Taking one's cue from the Alford Slot Antenna, let us propose the use of the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

To do this, we will terminate our 50 ohm coax with a series LC circuit of 50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

Very little of the energy supplied to our antenna will be radiated, and most
will be stored
as energy in the resonant LC circuit, until such time as the stored energy
outdoes the incoming energy,and
the LC circuit will now act as an energy source, sending it back down the
coax.

So, we'll have a short antenna, matched to its feeder, that is not radiating
all the power fed to it, and
is returning some of that power back down the coax.

(An even simpler case, although it would give a reactiv match, would be the
capacitor alone, but think
for one minute, why do you get the 90 degrees phase relationship between the
volts and current
in a capacitor? Because it acts like an energy source in its own right!

In rec.radio.amateur.antenna gareth wrote:
Taking one's cue from the Alford Slot Antenna, let us propose the use of the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

Slot antennas get added to the list of things Gareth doesn't understand.

A parallel plate capacitor is nothing like a slot antenna of any kind
and does not radiate like an antenna.

To do this, we will terminate our 50 ohm coax

Slot antennas are fed with waveguide.

with a series LC circuit of 50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

That would be two 25 Ohm resistors, one of which is slightly capacitive
and the other slightly inductive.

Very little of the energy supplied to our antenna will be radiated,

This part is correct; it will mostly be heat dissipated in the two
25 Ohm resistors in series.

and most
will be stored
as energy in the resonant LC circuit,

Nope, dissipated as heat in the resistors.

until such time as the stored energy
outdoes the incoming energy,and
the LC circuit will now act as an energy source, sending it back down the
coax.

Series LC circuits get added to the list of things Gareth doesn't understand.

So, we'll have a short antenna, matched to its feeder, that is not radiating
all the power fed to it, and
is returning some of that power back down the coax.

Gibberish.

(An even simpler case, although it would give a reactiv match, would be the
capacitor alone, but think
for one minute, why do you get the 90 degrees phase relationship between the
volts and current
in a capacitor? Because it acts like an energy source in its own right!

For a quarter cycle for AC.

Capacitors get added to the list of things Gareth doesn't understand.


--
Jim Pennino

On 9/14/2015 11:14 AM, gareth wrote:
Taking one's cue from the Alford Slot Antenna, let us propose the use of the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

To do this, we will terminate our 50 ohm coax with a series LC circuit of 50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

I think your intent and your values don't correspond. The +jX and -jX
terms are correct for the imaginary parts of the impedances, but you
don't want the capacitor and inductor to have 25 ohms each of
dissipative, real resistance do you? That would leave nothing for the
transmission resistance if you were to radiate any energy. So either
you would be designing an antenna at other than 50 ohms impedance, or
you are trying to design an antenna which doesn't radiate.


Very little of the energy supplied to our antenna will be radiated, and most
will be stored
as energy in the resonant LC circuit, until such time as the stored energy
outdoes the incoming energy,and
the LC circuit will now act as an energy source, sending it back down the
coax.

Not sure what you are trying to say with this. Why would the LC circuit
ever send power back down the feed line?


So, we'll have a short antenna, matched to its feeder, that is not radiating
all the power fed to it, and
is returning some of that power back down the coax.

The antenna you describe will be absorbing and dissipating nearly all
the energy it receives from the feed line as heat in the capacitor and
inductor.


(An even simpler case, although it would give a reactiv match, would be the
capacitor alone, but think
for one minute, why do you get the 90 degrees phase relationship between the
volts and current
in a capacitor? Because it acts like an energy source in its own right!

Not so much when it has a resistance of 25 ohms dissipating a huge
amount of power on each cycle.

--

Rick

"rickman" wrote in message
...
On 9/14/2015 11:14 AM, gareth wrote:
Taking one's cue from the Alford Slot Antenna, let us propose the use of
the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

To do this, we will terminate our 50 ohm coax with a series LC circuit of
50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

I think your intent and your values don't correspond. The +jX and -jX
terms are correct for the imaginary parts of the impedances, but you don't
want the capacitor and inductor to have 25 ohms each of dissipative, real
resistance do you? That would leave nothing for the transmission
resistance if you were to radiate any energy. So either you would be
designing an antenna at other than 50 ohms impedance, or you are trying to
design an antenna which doesn't radiate.

Oops! Yes, of course you are right; too much typing and not enough thought
:-)

What I was trying to picture was a series LC circuit presenting a
non-reacitve
impedance of 50 ohms.

"Brian Morrison" wrote in message
...
On Mon, 14 Sep 2015 15:37:11 -0000
wrote:

To do this, we will terminate our 50 ohm coax

Slot antennas are fed with waveguide.

Not always, it's very easy to feed them with microstrip or stripline.

And the Alford Slot, developed for 23 cm, was fed with coax.

Has Jimmy Pendulumino once more jumped in, in his haste to want
to be abusive, and again demonstrated his very limited knowledge?

In rec.radio.amateur.antenna Brian Morrison wrote:
On Mon, 14 Sep 2015 15:37:11 -0000
wrote:

To do this, we will terminate our 50 ohm coax

Slot antennas are fed with waveguide.

Not always, it's very easy to feed them with microstrip or stripline.

True, but certainly not coax.


--
Jim Pennino

On 9/14/2015 12:06 PM, gareth wrote:
"rickman" wrote in message
...
On 9/14/2015 11:14 AM, gareth wrote:
Taking one's cue from the Alford Slot Antenna, let us propose the use of
the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

To do this, we will terminate our 50 ohm coax with a series LC circuit of
50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

I think your intent and your values don't correspond. The +jX and -jX
terms are correct for the imaginary parts of the impedances, but you don't
want the capacitor and inductor to have 25 ohms each of dissipative, real
resistance do you? That would leave nothing for the transmission
resistance if you were to radiate any energy. So either you would be
designing an antenna at other than 50 ohms impedance, or you are trying to
design an antenna which doesn't radiate.

Oops! Yes, of course you are right; too much typing and not enough thought
:-)

What I was trying to picture was a series LC circuit presenting a
non-reacitve
impedance of 50 ohms.

So should I assume you intended to say the capacitor and inductor have
complex impedances of 0 ± jX ohms are are in series with the 50 ohm
radiation resistance? Or do you want the antenna to have a lower
impedance than 50 ohms?

--

Rick

wrote:

Slot antennas get added to the list of things Gareth doesn't understand.

Series LC circuits get added to the list of things Gareth doesn't understand.

Capacitors get added to the list of things Gareth doesn't understand.


Also on that list;

DSP
Tuning an FT101
Reaching further than a quarter mile on a car CB
Mounting a successful defence in court

--
STC // M0TEY // twitter.com/ukradioamateur

Slot antennas are fed with waveguide.

Not always, it's very easy to feed them with microstrip or stripline.

True, but certainly not coax.



For once Gareth is correct, slot antennas can be, and are, fed with
coax. A quick Google or look in the RSGB VHF/UHF Handbook with show
several designs, mostly for 23cms and above.

Jeff

"Jeff" wrote in message ...
For once Gareth is correct, slot antennas can be, and are, fed with
coax. A quick Google or look in the RSGB VHF/UHF Handbook with show
several designs, mostly for 23cms and above.
Those are not slot antennas but Alford Slot Antennas from the name of the
inventor.
I don't think it can be said that it is only the gap that radiates .
For me those antennas are more behaving like magnetic loops in horizontal
polarisation made of a very wide flat conductor
The slot is the capacitor and also a parallel transmission line
Both not supposed to radiate.