On Wed, 26 Sep 2007 16:56:27 GMT, "Wayne"
wrote:

It would be interesting to follow the change in VSWR as you add individual
radials. Some years ago, there was a discussion of adding radials until the
VSWR stopped increasing. This was based on an expected feedpoint resistance
of 36 ohms, and an assumption that the ground losses were 15 ohms or less.
So at the worst case, the VSWR was near 1:1. The best case, with a ground
loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1.

Hi Wayne,

This strategy may also have you stop adding radials too early when
your antenna presented 36 Ohms looking into a ground loss of 33 Ohms
(same SWR of 1.39).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
The best case, with a ground loss approaching 0 ohms, would
have a VSWR of 50/36= 1.39:1.
___________

A set of 120 buried radials each at least 1/4-wave long (free space value)
will produce an r-f ground resistance of around 2 ohms, maybe less in soil
with very good conductivity.

But in any case the impedance existing between the base of a series-fed
vertical monopole and the common point of the buried radials will depend on
the electrical height of the monopole which includes the ratio of its height
to its width, as well as the r-f resistance in the ground system itself.

Broadcast stations will install the tower and radials, measure the Z from
the tower base to the radials, and transform whatever that value is to 50
+j0 ohms using a network at the base of the tower.

Fewer radials will mean that the ground loss will increase, and system
radiation efficiency will decrease (even if the VSWR is 1:1)

RF

"Richard Clark" wrote in message
...
On Wed, 26 Sep 2007 16:56:27 GMT, "Wayne"
wrote:

It would be interesting to follow the change in VSWR as you add individual
radials. Some years ago, there was a discussion of adding radials until
the
VSWR stopped increasing. This was based on an expected feedpoint
resistance
of 36 ohms, and an assumption that the ground losses were 15 ohms or less.
So at the worst case, the VSWR was near 1:1. The best case, with a ground
loss approaching 0 ohms, would have a VSWR of 50/36= 1.39:1.

Hi Wayne,

This strategy may also have you stop adding radials too early when
your antenna presented 36 Ohms looking into a ground loss of 33 Ohms
(same SWR of 1.39).

73's
Richard Clark, KB7QHC

Right. I guess the better way to state it is to stop adding radials when
the r component stops dropping. I've always wanted to try that experiment,
but real estate considerations have prevented it.