Question about antennas

[bench]

Hello all,

I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

 

[John Miller]

bench wrote:

I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

Could it be.........that you're overlooking the relationship of the antenna
to ground?

--
John Miller
Email address: domain, n4vu.com; username, jsm

It is easier for a camel to pass through the eye of a needle if it is
lightly greased.
-Kehlog Albran, "The Profit"

 

[bench]

"John Miller" <me@privacy.net> wrote in message
news:c8e3hs$1di$1@n4vu.com...

bench wrote:
I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

Could it be.........that you're overlooking the relationship of the
antenna
to ground?

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?

--
John Miller
Email address: domain, n4vu.com; username, jsm

It is easier for a camel to pass through the eye of a needle if it is
lightly greased.
-Kehlog Albran, "The Profit"

 

[Rheilly Phoull]

"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

"John Miller" <me@privacy.net> wrote in message
news:c8e3hs$1di$1@n4vu.com...
bench wrote:
I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio
antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

Could it be.........that you're overlooking the relationship of the
antenna
to ground?

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?


--
John Miller
Email address: domain, n4vu.com; username, jsm

It is easier for a camel to pass through the eye of a needle if it is
lightly greased.
-Kehlog Albran, "The Profit"



Don't get caught up in terminology !!

There is a big difference between a circuit 'ground' and a radio 'ground.
I would think it best for you to read up on the theory rather than just ask
"how" here. The questions to ask here would be along the lines of help in
understanding what you are studying.
Best of luck

--
Regards ........... Rheilly Phoull

 

[Joe]

"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

"John Miller" <me@privacy.net> wrote in message
news:c8e3hs$1di$1@n4vu.com...
bench wrote:
I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio
antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

Could it be.........that you're overlooking the relationship of the
antenna
to ground?

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?


--
John Miller
Email address: domain, n4vu.com; username, jsm

It is easier for a camel to pass through the eye of a needle if it is
lightly greased.
-Kehlog Albran, "The Profit"

Hi bench,

I am not sure if this is an answer to your question, because I just started
studying RF stuff. I have my amateur radio tech license (just this past
Feb), and I have a copy of the Radio Amateur Handbook for 2004 (very
reasonable priced at US $24.95). I got my license so I could homebrew RF
stuff without getting into trouble with the FCC.

What you are describing is a 'vertical' antenna. I have used them in my
latest remote control project which uses 433MHz modules ( a ready made
transmitter and receiver which can be mounted on a pc board along with some
other stuff). So far I have tested the system with a loop type antenna and a
vertical (as you described).

The loop antenna is sort of a directional antenna and is just a loop of 20ga
wire with a capacitor coupled to the 'hot' end (the end which is connected
to the antenna pin on the module), and the other end connected to the
battery ground. I get pretty good range with it, but I get better range with
the vertical, which is a piece of 20ga wire sticking straight up out of the
transmitter box, and coupled to the antenna output of the module thru a
capacitor (per the AR handbook). The wire length overall is 15.5 cm which is
supposed to be close to a quarter of the wavelength of the radiation. I
experimented with the transmitter at different heights above the ground in
the backyard. The higher the transmitter antenna was, the better the range.
I think the 'ground' is actually the earth when you use a vertical antenna.
The closer it is to the earth, the less signal gets to the receiver. So the
earth is the ground, at least it seems that way from what I have observed.

In both experiments I used a vertical antenna (same, quarter wave) on the
receiver and kept the receiver at a constant height. The receiver flashed a
10mm jumbo LED when it received the signal so I could measure the distances.
I conducted most of the range measurement experiments close to dusk and
after dark so I could see the LED.

I am a hobbyist, so if my brain didn't interpret the results I observed
correctly, I hope someone on this forum who is more knowledgeable about
these things will correct me. I am still experimenting and open to opinions.

