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To Coil, or NOT to Coil; That is the question...

C

CDX-007

Transducer Fanatic
Jan 26, 2006
706
17
28
OK, I just rebuilt 2 Avanti Sigma 5/8 and found out that what Sacramento Sonny had said about "Some do, and some do not have coils" was correct.

I wonder if it was the earlier or the later production which had no internal coil? Did they get smarter or cheaper in their design & production?

Supposedly the version with the coil was quieter due to passing static to ground better as it had a DC path to ground through the coil.

Anyone know the real story here?

73-
 

When Antenna Specialists bought out Avanti, they upgraded the entire line of antennas.

The later versions (post A/S buy-out) came with the coils which made the Sigma 5/8 DC grounded. They had to change the physical length of the main radiator to compensate for the new coil.

A/S also strengthened the base assemble and added 2 more 1/4-20 bolts to hold the fiberglass insulator rigid, making the base thicker, and re-casting and strengthening the hubs (also used on the PDL-II and Moonraker beams).

The later model, with the coil, is the desired version.

If there is someone who knows more about Avanti antennas then me, please let me know! I'm always open to learning even more about these superior antennas!
 
Thanks Master Chief for that good info. :!:

I didn't know A/S bought Avanti, fascinating.

Do you also have the original length spec for the two versions?

:arrow: I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator...

- Possibly it was shortened for the new center of the 40 channel band...??? Straighten me out here if I'm
looking at this bass-ackwards.

The new ring made from some 20-year-old American made C0PPER tubing (with a much thicker wall than the crap currently available at Home Despot) is working out great.

The previous owner didn't use the three bolts which hold the bottom hub and, as you can guess, the weight of the coax jerked out the base, tearing out the feedline and unwinding the coil.

I got the S-5/8 because he decided the antenna was junk and gave it away!

I rewound the coil to fit, interesting challenge - resoldering that 6" feed wire, had to retap for a new SO-239 which it needed anyway.

Thanks for sharing the knowledge!

-73-
 
It is VERY possible that the original Avanti is shorter and I got the two mixed up. Now I'm going to have to pull it down and measure it. I posted pictures of the two coils a while ago right on this site!

Avanti Sigma 5/8

Pay attention to the feedpoint with the coil; it is LONGER than the original. Also, the longer base tube is for the antenna with the coil as is the fiberglass insulator.

Note that the ground radials can only ONLY mount in ONE location on the A/S version (above the three bolts)! This is a BIG mistake I see people make with the original versions of the Sigma 5/8; they mount the ground radials BELOW the single bolt.
 
CDX-007,
"I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator... " Oops, just backwards.
If a larger coil is added to the antenna, then more inductance is added. That means that if you want the thing to be resonant in the same place as before, then some inductance has to be subtracted from some place. That 'some place' is the 'whip' part of the antenna, so it's shorter, not longer. Not decreasing the 'whip's length would mean that the resonant frequency would go down. If you shorten the thingy just right the resonance stays in the same place.
- 'Doc
 
Master Chief said:
It is VERY possible that the original Avanti is shorter and I got the two mixed up. Now I'm going to have to pull it down and measure it. I posted pictures of the two coils a while ago right on this site!

Avanti Sigma 5/8

Pay attention to the feedpoint with the coil; it is LONGER than the original. Also, the longer base tube is for the antenna with the coil as is the fiberglass insulator.

Note that the ground radials can only ONLY mount in ONE location on the A/S version (above the three bolts)! This is a BIG mistake I see people make with the original versions of the Sigma 5/8; they mount the ground radials BELOW the single bolt.
Click to expand...

Well THANK YOU MC, - as I had 7 turns, not 8 as your excellent pic shows.


W5LZ said:
CDX-007,
"I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator... " Oops, just backwards.
If a larger coil is added to the antenna, then more inductance is added. That means that if you want the thing to be resonant in the same place as before, then some inductance has to be subtracted from some place. That 'some place' is the 'whip' part of the antenna, so it's shorter, not longer. Not decreasing the 'whip's length would mean that the resonant frequency would go down. If you shorten the thingy just right the resonance stays in the same place.
- 'Doc
Click to expand...

Well Doc, I think you're right, but, - the coil is going to ground not to the radiator, so it's an inductive path to ground from the transmitter in parallel not series with the radiator.
- I had to scratch my brain for a few on this one...
Normally I think this would cause the transmitter to see decreased inductive reactance as it now has two paths to follow.
Remember, this added coil is going to ground and not into the ring / radiator.

In order to rebalance the load reactances the antenna would have to offset that decreased inductive reactance by decreasing the capacitive reactance, or lengthing the antenna...right?
But wouldn't this cause a decline in the performance of the antenna as it would no longer be at an optimum resonant length?
I bet the resonance of the coil is far lower than the resonant frequency of the antenna to the degree that the coil's influence on the tuned length of the radiator should be negligible, otherwise the RF would have two paths, one to ground & one into the matching ring & radiator, and a 3dB loss in performance would be expected.
I think the coil is only there for static discharge and a DC path to ground, not for retuning or additional matching of the radiator.
If one version of the Sigma5/8 is shorter, (prolly the newer version) I'll bet it's to get the tuning closer the center of the newer 40 channel band.

...maybe? :)

73
 
CDX-007,
Well, I was right in what I said, but since I was 'looking' at the wrong coil (didn't actually look, "assumed" I knew which one I/we were talking about) what I said was wrong. In the pictures, the coil inside of the base tube does go to ground. It, along with the 'loop' attached to the vertical radiating element forms an inductive impedance matching circuit for the 5/8 wave antenna.
In one of the earlier posts, 'M.C.' said that because of the added coil, the length of the antenna had to be changed to 'make up' for the inductance of the added coil. And that is the reason for the difference in antenna length.
But, it doesn't have to be done that way if you are willing to adjust the respective sizes of the small added coil and the 'loop' till you have the correct relationship between the two. Being practical, that's a lot harder to do than just adjusting the vertical element's length (and that's what most manufacturors are after, quick and easy changes... otherwise it just costs too much, right?).
The over all affect is the same, sort of, speaking of 'resonance', of the whole antenna, not just the length of the vertical element. The difference in length just isn't all that much so the difference in performance (because of that length change) just isn't that much either. Some, but not much, and certainly not enough to be readily noticable due only to the length change.
- 'Doc

PS - If I'd pay more attention I wouldn't make 'assinine' mistakes, huh?
 
PS - If I'd pay more attention I wouldn't make 'assinine' mistakes, huh?
Click to expand...

Actually, I believe you were just taking off from my statement,
" I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator... " - as I was as clear as a bowling ball... :roll:
 
"I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator... " - as I was as clear as a bowling ball."

adding inductance electrically lengthens the radiator.
adding capacitance electrically shortens the radiator.
 
freecell said:
"I don't see why adding a correctly tuned coil would require shortening the radiator, in fact adding inductance should, if anything, require increased capacitance, adding length to the radiator... " - as I was as clear as a bowling ball."

adding inductance electrically lengthens the radiator.
adding capacitance electrically shortens the radiator.
Click to expand...

Sorry, Slixdexia. Sometimes I get things wackbirds. :rolleyes:
 
I just acquired one of these antennas. I am trying to rebuild it but the coil ends are in pieces, just wondering if anyone knows where to get this part or ideas on how to make my own. I also need to come up with the fiberglass insulator. I'm a newbie at all this so any help appreciated.
 

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