There is a scenario that you are both correct:
If the large vertical has quite high ground resistance in the beginning, the
base impedance can actually be greater than 50 ohms. As radials are added,
the impedance will drop and vswr will drop as well. That is until you reach the
50 ohm point where the vswr will rise as more radials are added.
73,
Gerald K5GW
In a message dated 12/5/2008 4:13:05 A.M. Central Standard Time,
jruing@ameritech.net writes:
ER... ah... um.... In my experience with a large vertical
monopole antenna, adding more radials seems to bring SWR
down.
As for the correct answer in theory... I am not entirely sure.
I based my advice on my experience adding radials and
testing the impact the same had on SWR in my case.
IN REALITY I THINK WE SHOULD BE TALKING ABOUT
GROUND RESISTANCE...
which I BELIEVE goes down with more radials - I am thinking
that in the ARRL Antenna book, Page 3-10.Table 1 (This is
in Chapter 3 of the 21st Edition 2007) - it shows that feed
point impedance in ohms for a vertical radiating element
goes down with the addition of more radials. The text
seems to say a reduction in Rg (Ground Resistance)
would require, more, longer radials. (page 3-9).
For the purposes of this discussion, I am making an
assumption that the Inverted L under scrutiny is, at least
mostly, a vertical radiator - and that most of the energy will
be radiated from the feed point (at the bottom of the L) on
up, and then, to a lesser degree, on out, so that at the far
end of the L, current is at minimum, and voltage is at highest.
This assumption is reinforced by the statement on Page 6-41
of the Antenna Book, that says "This antenna (i.e. inverted L)
is a form of top-loaded vertical, where the top loading is
asymmetrical. This results in both vertical and horizontal
polarization because the currents in the top portion do not
cancel like they might in a symmetrical-T antenna."
I read that with a good ground system, the impedance
may be as low as 12 ohms... and I believe it will go up
with a lousy ground plane, not down further... and my
own test measurements
According to ONrUN's book, the longer the vertical section,
the better the low angle DX radiation and the greater the
radiation resistance. He confirms my assumption that
"the bulk of the radiation comes from the bottom part of the
antenna, where the current is highest. Since it is a top-loaded
monopole, an inverted L requires a good ground system."
ON4UN/s Low Band DX-ing, page 9-73.
Over salt water, inverted L antennas exhibit a low 14 ohm
resistance.
Table 9-1 in ON4UN's book shows resistance decreasing with
more radials. Page 9-11.
On Page 9-32, he cites Peter Bobeck, DJ8WS (SK) as testing
vertical antennas and the impedance goes down as the number
of radials is increased.
NOW I AM NO EXPERT but this is what I am thinking.
I am certainly open to other ideas, but this is why I said
what I did.
Happy trails and 73 !!
============= Richards - K8JHR ===============
Tom Osborne wrote:
>
> Wouldn't adding more radials bring up the SWR?
================================================
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