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Some variations and construction ideas on the WA6SVT Omnidirectional Coaxial Collinear
By Kevin Custer W3KKC
Click here for the original WA6SVT article.
As stated earlier, I have been successful in building several versions of the WA6SVT collinear. I also have a few ideas that I think are worthy of presenting to you here. These ideas are not meant to demean Mike Collis' construction principles, they are presented here to give more ideas to the builder and allow other construction possibilities from available materials.
A note if you plan on using these ideas to build your own collinear - you may want to consider changing out your polo t-shirts or customized t-shirts and nice jeans for something you don't mind getting a little dirty before beginning any project!
Although Mike makes no reference to using this antenna on 2 meters or VHF, this article provides the approximate construction practice that Phelps Dodge (now Cellwave) uses to construct the all famous Stationmaster and Super Stationmaster style of fiberglass radome vertical omni's for VHF and UHF. I have used this article to construct antennas for 2M, 220, and 440 voice repeaters. They will also work well on digital and ATV.
Now onto my ideas, facts, and past construction practices for the collinear.
Step 1, and the "Concept of construction" states the calculation process of the brass element length. You might think from this calculation that the length of the antenna can be shorter by using solid dielectric instead of foam, however, remember it's not the number of elements that determines the gain, it's the overall length of the collinear. As Mike had stated, in the Concept, that he now uses RG-11 and RG-6 which is a foam dielectric cable. The use of foam cable allows a physically longer antenna using the same number of elements in design. Also, since this antenna is a radiating piece of coaxial feedline, the concept of losses should also be taken into consideration. Why? Solid dielectric cable has more loss than foam, and losses increase with frequency.
Remember these facts:
Since this antenna is end fed, the available signal for radiation is the greatest at the bottom of the antenna and diminishes as the signal goes toward the top. Why? As the bottom element receives the signal, some of the signal is lost to the next element because some of the energy was radiated (this is an antenna) and now is not available to be fed into the next element. Also, some of the energy was lost due to dielectric losses in the element, just as a piece of feedline has loss. This loss goes up in heat, and is not available for rf radiation from the antenna. This is why Mike uses different sizes of cable for high and low power antennas. The larger the element, the better the ability to dissipate heat. The use of solid dielectric cable on 2M and 220 is acceptable, however you may want to consider a foam type for UHF and beyond.
Step 2 suggests that you can build vertical beam downtilt into the antenna. When downtilt is desired you simply cut the element 2% shorter than the calculated length. This then makes the antennas resonant frequency higher than the operating frequency. Does this create SWR? Yes, and this can be trimmed somewhat with the adjustment of the decoupling sleeve. However, the suggested method of attaching the sleeve makes it difficult to adjust the sleeve easily. A better way of attaching the sleeve, to the brass feedline section, is to use a washer soldered into the top of the decoupling sleeve and solder a piece of brass tubing, one size larger than used to build the elements, into the washer allowing an inch or so inside the sleeve and above the washer. This will allow you to slide the decoupling sleeve, to the correct position, easily. When you are satisfied on the final position of the sleeve after tuning, simply solder the two brass tubes together at the upper joint. On UHF antennas I use a commode fill tube for the decoupling sleeve, available at the local hardware store and a U.S. twenty five cent piece fits into the tube perfectly. This is a cheap way of making a solderable washer, as it will only cost you a quarter. Make certain to use an older one in the 60's or 70's as not to get the newer cheap unsolderable aluminum ones.
The article suggests the use of hobby brass tubing in the construction of the antenna. I use a brand of brass tubing manufactured by K&S Engineering from Chicago Ill. This company makes brass tube in sizes from 1/16" to 5/8" in 1 foot and 3 foot lengths. Some RG-8 and RG-213 guts will sleeve into the 5/16" tubing very nicely. The 11/32" size can be used for the decoupling sleeve adjustment.
