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.