Now that is a handy observation and helpful formula. It is based on Tom's
point that the perimeter separation need be no more than .025 to .05
wavelength for 1/4 wave radials.
Now the question for more compromised situations:
What is the maximum allowed perimeter separation of 1/8 wave radials. In
other words, how many radials 1/8th wave long would be required to achieve
"close to" the same performance of 31 radials 1/4 wave long? Or....it is
just not possible to get "there" from "here". If 31 to 60 radials 1/4 wave
long approach 100% efficiency, how many radials 1/8 wave long will approach
100% efficiency. (Or more likely, just how many 1/8 wave radials will it
take to reach the point of diminishing return for the number of radials,
period, because no number of 1/8 wave or less radials will ever
significantly approach 100% efficiency)
The more generic question is something like a table (and I'm sure it's out
there somewhere, but I don't have the source material myself).
Radial Length #Radials for Point of Diminishing Return % or dB down from
100%
1/2w ?
???
1/4w 31
??? (not seen value)
1/8w ? (obviously lots more)
??? (terminal value << 100%)
1/16w ? (and even lots more)
??? (ditto except <<< 100%)
By terminal value, I mean you really can't approach 100% at all, and are
merely looking for the flat part of the efficiency curve and noting how many
dB down you are from the ideal 100%. << = much less, <<< much,much less
To summarize the questions I have from the table:
1. How many 1/2 wave radials are needed to get to the "flat" part of the
efficiency curve and how far below 100% efficiency is that.
2. How many 1/4 wave radials are needed to get to the "flat" part of the
efficiency curve and how far below 100% efficiency is that. (We know it is
probably 31 and certainly somewhere between 31 and 62, from Tom's excellent
post)
3. How many 1/8 wave radials are needed to .....you get my drift.
4. How many 1/16 wave....the drift continues.
I'm sure it could be questioned as to what really is the "flat" part of the
curve, but I would like Tom's observations using whatever criteria he
chooses. A response to this series of questions would really help anyone
installing a 1/4 wave vertical and ground mounting it.
"I only have room for "1/X" wavelength radials. How many do I need before
I'm wasting copper and effort, and how far down the efficiency curve am I
going to be?"
Thanks to Tom and Bill for stimulating my questions on this matter. It is
not just an academic exercise. I will be putting my full size 1/4w 40m
vertical back up in the spring and will be ground mounting it. Where I
choose to mount it is somewhat limited and knowing the compromises involved
(with an approximate numerical value), will be most helpful.
73,
hasan schiers, N0AN
schiers@netins.net
----- Original Message -----
From: Tom Rauch <w8ji@contesting.com>
To: <towertalk@contesting.com>; Bill Coleman <aa4lr@arrl.net>
Sent: Thursday, January 11, 2001 12:17 PM
Subject: Re: [TowerTalk] Re: Radials over salt water
> Hi Bill,
>
> > Hmm. 1/4 wave radials means a circle of 1/2 diameter. That's a perimeter
> > of pi/2 wavelength, which divided by .05 yeilds about 31 radials.
> > Similarly, .025 spacing at the ends is about 63 radials.
> >
> > If >63 radials is like a solid screen, I wonder why there's so much in
the
> > amatuer literature about installations with 120 or so radials.
> >
> > Bill Coleman, AA4LR,
>
> Good observation Bill.
>
> You hear that because people are parroting the FCC requirement,
> which is based on "twice as much as near-perfect is good enough".
> The FCC not only wants a system to work, they want it to have
> "headroom" if something happens over time and it deteriorates. If
> you read the RCA study that the FCC based its decision on, it
> confirms the rule of thumb I offered.
>
> As another point, BC stations almost always use ground screens
> near the base of the tower. Yet it does absolutely nothing for the
> signal, as long as the radials are sufficient in number. You can't
> make people not use ground screens, no matter how much proof
> you offer. It just "looks good".
>
> Another wive's tale is four elevated radials are noticeably more
> efficient than 120 radials on the ground. How can anything be
> "noticeably more efficient" than something that is already almost
> 100%??
>
> If they know that much about it, why don't they know 60 radials
> are an immeasurable amount less than 120 when the radials are
> 1/4 wl long? The truth is 30 radials on the ground are barely
> noticeable as being worse than perfect, and a lot less headache for
> RFI and lightning damage than any sparse counterpoise system.
>
> If you follow the general rule I outlined, you will be way out on the
> flat part of the efficiency curve. You would only use 120 radials if
> they were about 1/2 wl long, and you chose the narrowest end
> separation.
>
> Use more than that, and you are wasting copper. I know a couple
> people with 360 1/4 wl radials! They could have used 100 1/2 wl
> radials, and actually have picked up a couple tenths of a dB or so
> and used less wire (and spent less time installing the ground).
>
> I don't tell them that, cause it's a done deal. I listen to them telling
> people that, and hear the admiration in the other stations voice
> when they reply "no wonder you're so strong".
>
> Maybe we'd better keep this a secret Bill.
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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