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Author Topic: home-brew sigma 4 11 meter antenna  (Read 71430 times)
WB6BYU
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Posts: 13243




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« Reply #75 on: February 21, 2013, 07:16:15 PM »

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?
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KE1IZ
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Posts: 30




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« Reply #76 on: February 21, 2013, 08:19:11 PM »

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.
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KG8LB
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Posts: 237




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« Reply #77 on: February 25, 2013, 08:02:57 AM »

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 . Wink

  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 .

 
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MARCONI390
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Posts: 14




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« Reply #78 on: February 26, 2013, 09:50:07 AM »


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?
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WB6BYU
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Posts: 13243




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« Reply #79 on: February 26, 2013, 11:01:03 AM »

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.
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MARCONI390
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Posts: 14




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« Reply #80 on: February 27, 2013, 11:28:11 AM »

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.
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WB6BYU
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Posts: 13243




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« Reply #81 on: February 27, 2013, 11:43:53 AM »

Quote from: MARCONI390

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.
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KE1IZ
Member

Posts: 30




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« Reply #82 on: February 27, 2013, 07:43:08 PM »

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.
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KC6O
Member

Posts: 6




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« Reply #83 on: September 01, 2013, 01:33:26 PM »

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.

 Well, I find this thread quite interesting and since it's been a little over 6 months since your move Dale, have you had time to set up yet and test?

 And Donald, it intrigues me, the thought of using only a 1/4 wave phasing device to create a dual 1/2 wave collinear, since even your CST shows the high current bloom 1/4 wave below the tip, providing the high voltage point at it's tip.

 I'd expect if you add a 1/2 wave above it to create a collinear, shouldn't it always require a 180 degree phase delay, regardless of the antenna type beneath it, so long as the high voltage point is at the tip of the lower of the two?

 Seems to defy logic unless it is somehow causing a re-phasing of the lower antenna when adding a collinear section above, thus requiring only 90 degrees of phase lag.

 Now I'm wondering if your alleged phasing section isn't acting as an additional radiating section, adding 1/4 wave to the overall design causing a cancellation of lower and upper 1/2 wave radiators, leaving only a highly elevated 1/4 wave of radiating area at the top.

 How that would provide 4dB of gain would perhaps be the next mystery.
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WB6BYU
Member

Posts: 13243




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« Reply #84 on: September 01, 2013, 09:14:16 PM »

Quote from: KC6O

... it's been a little over 6 months since your move Dale, have you had time to set up yet and test?




No,  I don't have my station or work bench set up yet.  We finally finished selling the old
house this afternoon after 2 months of cleaning and repairs and 3 months on the market.
And it still will probably be a few months before I get things set up.

But this is a test that many hams can do - it would just require a step attenuator and a
rig with an S-meter to take a measurement within 1dB or so, and even less just to test
the basic idea to see what the match is like.  A tuned loop feeding a super-bright LED
makes a simple current indicator - mine is mounted on the business end of a plastic
fly swatter, and allows a quick determination of the current distribution.
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KE1IZ
Member

Posts: 30




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« Reply #85 on: November 14, 2013, 09:39:24 AM »

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.

 Well, I find this thread quite interesting and since it's been a little over 6 months since your move Dale, have you had time to set up yet and test?

 And Donald, it intrigues me, the thought of using only a 1/4 wave phasing device to create a dual 1/2 wave collinear, since even your CST shows the high current bloom 1/4 wave below the tip, providing the high voltage point at it's tip.

 I'd expect if you add a 1/2 wave above it to create a collinear, shouldn't it always require a 180 degree phase delay, regardless of the antenna type beneath it, so long as the high voltage point is at the tip of the lower of the two?

 Seems to defy logic unless it is somehow causing a re-phasing of the lower antenna when adding a collinear section above, thus requiring only 90 degrees of phase lag.

 Now I'm wondering if your alleged phasing section isn't acting as an additional radiating section, adding 1/4 wave to the overall design causing a cancellation of lower and upper 1/2 wave radiators, leaving only a highly elevated 1/4 wave of radiating area at the top.

 How that would provide 4dB of gain would perhaps be the next mystery.

Many months later and I'm still 100% confident in my field tests using a 90 degree phase shift to stack another 1/2 wave on this antenna. I too had problems understanding how this could be since we clearly see the peak voltage on the tip of the stock antenna.

I have the answer to this question. What we overlook is that the phase of the main vertical allowed to radiated has already been delayed by 90 degrees with respect to the source since the cone has confined this out of phase current.

