On Tue, 20 Nov 2007 16:23:45 -0800, (Dave Platt)
wrote:

Bailey gives two impedance figures - the "surge"
impedance and the feedpoint impedance

Re pg 340:
"By definition, the surge impedance of a conductor
for all practical purposes is that value of load
resistance which, if placed at the end of the
conductor, will completely prevent refelcted power
from being sent back into the conductor by
the load."

Less conductor periphery, more surge Z.

Some representative values:
Thin dipole 610 Ohms (Thin = #10 wire @ 200MC)
Dipole 420 Ohms (0.5" @ 200MC)
Thick Dipole 240 Ohms (Thick = 2.5" @ 200MC)
Vertical 260 Ohms (.25" @ 200MC)
Biconical 150 Ohms

YMMV

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Tue, 20 Nov 2007 16:23:45 -0800, (Dave Platt)
wrote:

Bailey gives two impedance figures - the "surge"
impedance and the feedpoint impedance

Re pg 340:
"By definition, the surge impedance of a conductor
for all practical purposes is that value of load
resistance which, if placed at the end of the
conductor, will completely prevent refelcted power
from being sent back into the conductor by
the load."

Less conductor periphery, more surge Z.

Some representative values:
Thin dipole 610 Ohms (Thin = #10 wire @ 200MC)
Dipole 420 Ohms (0.5" @ 200MC)
Thick Dipole 240 Ohms (Thick = 2.5" @ 200MC)
Vertical 260 Ohms (.25" @ 200MC)
Biconical 150 Ohms

YMMV

Ok, I give up. I've got a dipole in free space. I connect one end of the
load resistance to one end of the dipole. What do I connect the other
end of the resistor to?

If that's too tough, how about just a dipole a quarter wavelength above
ground?

Roy Lewallen, W7EL

Roy Lewallen wrote:
Ok, I give up. I've got a dipole in free space. I connect one end of the
load resistance to one end of the dipole. What do I connect the other
end of the resistor to?

How about a nice terminated rhombic?

If that's too tough, how about just a dipole a quarter wavelength above
ground?

How about an inv-V dipole 0.177 wavelength above ground?
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
"OK, I give up. I`ve got a dipole in free space. I connect one end of
the load resistance to one end of the dipole. What do I connect the
other end of the resistor to?"

Great question. You connect it to the inobtrusive convenient perfect
earth connection immediately present at the end of the terminating
resistor. Since this is likely impossible, you merely define you have
done the deed and calculate the results.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Roy Lewallen wrote:
"OK, I give up. I`ve got a dipole in free space. I connect one end of
the load resistance to one end of the dipole. What do I connect the
other end of the resistor to?"

Great question. You connect it to the inobtrusive convenient perfect
earth connection immediately present at the end of the terminating
resistor. Since this is likely impossible, you merely define you have
done the deed and calculate the results.

Another way to mentally view the surge impedance is with
an infinitely long wire for the dipole. It changes the
dipole from a standing-wave antenna into a traveling-
wave antenna. The ratio of V to I is constant and equal
to the characteristic (surge) impedance of the antenna.

Reg claimed the feedpoint (surge) impedance for an infinite
dipole would be about 1200 ohms, i.e. ~600 ohms in each
direction which roughly agrees with the formula 138*log(4D/d).
--
73, Cecil http://www.w5dxp.com

On Tue, 20 Nov 2007 21:38:55 -0800, Roy Lewallen
wrote:

If that's too tough, how about just a dipole a quarter wavelength above
ground?

Hi Roy,

Hmmm, too tough.

The math is all pretty ordinary and well within your capacity to
examine in your own copy of the book. So I won't belabor others with
that. However, as others do not have a copy, they might appreciate
some other benchmarks:

End Fed Half-Wave 600 Ohms
which, of course, suffers the same question;

Folded Quarter-Wave Dipole 260 Ohms
there, a resistor might comfortable be applied, but I don't know as I
have not tried it; yet and all, this is drawn from the same ordinary
math;

Folded Half-Wave Dipole 500 Ohms
same observation as with the Quarter-Wave and falling nearly on twice
the rating too - a coincidence of the math no doubt; (But I believe
there is a commercial variant of this that is popular with the MARS
group exactly for the reasons of not presenting SWR.)

Continuous Wire Array 300 Ohms
an oddity in the pantheon of styles, yet it too seems suitable to
resistive termination for testing "Surge Impedance;"

Rhombic Antenna 600 Ohms
here's one that is more familiar to all, and with the added bonus that
it is typically loaded with a resistor to exactly fulfill the
definition of "Surge Impedance" whose ordinary math dovetails with
experience.

But, Roy, I do recognize the terms "Surge Impedance" maybe a language
barrier with you as it certainly qualifies as an archaic term. Reggie
loved to carry this water and in spite of differences, his treatments
quite often got him results that were good first approximations, and
sometimes better.

If you really seek closure, I would suggest you could mine your own
copy of this reference for errors. As for its practical importance -
well, let's just say that this board would support only 2% of the
current traffic if it came to that.

73's
Richard Clark, KB7QHC