Also, you may want to post your question to a group called
'rec.radio.amateur.homebrew' if your ISP provides it.

hth,
Joe

 

[bench]

"Joe" <nuisancewildlife@nospamearthlink.net> wrote in message
news:siyqc.710$Tn6.526@newsread1.news.pas.earthlink.net...

"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

"John Miller" <me@privacy.net> wrote in message
news:c8e3hs$1di$1@n4vu.com...
bench wrote:
I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio
antenna
all what we have is a piece of metal, and the end bit which
physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

Could it be.........that you're overlooking the relationship of the
antenna
to ground?

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?


--
John Miller
Email address: domain, n4vu.com; username, jsm

It is easier for a camel to pass through the eye of a needle if it is
lightly greased.
-Kehlog Albran, "The Profit"


Hi bench,

I am not sure if this is an answer to your question, because I just
started
studying RF stuff. I have my amateur radio tech license (just this past
Feb), and I have a copy of the Radio Amateur Handbook for 2004 (very
reasonable priced at US $24.95). I got my license so I could homebrew RF
stuff without getting into trouble with the FCC.

What you are describing is a 'vertical' antenna. I have used them in my
latest remote control project which uses 433MHz modules ( a ready made
transmitter and receiver which can be mounted on a pc board along with
some
other stuff). So far I have tested the system with a loop type antenna and
a
vertical (as you described).

The loop antenna is sort of a directional antenna and is just a loop of
20ga
wire with a capacitor coupled to the 'hot' end (the end which is connected
to the antenna pin on the module), and the other end connected to the
battery ground. I get pretty good range with it, but I get better range
with
the vertical, which is a piece of 20ga wire sticking straight up out of
the
transmitter box, and coupled to the antenna output of the module thru a
capacitor (per the AR handbook). The wire length overall is 15.5 cm which
is
supposed to be close to a quarter of the wavelength of the radiation. I
experimented with the transmitter at different heights above the ground in
the backyard. The higher the transmitter antenna was, the better the
range.
I think the 'ground' is actually the earth when you use a vertical
antenna.
The closer it is to the earth, the less signal gets to the receiver. So
the
earth is the ground, at least it seems that way from what I have observed.

In both experiments I used a vertical antenna (same, quarter wave) on the
receiver and kept the receiver at a constant height. The receiver flashed
a
10mm jumbo LED when it received the signal so I could measure the
distances.
I conducted most of the range measurement experiments close to dusk and
after dark so I could see the LED.

I am a hobbyist, so if my brain didn't interpret the results I observed
correctly, I hope someone on this forum who is more knowledgeable about
these things will correct me. I am still experimenting and open to
opinions.

Also, you may want to post your question to a group called
'rec.radio.amateur.homebrew' if your ISP provides it.

My question is very general, not about radio amateurs. Do you have
a portable fm radio, if you do you will know that it has got what is
known as a telescoping antenna (I assume because it is opened
like a telescope). When I opened my radio, I saw one wire coming
out of the antenna and connecting to the circuit board. Now, ...
my question is this, if we want to apply a signal source to some
amplifier, we need a plus and a minus i.e. any voltage source
should have two connection points. So if the antenna is considered
as a signal voltage source, how can it have one wire. I consider
my question to be really focused so I think most people would
agree it is relevant here.

Thanks

hth,
Joe

 

[Jay Davis]

bench wrote:

My question is very general, not about radio amateurs. Do you have
a portable fm radio, if you do you will know that it has got what is
known as a telescoping antenna (I assume because it is opened
like a telescope). When I opened my radio, I saw one wire coming
out of the antenna and connecting to the circuit board. Now, ...
my question is this, if we want to apply a signal source to some
amplifier, we need a plus and a minus i.e. any voltage source
should have two connection points. So if the antenna is considered
as a signal voltage source, how can it have one wire. I consider
my question to be really focused so I think most people would
agree it is relevant here.

