home-brew sigma 4 11 meter antenna

[WB6BYU]

OK, then let me see if I understand what you mean...

Let's assume you have a 12" insulator and a 20" wire (just to make
the numbers easy.)  You connect the wire to the remainder of the
antenna at each end of the insulator.  Then (for example) you can
run the wires down to the center of the insluator, leaving 8" of
wire remaining.  (In practice the wires would be spaced further
apart, but the remainder would be used at the far end of the
stub, so there is no loss of generality.)  The remaining 8" of
wire is formed into a 4" stub, which is then wrapped around
the insulator in a circle.

Is that a reasonable description of how you have implemented
it?

[KE1IZ]

I think a picture is worth a thousand words here so I did a quick Google search of some simple phasing schemes. If you go to this link: http://www.hamradio.me/antennas/improving-the-super-j.html and look at figure C, it most closely resembles the phasing I used on the 2 meter version. The exceptions would be it is only 1/4 wavelength in electrical length and the top to bottom spacing is just about a foot.

[KG8LB]

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.  

  Many of us are a bit tired of the attitudes and on air practices that have migrated from CB to amateur radio as well .

  The fact remains however , that properly built  "11 Meter" type antennae designs have applications on ham bands . Sorting them from the useless is another matter .

 

[MARCONI390]

I'm hosting an antenna workshop this Saturday where other hams can come over
and build antennas, and that might be a good time to try it.  We'll have the ability
to measure antenna gain, and I think I can find my 2m current sniffer. 
Unfortunately the barn isn't heated, and the weather is supposed to be stormy,
so I suspect everyone will be in a hurry to finish up and go home.  After that I'm
packing up my equipment in preparation for a move to a new house, so it may be
months before I can get back to it (and the rest of the pile of projects that have
been stacking up.)


But as I understand it, your observation is this:  with a standard J-pole, or any
other common half-wave radiator, one can add a quarter wave shorted phasing
stub and a second half wave element and the radiation will be in phase, giving
a gain increase at the horizon.  But if the antenna is matched with a tapered
cone in place of the tapped matching stub of a J-pole, then the shorted phasing
stub has to be 1/8 wavelength long to achieve the same effect.  Is that correct?

If so, it shouldn't be difficult to build a common radiator structure so we can just
vary the shape of the matching stub at the base between a single parallel wire
and a 4-wire cone to see how much difference it makes.  One approach might be
to build the antenna on an SO-238 socket with the center radiator attached to a
banana plug in the center and the cone build on the outer shell of a PL-259 that
screws onto the outside.

Or we could build a phasing stub and extension that works when attached to the
top of a J-pole, and then try it on the cone version and see if it acts the same
way.

If I made an open cylinder out of some tin cans, would you expect it to act more
like the conical stub or parallel wires?  That would be an easy test to run once the
basic structure was in place, as would the "open sleeve" version with 4 parallel
wires.

Maybe I can get some of the other hams at work interested in the problem and
we can run some tests in the lab using the network analyzer, but that probably
would need to be scaled for 440, as the 2m version is getting a bit tall and
the ceiling may detune it.

Dale, did ya'll get the chance to consider the Sigma4 design at your get-to-gether this past weekend?

[WB6BYU]

Unfortunately, no, we didn't.  There were 5 people each wanting to
build a different antenna that I needed to help with, so I was stretched
pretty thin.

It will probably take me at least 6 months to get settled in the new
place, and then I'll see how the project list looks as to when I might
get to running the tests.

But it seems like a pretty straightforward experiment:  does the required
phasing change depending on the configuration of the feeding stub.  It
shouldn't be difficult for someone else to run the test.

[MARCONI390]

Thanks Dale.

Several of us have been discussing this Sigma4 idea for several years now, but it is still in the unresolved pile on my desktop.

Have you ever heard the term "non apparent collinear," as applied to antennas?

Please keep us posted.

[WB6BYU]

Have you ever heard the term "non apparent collinear," as applied to antennas?

Only with regard to a colinear wire made with thin wire so the neighbors
wouldn't notice it.

[KE1IZ]

The antenna is  a Jpole and has no relationship to the coaxial sleeve antenna. The bottom of the sleeve on a coaxial antenna is high impedance, the top is a current maxima- just the opposite of the Sigma 4. In addition the sleeve on the coaxial antenna has, because of skin effect opposite currents on the inside and outside.

The reason Herb flared the 1/4 wave arms on the Sigma IV was threefold fold:
1. It made it mechanically easier to build the antenna
2. It likely acts as a transmission line whose characteristic impedance increases as the separation increases. This would be more important if this were a more traditional transmission line with dielectric, as it would reduce the losses in this matching section.
3.It looked cooler

It could still benefit from a means to choke off common mode currents as there is no reason, the support mast and feedline would not have current on them. A quick EZNEC model will show this.
Model the antenna and then put a quarter wave wire from the bottom of the antenna extending downward (admittedly, a worst case scenario, but done to prove a point). That wire would represent the mast and/or the outside  of the coaxial shield. Rerun the model and look at the currents on that quarter wave wire.

Dale W4OP

I realize I really didn't address much of what Dale said directly so I'd like to do that here.

1) How is it mechanically easier to construct a much longer, 30 foot free standing radiator that eliminated one insulator to add several more that now support the cone?

2) If the purpose is to match the impedance why was there no success in arriving at a 50 ohm match and why did Herb decide to use a gamma to achieve the match? It's more likely that the design of this cone was primarily to confine the out of phase radiation on the vertical inside of it with the added benefit of a second current maxima point that is in a constructive phase.

3) I'll give you the fact that "It looked cooler" but this is not what Herb describes in his patent where he claims 1 db over a 5/8 wave groundplane.

The idea that a choke at the feedpoint would help or that adding a 1/4 wave counterpoise would show a worst case scenario are contradicted by the CST model and field tests. If you understand the CST representation of the current phase being radiated from the outside of the cone is one that constructively adds to the phase of the 1/2 wave above it, why would you think extending a counterpoise in any other direction would change this phase? Phase is determined by the electrical length from the source.