opinions on an antenna idea
 

Roy Lewallen wrote:
People have an extremely strong tendency to simplify the mass of
incoming data into simply digested and understood binary categories: Is
it good, or is it evil? Does the antenna work, or doesn't it?
...

Uhh, yeah, that sums me up pretty much. You think I should be ashamed?

JS

opinions on an antenna idea
 

Alan Peake wrote:

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan


If room temperature super-conductors were available, do you even realize
the shape antennas would take? My gawd man, share some of that material
here! The thought alone is inspiring!

Regards,
JS

opinions on an antenna idea
 

Alan Peake wrote:

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan

No. The answer can be found in any antenna textbook, because the
lossless short dipole is a very good platform to illustrate a number of
principles without the confounding additional consequences of loss.

Briefly,

-- The pattern of an infinitesimally short dipole is very similar to
that of a half wave dipole. The difference is due to the triangular
current distribution of the short dipole as opposed to the sinusoidal
current distribution of the half wave dipole. Because the patterns are
very similar and both antennas radiate all the applied power, the gain
of the two antennas is nearly the same. The short dipole's pattern is a
little fatter so it has slightly -- about a half dB -- less gain. But
the pattern of even an infinitesimally short dipole retains the basic
two-lobed dipole shape with around 1.7 dB gain over isotropic in its
favored directions.

-- The input resistance of the very short lossless dipole is very low
and the capacitive reactance very high. The resistance approaches zero
and the reactance negative infinity as the length approaches zero.
There's no comparison to an isotropic radiator, since the latter is a
purely fictional source with no even theoretical physical realization
and therefore no definable input characteristics.

-- The Q of the short dipole is very high, so the reactance varies very
rapidly with frequency. A matched short antenna would have an extremely
narrow bandwidth.

Most of these properties of the dipoles can easily be observed with the
free EZNEC demo program from http://eznec.com, and much more information
about the properties of the short lossless dipole can be found in any
antenna text.

Roy Lewallen, W7EL

opinions on an antenna idea
 

Roy Lewallen wrote:

...
-- The Q of the short dipole is very high, so the reactance varies very
rapidly with frequency. A matched short antenna would have an extremely
narrow bandwidth.
...

Roy Lewallen, W7EL


And, here is where a DLM antenna is nice, keep the coils of low Q and
bandwidth is "surprisingly wide."

Regards,
JS

opinions on an antenna idea
 

Alan Peake wrote:


Roy Lewallen wrote:
Alan Peake wrote:


If one were to find lossless material (superconductors?) for the
short antenna and it's corresponding matching network, what would
happen as the antenna became shorter and shorter compared with the
half-wave dipole? Would it simply approach an isotropic radiator?
Alan


No. The answer can be found in any antenna textbook,
...
etc.

Roy Lewallen, W7EL


Thanks Roy. Unfortunately, since I retired, I no longer have access to
Jasik, Kraus etc. So, thanks for the answer. I should have realised that
a dipole of any length is still a dipole and as such will not radiate
off it's ends. Mind you, Eznec shows the average dipole, less than
half-wave above ground, goes pretty close to an isotropic radiator for
all practical purposes :)
Alan



---------------


You appear to be extrapolating, if I see this correctly, that since all
of the radiation is believed to come from one end of the dipole, then
the rest of the antenna is merely acting as the necessary reactances and
resistance needed to obtain the proper feedpoint impedance at a given
frequency. True?

Following that line of reasoning, if the need for the aggregate
reactances/resistances can be eliminated via superconducting elements,
one will have just a single point source of radiation. Or, what is
commonly known as an isotropic radiator. I suspect that the plasma
antenna fellows are contemplating this too.

Ed, NM2K

opinions on an antenna idea
 

Alan Peake wrote:


Roy Lewallen wrote:
Alan Peake wrote:


If one were to find lossless material (superconductors?) for the
short antenna and it's corresponding matching network, what would
happen as the antenna became shorter and shorter compared with the
half-wave dipole? Would it simply approach an isotropic radiator?
Alan


No. The answer can be found in any antenna textbook,
...
etc.

Roy Lewallen, W7EL


Thanks Roy. Unfortunately, since I retired, I no longer have access to
Jasik, Kraus etc. So, thanks for the answer. I should have realised that
a dipole of any length is still a dipole and as such will not radiate
off it's ends. Mind you, Eznec shows the average dipole, less than
half-wave above ground, goes pretty close to an isotropic radiator for
all practical purposes :)
Alan



---------------


You appear to be extrapolating, if I see this correctly, that since all
of the radiation is believed to come from one end of the dipole, then
the rest of the antenna is merely acting as the necessary reactances and
resistance needed to obtain the proper feedpoint impedance at a given
frequency. True?

