On 2014-10-17 17:50:55 +0000, gareth said:

something unrelated to the OP


Can I go back to your original question, and perhaps recast it in a way
that is more useful for discussion?

I think we have to accept that once you can get energy *into* a small
antenna it radiates just as well as a large one, apart from resistive
losses which can be made fairly small with suitable materials.

A different but equally interesting question: is there any intuitive,
or simple mathematical, way of explaining why an electrically short
antenna couples into free space in such a way that its radiation
resistance is very low compared with a resonant antenna, and highly
reactive? Because this property is what makes short antennas hard to
use in practice. And must result from some property of its radiation
behaviour, conceivably related to your original postulate which I am
afraid I do not have the maths to understand.

--

Percy Picacity

On 18/10/14 11:17, Percy Picacity wrote:
On 2014-10-17 17:50:55 +0000, gareth said:

something unrelated to the OP


Can I go back to your original question, and perhaps recast it in a way
that is more useful for discussion?

I think we have to accept that once you can get energy *into* a small
antenna it radiates just as well as a large one, apart from resistive
losses which can be made fairly small with suitable materials.

A different but equally interesting question: is there any intuitive,
or simple mathematical, way of explaining why an electrically short
antenna couples into free space in such a way that its radiation
resistance is very low compared with a resonant antenna, and highly
reactive? Because this property is what makes short antennas hard to
use in practice. And must result from some property of its radiation
behaviour, conceivably related to your original postulate which I am
afraid I do not have the maths to understand.


You seem to be 'cross threaded' Roger, not to mentioned confusing your
IDs. Why not leave your socks off, now you've been outed.

Anyway, to the technical stuff.

I covered this earlier, in another thread.

Consider the Radiation Resistance, Loss Resistance, and reactive element
which determine the eff., and Zo.
(The reactive element represents the energy 'stored' in the field around
the antenna- just like the energy stored in an inductor or capacitor,
both reactive components.)

A short dipole, for example, will be a poor match but RRLR. Provided
the feeder loss is low, either by good matching or the use of low loss
feeder (assuming the PA is 'happy') then the overall losses are low and
the RF only has one place to go, to be radiated.

A short dipole has other issues, in particular if matching is used to
overcome the issue of the Zo, then the matching network plus antenna
will have a very narrow bandwidth (compared to a full sized dipole) and
adjustment will be essential to maintain efficiency if the frequency of
operation is changed.

Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential, nor
is maintaining a feeder SWR of 1.5, provided the PA can cope and feeder
loss can be tolerated/reduced (eg by using open wire feeder).

Those who don't really understand what is going on get all 'hung up' re
SWR. In fact it really causes two real issues. If the PA is forced to
operate outside of its safe area of operation, damage may result. (RF
does not 'go back into' the PA, as some think, the impedances are all
wrong for that) and feed loss. If the PA can 'cope' with what is being
asked of it and the feed is low loss (eg open wire), a high SWR on the
feeder is not really problem. In fact, adding a matching unit may
degrade overall performance.

As to the 'postulate', pure nonsense. Some terms he has read, thrown
together so as to try and give the impression he understands things.

When it this was pointed out, he got riled and start his tirade. As
normal. You know as well as I do the pattern, even if you refuse to
admit it due to you leftie bias against anyone who dares not to be a leftie.

On 2014-10-18 13:17:13 +0000, Brian Reay said:

On 18/10/14 11:17, Percy Picacity wrote:
On 2014-10-17 17:50:55 +0000, gareth said:

something unrelated to the OP


Can I go back to your original question, and perhaps recast it in a way
that is more useful for discussion?

I think we have to accept that once you can get energy *into* a small
antenna it radiates just as well as a large one, apart from resistive
losses which can be made fairly small with suitable materials.

A different but equally interesting question: is there any intuitive,
or simple mathematical, way of explaining why an electrically short
antenna couples into free space in such a way that its radiation
resistance is very low compared with a resonant antenna, and highly
reactive? Because this property is what makes short antennas hard to
use in practice. And must result from some property of its radiation
behaviour, conceivably related to your original postulate which I am
afraid I do not have the maths to understand.


You seem to be 'cross threaded' Roger, not to mentioned confusing your
IDs. Why not leave your socks off, now you've been outed.

