I would like one of those antennas!!

BTW, it should be patented!! It looks better than some I've read about!!

;-)

DD, W1MCE

W5DXP wrote:

Richard Harrison wrote:

What happens? Our loading, adjusted for 50 watts output, produces 50
volts at an in-phase current of 50 amps.


Hmmmm, 50 watts in, 2500 watts out. How much will you take for
that antenna, Richard? :-)

(Richard Harrison) wrote in message ...
Dr. Slick wrote:
"But, my point is that you can take a DC measurement anywhere on the
ideal lossless antenna and you will never see 50 Ohms anywhere, only
shorts."

True. D-C resistance of a lossless wire is zero. But, apply 50 watts r-f
power to an antenna adjusted to present 50-ohms resistance at a
particular frequency.

What happens? Our loading, adjusted for 50 watts output, produces 50
volts at an in-phase current of 50 amps.



That would be 2500 watts? I assume you mean 50 volts and 1 amp to
get 50 watts.



Radiation resistance can be readily measured by using an r-f bridge
instrument. The bridge and antenna are excited by a generator operating
at the same frequency the transmitter will use. The bridge will indicate
reactance in the antenna, if it is not resonant. The null detector for
the bridge is typically a good radio receiver.

Radiation resistance is real though it does not heat the antenna. Loss
resistance is real though it does heat the antenna and its surroundings.

A resistance is a volt to amp ratio in which amps are in-phase with the
volts.


The key here is IN-PHASE. If you read my other post, you and Roy
have clarified this for me: Both radiation resistance and dissipative
resistance are the real portion of the impedance.


A resistor is a special type of resistance in which the electrical
energy applied to the resistance is converted into heat.


With very little radiated energy.


Thanks for the input,

Slick

Dr. Slick wrote:
"The funny thing about this is that you cannot say that the 50 ohms in
the center of the chart is a "resistive" 50 ohms as there is very little
real resistance in the average antenna."

Resistance is defined as real. That is, current is instantaneously
proportional to the voltage.

Any efficient antenna has a high ratio of radiation resistance to loss
resistance.

Resistance is the ratio of in-phase voltage to current accepted by an
antenna. Part is made by loss in the antenna. part is made by radiation
from the antenna. They are often represented by an equivalent circuit of
two resistors in series.

Dr, Frederick Emmons Terman says of radiation resistance:
"This is the resistance that, when inserted in series with the antenna,
will consume the same amount of power as is actually radiated. ---it is
customary to refer the radiation resistance to a current maximum in the
case of an ungrounded antenna, and to the base of the antenna when the
antenna is grounded."

Best regards, Richard Harrison, KB5WZI

On Tue, 15 Jul 2003 10:22:02 -0500 (CDT),
(Richard Harrison) wrote:

Dr. Slick wrote:
"The funny thing about this is that you cannot say that the 50 ohms in
the center of the chart is a "resistive" 50 ohms as there is very little
real resistance in the average antenna."


Hi Richard,

This is seems to be a point of convergence for the derivation of many
fascinating and strange theories of fancy and speculation.

You point out in the remainder of your posting about the combination
of resistances (giving particular care to describe in terms of phase)
and yet many posters here fail to account for those same assortment of
R's available from real life.

One notable offering of measuring antenna (or load) impedance involved
the use of a thermometer to which I asked "what is the Z for a 1
degree rise?" I was not surprised to find no answer forthcoming even
when the premise was sound. Such is the shortfall of speculation in
the face of analytical enquiry. There is no corresponding shortfall
of opinion draped in the mantle of citations unfortunately.

These attempts to separate "real" resistance from other resistances
are challenged with volumes of formulaic recitation, and the absolute
resistance (yet another meaning) to merely stepping out into the field
with an OhmMeter (much less that same thermometer). Clearly,
resolution of these imponderables is not a target for some scribblers.

For many years there has been this effete distinction of there being
dissipative and non dissipative resistance (perhaps the basic, or
elemental concern of this thread; yet through hazy writing that agenda
remains elusive). The transmitter cannot possibly separate the two.
It thus remains for the target audience to resolve, but even they
cannot either unless the incident of gaining or losing this additional
resistance occurs in a short enough interval to allow its perception
(notably expressed in dB). If the researchers refuse to do some field
work, it will always remain among their mysteries of the sacrament.

73's
Richard Clark, KB7QHC

(Richard Harrison) wrote in message ...
Dr. Slick wrote:
"The funny thing about this is that you cannot say that the 50 ohms in
the center of the chart is a "resistive" 50 ohms as there is very little
real resistance in the average antenna."

Resistance is defined as real. That is, current is instantaneously
proportional to the voltage.

Any efficient antenna has a high ratio of radiation resistance to loss
resistance.



Ok, i stand partially corrected. I should have stated this:

"You cannot tell if the 50 Ohms reading on a Network analyzer into
a Black Box is a dissipative resistance like a dummy load, or if it is
a radiated resistance of a perfectly matched antenna. You don't have
that information."

Richard, I like your definition of real: "Resistance is defined
as real. That is, current is instantaneously proportional to the
voltage."

In other words, the V and I sine waves will be IN PHASE.

This clarifies it more for me. BOTH the radiation resistance and
the dissipative resistance of a dummy load are both "real" resistive
impedances.