Therefore it only takes another 90 degree shift to bring the second 1/2 wave element into phase with the radiation on the outside of the cone and 50% of the original top element. Using a full 180 degree phase shift would cause 50% of the added element length to buck the phase of the cone.

Another interesting aspect of this design that may be responsible for only requiring a 90 degree phase shift is its current distribution along the 3/4 wave stock radiator.

I have always expected as you move away from the feedpoint that we would find a nice 1/2 wave current node beginning at the feedpoint. If you pay attention to the CST model you will note this very much expected behavior is not at all what we see taking place.

For some reason beyond my ability to explain this design creates it's first current node along a short 1/4 wave and not the anticipated 1/2 wavelength of current. This can only be because the upward swept radials are having another effect we have not yet explained.
« Last Edit: November 14, 2013, 10:37:20 AM by KE1IZ » Logged
KE1IZ
Member

Posts: 30




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« Reply #86 on: November 14, 2013, 11:09:47 AM »

I just remembered that when you extend a radiator beyond 1/2 wavelength that the current inversion begins at the feedpoint. Just like a 5/8 wave has an 1/8 wave of inverted currents at the base. The difference here is the out of phase base current is extended to 1/4 wavelength and when the radials are also 1/4 wavelength and folded upwards, it created the "non apparent collinear" effect Cebik talked about.
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WB6BYU
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Posts: 13243




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« Reply #87 on: November 14, 2013, 03:07:41 PM »

I was thinking about this thread the other day, so I'm glad to see it popped back up again
so I don't have to go looking for it.  I still don't have my station set up at the new house,
and lots of other things to do in the meantime, but if I think about it enough beforehand
then running an experiment will go a lot faster when I do have a chance.

So let's recap where the discussion has progressed to, since it has stretched over time
quite a bit.  This is from memory - I'm open to correction, or other points to add.

The original hypothesis was that the Sigma 4 antenna using an inverted cone at the base
was somehow significantly different than a J-pole that used a section of parallel-conductor
line instead.  (There are other variants that we could throw in here, but we'll try to keep
it simple.)  One indication of this was that, when adding a colinear half wave on top of the
antenna, it required a 1/4 wave phasing section rather than the standard 1/2 wave phasing
section.

Does this seem like a relatively neutral statement of the issues, in a form that reasonably
could be proved one way or the other by physical experiments?

Should it matter whether the cone is formed of discrete wires or solid metal?  (I have
some cones on my AEA IsoPole that I may be able to press into service.)  Or, if made
from wire, whether the ends of the cone wires are connected together with a perimeter
wire or not?

So the first step is to see if the inverted cone version has any gain over a standard J-pole.
Now, J-poles vary a lot in performance depending on how they are built.  There are a number
of reasons, including currents flowing on the mast and coax.  Actually this applies to both
antennas.  So there will be some margin of error.  I would attempt to choke off extraneous
currents as much as possible, and see if the signal strength varies as I move the feedline around,
but we'll still have to agree how close is "close enough to the same" vs. "significantly
different".

But this is perhaps one of the easier tests to run once the antennas are built.  I have a receiver
with an audio S-meter that can detect differences less than 0.2dB, but even a ordinary FM mobile
rig should be able to see a 1 to 2dB difference (depending on the meter scale - most have only
a 10 to 12dB range from nothing to full scale regardless of the markings.)  A difference of 1 to 2
dB certainly should qualify as "significant" for this purpose.  So the setup would be to use a
standard transmitter feeding an antenna at one end of the yard, and the antenna under test
connected to the audio S-meter at the other end (about 90' away).  This allows me to keep
the coax very short, since the receiver is small and has an internal battery.  I'd swap antennas
back and forth to see which gives the strongest signal.  (I may need some way to compare the
two tones, such as switching back and forth between the two antennas quickly.)


The next step is where it gets interesting - the required phase shift method for adding a colinear
half wave.

To start with, I wouldn't normally choose the single turn coil (diagram C in the link) as my phasing
network - it is still going to contribute some out-of-phase vertically polarized radiation (depending
on the physical height of the coil) as well as some horizontally polarized signals (which may show
up in an EZNEC plot, but aren't going to help much on ground wave where the antenna at the
other end is vertical.)  A stub is better, but I remember somewhere seeing a study that a stub
or parallel tuned circuit didn't give a reliable 180 degree phase shift unless the electrical center
of it was grounded (in their case, using a  quarter wave horizontal ground radial, since it is a
tad inconvenient to run a ground wire in such cases.)

