Avanti Sigma4: An alternative view point

[wavrider]

Definitely some Gospel truth contained within that last post.

I am still hoping to be wrong about my expectations of the modded Salute losing to the .64 & 5/8 designs, because I want it to win. I like it's appearance better than a 4-radial CB looking thing on my roof. That 'ice cream cone' looks cool and now I wish I would've kept my LW-150 just in case all the monopole hoopla proves true.

- But mostly, I would LOVE to increase my omni performance since I find it the most used antenna in my airspace.

I don't see any reason a couple of obviously sharp operators such as Bob85 & Shockwave would be interested in propagating BS, nor do they seem the type to be too obtuse to tell a better performing antenna from naught so I actually have high hopes that I simply didn't find the right tuning for my old LW-150 waaaaaaaaaaaay back in '88... dang it, but that I WILL in a week or seven for Serge, thus spurring me on to fab one for myself from what else but...

Penetrator parts!


But I admit, I still have my doubts.

- We'll see soon enough.

Sirrio's new vector 4000 design claims 2DBD gain listed on their website, now it is getting some where, a true test result over the regular standard di pole, a full wave loop has 3dbd over a dipole, and it is just a wire.


Did some more research and found this blurb on another site, pretty well sums it up all in one.

*****5/8 Wave Vertical
There's nothing magical about the 5/8 proportion of a wavelength (62.5%), a whip this long has no special properties - a coil is needed to bring the antenna into resonance. It just happens to be close to the limit (0.64 of a wavelength) to which you can extend a single element (1/4 wave style) antenna before the radiation pattern breaks up in undesirable ways. By making the whip as long as possible compared to a 1/4 wave, we get a lower angle of radiation and a squashed radiation pattern (less signal goes upwards) and so a higher effective gain (3dB) than a 1/4 wave. Popular for mobile use, but they can lead to worse results in hilly areas as less signal gets up into the hills. If used on a handheld, you'll need to keep it vertical - which is why manufacturers usually provide 1/4 waves instead. 7/8 waves are slightly different, they are collinears - a 1/4 wave (2/8) combined with a 5/8 section, yielding a dB or so more than a 5/8 on it's own. ****End of quote

Except the Sigma is not a collinear antenna it is a shunt feed vertical.

So theory states the Sigma IV will have a whole 1DB gain
( maybe)over a 5/8 wave,1DB, do not know if I can hear that difference in my recieve.

So the real world comparision I posted of the 5/8 and the Sigma IV can be summed up to time of day, conditions, sun spots, etc. etc. as I heard the 5/8 stronger then the Sigma IV.

Great topic and really enjoyed the replies, comments, thanks to all.

 

[CDX-007]

- Thanks Waverider, I just read that same page a day or two ago, good stuff.

Over the years (decades!) I've tried a plethora of omni antennas and many offer a whole lot of difference in the signal strength at distances of 20-30+plus miles, even though they are all rated within a dB or so of each other.

I understand ground effects have some bearing on TOA and such, but when several S-units are lost simply by changing from, say, the Penetrator to an A99 or Starduster installed at 54' above ground, it seems there's a lot more to comparative antenna performance than near-field gain ratings!

I'm hoping the TOA on the 'ShockSigVectAluteWave IV000' will outperform the Penetrator at distances surpassing 40-60 miles because there are a few stations at that distance whom I can now barely hear as just part of the noise but whom I wish to hear clearly. - For me, that will be the real test.

To hell with the minuscule near-field gain measurement differences, if it allows me to hear these weaker stations such as Bill 501, Howie 444, or Doug 1955 clearly due to a better TOA or greater capture area then it's worth it!

'73

 

[Shockwave]

CDX,
I think my loop diameter is larger then Bob's because I started with a diameter that would get me as close as mechanically practical to the 20 degree angle suggested in the Avanti patent for maximum gain. Then I just adjusted the radial length to peak the gain and make the entire assembly equal to a 1/4 wave electrical length again.

I'm in the USA and Bob is in the UK. Our 11 meter AM/SSB bands in the USA are below the UK FM band. This could also account for the question of radiator length you asked about. Also keep in mind the point at which the gamma match contacts the main radiator will also influence apparent main radiator length to some degree. I find my tap point is lower then stock. I'll also mention on 11 meters this antennas SWR is not easy to set if you're not at least 36 feet above the ground. My resonance shifted slightly lower then desired when the antenna was installed higher.

Your comment about the collinear was a great idea we both thought was good. I built FM broadcast prototypes and even filed a design with the PTO years ago. The FM collinear antenna was called the Phazor. Bob has some pictures. No matter how you adjusted the radiator lengths and phasing section, you could not get the full expected 3 db increase in gain.

Best I could get was around a 1.5 db increase if everything was tuned perfect with the collinear. If not, all types of unexpected things could happen like strong directional lobes at the expense of uniform omni-directional gain. I believe the long 7/8 wave design is a poor choice for the bottom section of a collinear since much energy is radiated off it's surface before it can drive the next section. I later favored the phasing of multiple antennas rather then collinear stacking a single antenna to increase gain on VHF.

 

[bob85]

thanks for the pics

 

[CDX-007]

Thanks Shockwave for posting that info.
- Nice pics, too. Thanks Bob

I wonder if inverting the top 7/8, similar to collinear stacking of J-poles, would provide the full 3dB?

It would seem logical to me that any form of collinear stacking would tend to compress the 'donut' even more, resulting in what I expect would appear as gain at the horizon even though near field gain may not appear as that much improved...?

