Homemade Sigma 4 11 meter base antenna

[Marconi]

you mean the one found on this page?

Gain-Master - Vs. conventional Antenna

so you think they are one and the same?

i was thinking the sigma iv would have a 1dbd gain! oops i just answered my own question.

ok thanks marconi

Thanks GG, that's it. I don't think they are the same. I just wanted to see how this CST pattern for the 5/8 wave ground plane, compared with the Vector pattern we're talking about.

In the images below it shows the 5/8 GP pattern indicates the bottom is out-of-phase with the top 1/2 wave element, just as most would expect. This obviously suggest cancellation at this point on the radiator. Note the magnitude in red for the lower portion of the radiator, and how far out it pertrudes from the element. IMO, this is CST's approach to indicating magnitude, and the colors indicate (+ -) phase...as noted by the A/m scale attached.

The Vector model however shows the bottom is minimally radiating in-phase with the top radiator, just as some have claimed. But, to fairly consider the obvious low magnitude on the outside of the Vector radials compared to those on the 5/8 wave lower portion of the radiator has to be considered.

IMO, physics probably tell us we will never see a CMC free setup, and we will always see some CMC as a result, but in this case I estimate the Vector currents are at least 1/2 those on the 5/8 wave as indicated...even considering there is a notable difference in element lengths. So, the actual currents are probably a lot more than a 50% difference.

For us to know the true magnitude for these currents, we need to verify the real A/m current values not try and figure out the color scheme as precise.

IMO the currents on the radials are not 2.37 A/m as has been suggested, else the red color on the radials would be as pronounced as those on the radiator above.

These colors are graduated by color and the A/m scale on the 5/8 wave model is not set with power to the the same value as the one on the Vector model. So, we can't even compare the values directly...even if we could determine the gradation precisely.

At least we can see a difference if we consider the differences in the colors objectively. This is the same with Eznec where the red line suggest magnitude, but if you really want to know the values you have to refer to the Currents tabular log in Eznec.

I hope I made this understandable.

 

[gamegetter]

no they are not the same critters, but the effects are about the same and that's what i meant related to about the same gain in dbd, and that is why i was wondering about the data

 

[W5LZ]

... somewhere down the road a little light bulb will turn on and it'll be "I told you so" time. Have fun till then.
- 'Doc

 

[Shockwave]

... somewhere down the road a little light bulb will turn on and it'll be "I told you so" time. Have fun till then.
- 'Doc

I've been hoping the same thing for some time but sometimes it seems doubtful. Then again that is what we should expect when dealing with things that are "non apparent".

 

[BOOTY MONSTER]

... somewhere down the road a little light bulb will turn on and it'll be "I told you so" time. Have fun till then.
- 'Doc

if this is what you're referring to ......

Dam! Sounds remarkably like a 'J'-pole, doesn't it??
- 'Doc

that light bulb is a burnt out black light

 

[W5LZ]

Booty M',
I'm not saying it's a 'J'-pole. But it does behave remarkably like one doesn't it? So why would it do that, what are the similarities? And their differences?
Try looking at them in a different way/perspective. It's not easy, but it's certainly not impossible either. You might discover that there are some very interesting conclusions you can come to that don't 'fit' the 'normal' way of figuring things. The only 'mystery' or 'magic' to antennas is in how you 'look' at them.
That's all I'm going to say about it, you have to find out for your self for it to make any sense.
- 'Doc

 

[bob85]

this all started about 9 years ago with a post about the best performing vertical,
the j-pole camp claimed the 5/8 was king of the hill, i claimed the sigma/vector correctly setup would beat it,

the j-pole camp insisted the cone is only there to feed the upper 1/2wave and does nothing for radiation due to cancellation, that it was a 1/2wave and inferior to a 5/8wave,

they were wrong neither are true,
the vector works with or without the cone with some adjustment of gamma,
the cone does radiate in phase with the upper 1/2wave as shown in CST,

they were wrong about the j-pole too,
in an ideal world there would be no radiation from the short leg but due to some current imbalance at the top of the 1/4wave section it does have a little radiation from the short leg that causes the lob sided pattern,
you can see the effect in j-pole models,

the j-pole camp can't back-peddle and claim "we knew the short leg and skeleton sleeve had radiation and where that radiation came from"
their position on the lower 1/4wave's function was very clear,

the arrl explains what happens with the 3/4wave monopole using a 1/4wave open sleeve, i posted what they say in the sigma4 thread,

no its not a full blown co-linear, however it does seem to operate with a none apparent co-linear effect as CEBIK told me it could,

i don't dispute that the vectors cone radiation comes from the same mechanism as the j-pole short leg radiation, the skeleton & open sleeve antennas sleeve radiation even common mode radiation from the outer shield of your coax,

i disputed the true purpose and effect of the cone,

the cone is not there to feed the monopole,
its radiation unlike a simple j-pole is deliberate not coincidental to the design,
it is omnidirectional like a skeleton sleeve monopole,

imho there is a difference between getting the sleeve radiation via the same mechanism as a simple j-poles unwanted short leg radiation which the j-pole camp denied was there due to cancellation,

and deliberately designing an antenna to be more compact than a 5/8 ground-plane (30" wide vs 18ft wide) and produce more omnidirectional gain than a 5/8 ground-plane as the patent claimed,

imho the increased height of the upper 1/2waves current maxima and radiating cone both help give the vector an edge over a 5/8wave,
it is not a huge advantage,
i cannot guess how much of that advantage is due to the extra height and how much is due to the radiation from the cone but it certainly is not inferior to a 5/8 ground-plane as the j-pole camp claimed,

you may or may not agree, im ok with that,
vector users are happy to talk to the distant stations you can't hear on your 5/8 ground-planes