Thanks

http://www.electronics-tutorials.com/antennas/antenna-basics.htm

 

[Joe]

"Jay Davis" <test@windmeup.net> wrote in message
news:Q%Nqc.15790798$Of.2634598@news.easynews.com...

bench wrote:

My question is very general, not about radio amateurs. Do you have
a portable fm radio, if you do you will know that it has got what is
known as a telescoping antenna (I assume because it is opened
like a telescope). When I opened my radio, I saw one wire coming
out of the antenna and connecting to the circuit board. Now, ...
my question is this, if we want to apply a signal source to some
amplifier, we need a plus and a minus i.e. any voltage source
should have two connection points. So if the antenna is considered
as a signal voltage source, how can it have one wire. I consider
my question to be really focused so I think most people would
agree it is relevant here.

Thanks

http://www.electronics-tutorials.com/antennas/antenna-basics.htm

Thanks for the link, Jay. Very useful information there.

Joe

 

[bench]

"Joe" <nuisancewildlife@nospamearthlink.net> wrote in message
newsnPqc.2576$be.1089@newsread2.news.pas.earthlink.net...

"Jay Davis" <test@windmeup.net> wrote in message
news:Q%Nqc.15790798$Of.2634598@news.easynews.com...
bench wrote:

My question is very general, not about radio amateurs. Do you have
a portable fm radio, if you do you will know that it has got what is
known as a telescoping antenna (I assume because it is opened
like a telescope). When I opened my radio, I saw one wire coming
out of the antenna and connecting to the circuit board. Now, ...
my question is this, if we want to apply a signal source to some
amplifier, we need a plus and a minus i.e. any voltage source
should have two connection points. So if the antenna is considered
as a signal voltage source, how can it have one wire. I consider
my question to be really focused so I think most people would
agree it is relevant here.

Thanks

http://www.electronics-tutorials.com/antennas/antenna-basics.htm



Thanks for the link, Jay. Very useful information there.

Please share your answer to my question

Joe

 

[MG]

"bench" <bench@neti.com> wrote in message
news:c8e2ue$o72$1@news.epidc.co.kr...

Hello all,

I have a question about how do antennas actually work. My problem
is this : normally a voltage source has a plus terminal and a minus
terminal. Now, ... if we look at a standard fm telescopic radio antenna
all what we have is a piece of metal, and the end bit which physically
connects to the radio is also the voltage source. So do we have here
a one terminal voltage source?

The common node of your portable radio is an extended surface. Is the "GND"
plane of the PC board to which a lot of metal parts like connectors are
attached. All this stuff has a capacity just by sitting in the middle of the
empty universe, also has a capacitance to earth.
At RF frequencies this capacitance is sufficient to close the circuit to
earth or to establish a reference voltage anyway should earth be far away.

At 200kHz Long Waves a better GND is needed but at FM 100MHz even a small PC
board is enough.

MG

 

[L. Fiar]

"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?

I see your problem, it seems to be about perception. The antenna
signal does use a ground, and that ground is part of the antenna
system... you just have to recognise it.

A loop antenna should be easy. Being balanced, it does not have a
ground, the two ends are equal but opposite and DC coupled.

The basic dipole antenna is also balanced, with two sections, each
having an equal but opposite signal upon it.

With the ground plane antenna, this may appear to have just a single
vertical element, but has an image reflected in the ground. The
reference point is the ground.
Your problem is in the concept of that ground, which may be true
ground or an artificial ground made up of a plate, radials, etc.
In the case of a car, the vehicle body is that ground. There are
antennas that have radials pointing out from the base of the
antenna - this is then the ground.
Efficiency is traded off against practicality and, in some cases, the
ground may be as small and simple as the radio ground. Not the most
efficient situation, but practical for portable equipment.

There are several radio handbooks available, which may go into more detail
on RF radiation, propagation, and antenna theory. The Internet is fine, but
nothing beats a good technical book.

 

[L. Fiar]

"Joe" <nuisancewildlife@nospamearthlink.net> wrote in message
news:siyqc.710$Tn6.526@newsread1.news.pas.earthlink.net...

