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The Avanti Sigma 5/8

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Old 02-14-2006, 11:07 AM
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Default The Avanti Sigma 5/8


I have uploaded pictures of the base assemblies of 2 versions of the Sigma 5/8. One is after the Antenna Specialist (A/S) buy-out and the other is an original Avanti product.

You will notice that Antenna Specialist, with their deeper resources, mechanically upgraded the antenna from it's earlier design. Pay special attention to the two hub assemblies. The newer one (with cast #1007) is THICKER around the hole for the base tube. The older hub is cast #MR101.

The base tube was also lengthened and double walled. The inner tube of the A/S part has the same diameter (1 5/8 x .058 wall) as the older base tube. The outer tube is now 1 3/4 x 18"

The feedpoints are completely different. A/S added a coil which made the Sigma 5/8 DC grounded for the first time! They also used 3 bolts around the bottom to secure it to the base tube. The length of the 10 guage wire (printed "Phelps Dodge" on the insulation) was also increased when the coil was added. You can also see the longer fiberglass tube. The top section of the main radiator was also shorten approx 6" to bring the electrical length of the antenna back to 5/8.

From the center conductor of the coax jack, the signal goes up the orange wire to the top aluminum block. The bottom of the loop attaches here. The signal travels around the loop an into the base of the main radiator. Inside the fiberglass tube, there is a 1" gap between the base of the main radiator and the aluminum block. This is Avanti's elevated feed system I've spoken of in the past. The main radiator is NEVER surrounded by any metal at ground potential (unlike the Penetrator).

The base of the main element has an aluminum boss pressed in and tapped to accept the bolt that secures the top of the loop. The bolt head you see INSIDE the tube is a plug to keep the water out of the feedpoint. There is a small hole in the tubing just above this plug to let the water out.

Antenna Specialist has machines that punched the slots into the tubing. Avanti used a band saw. The last pictures shows a couple of original Sigma IV antennas, some new Alcoa 6061-T6 aluminum (.058 wall x 12' each, from 1/2" to 1 3/4"), and some old ham radio antennas I've remove from towers for other people. This is only abou 1/10th of the aluminum I have at home.

I've done my best to illustrate the two versions of the Sigma 5/8. If you have any questions, please ask!


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Old 02-14-2006, 01:11 PM
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Again MC, very good work. You just don't see that kind of attention to detail that Avanti showed in design, very often anymore and in our hobby for sure.
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Old 02-14-2006, 01:22 PM
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good pics mc they bring back some happy memories, when did AS buy avanti, every sigma iv i have seen since the late 70's have had the machined slots, they changed the basket separators from fibergalss to plastic sometime in the early 80's i would guess :?:
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Old 02-14-2006, 01:36 PM
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I'm not sure when the buy-out came about. My first Sigma IV had the plastic spreaders. The two I have now have the fiberglass spreaders with the clamps that rust and fall apart.

I keep my eyes peeled for old Avanti antennas and grab them every chance I get.

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Old 02-14-2006, 03:05 PM
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which spreaders are original avanti, my first ( late 70's ) had flat fibreglass with stainless clips, others i have owned used the plastic type?, we always thought the plastic were the later type and not as strong?.
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Old 02-14-2006, 07:34 PM
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The original spreaders on the Sigma IV were flat pieces of fiberglass with plated hardware. I don't think they ever used stainless hardware on anything. The clamps used with the fiberglass spreaders would rust and break!

I liked the three plastic spreaders. They worked well and I never had one fail.

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Old 02-14-2006, 08:25 PM
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you could be right, i never saw the clips rust but the bolts used to rust away and parts would start falling off.
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Old 04-10-2009, 09:55 AM
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Quote:
Originally Posted by Master Chief View Post
I have uploaded pictures of the base assemblies of 2 versions of the Sigma 5/8. One is after the Antenna Specialist (A/S) buy-out and the other is an original Avanti product.

You will notice that Antenna Specialist, with their deeper resources, mechanically upgraded the antenna from it's earlier design. Pay special attention to the two hub assemblies. The newer one (with cast #1007) is THICKER around the hole for the base tube. The older hub is cast #MR101.

The base tube was also lengthened and double walled. The inner tube of the A/S part has the same diameter (1 5/8 x .058 wall) as the older base tube. The outer tube is now 1 3/4 x 18"

The feedpoints are completely different. A/S added a coil which made the Sigma 5/8 DC grounded for the first time! They also used 3 bolts around the bottom to secure it to the base tube. The length of the 10 guage wire (printed "Phelps Dodge" on the insulation) was also increased when the coil was added. You can also see the longer fiberglass tube. The top section of the main radiator was also shorten approx 6" to bring the electrical length of the antenna back to 5/8.

From the center conductor of the coax jack, the signal goes up the orange wire to the top aluminum block. The bottom of the loop attaches here. The signal travels around the loop an into the base of the main radiator. Inside the fiberglass tube, there is a 1" gap between the base of the main radiator and the aluminum block. This is Avanti's elevated feed system I've spoken of in the past. The main radiator is NEVER surrounded by any metal at ground potential (unlike the Penetrator).

The base of the main element has an aluminum boss pressed in and tapped to accept the bolt that secures the top of the loop. The bolt head you see INSIDE the tube is a plug to keep the water out of the feedpoint. There is a small hole in the tubing just above this plug to let the water out.

Antenna Specialist has machines that punched the slots into the tubing. Avanti used a band saw. The last pictures shows a couple of original Sigma IV antennas, some new Alcoa 6061-T6 aluminum (.058 wall x 12' each, from 1/2" to 1 3/4"), and some old ham radio antennas I've remove from towers for other people. This is only abou 1/10th of the aluminum I have at home.

I've done my best to illustrate the two versions of the Sigma 5/8. If you have any questions, please ask!
Hey Master Chief - I never knew A/S produced the Sigma5/8, and this is very interesting info.

I'm wondering if you could reload those pictures.

I have what is evidently an A/S Sigma5/8 as it has the inner coil for DC grounding, but when I received it the coil had deformed due to the Einstein who installed it before I got it leaving out the 3 bolts which held the bottom part of the hub with the SO-239.
When his single round of tape fatigued, the weight of the coax pulled the coil out and down until it's now almost a piece of straight 14 gauge.

I need to rewind a fresh coil but I can't tell how many turns it requires to bring it back to factory stock.

Any help would be appreciated.

73

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