Anyway, to the technical stuff.

I covered this earlier, in another thread.

Consider the Radiation Resistance, Loss Resistance, and reactive
element which determine the eff., and Zo.
(The reactive element represents the energy 'stored' in the field
around the antenna- just like the energy stored in an inductor or
capacitor, both reactive components.)

A short dipole, for example, will be a poor match but RRLR. Provided
the feeder loss is low, either by good matching or the use of low loss
feeder (assuming the PA is 'happy') then the overall losses are low and
the RF only has one place to go, to be radiated.

A short dipole has other issues, in particular if matching is used to
overcome the issue of the Zo, then the matching network plus antenna
will have a very narrow bandwidth (compared to a full sized dipole) and
adjustment will be essential to maintain efficiency if the frequency of
operation is changed.

Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential,
nor is maintaining a feeder SWR of 1.5, provided the PA can cope and
feeder loss can be tolerated/reduced (eg by using open wire feeder).

Those who don't really understand what is going on get all 'hung up' re
SWR. In fact it really causes two real issues. If the PA is forced to
operate outside of its safe area of operation, damage may result. (RF
does not 'go back into' the PA, as some think, the impedances are all
wrong for that) and feed loss. If the PA can 'cope' with what is being
asked of it and the feed is low loss (eg open wire), a high SWR on the
feeder is not really problem. In fact, adding a matching unit may
degrade overall performance.

As to the 'postulate', pure nonsense. Some terms he has read, thrown
together so as to try and give the impression he understands things.

When it this was pointed out, he got riled and start his tirade. As
normal. You know as well as I do the pattern, even if you refuse to
admit it due to you leftie bias against anyone who dares not to be a
leftie.

All very true, and commonplace to most of us, but doesn't address the
question as to *why* the short antenna has such inconvenient properties.

--

Percy Picacity

On 18/10/14 14:34, Percy Picacity wrote:
On 2014-10-18 13:17:13 +0000, Brian Reay said:

On 18/10/14 11:17, Percy Picacity wrote:
On 2014-10-17 17:50:55 +0000, gareth said:

something unrelated to the OP


Can I go back to your original question, and perhaps recast it in a way
that is more useful for discussion?

I think we have to accept that once you can get energy *into* a small
antenna it radiates just as well as a large one, apart from resistive
losses which can be made fairly small with suitable materials.

A different but equally interesting question: is there any intuitive,
or simple mathematical, way of explaining why an electrically short
antenna couples into free space in such a way that its radiation
resistance is very low compared with a resonant antenna, and highly
reactive? Because this property is what makes short antennas hard to
use in practice. And must result from some property of its radiation
behaviour, conceivably related to your original postulate which I am
afraid I do not have the maths to understand.


You seem to be 'cross threaded' Roger, not to mentioned confusing your
IDs. Why not leave your socks off, now you've been outed.

Anyway, to the technical stuff.

I covered this earlier, in another thread.

Consider the Radiation Resistance, Loss Resistance, and reactive
element which determine the eff., and Zo.
(The reactive element represents the energy 'stored' in the field
around the antenna- just like the energy stored in an inductor or
capacitor, both reactive components.)

A short dipole, for example, will be a poor match but RRLR. Provided
the feeder loss is low, either by good matching or the use of low loss
feeder (assuming the PA is 'happy') then the overall losses are low
and the RF only has one place to go, to be radiated.

A short dipole has other issues, in particular if matching is used to
overcome the issue of the Zo, then the matching network plus antenna
will have a very narrow bandwidth (compared to a full sized dipole) and
adjustment will be essential to maintain efficiency if the frequency
of operation is changed.

Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential,
nor is maintaining a feeder SWR of 1.5, provided the PA can cope and
feeder loss can be tolerated/reduced (eg by using open wire feeder).

Those who don't really understand what is going on get all 'hung up'
re SWR. In fact it really causes two real issues. If the PA is forced
to operate outside of its safe area of operation, damage may result.
(RF does not 'go back into' the PA, as some think, the impedances are
all wrong for that) and feed loss. If the PA can 'cope' with what is
being asked of it and the feed is low loss (eg open wire), a high SWR
on the feeder is not really problem. In fact, adding a matching unit
may degrade overall performance.