I think the 'ground' is actually the earth when you use a vertical
antenna.

It can be the earth, but there are antennas that use an artificial ground...
maybe a plate, radials, or something similar.

The closer it is to the earth, the less signal gets to the receiver. So
the
earth is the ground, at least it seems that way from what I have observed.

There can be a couple of reasons for this...

First of all, view point - the antenna will be clear of obstructions that
may cut down the signal, and you can see further around the earth from
higher up.

Secondly, the distance between an antenna and it's ground will affect both
radiation resistance and radiation pattern. At certain heights, more signal
is radiated up into the sky, but at the correct height more goes out where
you want it - across the earth.

Also, you may want to post your question to a group called
'rec.radio.amateur.homebrew' if your ISP provides it.

Also, there are many more Amateur groups, like:
rec.radio.amateur.antenna
rec.radio.amateur

 

[bench]

"L. Fiar" <_@_._> wrote in message news:40ac532e.0@entanet...

"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?

I see your problem, it seems to be about perception. The antenna
signal does use a ground, and that ground is part of the antenna
system... you just have to recognise it.

A loop antenna should be easy. Being balanced, it does not have a
ground, the two ends are equal but opposite and DC coupled.

The basic dipole antenna is also balanced, with two sections, each
having an equal but opposite signal upon it.

With the ground plane antenna, this may appear to have just a single
vertical element, but has an image reflected in the ground. The
reference point is the ground.
Your problem is in the concept of that ground, which may be true
ground or an artificial ground made up of a plate, radials, etc.
In the case of a car, the vehicle body is that ground. There are
antennas that have radials pointing out from the base of the
antenna - this is then the ground.
Efficiency is traded off against practicality and, in some cases, the
ground may be as small and simple as the radio ground. Not the most
efficient situation, but practical for portable equipment.

There are several radio handbooks available, which may go into more detail
on RF radiation, propagation, and antenna theory. The Internet is fine,
but
nothing beats a good technical book.

Believe me I have looked in books, they do not have
what I am asking. Even the link posted here talks
about antennas but does not answer my question.
Again, I have seen many interesting
answers here so I will reply here rather than reply to
all the senders. Firstly people seem to be talking about
loop antennas. Can we get a fix on terminology, is
a loop antenna the same as an fm telescopic antenna
because if it isn't then sorry but I am not really
interested in loop antennas.

Now, let me explain, ... if you connect a coax cable
to a t.v. vhf antenna, the antenna has two points and
this is very sensible, since the voltage is induced on
the two ends of this ellipse-like loop. Now, ... with
my fm radio there is this piece of extendable antenna
and it seems that there is one wire coming out of it
if one opens the cover and looks inside. Supposing
I wanted to connect a coax to this antenna, I have
only one wire, this is my question, where is the other
wire because a voltage must be between two wires.

Now, ... I think your answer was that the circuit
board, being made up of a lot of copper wires,
becomes the other polarity of the antenna, is that
what you mean, if so, this may make sense
because also in the Half Wave Dipole Antenna
as seen in Jay's link the two halfs of the antenna
are not physically connecetd.

I think the thing that is confusing me is that in t.v.
vhf antennas both sides of the output come from
the same piece of metal, i.e. they are physically
conneceted, whereas with a Half Wave Dipole
Antenna the two sides are physically isolated,
there is something not intuitive about this.

 

[John Miller]

bench wrote:

I think the thing that is confusing me is that in t.v.
vhf antennas both sides of the output come from
the same piece of metal, i.e. they are physically
conneceted, whereas with a Half Wave Dipole
Antenna the two sides are physically isolated,
there is something not intuitive about this.

I understand your perplexity. When you try to figure it out based on an
understanding of how DC behaves, it makes no sense at all.

RF is a different breed of cat. A length of conductor, continuously excited
by RF of a sufficiently high frequency relative to its length, will have
different AC voltages and currents along its length, ranging all the way
down to zero volts or amps. So you can see why DC thinking can mislead you
in an RF world.