In this case, I'd probably take a model of the inverted cone antenna and a half wave extension
and build some different phasing schemes to see how they work.  A simple quarter wave stub
is the easiest, but if the two sections of radiator were made of pipe with an insulator between
them then it wouldn't be difficult to swap out various sorts of phasing stubs and adjusting
them for minimum SWR and/or maximum radiation.  By comparing the length of a standard
stub to that of your single-turn coil (and possibly running the test on a standard j-pole or
some other antenna as well) it would tell us a lot about the differences and whether they
were due to the antenna itself or the way we had built the phasing unit.


Does this sound like a reasonable approach to answer the questions?  Is there something
that I'm missing that needs to be included?
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KE1IZ
Member

Posts: 30




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« Reply #88 on: November 14, 2013, 04:55:51 PM »

I think you have a basic outline of the exact steps I used to determine it is a 270 degree phase corrected radiator. Some things to add are that the cone can be made from wire and it is not required to form a loop around the top. Although this will effect the dimensions of the cone because removing the loop will make it appear electrically shorter. The radials will have to be lengthened to compensate for the removal of the loop.

It also appears impossible to make any phase shift section for this antenna that would not radiate. That would require the phase shift to be long enough that it contained opposing phases within its length that could be orientated to cancel the currents. At only 90 degrees this will not be possible.

The shorted stub is ideal for measuring the exact length of the required phase shift but provides a horribly distorted radiation pattern as a result of its uneven radiation. This is why the helical loop would be preferred from a performance standpoint.

While these field tests may be time consuming to perform, if they are done with any reasonable level of care or scientific method, you will be surprised to arrive at the same conclusion I have.
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WB6BYU
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Posts: 13243




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« Reply #89 on: November 14, 2013, 10:10:39 PM »

I'm sure there was something else I was supposed to be doing this evening...


But instead I spent it modeling the antenna on EZNEC.  So far nothing that isn't
exactly as I would expect it to be, but the point is to confirm it.

I have an old commercial heavy-duty ground plane antenna - think the radiator
is 3/8" solid aluminum.  Looks like I can attach the radials to the outside of the
lower portion using hose clamps and some #12 solid copper wire.  That gives us
the bottom section.

Then I'll check my supply of scrap aluminum tubing to see if I can make a half
wave extension that will go on the top of it.  I can test that by assembling the
ground plane in the normal manner and attaching the added 1/2 wavelength - the
pattern won't be optimal, but I should be able to adjust the length until the
SWR curve dips at the same frequency as before.

Then I install the wire cone and make another half wave extension to resonate
at the same frequency again - this is the radiator for the cone antenna.  With
the right angle of the cone wires it looks like I should be able to get a good
match to 50 ohms.  So far, so good.

Now I can measure this antenna against a J-pole or other dipole, and EZNEC
predicts that they should be about the same gain, because both of them are
half wave radiators:  I just have to make sure that I mount the effective centers
of the radiator at the same height above ground for my measurements.


So, here is the question:  what does your theory about the current distribution
predict would happen if I then add the other half wave element to the top of the
first one without any phase shift?  The radiation is out of phase, of course, as it
would be in any end-fed full-wave wire, but should the SWR curve remain centered,
or does the unexpected current distribution change the resonant frequency?

The next step would be to make a 1/4 wave phasing stub and put it between the
sections, then check the SWR again.  I'm assuming that, if I get the wrong stub
length, that the antenna will no longer be resonant, and the SWR would immediately
show the difference without the need to measure the signal strength.  I would check
out the phasing stub with the extension above it using the original ground plane -
again, I should be able to add it without shifting the point of minimum SWR (though
the minimum SWR may increase to 2 : 1 or so.)

So basically I check out my top half wave and the phasing stub both on the original
ground plane, then convert the ground plane into the bottom cone and add a half
wave radiator to make the conical J-pole.  Once that is adjusted for minimum SWR
at the test frequency, I add the second half wave radiator (with or without the
matching stub - it shouldn't matter for this test) and again check where the dip
in the SWR curve is.


Does that seem like a reasonable approach?


It may be easier to do the whole thing with #12 solid copper wire, but it wouldn't
be stiff enough to hold up the top section and matching stub.  I might find a piece
of pipe or fiberglass tubing that I can secure the wire to to hold it up.
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