Step 7 suggests that a quarter wave whip be placed on top of the last coax element. I simply prepare the last element by folding the center conductor over the top of the brass tube and then I cut a piece of brass tube that is one size larger than the elements and slightly longer than the quarter wavelength. Slot the end of the tube to allow the center conductor to be folded over while sleeving a ¼" of the larger tube over the last element. Ensure that a full ¼ wavelength of open tube extends past the end of the shorted ¼ wave coax element. Solder the tube and center conductor to the top of the last element. This makes a more rigid top element that can be supported from the top of the fiberglass or PVC pipe. If you are crafty, you can build the last ¼ coax element and the top ¼ wave whip element all from one piece of tubing. Simply drill a small hole in the side of the tube to allow the center conductor to be pulled through the side and solder. The hole will be drilled at the intersection as if two separate elements. Ensure that the dielectric extends to the hole and a full ¼ wave in air exists above the hole. Another method for the top ¼ wave element is to use a piece of brass all-thread. You can solder the all-thread paralleling the full length of the last ¼ wavelength coax element. Allow the full ¼ wavelength whip to extend past the shorted end of the last element. This practice will allow you to use a PVC pipe cap to carry the weight of the antenna. Drill the center of the cap and allow the brass all-thread to extend past the end of the radome. Solder the brass all-thread rod to the last element before you stuff the dielectric and center conductor into the element, as not to melt the last coax element with too much heat. Place a brass or stainless nut above the pipe cap to allow the all-thread bolt to carry the weight of the antenna. If you have the antenna attached at the bottom, Do Not string the elements too tight, with the nut, as this will cause the elements to break at the joints. Use an "O" ring or some silicon RTV to weatherproof the area at the top where the brass threads come out.
Steps 11 and 12 suggest methods of mounting the antenna into PVC or fiberglass pipe. I have successfully dismantled old commercial fiberglass antennas and used the radome for my antenna project. This makes an excellent home for the new antenna. Some of these style antennas have a large brass top that is pointed and secured to the top of the fiberglass. I have drilled the brass cap to allow the top ¼ wave tube whip to be soldered into the cap. This allows the weight of the antenna to hang, instead of the weight pushing to the bottom. Also, the large aluminum sleeve that the commercial antennas have for mounting to the mast is actually the decoupling sleeve for the antenna. This usually has three allen head screws in the top to secure the inside of the antenna to the sleeve. I have been successful in saving this part of the commercial antenna and adapting it mechanically to the homemade collinear. Commercial antenna construction differs from one manufacturer to another and this may not be a possibility with all types. The stationmaster uses a matching scheme in the bottom of the antenna that no one seems to understand. I have a drawing of a stationmaster bottom and the machine work and materials are fabulous, but I do not understand their function or purpose. When I build an antenna, using the stationmaster radome, I gut the bottom machined section and apply the parts to the new antenna design. The most important part of the antenna, in my opinion, is the decoupling sleeve. In the stationmaster, this is also the bottom aluminum mounting sleeve. Usually ground radials are attached to the top of this sleeve. Ground radials, on a sleeve decoupled antenna, do nothing as the sleeve is far better to decouple the feedline than radials anyway. Ground radials will also radiate signal in the horizontal plane taking away from the clean vertical pattern and elevating the brewster angle. I simply don't use them. I have built antennas using the sleeve as in the article and then attaching this sleeve to the stationmaster sleeve connecting both sleeves together at the top. My theory on this practice comes from looking at a C-Band satellite feedhorn. If you look at the feedhorn as a choke, which it is, you can visualize several decoupling sleeves one inside the other all connected at the top. Are two sleeves better than one? Good question, however I have had excellent results with this practice, and satellite tv feedhorns have more than one ring (sleeve.) I simply take the large top "washer" that has the holes drilled and tapped for the 3 screws and desolder all from it. I then drill the washer out to allow the new decoupling sleeve to be inserted. I solder the washer to the top of the new decoupling sleeve. This allows you to thread the antenna into the radome and secure the washer to the original sleeve with the 3 recessed screws. The local Motorola shop usually has a few old Stationmaster types that were removed due to being broken internally and no longer duplex well. I buy these antennas for little to nothing and use the radome for my own use.
Jim Hartzell WA3UQD (callsign /at/ consolidated /dot/ net) offers these suggestions in testing the antenna:
Mounting larger arrays near the ground (less than 1 1/2 times the length of the antenna) causes a false reading of the SWR (it reads higher). If the antenna is tested in the horizontal position it need only be greater than 1 wavelength above the ground on non-metallic supports, the area still must be clear of other objects.
If you have enjoyed this article and/or these construction notes, or if you have any questions or comments about this antenna or construction project, Please email me at kuggie /at/ kuggie /dot/ com and on the subject line please include something ot the effect of "Questions about the WA6SVT collinear antenna".
Click here for the original WA6SVT article.
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Article text © September 1998 Kevin K. Custer W3KKC
Artistic layout and hand-coded HTML © Copyright 2006 and date of last update by by Mike Morris WA6ILQ
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.