--
John Miller
Email address: domain, n4vu.com; username, jsm

I get my exercise acting as pallbearer to my friends who exercise.
-Chauncey Depew, who lived to ~94 years of age.

 

[Frank Raffaeli]

"bench" <bench@neti.com> wrote in message news:<c8i27c$5ov$1@news.epidc.co.kr>...

"L. Fiar" <_@_._> wrote in message news:40ac532e.0@entanet...
"bench" <bench@neti.com> wrote in message
news:c8e4ib$p61$1@news.epidc.co.kr...

But the radio is not connected to ground, the ground of the radio is the
negative of the battery, isn't it ?

I see your problem, it seems to be about perception. The antenna
signal does use a ground, and that ground is part of the antenna
system... you just have to recognise it.

A loop antenna should be easy. Being balanced, it does not have a
ground, the two ends are equal but opposite and DC coupled.

The basic dipole antenna is also balanced, with two sections, each
having an equal but opposite signal upon it.

With the ground plane antenna, this may appear to have just a single
vertical element, but has an image reflected in the ground. The
reference point is the ground.
Your problem is in the concept of that ground, which may be true
ground or an artificial ground made up of a plate, radials, etc.
In the case of a car, the vehicle body is that ground. There are
antennas that have radials pointing out from the base of the
antenna - this is then the ground.
Efficiency is traded off against practicality and, in some cases, the
ground may be as small and simple as the radio ground. Not the most
efficient situation, but practical for portable equipment.

There are several radio handbooks available, which may go into more detail
on RF radiation, propagation, and antenna theory. The Internet is fine,
but
nothing beats a good technical book.


Believe me I have looked in books, they do not have
what I am asking. Even the link posted here talks
about antennas but does not answer my question.
Again, I have seen many interesting
answers here so I will reply here rather than reply to
all the senders. Firstly people seem to be talking about
loop antennas. Can we get a fix on terminology, is
a loop antenna the same as an fm telescopic antenna
because if it isn't then sorry but I am not really
interested in loop antennas.

Now, let me explain, ... if you connect a coax cable
to a t.v. vhf antenna, the antenna has two points and
this is very sensible, since the voltage is induced on
the two ends of this ellipse-like loop. Now, ... with
my fm radio there is this piece of extendable antenna
and it seems that there is one wire coming out of it
if one opens the cover and looks inside. Supposing
I wanted to connect a coax to this antenna, I have
only one wire, this is my question, where is the other
wire because a voltage must be between two wires.

Now, ... I think your answer was that the circuit
board, being made up of a lot of copper wires,
becomes the other polarity of the antenna, is that
what you mean, if so, this may make sense
because also in the Half Wave Dipole Antenna
as seen in Jay's link the two halfs of the antenna
are not physically connecetd.

I think L. Fiar answered your question very well. You have it exactly
right. There is no monopole antenna. If your FM radio were reduced to
the size of a pea, but the telescoping antenna stayed the same, it
would cease to be effective as an antenna.

If you put a piece of metal floating in the air, it is passive: it
resonates at multiples of it's length, just like an acoustic
resonator. If it is a perfect conductor, it absorbs no power. At 1/2
wavelength, the voltage is highest at the ends (and 180 degrees out of
phase), and the current is highest in the middle.

If you want to make an antenna out of it, you neeed to break it in the
middle and tap into that current flow. The current flows into a load
(the receiver) and now, the antenna absorbs power.