 

[northern35s]

I decided to do some rudimentary modelling in MMANA, I took a standard 5/8 WL model that I use as a basis for antenna construction and extended the element to 3/4WL, as predicted the gain increased, however that gain was at pretty useless angles, a fundamental problem with a 3/4WL monopole, I added 1/4WL radials, which again increased gain, but did little to lower the max angle of radiation, however once I angled the radials up, similar to the Vector/Sigma, the max angle of radiation dropped like a stone, the gain was slightly less, but overall a much more useful antenna and for that reason alone the design has merit.

I didn't end up with an exact copy of the Sigma (lazy I guess ), but it was close


One thing I do wonder is whether the ring at the top of the cone adds capacitance?

 

[Shockwave]

One thing I do wonder is whether the ring at the top of the cone adds capacitance?

I'm not sure about its purpose being to add capacitance. The patent says the loop increases the usable bandwidth. My field tests showed the antenna works nearly as good without the loop with minor distortion in the pattern. When the loop is removed some peaks in gain can be noted on the four sides with radials. Once the loop is added you can spin the antenna and not see any change in gain because the loop smoothes this out. It also adds mechanical strength to the design.

 

[Shockwave]

In an ideal world there would be no radiation from the short leg but due to some current imbalance at the top of the 1/4wave section it does have a little radiation from the short leg that causes the lob sided pattern, you can see the effect in j-pole models.

The j-pole camp can't back-peddle and claim "we knew the short leg and skeleton sleeve had radiation and where that radiation came from"
their position on the lower 1/4wave's function was very clear.

Bob, I wonder if the J-Pole camp tilted the 1/4 wave leg on their J-Poles outwards from the vertical, what they would think of the increased lopsided effect in the pattern? Would they then connect the increase in radiation from the stub to the fact the elements are no longer parallel?

 

[gamegetter]

according to the patent i read, the loop besides increasing bandwidth, also shortens the electrical length of the radials.

 

[northern35s]

I'm not sure about its purpose being to add capacitance. The patent says the loop increases the usable bandwidth. My field tests showed the antenna works nearly as good without the loop with minor distortion in the pattern. When the loop is removed some peaks in gain can be noted on the four sides with radials. Once the loop is added you can spin the antenna and not see any change in gain because the loop smoothes this out. It also adds mechanical strength to the design.

I spend most of my radio time working mobile (amateur frequencies) and one way to increase efficiency, and bandwidth, is to add a capacitance hat to a shorter than 1/4WL mobile antenna, with this in mind and if the ring does indeed increase bandwidth, then the case for capacitance is looking good

Just read Gamegetter's post, I'm sold on capacitance

 

[bob85]

possibly if they read what cebik said caused the radiation in the parallel j-pole.

Bob, I wonder if the J-Pole camp tilted the 1/4 wave leg on their J-Poles outwards from the vertical, what they would think of the increased lopsided effect in the pattern? Would they then connect the increase in radiation from the stub to the fact the elements are no longer parallel?

 

[The DB]

in an ideal world there would be no radiation from the short leg but due to some current imbalance at the top of the 1/4wave section it does have a little radiation from the short leg that causes the lob sided pattern, you can see the effect in j-pole models,

Just a question on this. The radiation pattern being slightly stronger on the side of the stub, is the stub in phase with the radiator, or is it out of phase and some other mechanic also at play?


The DB

 

[Marconi]

I decided to do some rudimentary modelling in MMANA, I took a standard 5/8 WL model that I use as a basis for antenna construction and extended the element to 3/4WL, as predicted the gain increased, however that gain was at pretty useless angles, a fundamental problem with a 3/4WL monopole, I added 1/4WL radials, which again increased gain, but did little to lower the max angle of radiation, however once I angled the radials up, similar to the Vector/Sigma, the max angle of radiation dropped like a stone, the gain was slightly less, but overall a much more useful antenna and for that reason alone the design has merit.

I didn't end up with an exact copy of the Sigma (lazy I guess ), but it was close

One thing I do wonder is whether the ring at the top of the cone adds capacitance?

35, a while back I was modeling a 3/4 wave radiator to try and see how the radials affected the pattern. I used a model I had that was labeled (Imax 18'). I did about the same modeling as you described, but I was using Eznec.

My 5/8 wave was just a 22.5' wire with no radials that I labeled (Imax 18'). I extended it to 27' as a 3/4 wave and labeled it (.75 w 18'). I then added 4 x 106" radials and labeled it as (.75 w 18' GPK). Then I slanted the radials up to about 20* degrees from the radiator and labeled it (.75 w 18' GPK slanted).

Over the process I saw the 3/4 wave pattern go to pot...until I slanted the radials up, just as you described. In the end, the 3/4 wave antenna pattern with slanted radials...ended up dropping back down in angle and gain to about the same as the original Imax without radials.

So, I thought it might be interesting to see what happens adding radials to the 5/8 wave Imax model, so I did a model with 4 x 72" horizontal and slanted down radials to complete the comparisons for the 5/8 wave to the 3/4 wave variations.

Here is an overlay for those results at 18' feet with no mast.

View attachment .625 wave vs. .75 wave.pdf