Dave,
If you are going to lay the radials on the ground and the vertical is ground
mounted, then I think you would be very happy with the moderate performance of
16 radials, 1/4 wave or so on the lowest band. There is NO RESONANCE effect for
radials that are on the ground or shallowly buried. If you elevate the radials
they must be individually tuned for a 1/4w for each band.
The spec for 3 on each band, at 1/4 wave long implies the antenna is NOT going
to be ground mounted, but rather will be supported a significant distance in
the air, thus becoming a "ground plane" antenna. Your performance with 3 tuned
elevated radials on each band should be "decent", although far from ideal. The
primary variable for elevated radials is how high! The higher you get the
antenna as a portion of a wavelength, the more efficient your elevated tuned
radials become. If you get up 20 to 25' on 40m, the performance can be quite
good. On 80m it won't be as good.
Let's not get overly lost in the Take Off Angle (TOA), commonly called angle of
radiation. The key is how much power is put into the lobe of radiation centered
at the TOA, not the TOA itself. The reason I say it this way is if you have
very high ground return losses (poor radial field), your TOA will still be
low...but there will be very little power going into that TOA because it has
been eaten up with ground losses.
For this reason, a lot of people opt for the elevated ground plane with tuned
radials. The mistake they make, however, it not getting the base of the
elevated vertical high enough to cause the tuned elevated radials to "mask" the
high losses of the earth beneath it.
Again, for what you are talking about (moderate operational success, as opposed
to ideal), 3 tuned radials per band with a height of 15' might work out just
fine especially on 40m and 20m. The nice thing is, you can always add more!
73,
...hasan, N0AN
----- Original Message -----
From: Dave Tipton
To: Jim Lux ; hasan schiers ; Bill Coleman ; Dave Tipton
Cc: towertalk@contesting.com
Sent: Friday, June 16, 2006 12:16 PM
Subject: Re: [TowerTalk] re Radials
Well.. I have an old 5BTV (Hustler) that I'm going to put up this weekend...
It is spec'd for 1/4 wave radials on each band, telling me that I need to have
3 on each band...
MY question... If I were to lay down 8 radials on 80, 80 on 40 and 8 on 20
(The only 3 bands I'm really interested in), do you think this would give me
sufficient performance to rely on it for some medium distance work? I'm not
going for big DX here, but I'd take it if I heard it.. I now have an 800 watt
amp in the mix too, so I'm going for as near resonant as I can get.
Dave
Jim Lux <jimlux@earthlink.net> wrote:
At 06:52 AM 6/16/2006, hasan schiers wrote:
>Actually, they can be MUCH shorter than .22 wl, as long as you have enough
>of them. Many more shorter is quite a bit better than many fewer longer.
>
>There are free programs by G4FGQ, RADIAL2 is the name I think, but just
>google G4FGQ and you will get to his site and free downloads. The radial
>program is a good one and answers all the questions being raised. It lets
>you manipulate number/length/freq and ground characteristics to show
>efficiency. VERY HANDY!
Except that Reg's program isn't necessarily a very good model of
radials. He's modeling the radial field as a bunch of lossy transmission
lines (which is sort of fine), but doesn't account for the transfer of
energy into the surrounding soil, some of which reradiates, as opposed to
being simply loss in an equivalent loss resistance. Reg has some
explanation of what he's doing in the notes in the program, but doesn't
publish his algorithms or source code, so it's tough to figure exactly what
he's doing (and I'm not patient enough to just run lots of cases and
reverse engineer it). It's also not clear how Reg is modeling wires close
to the surface, where a significant part of the field crosses the
ground/air boundary.
That said, it might be a handy way to tinker around and gain some
understanding of the effect of various radials.
If you really, really want to model radials, you're probably best off to
fork out the bucks for a copy of NEC4 and do it right. (NEC4 does model
buried wires, and has been rigorously validated, and all the gory details
are published) Otherwise, just use the "throw lots of wire out, don't worry
about the exact length, and more radials is better than fewer" approach.
Jim
>The data I got is completely consistent with the ARRL publications that
>describe how to get minimum loss for a given wire length available, as well
>as with G4FGQ's program. I do think Reg's program is a bit optimistic, but
>it points one in the right direction. I have full faith in the studies
>published in the ARRL Antenna Handbook on how to optimize a radial field,
as
>my actual measurements agree very strongly with the article.
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