- Unfortunately that would be ~60' tall for 10m & 11m

Damn, I haven't been this enthused about an antenna project in years!

- Tanx Guyz!

 

[wavrider]

Damn, I haven't been this enthused about an antenna project in years!

- Tanx Guyz![/QUOTE]

I also am enthused about this project and follow it with open ears to any modification that results in gain and lower angle of radiation.

I have a full size 1/4 wl 75 meter vertical for phone dx, and if adding some upward pointing radials will increase the gain then I am all for that, easier to prove on an 11 meter vertical than that monster vertical so if and when the results ever come out proving the raised elemants do actually make a lower angle of radiation I will wrestle with that 75 meter vertical.

The 1/4 wl works great now, take into consideration the wave length and full legal limit power, but if I can lower the take of angle then big fun.

 

[Shockwave]

Damn, I haven't been this enthused about an antenna project in years!

- Tanx Guyz!

I also am enthused about this project and follow it with open ears to any modification that results in gain and lower angle of radiation.

I have a full size 1/4 wl 75 meter vertical for phone dx, and if adding some upward pointing radials will increase the gain then I am all for that, easier to prove on an 11 meter vertical than that monster vertical so if and when the results ever come out proving the raised elemants do actually make a lower angle of radiation I will wrestle with that 75 meter vertical.

The 1/4 wl works great now, take into consideration the wave length and full legal limit power, but if I can lower the take of angle then big fun.[/QUOTE]

The upward swept radials will not work on your 75 meter 1/4 wave vertical. That radial system only works on 3/4 and 7/8 wave verticals. Can you imagine that on 75 meters? The radials alone would be as tall as the 1/4 wave is now.

 

[wavrider]

The upward swept radials will not work on your 75 meter 1/4 wave vertical. That radial system only works on 3/4 and 7/8 wave verticals. Can you imagine that on 75 meters? The radials alone would be as tall as the 1/4 wave is now.[/QUOTE]


Yes goes without saying, but if the antenna is coil loaded, and the radials are coil loaded etc etc,

 

[CDX-007]

...I'm beginning to feel like I should have "Wave" somewhere in my username

I was just smiling as I imagined a .82 for 3.796mHz

Anyone got a balloon?

 

[bob85]

shockwave, just an thought,

if the sigma style antenna had an almost none radiating lower 1/4wave and you phased a second radiating element on top you would expect similar results to the super j-pole,
im not so sure the super j does provide the claimed extra 3db gain with such a small vertical separation between radiating elements but that is what is claimed in the few texts i have seen,

on the other hand if the lower 1/4wave does radiate combining in a constructive manner with the upper 1/2-5/8wave you would never see a 3db increase by phasing a second 1/2-5/8wave on top,

could your 1.5db measurement shed any light on how much the radial sleeve does or does not contribute to the overall gain,

i wonder how much gain the second element really adds in the super j-pole.

 

[bob85]

for your amusement i thought i would plot antenna mode currents with constant diameter wires using my ezbob software

 

[wavrider]

...I'm beginning to feel like I should have "Wave" somewhere in my username

I was just smiling as I imagined a .82 for 3.796mHz

Anyone got a balloon?

Weather balloon??

In my dreams (smile).

There is a JA station with a full size 80 meter 4 element beam 1/2 wl in height on a tower with a rotor, heard he lost it in a typhoon though.

ON4UN has a full 1/4 wave vertical for 160 meters.

thought process is, 1/4 wl vert. loading coil for another 1/4 wl now have 4/8 add another 1/8 in top hat cap loading should give a 5/8 wl vert.

The radials will be 1/8 wl with coils making it another 1/8 wl or a 1/4 wave.

Probaly be to lossy with the coils to show any efficiency, oh well I sure enjoy the 75 meter DX, just cant think of any way to improve the performance of that 1/4 wl vert due to size of antenna.

 

[Toll_Free]

This thread is STILL going?

Talked to WW11 the other day, he reminded me of this site.

Then I popped into antennas, and here we all are, STILL debating this antenna.

Mines STILL up, having survived 3 winters at 6500 feet and 100+ MPH winds. N6NB.com is located about 1500 feet higher up the mtn than I was.

--Toll_Free

 

[CDX-007]

...thought process is, 1/4 wl vert. loading coil for another 1/4 wl now have 4/8 add another 1/8 in top hat cap loading should give a 5/8 wl vert.

The radials will be 1/8 wl with coils making it another 1/8 wl or a 1/4 wave.

Probaly be to lossy with the coils to show any efficiency, oh well I sure enjoy the 75 meter DX, just cant think of any way to improve the performance of that 1/4 wl vert due to size of antenna.

I think you'll need about another 1/8wl in the bottom phasing coil. I have an old one-piece SigmaII for 2m made from stainless steel and having measured the single-turn phase matching coil on the bottom it appears the entire antenna including match is 3/4wl total.

Shunt feed it so you bleed off the high voltage which will occur on an antenna of that size.

I like your idea and am brain farting how I might erect one like it here, though I might opt for more antenna in the horizontal for better NVIS since I mostly talk to stations within 1000 miles.

Interesting curves, Bob. Which of the three bottom curves best represent the current nodes?

Good to see you back, Toll_Free. Is yours modded, and if so, do you have measurements including the Gamma, and for what center frequency?

73zzz

 

[bob85]

all three, you just have to squint a little