Antenna Systems page
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One ham's experiences with
the WA6SVT antenna
This page was developed by WA6ILQ from an email posted by Paul Kelley N1BUG
on the Repeater-Builder Mailing List
Thanks go to Paul for permission to present this article.
Click here for the original WA6SVT antenna construction project.
Recently when I said I was building a WA6SVT antenna some folks wanted my evaluation of it when it was completed. To make my experiences helpful to the whole ham community, here's a writeup. Rather than my sending out multiple individual emails, benefitting only those hams at that time, I decided to let Kevin post it here for both current and future readers.
I built a ten element UHF version. One minor concern: I couldn't change the SWR at all by moving the decoupling sleeve up or down. However, it's OK since the SWR minimum is exactly on my design frequency... Impressive.
I was actually shocked to find the SWR did not change appreciably between top mounting, side mounting at 1/2 or 1/4 wavelength. I expected it to "complain" about the nearby tower when side mounted. It doesn't even seem to notice.
Unfortunately I don't have much to compare its performance to. The previous antenna was a Cushcraft 4-pole at the same height. The coaxial collinear provides significantly better coverage, leading me to question whether the Cushcraft was performing to its full potential on my tower. It may have been suffering as a result of proximity to the VHF antenna.
The performance of the collinear exceeds my expectations. I have a VHF repeater at the same site (1 dB less power into a 5 dB gain commercial StationMaster). Here are some observations comparing the coverage on VHF to that on UHF with the new 10 element collinear. Over flat terrain VHF and UHF coverage is nearly identical. In rolling hills VHF is generally better, as you might expect. In mountainous terrain VHF sometimes has the edge, while in other spots the UHF wins hands down. I wish I could offer more meaningful comparisons. I'm quite happy with this antenna and plan to build more of them for other sites and applications.
I made one change in the overall construction: I used a slightly different approach to supporting the weight of the antenna inside the radome. Perhaps this will be of interest to someone, perhaps not. I will try to describe it, but this is where a picture would help.
Refer to step 8 in the construction article. For 445 MHz the half wave brass tube would be 13.25". I made mine about 17" so the soldered connection to the RG-213 shield would be below the decoupling sleeve.
After soldering the feedline braid to the brass tube, I put a 2" long piece of 1" diameter heavy wall heat shrink (the type lined with hot melt adhesive) over the joint and shrunk it in place. After letting that cool, I placed a 1" length of the same shrink tubing over the upper portion of the first piece. After shrinking and cooling, the bottom edge of this second piece provides a "shoulder" which can be used to support the weight of the antenna.
I made a round disc from a piece of 1/4" thick delrin plate (other materials could be used) with a hole in the center just large enough to slip over the inner piece of heat shrink. I also drilled a couple of 1/4" holes in the disc for ventilation.
Once the antenna is in the radome, 3 or 4 small holes can be drilled through the radome into the edge of this disc, so that it can be secured into the radome with screws. Alternatively, the screws can protrude through the radome just below the disc to that it rests on top of the screws to carry the weight of the antenna. Be sure the screws are not long enough to damage the feedline. This method supports the weight from the bottom while keeping the antenna base centered in the radome.
And I'll definitely shoot some photos when I build the next one.
de Paul, N1BUG
Notes from the Editor (WA6ILQ): Rather than drilling "a couple of
1/4" holes in the disc for ventilation" I'd drill a half-dozen 1/8"
holes (or even more holes of an even smaller diameter) to provide
the same ventilation but the smaller holes would act like screening
to keep out the spiders and other local critters.
Lastly, this is an ideal antenna to install inside an old Stationmaster or Super-Stationmaster radome. Most 2-way shops replace their repeater antennas when they develop the "cracklies" while transmitting, sometimes called "duplex noise" - and rebuilding an antenna is not time-vs.-cost-effective for a commercial shop. You can sometimes get a good radome for hauling it away (and it doesn't matter if you are building an antenna for a completely different band - if you think about it a fiberglass radome from a 160 MHz antenna will work perfectly well at 146, 220, 440 or even 900 MHz - you're just going to use it as a big fiberglass tube with a top cap and a bottom mounting sleeve. And don't pass it up if it is old and weaterbeaten and shedding fiberglass... as long as the tube is structurally sound it can be recoated... see the Cellwave article on antenna painting.)
Antenna Systems page
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Original text (the white background) © Copyright Paul Kelly N1BUG 2004
Hand coded HTML by Mike Morris WA6ILQ
This page last updated 31-October-2004
The information presented in and on these conversion pages is © Copyrighted 1995 - 2004 by Kevin Custer W3KKC and multiple originating authors.