Cecil Moore wrote:
AI4QJ wrote:
... it is plain and simple "intuitive" once you know that current
changes along the electrical "degree length" in an unloaded antenna,
the same should happen in the degree length loaded coil.

Unfortunately, both sides cannot be right but both sides
are still illustrated as fact in the ARRL Antenna Book.
There's one graphic that shows the drop in amplitude
through a loading coil and another that shows no change.
Apparently, the ARRL doesn't know what happens so they
show both possibilities as technically correct.

Also, as indicated, the pictures do say 1000 words and it also looks
like W8JI ended up agreeing with you after you pointed out the same
effect at "ON4UN's Low Band DXing", 3rd Edition, on page 9-34.

Unfortunately, it is rumored that W8JI has talked ON4UN
into changing that in the latest edition. I emailed ON4UN
about it but got no reply.

It has been changed. There is no longer any discussion of "degrees",
only "current".

73,
Gene
W4SZ

Gene Fuller wrote:
Cecil Moore wrote:
Unfortunately, it is rumored that W8JI has talked ON4UN
into changing that in the latest edition. I emailed ON4UN
about it but got no reply.

It has been changed. There is no longer any discussion of "degrees",
only "current".

It is too bad that W8JI has that much political power.
--
73, Cecil http://www.w5dxp.com

Gene Fuller wrote:
Cecil Moore wrote:
AI4QJ wrote:
... it is plain and simple "intuitive" once you know that current
changes along the electrical "degree length" in an unloaded antenna,
the same should happen in the degree length loaded coil.
Unfortunately, both sides cannot be right but both sides
are still illustrated as fact in the ARRL Antenna Book.
There's one graphic that shows the drop in amplitude
through a loading coil and another that shows no change.
Apparently, the ARRL doesn't know what happens so they
show both possibilities as technically correct.


Every author has a problem in drawing those diagrams, because we are
trying to draw too many things at the same time: physical height,
electrical height, loading coils, current distributions and voltage
distributions. It doesn't matter which viewpoint we are trying to
illustrate, it is still impossible to draw *all* of those things
truthfully to scale on the same diagram.

When comparing the full quarter-wave against the mobile whip, we have to
choose: do we draw the two antennas to true physical scale; or do we
use an 'electrical' scale of 0 to 90deg? Whichever one we choose, the
scale for the other on becomes grossly distorted, and this is what leads
to confusion.

Every author has trouble with this. Illustrations by different authors
attempt to square the circle in different ways, but none of them ever
can succeed because it fundamentally cannot be done.

ARRL publications are no exception, and a further complication is that
the handbook compilations tend to re-use illustrations from individual
articles by different authors. So please don't read too much into the
mixture of drawing styles - the reasons are often more historical than
technical.

Also, as indicated, the pictures do say 1000 words and it also looks
like W8JI ended up agreeing with you after you pointed out the same
effect at "ON4UN's Low Band DXing", 3rd Edition, on page 9-34.
Unfortunately, it is rumored that W8JI has talked ON4UN
into changing that in the latest edition. I emailed ON4UN
about it but got no reply.

It has been changed. There is no longer any discussion of "degrees",
only "current".

Well, not quite. The 4th edition does use degrees for the electrical
lengths of the plain unloaded sections (which is valid from everyone's
point of view); but it no longer implies that the loading coil
"replaces" any number of degrees.

I don't know the detailed history behind that change, but I do know one
thing: ON4UN is not a man to be swayed by "political" influence. The
change in the 4th edition would be because he was challenged to look
again at the *technical* issues, and then he made up his own mind.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
The 4th edition does use degrees for the electrical
lengths of the plain unloaded sections (which is valid from everyone's
point of view); but it no longer implies that the loading coil
"replaces" any number of degrees.

"Replace" seems to mean different things to different
people so it is not a good word to use without a stated
definition. It would probably be better to say the loading
coil "occupies" a certain number of degrees in a loaded
antenna.

The number of degrees occupied by the coil varies but it
is in the tens of degrees for a 75m mobile loading coil.
Here is an EXCEL file that computes the Z0 and VF of a
loading coil assuming it meets the "less than 1" test
included in the computation. Of course, the results
are only approximate since some secondary effects, such
as wire diameter, are ignored.

http://www.w5dxp.com/CoilZ0VF.xls

The VF of a 75m Texas Bugcatcher coil is ~0.02 at 4 MHz.
Since it is ~7 inches long, it occupies ~43 degrees of
antenna. The stinger occupies ~10 degrees so the coil
indeed does not "replace" 80 degrees of antenna. It
*occupies* 43 degrees of the antenna. The rest of the
necessary phase shift, 90-43-10 = 37 degrees, occurs at
the coil to stinger impedance discontinuity where the
Z0 of the coil is ~4000 ohms and the Z0 of the stinger
is ~400 ohms. A 10/1 ratio of Z0s causes a considerable
phase shift in the traveling waves, not in the standing-
waves.

One side of the argument recognizes only the phase shift
through the coil. The other side of the argument recognizes
only the phase shift at the top of the coil. Both sides
are partially right and partially wrong. Interestingly,
the truth lies just about half way in between the two
rail arguments. About half of the "missing degrees" are
contributed by the part of the antenna *occupied* by the
coil while the rest is contributed by the impedance
discontinuity between the coil and the stinger.

I don't know the detailed history behind that change, but I do know one
thing: ON4UN is not a man to be swayed by "political" influence. The
change in the 4th edition would be because he was challenged to look
again at the *technical* issues, and then he made up his own mind.

If he changed his mind based on experiments using standing-
wave current measurements, he is still wrong. I have tried
to contact him using his ARRL email address, but got no
reply.
--
73, Cecil http://www.w5dxp.com