlier, anything longer than .64 (5/8 is .625) exhibits pattern distortion; go back and read what I wrote before.

I also stated that its MY opinion that only the top 1/2 wave of the Sigma IV radiates, claiming the bottom 1/4 wave is part of the matching network. Using this same principal, the 7/8 version is a 5/8 over a 1/4 (5/8 + 2/8 = 7/8 ), where the Sigma IV would be a 1/2 over a 1/4 (2/4 + 1/4 = 3/4) This is to say that we have discussed this point out here. Am I right? I can't say for certain, but it all makes sense!

 

[W5LZ]

'M.C.',
Looking at it strictly from measurable dimensions your guesses about those antenna's length are close enough. But when you look at them 'electrically', the parts that sort of 'fold back' on the antenna, don't really count as 'length'. One example is that 'J' pole. It radiates just like a 1/4 wave antenna does (except for some minor pattern distortion due to that 'folding back' thingy). So, it's just a matter of 'how' you look at it.
The bottom line for any antenna is where it radiates the signal. If it puts it where you want it (and vis-versa for hearing). The 'other half' of any useful antenna is how easy/difficult it is to match to the rest of the system. Several ways of doing that, just a matter of what you're most comfortable with.
- 'Doc

PS - Finding one of those antenna modeling programs is a very nice idea. Learning to use it is even better!

 

[Marconi]

yes 3/4 and yes 7/8 or .82 just not a conventional endfed with transformer feed otherwise my 5/8 would whip it,
your right i do LOVE em but i would like to try an astroplane at this location even though the astro was not as good as my sigma when i tried them at home in an open field i feel it may do the trick here while i have to keep the antenna low relative to my surroundings,
i realise the surroundings also hurt the performance of my 5/8 even when mounted at 34+feet theres still too much crap and buildings near to the antenna to be ideal, the surroundings here are better suited to the avanti design i guess.

Bob, when you compared the AP and the Sigma before, how did the installed heights compare?

Is it your thinking that the AP installed low at your current location will somehow punch a signal thru nearby objects better than the Sigma or that it somehow will get the signal over these objects better?

In your last statement where you compared the 5/8 to the Sigma you claim the Sigma to perform better. Do you think this was because the Sigma has a smaller radial diameter (footprint)and therefore is affected less by the stuff nearby or what?

"Doc, what happens with the RF in the bottom of a J-Pole when the two elements are parallel and close together where one element has a + current flowing and the other has a - current flowing? Maybe it is not zero RF generated, but don't we see RF cancellation under this condition? Thus isn't this 1/4 wave section at the bottom simply a feeder for the end fed element above it, and therefore contribuites very little to the RF generated via the feeder?

"Doc you also once commented that a gamma match will only raise the feed point impedance and thus it would not tune a single 1/2 wave dipole element like a yagi element. If that is so, then how do the Sigma IV, the JoGunn Ground Plane, and the Wolf .64 work using gammas? Don't these particular elements without a matching device present a rather high impedance at the base?

I believe it is the action of a gamma to only add some inductive reactance to a shortened element and that the physical design allows for some reasonable adjustment along the element to a tap point where 50 ohms might exist. Maybe this dual adjustment can cause tuning problems, but can't it also be effective and reasonably lossless in matching the element?

 

[bob85]

marconi im not sure about exact heights, more than likely i would have put the hoop of the astro level with the bottom of the sigma,

not punch through but raise the point of maximum radiation and have a lower angle than the groundplane mounted at the same low height, i can set the antenna tips the same,

dont know about smaller footprint been the cause but the sigma does work very well when mounted low compared to other antennas,

what about a sigma style with a cap hat? do you think that may give any advantage over a stock sigma?

 

[Marconi]

In a practical sense, if we are comparing antennas, then getting them both at eqaul heights to the feed point is probably the best procedure. However, the AP was designed and probably reported with the maximum tip height in mind when comparing features. So, maybe having the tip heights in mind is the only way to bear out Avanti's claims. I compared mine with the tip heights equal and I saw things pretty much equal among them all. Even so, my comparing a 12' antenna to a 24' antenna was impressive in this configuration.

I also don't believe there is much truth in the RF pattern that Avanti suggests in their ads, with the beam going over the tops of all the buildings around. All of these 11 meter vertical antennas should produce very low angle patterns and I don't see or hear much difference.

The top hat on the AP will likely have some postitive affect on the antenna because it is 50% of the top radiating element, but I doubt a little wire on top of the Sigma, the I-10K, or the Wolf .64 will make one bit of difference.

 

[W5LZ]

Marconi,
"Doc, what happens with the RF in the bottom of a J-Pole when the two elements are parallel and close together where one element has a + current flowing and the other has a - current flowing? Maybe it is not zero RF generated, but don't we see RF cancellation under this condition? Thus isn't this 1/4 wave section at the bottom simply a feeder for the end fed element above it, and therefore contribuites very little to the RF generated via the feeder?"

Is it simply a feeder? Not that alone, it's also the 'other half' of the antenna. There's always two 'halves', the element radiating has to 'work' against something or there's no radiation. By tapping that bent 'other half' in the right places an acceptible input/matching impedance is produced. The same thing can be done if that bent 'other half' is straightened. Varying the feed point changes the antenna's input impedance.

"Doc you also once commented that a gamma match will only raise the feed point impedance and thus it would not tune a single 1/2 wave dipole element like a yagi element. If that is so, then how do the Sigma IV, the JoGunn Ground Plane, and the Wolf .64 work using gammas? Don't these particular elements without a matching device present a rather high impedance at the base?"

Yes, they do have a higher than 50 ohm input impedance. That gamma match also has inductive and capacitive reactance characteristics that can act the same way a 'lumped' inductive or capacitive reactance acts like. If the right proportions of the appropriate reactances are added at the feed point, it not only cancels out the reactances of the antenna, it also can change the ~apparent~ resistance of the antenna. Just like adding that "hook" at the bottom of a 'J'-pole does. The places where the feed line connects with that 'hook' determine the type and amount of reactances displayed.
As for a gamma match "always" raising impedances, well, while it's very typical of what gamma matches do, that's a "generalized" statement, and as such, never always true (just most of the time ). I know it sounds like a 'cop out' but it really isn't. My problem is in trying to explain it, and it's been way too long since I've had to Sort of like trying to remember which fork to use when eating 'properly. Haven't had to worry about that since I was taught so don't really remember for sure. (Hey! I make much better excuses than that, so you ought'a know it's true.) It involves phasing and I just ain't going there. Hate the taste of worms!
- 'Doc

 

[freecell]

"Don't these particular elements without a matching device present a rather high impedance at the base?"

no, the value of impedance present is less than 50 ohms resistive.

 

[W5LZ]

Nice way of confusing the issue, congratulations!
The 'resistive' portion of an input immpedance certainly doesn't imply a good/decent match if the reactive portion of that input impedance isn't manipulated, at least in the case of most non-1/4 wave end fed antennas (or odd multiples there of). What if there is no reactive components? Patent that thing! You got a miracle.
- 'Doc

 

[freecell]

"Don't these particular elements without a matching device present a rather high impedance at the base?"

no, the value of impedance present is less than 50 ohms resistive.