Avanti Sigma4: An alternative view point

[HomerBB]

I'm not sure I've have the answer to this problem but I have some observations from my FM antennas. It is true that the placement and diameter of the gamma match can shift the impedance. I have used gamma matches on this design that ranged from 3/8 to 1 inch diameter and could always find a perfect match. On the high power models I use a large diameter gamma to accommodate the Teflon insulator. The antenna also uses a DIN connector that must be spaced out from the base of the antenna so that hardline can be used. This places a fat gamma almost parallel to the radiator and still shows no problems with matching or performance. Tilt it too close to the radials and that will mess things up quickly.

One question I have is what material is the round radial support disk made from? I'm talking about the disk with the four holes that the radials pass through. I assume this is an insulator because it can not be conductive. Also when tuning your gamma do you find the lowest VSWR with the match set at any of it's extremes? For example, is the rod all the way inserted into the lower tube? Is the tap point between the gamma and main radiator all the way at the top of the gamma rod? Does the antenna have a good VSWR on any frequency you can check?

Your radials do look too close to the radiator at the base. Bend the four large wire connectors you have holding the radials at the base so the radials maintain the same angle all the way to the loop. I don't think adding spacers here will help. It has more to do with the sharper angle in the lower part of the radials due to the bend. Your wire connectors place the base of the radials parallel to the radiator. Bending the connectors to match the desired angle of the radials will improve this. Although it still may not be the entire problem.

Making a gamma with a 36 inch 3/8 rod and 10 inch 1/2 tube should have more then enough capacitance and length to match this antenna. The tap point from rod to radiator is typically around 27 inches up from the connector bracket. If you correct the angle of the radials at the base, your radial support disk is an insulator, and your gamma is close to these specs, I'm at a loss as to why you can't match it. You can change element lengths on this antenna by feet, making the antenna completely ineffective but yet the gamma can still be tuned to provide a perfect match in nearly every case.

The spacer disk is a non-conductive plastic material. It is the end of an empty wire spool. I will fix the angle of the wire connectors holding the four radials to get a consistent angle from end to end. I am prepared to insert spacers for a more distant standout at the radial mount points, but can do the bent to proper angle fix first.
And I can make the gamma to the suggested specs.
I have noticed more SWR response at the two extremities you ask about, but so far the closest I've gotten is about 3.1:1 well above the 27.205 I'm reaching for as a low point. With some adjustments I get only from a 4___ thru 6___ SWR.

Thanks SW

 

[Shockwave]

Before we reached an agreement with Sirio to custom manufacture our FM antennas, I use to make them all by hand from copper tubing. The bottom ends of the radials were flattened and formed around the radiator. I soldered the radials directly to the radiator and this worked perfectly in terms of matching and gain. NEC suggests that a drop in gain will be seen if the spacing is beyond about 1 inch at the base. The more you move the radials out at the base the more the design mimics the skeleton monopole. Which interestingly claims to have no gain over the standard J-Pole.

 

[HomerBB]

Yes, Bob,
Thanks for these dimensions. They are very helpful.

 

[HomerBB]

. . . I soldered the radials directly to the radiator and this worked perfectly in terms of matching and gain. NEC suggests that a drop in gain will be seen if the spacing is beyond about 1 inch at the base. The more you move the radials out at the base the more the design mimics the skeleton monopole. Which interestingly claims to have no gain over the standard J-Pole.

Got it.

 

[CDX-007]

Very much a better adjective as surface wave covers both ground wave and line of sight comms,which predominate at 27 mhz.At higher hf frequencies the actual ground wave portion is very limited due to ground absorbtion.

tropospheric ducting would come under skywave propagation like sporadic e,f1 and f2 layer propagation,auroral scatter modes etc,theres also a type of ducting found close to water,which for arguments sake i'd call sea ducting,similar effect to tropospheric ducting,all of which can take place on 11m.



I'm very sorry to hear of the loss of your best friend,had i known i personally would have been a lot less stroppy in previous posts,my sincerest condolences and sorry for being a retard in a tough time for you.

'Stroppy' - now there's a new one on me. I had to look it up. When I did I realized that you weren't the only one who was being a bit 'Stroppy'.

No worries, it's all in fun and I shouldn't have even been posting those first few days due to having had almost no sleep and feeling the loss of my friend so hard.

And thanks for all the measurements, I'm now thoroughly confused!

73

 

[Marconi]

'Stroppy' - now there's a new one on me. I had to look it up. When I did I realized that you weren't the only one who was being a bit 'Stroppy'.

No worries, it's all in fun and I shouldn't have even been posting those first few days due to having had almost no sleep and feeling the loss of my friend so hard.

And thanks for all the measurements, I'm now thoroughly confused!