Thanks,

Slick

Dr. Slick wrote:
"You cannot tell if the 50 Ohms reading on a Network analyzer into
a Black Box is a dissipative resistance like a dummy load, or if it is
a radiated resistance of a perfectly matched antenna. You don't have
that information."

Conversion of RF energy to heat can be measured. Conversion of RF energy
to EM radiation can be measured.
--
73, Cecil http://www.qsl.net/w5dxp



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W5DXP wrote in message ...
Dr. Slick wrote:
"You cannot tell if the 50 Ohms reading on a Network analyzer into
a Black Box is a dissipative resistance like a dummy load, or if it is
a radiated resistance of a perfectly matched antenna. You don't have
that information."

Conversion of RF energy to heat can be measured. Conversion of RF energy
to EM radiation can be measured.


Agreed. But a Black Box to me implies you have limited
information from it. My point is that if someone gives you an
impedance plot of a resistive 50 Ohms, you will not be able to tell if
it is dissipative (lossy) or radiated resistance.

I was just reading that Joseph Carr calls radiated resistance as
a sort of "ficticious" resistance. I'm sure many here would argue
this description, but it kinda makes sense to me.


Slick

I'd be one of the people arguing. Radiation resistance fits every
definition of resistance. There's no rule that a resistance has to
dissipate power. The late Mr. Carr was quite apparently confusing
resistance with a resistor, a common mistake.

Why not call radiation resistance "real" resistance and loss resistance
"ficticious"? Makes just as much sense as the other way around -- that
is to say, none.

Roy Lewallen, W7EL

Dr. Slick wrote:
W5DXP wrote in message ...

Dr. Slick wrote:

"You cannot tell if the 50 Ohms reading on a Network analyzer into
a Black Box is a dissipative resistance like a dummy load, or if it is
a radiated resistance of a perfectly matched antenna. You don't have
that information."

Conversion of RF energy to heat can be measured. Conversion of RF energy
to EM radiation can be measured.



Agreed. But a Black Box to me implies you have limited
information from it. My point is that if someone gives you an
impedance plot of a resistive 50 Ohms, you will not be able to tell if
it is dissipative (lossy) or radiated resistance.

I was just reading that Joseph Carr calls radiated resistance as
a sort of "ficticious" resistance. I'm sure many here would argue
this description, but it kinda makes sense to me.


Slick

(Dr. Slick) wrote in message . com...
....
Why do i ask all this? Well, if you believe that complex
impedance measurements (series equivalent) by MFJ antenna analyzers
are not completely inaccurate, then it appears that two 1/4 watt 100
Ohm resistors in parallel (lead lengths short) are a much more
consistent 50 Ohms over the VHF band than almost all the higher power
dummy loads we have tested.

Problem is, the high power dummy loads will vary from 52 to 45
"real" ohms depending on the frequency, with the "real" part of the
impedance getting lower with increasing frequency, so it doesn't seem
to be a "skin effect". The spread gets much worse when you put a 3'
jumper coax in between, and even more worse when you add a power/swr
meter. Then the "real" Ohms will be from 65 to 35 ohms, with the max
and mins not correlating with frequency at all, and the stray
reactances will be much more too, but just as varied with frequency.
So much for "50 ohm" jumper cables! I suppose they are as close as
they can get them for a particular price.


My theory is that the "real" part of the impedance is mainly the
truly resistive 50 ohms of the dummy load at low frequencies around 10
MHz or so...but as you go up in frequency, the parasitics of the dummy
load and the coax jumper cable will cause "radiation" resistance to be
mixed in with this truly real 50 ohms, giving us readings all over the
map.


What do you folks think?

With just a bit of experience in this area, I think you need to find a
way to control your experiments more carefully so you KNOW what's
going on, to a better approximation. I'm quite sure it's possible to
build fairly high power dummy loads--at least high enough power to
handle legal amateur transmitter outputs--that present much better
matches than you describe. Our most usual problem in calibrating
precision instruments at high frequencies is in finding cables which
are really close to 50 ohms and are also practical for routine tests.
Some of the other parts such as power splitters present problems, too.
But we've gotten pretty good at figuring out just where the errors
creep in, through a combination of analysis and experiments. For
sure, our work builds on a tremendous amount of work that has gone on
in the past in the area of precision RF measurements. We currently
work at frequencies from DC to several GHz, and worry in the GHz range
about errors equivalent to an ohm or so...less down at 100MHz and
below.

But for ham applications, do you really care about all this? There's
probably not a need to, if you're just trying to get power to an
antenna, but if you care because you simply want to learn how to build
a precision system and make precision measurements, that's fine too.
You should be able to find lots of info on the web about that...try,
for example, to find Hewlett-Packard/Agilent ap notes on RF
measurements and calibrations.

Cheers,
Tom

(Tom Bruhns) wrote in message m...

But for ham applications, do you really care about all this? There's
probably not a need to, if you're just trying to get power to an
antenna, but if you care because you simply want to learn how to build
a precision system and make precision measurements, that's fine too.
You should be able to find lots of info on the web about that...try,
for example, to find Hewlett-Packard/Agilent ap notes on RF
measurements and calibrations.

Cheers,
Tom


When you are trying to design a 9 element Chebychev low-pass
filter, it becomes important to test it will a 50 Ohm dummy load that
doesn't drift all over the place at higher frequencies.


Slick