Following that line of reasoning, if the need for the aggregate
reactances/resistances can be eliminated via superconducting elements,
one will have just a single point source of radiation. Or, what is
commonly known as an isotropic radiator. I suspect that the plasma
antenna fellows are contemplating this too.

Ed, NM2K

opinions on an antenna idea
 

Ed Cregger wrote:
Alan Peake wrote:


Roy Lewallen wrote:
Alan Peake wrote:


If one were to find lossless material (superconductors?) for the
short antenna and it's corresponding matching network, what would
happen as the antenna became shorter and shorter compared with the
half-wave dipole? Would it simply approach an isotropic radiator?
Alan


No. The answer can be found in any antenna textbook,
...
etc.

Roy Lewallen, W7EL


Thanks Roy. Unfortunately, since I retired, I no longer have access to
Jasik, Kraus etc. So, thanks for the answer. I should have realised
that a dipole of any length is still a dipole and as such will not
radiate off it's ends. Mind you, Eznec shows the average dipole, less
than half-wave above ground, goes pretty close to an isotropic
radiator for all practical purposes :)
Alan



---------------


You appear to be extrapolating, if I see this correctly, that since all
of the radiation is believed to come from one end of the dipole, then
the rest of the antenna is merely acting as the necessary reactances and
resistance needed to obtain the proper feedpoint impedance at a given
frequency. True?

Following that line of reasoning, if the need for the aggregate
reactances/resistances can be eliminated via superconducting elements,
one will have just a single point source of radiation. Or, what is
commonly known as an isotropic radiator. I suspect that the plasma
antenna fellows are contemplating this too.

Ed, NM2K


-------------


Bellsouth, now AT&T, is back to double posting everything again. Was
having problems with their DSL service all last night. Wish they would
get their act together.


Ed, NM2K

opinions on an antenna idea
 

The answer here, as it is to so may binary questions, is that it behaves
in some ways like one, some ways like the other, and some ways like
neither.
..
..
..

Roy Lewallen, W7EL

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan

opinions on an antenna idea
 

Ed Cregger wrote:

You appear to be extrapolating, if I see this correctly, that since all
of the radiation is believed to come from one end of the dipole, then
the rest of the antenna is merely acting as the necessary reactances and
resistance needed to obtain the proper feedpoint impedance at a given
frequency. True?

No. It's not true that all the radiation "comes from one end of the
dipole". Extrapolation from that mistaken premise will lead to invalid
conclusions.

Following that line of reasoning, if the need for the aggregate
reactances/resistances can be eliminated via superconducting elements,
one will have just a single point source of radiation. Or, what is
commonly known as an isotropic radiator. I suspect that the plasma
antenna fellows are contemplating this too.

And there's the first one. . .

Roy Lewallen, W7EL

opinions on an antenna idea
 

Roy Lewallen wrote:
Ed Cregger wrote:

You appear to be extrapolating, if I see this correctly, that since
all of the radiation is believed to come from one end of the dipole,
then the rest of the antenna is merely acting as the necessary
reactances and resistance needed to obtain the proper feedpoint
impedance at a given frequency. True?

No. It's not true that all the radiation "comes from one end of the
dipole". Extrapolation from that mistaken premise will lead to invalid
conclusions.

Following that line of reasoning, if the need for the aggregate
reactances/resistances can be eliminated via superconducting elements,
one will have just a single point source of radiation. Or, what is
commonly known as an isotropic radiator. I suspect that the plasma
antenna fellows are contemplating this too.

And there's the first one. . .

Roy Lewallen, W7EL


-------------


You are the acknowledged expert here (we're not worthy!!!).

What is the flaw in the proposed thinking? You have to admit that lots
of the commercial antenna companies and ham publications either do, or
used to, emphasize the point that "most of the radiation of a 1/4 wave
ground plane antenna (half of a half wave) occurs near the feed point".

Instead of just saying, no, this thinking is incorrect, how about
teaching your students (includes me) precisely what is wrong with this
line of thinking. Not at the engineering level necessarily (oodles of
formulas), but in the analog/real world level.

Please?

Be merciful, oh great one. I'm on enough prescription drugs to put half
a football team to sleep, so, occasionally, I get quite tangential to
the topic at hand. I hope this isn't one of those times. G

Thank you, oh merciful one.


Ed, NM2K