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Old August 8th 03, 12:21 PM
Dr. Slick
 
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Default Supporting theory that Antennas "Match" to 377 Ohms (Free space)

Hi,

I looked through Albert Shadowitz's "The Electromagnetic Field",
and found on page 554 support for my original statement that antennas
"match" to the impedance of free space (377 Ohms).

I didn't buy the book, but professor Shadowitz did write on this
page about how creating antennas to most efficiently transfer power to
free space is a similar problem to matching a circuits source to its
load. He goes on to make a short comparison between source/load
impedances to an antenna matching to the impedance of free-space.

Food for thought and no doubt, debate.


Slick

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Old August 8th 03, 03:09 PM
WB3FUP \(Mike Hall\)
 
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And another good reason the folded dipole FM antenna works so great. There
is actually science to support use of a 300 ohm antenna. who'd a thunk it.

--
73 es cul

wb3fup
a Salty Bear

"Dr. Slick" wrote in message
m...
Hi,

I looked through Albert Shadowitz's "The Electromagnetic Field",
and found on page 554 support for my original statement that antennas
"match" to the impedance of free space (377 Ohms).

I didn't buy the book, but professor Shadowitz did write on this
page about how creating antennas to most efficiently transfer power to
free space is a similar problem to matching a circuits source to its
load. He goes on to make a short comparison between source/load
impedances to an antenna matching to the impedance of free-space.

Food for thought and no doubt, debate.


Slick



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Old August 8th 03, 03:46 PM
Yuri Blanarovich
 
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Hi,

I looked through Albert Shadowitz's "The Electromagnetic Field",
and found on page 554 support for my original statement that antennas
"match" to the impedance of free space (377 Ohms).

I didn't buy the book, but professor Shadowitz did write on this
page about how creating antennas to most efficiently transfer power to
free space is a similar problem to matching a circuits source to its
load. He goes on to make a short comparison between source/load
impedances to an antenna matching to the impedance of free-space.

Food for thought and no doubt, debate.


Slick



Yep,
I tried to question that few moons back, but was "convinced" here that it is
not important.
Generally antenna exhibits all kinds of impedances along its length. I was
reasoning that antenna having its lowest impedance higher or closer to 377 ohm
should have better efficiency in coupling to the space (air). So loops and
folded dipoles should be better in that respect. K8CFU et al, when doing
experiments with verticals and radials, found that folded monopole measured
higher signal levels (over simple monopole) than expected.
Any progress since then?

Yuri, K3BU
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Old August 8th 03, 04:00 PM
Richard Harrison
 
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Dr. Slick wrote:
"---creating antennas to most efficiently transfer power to free space
is a similar problem to matching a circuit`s source to its load. (Quote
from Shadowitz)"

Reciprocity rules in antennas. Kraus has an Apendix D (Absorbing
Materials) to his "Antennas For All Applications".

Kraus says:
"The use of space cloth (Z=377 ohms per square) placed lambda/4 from a
reflecting plane was invented by Winfield Salisbury (1) at Harvard Radio
Research Laboratory during WW-2 ---." (Shades of stealth)

However, a century of antenna experimentation has not revealed a
practical need to especially design an antenna to match its radiation to
a resistive 377 ohms.

Most effective radiation occurs when the antenna circuit is matched and
has no loss. 100% of the energy accepted is radiated.

Best regards, Richard Harrison, KB5WZI

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Old August 8th 03, 04:35 PM
Reg Edwards
 
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An antenna's radiating efficiency has nothing whatever to do with the
impedance of its feedline, or whether it's matched to it or not.

Now I suppose somebody will drag in the irrelevant matter of SWR on the
feedline.
---
Reg, G4FGQ




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Old August 8th 03, 05:10 PM
W5DXP
 
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Richard Harrison wrote:
Most effective radiation occurs when the antenna circuit is matched and
has no loss. 100% of the energy accepted is radiated.


In the real world for the same size wire on HF, a folded dipole should
be slightly more efficient than a dipole because of lower I^2*R losses.
--
73, Cecil, W5DXP

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Old August 8th 03, 05:18 PM
W5DXP
 
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Reg Edwards wrote:
An antenna's radiating efficiency has nothing whatever to do with the
impedance of its feedline, or whether it's matched to it or not.

Now I suppose somebody will drag in the irrelevant matter of SWR on the
feedline.


Actually, you brought up the subject. :-) The feedline's power transfer
efficiency is just as important as the antenna's radiating efficiency.
--
73, Cecil, W5DXP

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Old August 8th 03, 07:23 PM
Reg Edwards
 
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Now I suppose somebody will drag in the irrelevant matter of SWR on the
feedline.


Actually, you brought up the subject. :-) The feedline's power transfer
efficiency is just as important as the antenna's radiating efficiency.
--

============================
Cecil, you forgot the efficiency of the PA DC power supply. There's much
more energy wasted there than what's lost in the feeder.

Sort out your power budget. ;o)
---
Reg.


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Old August 8th 03, 07:53 PM
Richard Clark
 
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On Fri, 8 Aug 2003 10:09:01 -0400, "WB3FUP \(Mike Hall\)"
wrote:

And another good reason the folded dipole FM antenna works so great. There
is actually science to support use of a 300 ohm antenna. who'd a thunk it.


Hi Mike,

What you describe is the feedpoint Z not the antenna Z which to all
intents and purposes is not far from the original, single-wire dipole.

73's
Richard Clark, KB7QHC
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Old August 8th 03, 08:08 PM
W5DXP
 
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Reg Edwards wrote:
Actually, you brought up the subject. :-) The feedline's power transfer
efficiency is just as important as the antenna's radiating efficiency.


Cecil, you forgot the efficiency of the PA DC power supply. There's much
more energy wasted there than what's lost in the feeder.


My DC power supply is a 12 VDC marine battery charged by a solar
panel. What's the efficiency of free energy? :-)

Sort out your power budget. ;o)


Actually, I don't much care about the efficiency of the electronics.
60 Hz energy is cheap. I am much more interested in getting the
generated RF into the Æther.
--
73, Cecil, W5DXP



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