As to the 'postulate', pure nonsense. Some terms he has read, thrown
together so as to try and give the impression he understands things.

When it this was pointed out, he got riled and start his tirade. As
normal. You know as well as I do the pattern, even if you refuse to
admit it due to you leftie bias against anyone who dares not to be a
leftie.

All very true, and commonplace to most of us, but doesn't address the
question as to *why* the short antenna has such inconvenient properties.


Really?

Why not think about what you need to change in the above to change things.

Then, in the physical world, what you would need to change to realise
those changes.

All quite logical then.

On 2014-10-18 14:19:53 +0000, Brian Reay said:

On 18/10/14 14:34, Percy Picacity wrote:



All very true, and commonplace to most of us, but doesn't address the
question as to *why* the short antenna has such inconvenient properties.


Really?

Why not think about what you need to change in the above to change things.

Then, in the physical world, what you would need to change to realise
those changes.

All quite logical then.

I don't think you have said a great deal there; anyone got any
suggestions? I am prepared to accept that there may not be any useful
analogy for what is going on in the maths governing the behaviour of
radiators, but does anyone think there is a simple way to explain it?
I suspect this is really what Gareth was trying to describe in his
original post but I don't think that explains it to me.

--

Percy Picacity

Setting aside your deep-seated need to resort to gratuitous abuse at the
bottom of your post, you seem to be confused, for the discrepancy in
radiation
has got nothing whatsoever to do with the feeder or with Z0.

"Brian Reay" wrote in message
...

Consider the Radiation Resistance, Loss Resistance, and reactive element
which determine the eff., and Zo.
(The reactive element represents the energy 'stored' in the field around
the antenna- just like the energy stored in an inductor or capacitor, both
reactive components.)

No, it does not, for a 1/2 wave dipole has such energy stored in the near
field,
but it is not reactive.

The apparent reactivity is because the wave reflected from the end of a
shprt antenna
is well out of phase with the feed.


A short dipole, for example, will be a poor match but RRLR.

That is not true, especially in the case of those down at 137 kHz, where the
loss resistance is higher than the apparent radiation resistance by an order
of magnitude, which is why the published designs feed 1kW from the TX to
get 1W erp.

Provided the feeder loss is low, either by good matching or the use of low
loss feeder (assuming the PA is 'happy')

The feeder has now to do with the performance of a short antenna.


the RF only has one place to go, to be radiated.

Or reflectd back from the open end, just as happens with the standing
wave that is the 1/2 wave dipole. A short antenna does not have a standing
wave.

A short dipole has other issues, in particular if matching is used to
overcome the issue of the Zo, then the matching network plus antenna will
have a very narrow bandwidth (compared to a full sized dipole) and
adjustment will be essential to maintain efficiency if the frequency of
operation is changed.

not relevant.

Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential, nor
is maintaining a feeder SWR of 1.5, provided the PA can cope and feeder
loss can be tolerated/reduced (eg by using open wire feeder).

not relevant


Those who don't really understand what is going on get all 'hung up' re
SWR. In fact it really causes two real issues. If the PA is forced to
operate outside of its safe area of operation, damage may result. (RF does
not 'go back into' the PA, as some think, the impedances are all wrong for
that)

Nonsense. the reflected energy can cause the maximum operating
characteristics
of the PA to be exceeded. How else could damage be caused?


As to the 'postulate', pure nonsense. Some terms he has read, thrown
together so as to try and give the impression he understands things.

Once again, it is you who originates the gratuitous abuse. You really cannot
help yourself, can you, Sonny?


When it this was pointed out, he got riled and start his tirade.

Untrue.

As normal. You know as well as I do the pattern, even if you refuse to
admit it due to you leftie bias against anyone who dares not to be a
leftie.

Oh, for god's sake, sonny, that is completely random and out of place.

"Brian Reay" wrote in message
...

As to the 'postulate', pure nonsense. Some terms he has read, thrown
together so as to try and give the impression he understands things.

Listen, Sonny, I graduated in electronics, specialising in computer and
communication engineering when you were still sobbing and screaming
in your soiled nappies.

Despite what you have said at length, largely a wall-of-text of diversionary
irrelevancies,
you have not ventured anything apposite to the discussion.