Bill wrote:
Telamon wrote:

In article , Bill wrote:


Telamon wrote:



For average earth conductivity and a 22 gauge wire the height above
ground for 500 ohms impedance would be less than 5 foot and most
likely you would want it around 2 to 3 feet off the ground.


At what frequency did you calculate this?
-Bill



The impedance of the wire is not dependent on frequency.

Z= 138 * log (4* height / wire diameter)

Don't confuse a physical property of the wire with reactance.

Well, you're correct, but. There's more to the antenna than the natural
impedance of the wire alone. You have to look at the 'feedpoint'
impedance which is totally different and thats where you'll find the
reactance which cannot be ignored in actual practice.
Z=R+jX
Thats where frequency gets into the picture and gives you a number to
work with when matching the antenna to your radio.
This is Smith Chart 101...(which I never did too well with)


The fixed *characteristic* impedance of the wire is key to understanding
the feedpoint impedance. If you choose Z0 of your Smith chart to be
equal to the characteristic impedance of the wire antenna, you'll find
that the feedpoint impedance makes a spiral about the center of the
chart as the frequency is varied. This means that the characteristic
impedance (the center point of the spiral) is the best *frequency
independent* match to the wire.

Since for practical configurations the formula Telamon quoted above
yields characteristic impedances in the range of 300-700 ohms, many
receivers have ~500 ohm inputs and many of us use 9:1 matching
transformers when using coax feed.

See http://anarc.org/naswa/badx/antennas/SWL_longwire.html

-jpd

dxAce wrote:


Yes, why try to belabour the point?

I recognized Telemon's antenna formula as something very much like the
transmission line formula. I'm not sure how it applies to resonant
receiving/transmitting end fed wires. If it does, I'd like to learn
something.

But, generally, I don't see much point in trying to caluclate a receiving
antenna's impedance.


He'll do just fine to plug the dang wire
into the 500 ohm input. If he wants or needs to do better he can
improvise a
matching transformer, keep his antenna away from the house or whatever
and then
feed the 50 ohm port.

That's right.


This ain't rocket science, though a few minor details can enhance
performance.

dxAce


Not only that, but the rocket scientists stay stuck on the ground!

Frank Dresser

In article
,
"Frank Dresser" wrote:

dxAce wrote:


Yes, why try to belabour the point?

I recognized Telemon's antenna formula as something very much like
the transmission line formula. I'm not sure how it applies to
resonant receiving/transmitting end fed wires. If it does, I'd like
to learn something.

Yes. Because it is a single wire (Marconi type) antenna the RF
reference is the ground so that is where the simplified equation comes
from. Air is the dielectric (1) where the two conductors are the wire
and ground under the wire. The distance between the two conductors and
the size (diameter) of the conductors determine the characteristic
impedance of the path.

But, generally, I don't see much point in trying to caluclate a
receiving antenna's impedance.

The original poster specifically asked the question.

He'll do just fine to plug the dang wire into the 500 ohm input. If
he wants or needs to do better he can improvise a matching
transformer, keep his antenna away from the house or whatever and
then feed the 50 ohm port.

You bet.

That's right.


This ain't rocket science, though a few minor details can enhance
performance.

I just wanted to make the point that if people want to use the Marconi
type antenna that the return is ground. Ground is the other half of the
antenna. There are always two elements to an antenna because the RF
needs to complete a loop just like a battery in a DC circuit. You have
to connect both sides of a battery to a circuit for it to work,
everybody understands that but people forget that an RF circuit needs
to complete a circuit loop in a similar way.

Not only that, but the rocket scientists stay stuck on the ground!

Other than time spent in a few plane trips I'm earth bound.

--
Telamon
Ventura, California

Frank Dresser wrote:

I recognized Telemon's antenna formula as something very much like the
transmission line formula. I'm not sure how it applies to resonant
receiving/transmitting end fed wires. If it does, I'd like to learn
something.

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that current
distribution. You can get the antenna impedance by calculating the
impedance of a lossy transmission line (with loss equal to the
radiation) with the assumed current distribution. You get the reception
properties by reciprocity.

Not only that, but the rocket scientists stay stuck on the ground!

