In message , Dave Platt
writes
In article ,
Ian Jackson wrote:

I would have thought that the feed impedance of a dipole at a wide range
of frequencies/lengths (ie 'very short' to 'very long') would have been
fairly typical rule-of-thumb required information for those interested
in antennas. However, it does not seem to be!

Oh... if rule-of-thumb is good enough for your needs, then it's not
too difficult to summarize. There's a nice chart on page 2-3 of the
ARRL Antenna Book.

You should consider the resistive, and reactive portions of the
feedpoint impedance separately.

The resistive part rises from zero, up through a nominal 50 ohms or so
at resonance (just under 1/2 wavelength), up to several thousand ohms
at second (or anti-) resonance. If you plot the impedance-vs.-
resistance relationship with the doublet length on a linear scale and
the resistance on a logarithmic scale, it's not too far from being a
straight line through much of this range.

Between second and third resonance, the resistance drops back down to
around 100 ohms... between third and fourth, up to several thousand
ohms again, and so forth. As the doublet continues to get longer, the
feedpoint resistance oscillates between low (odd-resonant) and high
(even- or anti-resonant) values, with the oscillation becoming less
and less as the doublet gets longer (think of a damped sine wave). In
theory it'll eventually settle down to 377 ohms.

The reactive portion of the impedance also oscillates as the doublet
gets longer and longer. Between an even-numbered and odd-numbered
resonance it's capacitive, dropping from thousands of ohms of
negative reactance, to zero at the odd resonance. It then becomes
inductive, rising to several thousand ohms just before the next even
(anti-) resonant length is reached. As the even-numbered resonance
length is passed it falls abruptly from very positive (inductive) to
very negative (capacitive), and then begins to return slowly to zero
at the next odd resonance.

These excursions from positive (inductive) to negative (capacitive)
continue, and also fall in their absolute value as the doublet gets
longer and longer. Once the doublet is "sufficiently long" its
reactance pretty much vanishes and it looks like a 377-ohm resistance.

Near the resonant lengths, the value of the reactance is changing
rather more rapidly than the value of the resistance.

The same basic principles apply fairly well to doublets that aren't in
free space, but ground reflections, mutual coupling with other antenna
elements, etc. have a big effect on the actual values. Few of us
have the luxury of stringing up an 80-meter longwire doublet in free
space, alas :-)


Yes, rule-of-thumb is more than good enough for me! I has a sneaky
feeling that the feed impedance would end up at 377 ohms (impedance of
free space).

Many years ago, from some tables compiled by one of the many Wu's
involved with antenna theory and design, I plotted Zin vs antenna length
on a Smith chart. As the spiral progressively wound its way inwards with
increasing antenna length, it seemed that it was heading for something
between 200 and 600 ohms, so I thought to myself, "377 ohms?"
Unfortunately, the table stopped when the antenna was about 5
wavelengths. I haven't seen similar tables since.
--
Ian

On Oct 20, 5:57*pm, Jim Lux wrote:
....
Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. *And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

I haven't noticed in this thread any reference to Ronold W. P. King's
work. His writings should give more insight into the subject, if you
can get deeply enough into them. It's discussed empirically in
"Transmission Lines, Antennas and Waveguides," (with lots and lots of
interesting graphs showing the effect from various viewpoints) but you
can probably go deeper into the theory than you need in his other
books and papers on linear antennas.

Cheers,
Tom

K7ITM wrote:
On Oct 20, 5:57 pm, Jim Lux wrote:
...
Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

Which was the original intent of my comment. Fat radiators are shorter
at resonance than thin ones, and the details of why are not simply
explained by something like "capacitance effects", although such an
explanation may sort of work over a limited range.

Jim Lux wrote:
K7ITM wrote:
On Oct 20, 5:57 pm, Jim Lux wrote:
...
Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

Which was the original intent of my comment. Fat radiators are shorter
at resonance than thin ones, and the details of why are not simply
explained by something like "capacitance effects", although such an
explanation may sort of work over a limited range.

Sorry, been away for a while, but I'm back.

Certainly the capacitance may play some small part. But does added
capacitance increase bandwidth to the extent - or at all - that is
achieved by the cage or very thick dipole?


Richard Harrison's reference to Baily regarding velocity is interesting.
Why would the velocity be less at increased width? And would that
increase the Bandwidth?


- 73 de Mike N3LI -

Michael Coslo wrote:
Jim Lux wrote:
K7ITM wrote:
On Oct 20, 5:57 pm, Jim Lux wrote:
...
Some might argue, though, that the reason the effective velocity is
less
is because the sqrt(1/LC) term is smaller because C is bigger
because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

On the other hand, the propagation velocity of coaxial cable of
constant outer conductor ID is independent of the inner conductor
diameter, even though the capacitance per unit length increases as the
inner conductor diameter is increased. Clearly one must be careful
about attributing the effect to a single cause like increased
capacitance.

