WA4SZE wrote:
"I have shunt fed a 120 foot high Rohn 25G tower."

The Rohn has a 1-ft face, so the h/d is about 120. That`s OK. What`s
wacko is a 23-degree tower over (8) 200-ft radials at 529 KHz. Ground
connection resistance is high and eating up all the signal.

Shunt - feeding is OK. Bill Orr and Stu Cowan give feed capacitors for
scalimg in "All About Vertical Antennas".

Brown, Lewis and Epstein would be disappointed with your radials. Shoot
for the broadcast practice of (120) evenly distributed around from the
tower base.

A short tower radiates almost as well as a 1/4-wave. but it has a very
low radiation resistance so can`t tolerate any loss resistance.

Kraus gives advice for Electrically Small Antennas in the 3rd edition of
"Antennas". Page 710 says:
"To increase the radiation efficiency requires an increase in the
radiation resistance Rr or a decrease in the loss resistance Rl or both.

The SWR of a dummy load usually looks fine, but radiation is just
incidental.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
Brown, Lewis and Epstein would be disappointed with your radials. Shoot
for the broadcast practice of (120) evenly distributed around from the
tower base.
_________

BL&E's 1937 measurements show (Fig 30) that a vertical monopole of 25 to 90
electrical degrees used with 113 buried radials each of 0.412 wavelength
produced a measured groundwave field within a few percent of the theoretical
maximum for such radiators over a perfect ground (notwithstanding that the
conductivity at their test site was around 4 mS/m).

In Fig 32 of that paper it can be seen that if the 113 radials are only
0.274-wavelengths long, then at the 25-degree electrical height of this Rohn
tower, the measured field was about 79% of theoretical field over a perfect
ground.
..
So it's not just the number of radials that is important, but also their
length.

The referenced figures are linked below, under the "fair use" provisions of
copyright law.

http://s62.photobucket.com/albums/h8...BLERadials.gif

RF

Here is the correct link to BL&E Figs 30 and 32.

http://i62.photobucket.com/albums/h8...ndERadials.gif

On Sat, 7 Jul 2007 16:02:11 -0500, "Richard Fry" wrote:

Here is the correct link to BL&E Figs 30 and 32.

http://i62.photobucket.com/albums/h8...ndERadials.gif

Got it, Richard, but I see the two figs are simply from their 1937 IRE paper.

Thanks anyway.

Walt

On Sat, 7 Jul 2007 15:53:36 -0500, "Richard Fry" wrote:

The referenced figures are linked below, under the "fair use" provisions of
copyright law.

http://s62.photobucket.com/albums/h8...BLERadials.gif

RF

Richard, I just now tried to access your link above, but it says the file is no longer available. Do you have
any other source of this data? I worked with BL&E, so I'm kinda partial to having all the data from their 1936
experiment that I can find. I have their 1937 IRE paper.

Walt, W2DU

"Walter Maxwell"
Richard, I just now tried to access your link above, but it says the file
is no longer available. Do you have any other source of this data? I
worked with BL&E, so I'm kinda partial to having all the data from their
1936 experiment that I can find. I have their 1937 IRE paper.
_________

Walt -

I showed the working link in another post I made in followup, which should
work for you.

The link (again) is

http://i62.photobucket.com/albums/h8...ndERadials.gif

But if you have their 1937 paper, you already have the figures I referred
to.

RF

Richard Fry wrote:
"So it`s not just the number of radials that is important, but also
their length."

True. I don`t have B.L. and E`s work to refer to but do recall an
observation that to get the best ground connection for the least copper
it might be wise to cut the radials in half so that their number might
be doubled.

The FCC standard is 120 radials, each 1/4-wavelength long, which may
seem extreme but it produces a near perfect ground connection.

Fortunately, the length of radials does not need to be increased in
direct proportion to wavelength below the broadcast band as skin effect
varies with the square root of the frequency so as we go lower in
frequency we need to increase length of the radials by the square root
of the wavelength to keep the resistance of our contact constant.

Best Regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
Fortunately, the length of radials does not need to be increased in
direct proportion to wavelength below the broadcast band as skin effect
varies with the square root of the frequency so as we go lower in
frequency we need to increase length of the radials by the square root
of the wavelength to keep the resistance of our contact constant.
____________

Here is a quote about this from RADIO ANTENNA ENGINEERING
by Edmund Laport:

"The distance from the antenna at which returning ground currents are of
such a low value as to be negligible is of the order of 0.5 wavelength."

At least across the AM broadcast band 530-1700 kHz, Laport, Terman, Kraus,
and Balanis show _no_ dependence of the lengths of buried radials with
frequency -- except, of course, that radials used with lower frequency
systems need to be physically longer to reach the desired radius from the
monopole, in free-space wavelengths.

The FCC uses a computer program (linked below) to calculate the groundwave
inverse distance field at 1 km and 1 mile from a MW monopole, based on the
radiator height and the number/length of buried radials.

On-line users of this program can enter their own system parameters to see
their effects on the radiated field. The program does restrict entries to
the minimum values acceptable to the FCC for commercial AM broadcast
stations.

== NOTE: The FCC program applet has no entry block for frequency.

Here is the link. The applet starts at the bottom of that web page.

http://www.fcc.gov/mb/audio/bickel/figure8.html

RF

== NOTE: The FCC program applet has no entry block for frequency.
_________

Yes it does (sorry), but ...

== NOTE: The FCC program results are essentially the same regardless of
frequency for all systems of a given radiator height and radial length (in
wavelengths), when using the same number of radials.

RF