Good thing. Even when the rocket fails and destroys your payload, you
get to go home and hug your wife and kids. It's happened to me twice so
far (HETE-1 on a Pegasus in 1996, and ASTRO-E1 on a M-V in 2000).

-jpd

In article ,
John Doty wrote:

Frank Dresser wrote:

I recognized Telemon's antenna formula as something very much like
the transmission line formula. I'm not sure how it applies to
resonant receiving/transmitting end fed wires. If it does, I'd
like to learn something.

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that
current distribution. You can get the antenna impedance by
calculating the impedance of a lossy transmission line (with loss
equal to the radiation) with the assumed current distribution. You
get the reception properties by reciprocity.

It boils down to this, smaller RF current loops radiate less
effectively. The wire will become a better antenna the higher it is off
the ground.

If the wire was vertical instead of horizontal then it would not look
like transmission line where the inductance and capacitance are evenly
distributed over its length.

--
Telamon
Ventura, California

Telamon wrote:
In article ,
John Doty wrote:


Frank Dresser wrote:


I recognized Telemon's antenna formula as something very much like
the transmission line formula. I'm not sure how it applies to
resonant receiving/transmitting end fed wires. If it does, I'd
like to learn something.

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that
current distribution. You can get the antenna impedance by
calculating the impedance of a lossy transmission line (with loss
equal to the radiation) with the assumed current distribution. You
get the reception properties by reciprocity.


It boils down to this, smaller RF current loops radiate less
effectively.

Yes, but what does that have to do with the discussion above?

The wire will become a better antenna the higher it is off
the ground.

Often true. Nevertheless, a low Beverage can be an extremely effective
antenna.


If the wire was vertical instead of horizontal then it would not look
like transmission line where the inductance and capacitance are evenly
distributed over its length.


Actually, the Schelkunoff approximation works quite well in that case.
The field near the wire is not strongly affected by its orientation. The
characteristic impedance varies only logarithmically with distance from
ground, so that except for a modest bump in the immediate vicinity of
ground, the inductance and capacitance per unit length are nearly
constant. If this was not true, the Schelkunoff approximation would be
nearly useless.

-jpd

None of the esoterica below should worry anyone who just wants to hook up a
wire to his radio and listen to shortwave stations.


"John Doty" wrote in message
...

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that current
distribution. You can get the antenna impedance by calculating the
impedance of a lossy transmission line (with loss equal to the
radiation) with the assumed current distribution. You get the reception
properties by reciprocity.

OK, but if the transmission line analogy holds, shouldn't the unterminated
antenna look like a lossy stub? If a stub is open, it will look like an
open or short at certain frequencies, and some sort of reactance at others.
Of course, if the transmission line/antenna is terminated with it's
charactistic resistance, it will look flat.

Since the formula isn't frequency sensitive, I was wondering if it was for
terminated wires.


Good thing. Even when the rocket fails and destroys your payload, you
get to go home and hug your wife and kids. It's happened to me twice so
far (HETE-1 on a Pegasus in 1996, and ASTRO-E1 on a M-V in 2000).


Have you seen the movie "Cape Canaveral Monsters"?

Frank Dresser

Frank Dresser wrote:
None of the esoterica below should worry anyone who just wants to hook up a
wire to his radio and listen to shortwave stations.


"John Doty" wrote in message
...

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along the
wire and not radiate. This leads to Schelkunoff's approximation: you
calculate the current distribution along the antenna as if it was a
transmission line, and then calculate the radiation due to that current
distribution. You can get the antenna impedance by calculating the
impedance of a lossy transmission line (with loss equal to the
radiation) with the assumed current distribution. You get the reception
properties by reciprocity.


OK, but if the transmission line analogy holds, shouldn't the unterminated
antenna look like a lossy stub?

Exactly!

If a stub is open, it will look like an
open or short at certain frequencies, and some sort of reactance at others.

If it's lossy, the terminal impedance of an open stub will have a
resistive component at all frequencies. A good first approximation for
an inverted L is a 500 ohm line terminated with a 5000 ohm load. Works
pretty well for wires with lengths in the range from 1/4 wavelength to
several wavelengths.

Of course, if the transmission line/antenna is terminated with it's
charactistic resistance, it will look flat.

When you match to the characteristic impedance, you get a nearly flat
frequency response, but it's down by a few dB from what you'd get by
matching to the terminal impedance.