Which was the original intent of my comment. Fat radiators are
shorter at resonance than thin ones, and the details of why are not
simply explained by something like "capacitance effects", although
such an explanation may sort of work over a limited range.

Sorry, been away for a while, but I'm back.

Certainly the capacitance may play some small part. But does added
capacitance increase bandwidth to the extent - or at all - that is
achieved by the cage or very thick dipole?

Nope.. that's why "increased capacitance" is a bad model.



Richard Harrison's reference to Baily regarding velocity is interesting.
Why would the velocity be less at increased width? And would that
increase the Bandwidth?

larger C per unit length makes 1/sqrt(LC) smaller
no for the BW

Mike, N3LI wrote:
"Why would the velocity be less at increased (antenna element) width?"

Let B = the phase velocity on the antenna element, in radians per unit
length. 2pi/B = wavelength on the element.
Therefore, 2pi/B=velocity of phase propagation.
Due to the behavior of of open-circuited transmission lines and
open-circuited antennas:
B=2pif times sq.rt. of LC radians / unit length.

2 pi f / B = velocity of propagation.

It is intuitive that a fat antenna element has more L & C than a thin
element and thus a lower velocity of propagation.

Best regards, Richard Harrisob, KB5WZI

"Richard Harrison" wrote in message
...
Mike, N3LI wrote:
"Why would the velocity be less at increased (antenna element) width?"

Let B = the phase velocity on the antenna element, in radians per unit
length. 2pi/B = wavelength on the element.
Therefore, 2pi/B=velocity of phase propagation.
Due to the behavior of of open-circuited transmission lines and
open-circuited antennas:
B=2pif times sq.rt. of LC radians / unit length.

2 pi f / B = velocity of propagation.

It is intuitive that a fat antenna element has more L & C than a thin
element and thus a lower velocity of propagation.

Best regards, Richard Harrisob, KB5WZI


Hmmmm... my straight wire inductance equation from the ARRL handbook
indicates smaller wire diameters have larger inductance.

???

73,
John

On Oct 22, 9:52*pm, "John KD5YI" wrote:
"Richard Harrison" wrote in message

...



Mike, N3LI wrote:
"Why would the velocity be less at increased (antenna element) width?"

Let B = the phase velocity on the antenna element, in radians per unit
length. 2pi/B = wavelength on the element.
Therefore, 2pi/B=velocity of phase propagation.
Due to the behavior of of open-circuited transmission lines and
open-circuited antennas:
B=2pif times sq.rt. of LC radians / unit length.

2 pi f / B = velocity of propagation.

It is intuitive that a fat antenna element has more L & C than a thin
element and thus a lower velocity of propagation.

Best regards, Richard Harrisob, KB5WZI

Hmmmm... my straight wire inductance equation from the ARRL handbook
indicates smaller wire diameters have larger inductance.

???

73,
John

Not surprisingly, that's what E&M texts say too--or leave as an
exercise. With a larger diameter, there's less net magnetic field for
a given current, so less energy stored, so less inductance.

Cheers,
Tom

In article , Richard Clark
wrote:

Lest there be any confusion: an antenna IS a transmission line.


Hello, and I think one would have to include two antennas and the
intervening medium(s) for the above statement to make sense. In any
event, the behavior of an antenna-medium-antenna as a passive 2-port
device can be considered as a transmission line at a given frequency. The
"loss" associated with this topology can be mitigated by keeping the two
antennas within a near, rather than far, field separation. Over a range
of frequencies the behavior of this 2-port can easily differ from that of
a transmission line, though.

In some electromagnetics textbooks an antenna is developed mathematically
via the gradual unfolding of a twin-lead transmission line. And many hams
know that a quick and dirty dipole can be created by simply folding the
braid back on a length of coax so that the braid and the exposed center
conductor become the radiating elements.

A more correct statement might be that a transmission line can be an antenna.
This can include unintended radiotion (e.g. RF flowing on the outside of
caox due to imbalance and/or stray coupling) or intended such as Andrew's
"Radiax" brand of leaky transmission line for installation in tunnels and
elevator shafts as a convenient means to extend the reach of over-the-air
broadcasts. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Richard Harrison wrote:
Mike, N3LI wrote:
"Why would the velocity be less at increased (antenna element) width?"

Let B = the phase velocity on the antenna element, in radians per unit
length. 2pi/B = wavelength on the element.
Therefore, 2pi/B=velocity of phase propagation.
Due to the behavior of of open-circuited transmission lines and
open-circuited antennas:
B=2pif times sq.rt. of LC radians / unit length.

2 pi f / B = velocity of propagation.

It is intuitive that a fat antenna element has more L & C than a thin
element and thus a lower velocity of propagation.


I thought that the inductance tends downward as the diameter of the wire
increases. I can understand your calculation after the wavelength part,
but don't quite get the increased inductance part.


- 73 de Mike N3LI -