As the current in each radial is small, even at legal max power, you can
use insulated #18 which is available on very large spools at ebay. Last
spool I bought had 15,000+ feet for less than $20 SHIPPED.
Don
N8DE
> This thread was perfect timing for me as I was waiting for some cooler wx
> before starting my vertical project. I still am undecided whether to go
> elevated or not. In case I do go ground mounted, does anyone have a good
> source for very inexpensive wire? I like to use #16 insulated wire but
> have not found a good source. I know I can get #14 insulated from Lowe's
> etc for around $25/500' and was hoping for even a better price for some
> #16. Any ideas?
>
> Gedas, W8BYA
>
> e-mail w8bya@mchsi.com
> online gallery http://gedas.cc
> web page http://www.w8bya.com
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> To: "K8RI on Tower talk" <k8ri-tower@charter.net>;
> <towertalk@contesting.com>
> Sent: Tuesday, October 04, 2005 5:56 PM
> Subject: Re: [TowerTalk] Elevated vertical
>
>
>> > Elevated radials SHOULD have the feedpoint decoupled
> through
>> > a common mode choke. if you ground the radial common
> point
>> > is causes increased loss as well as current on the
> feedline
>> > shield.
>>
>> How would you go about this with an antenna like the
> AV-640 which has a
>> built in tuning network that is grouneded. Isolate the
> entire antenna from
>> the tower, feed it through a choke, and atach the radials
> to the bottom of
>> the antenna where it would normally atach to the mast
> driven into the
>> ground?
>
> To be truly unbalanced a system should have equal and
> opposite currents flowing into the shield and center
> conductor at both ends, and the shield has to have
> essentially zero voltage to the environment around the
> antenna including the feedline.
>
> Any vertical antenna with anything less than a large radial
> system has significant voltage on the radial common-point
> with respect to the surrounding of the antenna. That
> includes earth, the feedline, and so on. This is true even
> with 1/4 wl groundplanes using four full size radials.
> Substantial pattern and SWR changes occurred when feedline
> length was altered in a commercial 40 MHz groundplane I
> designed. The cure was to isolate the radials from the mount
> and decouple the feedline with a common mode choke.
>
> One of the things I didn't initially believe when I read the
> first article (based only on models) was an earth path would
> reduce antenna efficiency when using elevated radials. When
> I actually tested systems, I found it was true.
>
> Small groundplane verticals are a worse case situation. They
> are simply nasty for common mode feedline currents. Most of
> the small groundplane antennas like the 640 include
> decoupling, but it is often not enough. When I worked for
> Ameritron the majority of "I bought and amp and everything
> in my shack goes nuts when I use it" complaints were with
> these small groundplane verticals, even though they had some
> factory decoupling.
>
> I'm not sure how a person could cure every antenna like
> that. We generally advised a large common mode feedline
> choke before the feed cable entered the shack as well as
> keeping the feedline away from other conductors between the
> antenna and that choke.
>
> I don't know how significant a problem this is, it varies
> with every installation.
>
> I'm only pointing out small radial systems are a headache
> for a number of reasons. They are frequency sensitive, they
> radiate like crazy in the area around the radials, they
> couple to everything around them, they produce feedline
> common mode problems. In my tests and every BC station I've
> seen or heard of where the radials were actually A-B'ed
> against a regular system of 50 or more radials, and as my
> friends have changed them, the results were always that a
> conventional system was better.
>
> Even if it isn't better, it is virtually impossible for a
> conventional system with 50 or so radials to be worse. With
> 50 or so radials efficiency is in the high 90% range.
> That's true no matter what type of soil is under the
> radials, if the 50 radials are shallow buried, or if they
> are elevated. Conventional systems are not critical. They do
> not have common mode or near field coupling issues. In
> addition to RF advantages they also work well for lightning
> protection of equipment.
>
> I'm very skeptical of any system that is reported to clearly
> beat a system known and proven to have nearly 100% of the
> maximum possible field strength. Of course it is work to
> install 50 ground wires, and that alone is good reason to
> not use them if ease of installation outweighs the
> electrical disadvantages of small counterpoise systems.
>
> 73 Tom
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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