Since the formula isn't frequency sensitive, I was wondering if it was for
terminated wires.

The characteristic impedance isn't frequency sensitive. The terminal
impedance is. Just like a stub.



Good thing. Even when the rocket fails and destroys your payload, you
get to go home and hug your wife and kids. It's happened to me twice so
far (HETE-1 on a Pegasus in 1996, and ASTRO-E1 on a M-V in 2000).



Have you seen the movie "Cape Canaveral Monsters"?

Nope.

-jpd

"John Doty" wrote in message
...
[snip]


Since the formula isn't frequency sensitive, I was wondering if it was
for
terminated wires.

The characteristic impedance isn't frequency sensitive. The terminal
impedance is. Just like a stub.


I gotcha. I thought the formula was supposed to show a flat terminal
impedance from an end fed wire. I was confused.

By the way, Reg Edwards has some small design programs on his website. One
of them is on topic for this thread. It's called ENDFEED:

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"



Good thing. Even when the rocket fails and destroys your payload, you
get to go home and hug your wife and kids. It's happened to me twice so
far (HETE-1 on a Pegasus in 1996, and ASTRO-E1 on a M-V in 2000).



Have you seen the movie "Cape Canaveral Monsters"?

Nope.


CCM features bickering aliens and both radio and radium! Who could want
anything more? Aside from the narrow minded goofs who list CCM as the worst
movie ever made, that is. I've seen CCM at least half a dozen times and I'd
be watching it right now, if it was on TV. Unless "Hot Rods to Hell" was
on. Anyway, there's a wordy synopsis of CCM he

http://www.jabootu.com/acolytes/bnotes/ccmonsters.htm

Frank Dresser

In article ,
John Doty wrote:

Telamon wrote:
In article ,
John Doty wrote:


Frank Dresser wrote:


I recognized Telemon's antenna formula as something very much like
the transmission line formula. I'm not sure how it applies to
resonant receiving/transmitting end fed wires. If it does, I'd
like to learn something.

For a wire antenna, the field configuration near the wire is very
similar to the field inside a coaxial cable. Unsurprisingly, it has
similar behavior: the bulk of the energy tends to propagate along
the wire and not radiate. This leads to Schelkunoff's
approximation: you calculate the current distribution along the
antenna as if it was a transmission line, and then calculate the
radiation due to that current distribution. You can get the antenna
impedance by calculating the impedance of a lossy transmission line
(with loss equal to the radiation) with the assumed current
distribution. You get the reception properties by reciprocity.


It boils down to this, smaller RF current loops radiate less
effectively.

Yes, but what does that have to do with the discussion above?

Well the original question asked what the characteristic impedance of
the antenna was and its connection to the radio. Since this is a SWL
news group the antenna questions here tend to be "what is the most
effective way to string a long wire" so I can get good reception so I
attempted to relate the antenna height to the discussion pointing out
yet again in another way why higher is better.

The wire will become a better antenna the higher it is off the
ground.

Often true. Nevertheless, a low Beverage can be an extremely
effective antenna.

Yes, but a Beverage antenna to be effective has to be much longer than
a random wire in general. The Beverage is a good antenna if you have
the real estate in the needed direction of the desired station. Most
people do not want a directional antenna for SWL listening.

If the wire was vertical instead of horizontal then it would not
look like transmission line where the inductance and capacitance
are evenly distributed over its length.


Actually, the Schelkunoff approximation works quite well in that
case. The field near the wire is not strongly affected by its
orientation. The characteristic impedance varies only logarithmically
with distance from ground, so that except for a modest bump in the
immediate vicinity of ground, the inductance and capacitance per unit
length are nearly constant. If this was not true, the Schelkunoff
approximation would be nearly useless.

I was just trying to illustrate the reason why a formula for the
characteristic impedance of a horizontal random or long wire "looks"
like a formula for a transmission line with two parallel elements and
how the same wire vertically does not look the same because you no
longer have the constant or even distribution of the inductance of the
wire over its length along with the same capacitance to ground over its
length.

If the wire is vertical the capacitance to ground to a point on the
wire becomes smaller as you move up the wire toward the top. You no
longer have the equal distribution of capacitance and inductance over
its length.

--
Telamon
Ventura, California