73

Hey 007, I don't know what Stroppy means either, but now you sound like me regarding this topic.

What's the problem now?

 

[CDX-007]

Hey 007, I don't know what Stroppy means either, but now you sound like me regarding this topic.

What's the problem now?

Oh there's several possibilities here :blink: but I'm thinking I'll begin with the 27.7" ¾ wave version and go up from there.

 

[Marconi]

Oh there's several possibilities here :blink: but I'm thinking I'll begin with the 27.7" ¾ wave version and go up from there.

That is the exact length of the old three radial Antenna Specialists Sigma 4 from the bottom of the hub to the tip.

007, could you also post the overall/exposed dimensions for your 31' foot old styled Vector like Bob and 200 did with the New Vector. I would like to have that info for my file.

 

[HomerBB]

I took the overall measurements posted by Bob for the new Vector 4000 and set my total length to them. Of course, I do not have the open cage ball for the top, so I just took the length in with tubing. I ended up around 28' 2" overall.
Next I bent out the radial connectors to what I feel approximates the proper 30* needed, and remounted them to the center radiator at 13 3/4" from the bottom of the antenna.
I checked the radial loop for proper size, and found it a little large, reduced its overall length and repositioned the radials on the loop to exact quarters of the loops circumference. because the plastic circular standoff is only so big, it ended up beneath the center gamma rod strap with the new radial angle.

I made a new gamma to the dimensions posted by bob for the new V4k, and found the center rod a little short to bring down the match closer to ideal. You can see in the posted photo that the strap is at the extreme end of the gamma rod. I used a 36" rod and had to move my strap up to 35" to get the SWR down.

I did not like the bottom of the gamma tube being higher than the mount points of the radials so I reluctantly used a nut that isn't stainless steel to put it lower . I'll have to replace it tomorrow.
(bringing the bottom of the gamma sleeve down 1", and the strap up 1" took in the difference in the 36" gamma rod to the 34" rod.)

Everything done seems to have had a positive effect. Results for now at 12' above ground isolated from the mounting mast and no coax choke beneath the feedpoint connector:

28.755 ---------- 5.2:1
28.305 ---------- 1.8:1
27.855 ---------- 1.2:1
27.555 ---------- <1.1:1
27.405 ---------- 1.1:1
27.205 ---------- 1.2:1
26.965 ---------- 1.5:1
26.515 ---------- 2.5:1
26.065 ---------- 4.0:1
25.615 ---------- 5.2:1
25.165 ---------- 6.0:1

At this point it is a little bit more narrow banded than the 5/8 wave from which it has gotten its parts, but certainly usable well within the range of frequencies that I talk. When nothing's happening re DX I monitor 19. When DX rolls I mostly run from 20 to 40 and a little above, preferring 38LSB.
I did briefly talk to a station west of me 10 miles away, another northwest of me 30 miles away, an yet another south of me 10 miles. I received good reports from all of them. I wish I had more technical locals to give me more than "a room full of you" and "blowing smoke" I'll see if I can get someone to mobile out a few miles and see what happens one day soon.

Next I've got to get it up in the air. I'll have to start on that tip over tower tomorrow after work.

Thanks, Everyone.
What other suggestions might be in order for this thing? any?

Perhaps when the new Vector 4000 goes up the SWR results can be posted for comparison. . .

 

[Marconi]

Very good work Homer.

My best SWR bandwidth for my Sigma 4 was 2.3 mhz. I'm not sure what it was on Bob's super tuned Vectors, but he said he did see it improve as the antenna performed better.

I think you might expect to see a better bandwidth when recording the SWR bandwidth as apposed to when using an analyzer, so I chart both. I saw my BW improve just by using a coaxial choke directly at the feed point. It also produced a better curve what was very smooth and bowl shaped @ 100 khz step rate.

You mention that the radial support device is below the contact point of the gamma dog bone. I can't tell you if that makes a difference or not, but one might conclude that such an electrically neutral insulating device probably makes little or no difference. However, if you look at several of the manuals available on the INTERNET they all seem to find a reason to give a dimension for its placement along the radials. In fact on the LW150 antenna instruction sheet, which is almost void of dimensions, it shows this one dimension and maybe one other. I could never figure that out. So, maybe that placement might be something to consider fixing in your case.

I don't know if you took the chance to check between the changes you made, but it you did were you able to tell which change made the difference?

Have you been able to compare it to your 1/2 wave sleeved radiator?

Very good work Rube!

 

[HomerBB]

Very good work Homer.

My best SWR bandwidth for my Sigma 4 was 2.3 mhz. I'm not sure what it was on Bob's super tuned Vectors, but he said he did see it improve as the antenna performed better.