In the case of a 1/4 wave antenna with ground plane or ground radials
.... look at it like this: Place a wire vertically on a reflective
surface and look at the wire and it's reflection. You will see the
dipole. At radio frequencies, the radials (or your radio) look like a
mirror.

hth

Frank Raffaeli
http://www.aomwireless.com/

 

[Joel Kolstad]

Jay Davis <test@windmeup.net> wrote:

bench wrote:

My question is very general, not about radio amateurs. Do you have
a portable fm radio, if you do you will know that it has got what is
known as a telescoping antenna (I assume because it is opened
like a telescope). When I opened my radio, I saw one wire coming
out of the antenna and connecting to the circuit board. Now, ...
my question is this, if we want to apply a signal source to some
amplifier, we need a plus and a minus i.e. any voltage source
should have two connection points. So if the antenna is considered
as a signal voltage source, how can it have one wire. I consider
my question to be really focused so I think most people would
agree it is relevant here.

http://www.electronics-tutorials.com/antennas/antenna-basics.htm

That web site doesn't really answer his question.

Jay: The 'other wire' (ground) is some ill-defined mash of the circuit
ground in the radio. If you're familiar with the idea of an antenna above a
ground plane (and a 'whip' antenna as a voltage source with its ground
referenced to that plane), they just start thinking of taking that ground
plane and making it finite and smaller and smaller until it's, uh, all the
ground traces of the radio's circuit board. At that point, the exact
radiation pattern is anyone's guess, although often times whip antennas are
electrically short and the radiation pattern is still something resembling a
bagel.

Another way to think of this: Say you have a dipole antenna that you're
using as a 'voltage generator' by taping the middle of it, Vant+ and Vant-
(like figure 3 on the web site listed). Now start sliding the the tap point
down... further, further, further, until you're all the way at one end!
This is called an 'end fed' antenna (see:
http://www.qsl.net/aa5tb/efha.html) and can be analyzed rigorously.

---Joel

 

[Joel Kolstad]

bench <bench@neti.com> wrote:

I think the thing that is confusing me is that in t.v.
vhf antennas both sides of the output come from
the same piece of metal, i.e. they are physically
conneceted, whereas with a Half Wave Dipole
Antenna the two sides are physically isolated,
there is something not intuitive about this.

A 'classical' way to think about this is that the loop antenna is being
coupled to inductively (a current is flowing around the loop) whereas with
the dipole the antenna is being coupled to capacitively. Think about what
happens if you have, say, a sine wave generator connected to a capacitor,
then a load resistor, and then another capacitor back to the generator: The
sine wave will 'go through' the capacitors and appear across the resistor,
even though there's one two plates involved in (each) capacitor. Now, move
the plates a thousand miles away and everything is (to a very crude
approximation) still the same other than changes in magnitude, phase, etc.

The bottom line is that if you stick any 'bits of metal' in space and
connect them together through a load, the fields in the air will start
inducing currents through the load. Exactly how 'well' that occurs depends
on the geometry of those 'bits of metal,' and the well-known cases of
dipoles can be readily analyzed. You cannot realistically get anything more
than an educated guess as to the _exact_ radiation pattern of, say, the
antenna on a cell phone... although with a computer with lots of horsepower
and memory and time on your hands, you can solve Maxwell's equations
directly and figure it out.

---Joel

 

[Joel Kolstad]

Frank Raffaeli <SNIPrf_man_frTHIS@yahoo.com> wrote:

I think L. Fiar answered your question very well. You have it exactly
right. There is no monopole antenna. If your FM radio were reduced to
the size of a pea, but the telescoping antenna stayed the same, it
would cease to be effective as an antenna.

I beg to differ... end-fed antennas (just about) fit this description and do
work well. See the link posted previously:
http://www.qsl.net/aa5tb/efha.html . This doesn't stop people from claiming
that end-fed antennas shouldn't work at all... the main difficulty being
that, while the "ideal" end-fed antenna has infinite impedance and therefore
can't accept/output power, real end-fed antennas (or any antenna fed at the
minimum current/voltage maximum) tends to have an impedance more along the
ballpark of some kilohms, which can actually be more desirable than 50 ohms
at times.

