Here's my take on it:
Economic tradeoffs:
Considering that 14THHN is about
2 cents a foot at Home Depot, etc.,
how much are you earning per hour
to make all those solder connections
to save $50 or $100 worth of wire?
If you use scrounged hook up wire, etc.
the cost of the wire is completely
negligible.
Technical issues:
Everyone knows about the rule that the
outer tips of the radials should be spaced
.025 wl (or some such number). It may
seem as if this spacing would also be
sufficient closer in. However, I
believe that closer in you need closer
spacing because the current density is
higher. My experience is that you need
60 radials that go all the way into the
antenna, no matter what is going on farther
out. Thus forking doesn't make any sense
unless you have 120 or more radials at
the periphery.
I base this on measurements of verticals
I made over my 3x3 ft wire grid on 40 meters.
The grid fills a circle 230 feet in diameter.
Measuring the impedance of a quarter wave
vertical, I found that I needed 60 radials
starting at the antenna itself to get down
to 36 ohms. I also tried 120 radials, but
it made no difference (not surprisingly).
Fewer radials resulted in 40 to 50 something
ohms.
This is in spite of the fact that I soldered
the ends of the radials into the nearest
grid crossing. The antenna was at the
exact center of the grid. Simply tying
the antenna into the center grid crossing
w/o radials resulted in about 60 ohms driving
impedance.
The more I study this problem, the more I agree
with the common advice of using 60 1/4 wl radials,
nothing fancy. You just can't go wrong with that.
Rick N6RK
Rick Karlquist N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com
> -----Original Message-----
> From: towertalk-admin@contesting.com
> [mailto:towertalk-admin@contesting.com]On Behalf Of Pat Rundall
> Sent: Wednesday, February 27, 2002 12:05 PM
> To: towertalk@contesting.com
> Subject: [Towertalk] Radial "fan out" summary
>
>
> Thanks to those of you who responded with information
> on radial "fan out". Here's what I learned:
>
> - apparently, this had been discussed before on tower
> talk using the term "forking" (which makes as much
> sense as 'fan out' I suppose):
> http://lists.contesting.com/_towertalk/200101/msg00300.html
>
> - the group never did reach consensus as to whether
> forking aka fanning out radials made sense. A few
> responses to that earlier thread doubted the
> practicality. Further study/modeling is warranted. In
> the earlier thread, the proposal was to "fork" about
> halfway out (at .125 wl) - I was hoping to fan-out as
> soon as the radial hit the ground.
>
> - One thought was that because the highest current was
> near the antenna, the most radials should be there -so
> fan-out would be ineffective because of the reduced
> number of radials at the mast. Again, IMO modeling and
> field tests are needed to bear this out.
>
> - A few commented that the fan-out idea probably
> wouldn't work (gut feelings, etc.) but hadn't tried
> it.
>
> - Another tip was to keep the slope of the radials as
> close to horizontal as possible.
>
> Once again, thanks for the great info this board
> provides.
>
> Pat, N0HR
>
>
> =====================================================
> Check out MobileLog, the Ham Radio logging package
> for PocketPC users! http://n0hr.tripod.com
> =====================================================
>
>
>
>
> .
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Greetings - Send FREE e-cards for every occasion!
> http://greetings.yahoo.com
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
|