I think you might expect to see a better bandwidth when recording the SWR bandwidth as apposed to when using an analyzer, so I chart both. I saw my BW improve just by using a coaxial choke directly at the feed point. It also produced a better curve what was very smooth and bowl shaped @ 100 khz step rate.

i will put the coax choke inline today. In virtually every installation I've used one it helped to produce a wider, smoother SWR curve for me, too.

You mention that the radial support device is below the contact point of the gamma dog bone. I can't tell you if that makes a difference or not, but one might conclude that such an electrically neutral insulating device probably makes little or no difference. However, if you look at several of the manuals available on the INTERNET they all seem to find a reason to give a dimension for its placement along the radials. In fact on the LW150 antenna instruction sheet, which is almost void of dimensions, it shows this one dimension and maybe one other. I could never figure that out. So, maybe that placement might be something to consider fixing in your case.

I have another larger plastic disc I can use here, but I have to measure it to see if it is larger enough to get above the dog bone. If not, I'll have to fabricate the item somehow.

I don't know if you took the chance to check between the changes you made, but it you did were you able to tell which change made the difference?

The only changes I measured between were the diameter of the radial ring and placement of the radials at exact quarters of the ring circumference, and the length of the gamma rod and lowering of the gamma sleeve to equal elevation with the mount points of the radials.

Taking the antenna up and down so often caused the four radials to skewer out or equal placement along the ring circumference. Straightening them out brought the SWR down to where it is, but higher into the freeband than I wanted. When I discovered moving the dog bone up along the gamma rod and center radial moved the center of the SWR curve down, but extending the rod screwed it up, I decided to try a longer rod that could sit inside the gamma sleeve as it was but give me more room to take the dog bone higher up the radiator, this drove the low point of the SWR down the band, I I had a longer gamma rod I'd see what could happen next. I can only surmise that the open cage ball on op I don't have has some effect on electrical length my straight shaft doesn't. Do you think a cross wire hat would help.

Have you been able to compare it to your 1/2 wave sleeved radiator?

Yes. The Qv4k is better than the sleeve now in the same way as it was the other way around before. I am anxious to get it higher now.

Very good work Rube!

Are you kidding? This has been a forum wide community project. We've done this together, but I get to play with the antenna.

 

[CDX-007]

Marconi, I'd be glad to measure it but it came with no paperwork. It appears to be about 33' total tubing so I'll wager it to be a 7/8.

There seem to be several lengths posted which are possibilities. I recall the one I tried out being 26' 7" but I may be remembering it a foot short, after all it was 23 years ago...

If anyone happens to read this whom also has the manual for the Salute 7/8 - I'd love to have a copy.

This is the one with the silly alloy hub atop the 5/8" OD upper radiator tube into which is horizontally threaded a single allen about the size of that found in a Wilson 1000 collar, and which is meant to secure the top 6.5' of 5/16 - 3/16 tubing.

I plan to cut the top just below that piece of sh... uh, alloy hublet, lose the 5/16 - 3/16 and redo with 1/2" - 3/8" tubing.

I'm also working on removing the screws intended for securing each section to the one below it, and replacing with a vertical slit about 2.5" long x 1/8" wide (table saw) and a pair of Breeze, ABA or Tridon hose clamps at the top of each tube, like the Penetrator / I-10K construction design.
Screw screws! - I'm

-king tired of seeing sections which have fallen down inside of the larger one beneath it...

 

[bob85]

not much info available on the saliut, why they call it a 3/4wave when its 9.5 mtrs long is beyond me, even taking into account the electrical shortening as the radials are swept upwards it still don't tally, its virtually identical to the 7/8 vector,

http://www.cbtricks.com/ant_manuals/cte/sigma4_copy.pdf

 

[Shockwave]

Homer, lowering the plastic disk that supports the radials will not hurt the performance of the antenna although it may make adjusting your gamma harder. In fact the lower the disk is the less RF voltage will be across it. Placing the plastic supports too high on the radials can cause them to arc over the insulator at high power.

 

[CDX-007]

not much info available on the saliut, why they call it a 3/4wave when its 9.5 mtrs long is beyond me, even taking into account the electrical shortening as the radials are swept upwards it still don't tally, its virtually identical to the 7/8 vector,

http://www.cbtricks.com/ant_manuals/cte/sigma4_copy.pdf

Well, it does have about 16" of mounting length below the radial/ gamma hub, but that would still be well over 27' 7".

Whats your radiator length Bob, from hub to tip, ring & radial dimensions, and what is yours topped with?
You seem to have excellent performance and have compared yours to many others so I'd be inclined to copy your length for starters. I've already had one 26'-27' 7".

Might be good to know your mast length, and did you wind a choke or use ferrite for decoupling?