You can't get that 'some kilohms' answer from the 'ideal dipole' equations
with simplifications, however (there's where the infinite impedance answer
comes from)...you need to solve something like Hallen's or Pocklington's
integral equation, which -- since they can only be done numerically
anyway -- immediately leads one to just sit down and use NEC or a similar
computer program to perform the analysis.

---Joel

 

[Frank Raffaeli]

"Joel Kolstad" <JKolstad71HatesSpam@Yahoo.Com> wrote in message news:<c8jsvq$tq0$1@news.oregonstate.edu>...

Frank Raffaeli <SNIPrf_man_frTHIS@yahoo.com> wrote:
I think L. Fiar answered your question very well. You have it exactly
right. There is no monopole antenna. If your FM radio were reduced to
the size of a pea, but the telescoping antenna stayed the same, it
would cease to be effective as an antenna.

I beg to differ... end-fed antennas (just about) fit this description and do
work well. See the link posted previously:
http://www.qsl.net/aa5tb/efha.html . This doesn't stop people from claiming
that end-fed antennas shouldn't work at all... the main difficulty being
that, while the "ideal" end-fed antenna has infinite impedance and therefore
can't accept/output power, real end-fed antennas (or any antenna fed at the
minimum current/voltage maximum) tends to have an impedance more along the
ballpark of some kilohms, which can actually be more desirable than 50 ohms
at times.

You can't get that 'some kilohms' answer from the 'ideal dipole' equations
with simplifications, however (there's where the infinite impedance answer
comes from)...you need to solve something like Hallen's or Pocklington's
integral equation, which -- since they can only be done numerically
anyway -- immediately leads one to just sit down and use NEC or a similar
computer program to perform the analysis.

Hi Joel,

It's true that my example did not account for end-fed antennas, where
the feed is at (or near) the maximum voltage point; however, if the
circuitry and feedline attached to the antenna were reduced to "pea"
size, it would still cease to be an effective antenna because the
voltage gradient along the "pea" would be nearly zero, thus giving the
antenna nothing to drive against. In this very hypothetical example,
the antenna would still be ineffective. In the real world, there is
always something attached to the pea (or the receiver or transmitter
has significant size relative to a wavelength), thus completing the
total antenna. In the usual case, an end-fed antenna would be as
_practical_ as a center-fed antenna.

The feedline and radio contribute to the total aperture, and therefore
become part of the antenna, unless the feed is balanced and matched.

Frank Raffaeli
http://www.aomwireless.com/

 

[L. Fiar]

"bench" <bench@neti.com> wrote in message
news:c8i27c$5ov$1@news.epidc.co.kr...

I think your answer was that the circuit board,
being made up of a lot of copper wires, becomes
the other polarity of the antenna, is that
what you mean,

Yes.
It is quite common to use ground rods or a car body
as the ground element, but that is not practical with
a portable radio. So, you make do with what
you can carry around.

if so, this may make sense because also in the Half
Wave Dipole Antenna as seen in Jay's link the two
halfs of the antenna are not physically connecetd.

No connection as far as DC is concerned, but there is capacitance.

To compare with a loop antenna...
As far as DC is concerned, you can see a short circuit. As far as the RF is
concerned, that loop has an impedance.
In the case of a dipole, you see an open circuit, but the RF sees 75 Ohms
between the elements.

For the ground plane antenna, at DC it is an open circuit, but the impedance
to the RF signal very low, lower than a dipole. Where an antenna uses
radials for the ground, the angle of these radials affects the impedance.


I may have found a Web reference for what you are looking
for...

<quote>
How can we axplain the fact that you can drive current
into a dipole when the ends are open circuited...
</quote>
The document goes on to ask:
"What about a monopole, does it need just one
part to radiate?"

http://www.hottconsultants.com/pdf_files/dipoles-1.pdf
The site is directed at EMC issues, which is why it then goes on to refer to
EMC.

There's a couple more parts to the dipole document on that site:
http://www.hottconsultants.com/pdf_files/dipoles-2.PDF
http://www.hottconsultants.com/pdf_files/dipoles-3.PDF