Wire thicknes versus bandwith
 

Hi All

Is there a web site where I can read about the relation between the
thickness of the wire and the bandwith.

Thanks in advance

73 Per OZ1EQC

Is there a web site where I can read about the relation between the
thickness of the wire and the bandwith.
================================

Antenna bandwidth changes very slowly relative to wire diameter. It is best
considered in terms of antenna Q at 1/2-wave or 1/4-wave resonance.

Approximately -

Q = 1.7 * [ Ln( Lambda / 2 / Diameter ) - 1]

where Lambda is free-space wavelength and conductor diameter is in the same
units. Ln is the logarithm to the base e.

The formula applies up the point where conductor diameter is about 1/6 of
dipole antenna length.

Knowledge of antenna bandwidth is not of great practical use at HF because
the Q of the feedline plus tuner has at least the same effect on the overall
system bandwidth as the antenna.

Download program DIPCAGE2 from website below to see how bandwidth changes
with effective diameter of a dipole cage antenna.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

"Per Bekker-Madsen" wrote in message
...
Hi All

Is there a web site where I can read about the relation between the
thickness of the wire and the bandwith.

Thanks in advance

73 Per OZ1EQC


Per,

Going from 2 mm to 10 mm wire is not going to make a dramatic increase in
bandwidth. If you are trying to cover all of 75/80 meters, for instance,
with an SWR less than 2, you are better off having two parallel dipoles. You
would have to play with EZNEC to get it right, but you need the dipoles to
be resonant at roughly 3800 and 3600.

Tam/WB2TT

Knowledge of antenna bandwidth is not of great practical use at HF
because
the Q of the feedline plus tuner has at least the same effect on the
overall
system bandwidth as the antenna.

I'm sorry, you lost me there Reg. How does the Q of a modern
broadband amplifier feeding a nom. 75 ohm feedline contribute more to
system Q than an 80 meter 1/2 wave dipole made from 2mm wire -- or for
that matter even one that's a half-meter diameter cage (though by then
it doesn't much matter for ham use)?

Cheers,
Tom
================================
Tom, who said anything about 75-ohm lines? And there's still a tuner, with
FIXED settings, to contend with.

In any case an antenna can present a match to a line at only one frequency
in the band. The transmission line transforms the mismatch at the antenna
to something else at the tuner and something yet again at the transmitter.

Lines and tuners have lots of inductive reactance, lots of capacitative
reactance, but not a lot of resistance. Which are just as much a part of
the system as the antenna, if not the greater part. Q =
reactance/resistance. Bandwidth is proportional to 1/Q

What the transmitter sees, even with a precisely known antenna bandwidth, is
anybody's guess.
----
Reg, G4FGQ

Hello,

What does "Q" actually stand for as i can't find anyone that can explain
this. I know many people repeat paragraphs of books without understanding
them!
Are you stating "Q" is reactance divided by resistance or "Q" = reactance OR
resistance. You obviously know what you mean but it isn't too clear the way
you're explaining it.

What is "Q" ? So far on this group I have had many answers such as it means
"quality" "goodness" "resitance" etc, but no one can give a similar answer
to anyone else. Does anyone really know or is it a made up term.


"Reg Edwards" wrote in message
...
Knowledge of antenna bandwidth is not of great practical use at HF
because
the Q of the feedline plus tuner has at least the same effect on the
overall
system bandwidth as the antenna.

I'm sorry, you lost me there Reg. How does the Q of a modern
broadband amplifier feeding a nom. 75 ohm feedline contribute more to
system Q than an 80 meter 1/2 wave dipole made from 2mm wire -- or for
that matter even one that's a half-meter diameter cage (though by then
it doesn't much matter for ham use)?

Cheers,
Tom
================================
Tom, who said anything about 75-ohm lines? And there's still a tuner,
with
FIXED settings, to contend with.

In any case an antenna can present a match to a line at only one frequency
in the band. The transmission line transforms the mismatch at the antenna
to something else at the tuner and something yet again at the transmitter.

Lines and tuners have lots of inductive reactance, lots of capacitative
reactance, but not a lot of resistance. Which are just as much a part of
the system as the antenna, if not the greater part. Q =
reactance/resistance. Bandwidth is proportional to 1/Q

What the transmitter sees, even with a precisely known antenna bandwidth,
is
anybody's guess.
----
Reg, G4FGQ

What does "Q" stand for or represent?
==========================

Q is a universal measure of the selectivity, the sharpness of tuning, of a
tuned circuit at resonance.

It is also the ratio of reactance to loss resistance of a coil or capacitor
which forms part of a tuned circuit. Numerically, it is the ratio Q = X
ohms / R ohms. It is a measure of the "Quality" of an L or C component.

Values of low Q are 20 or less. Typical values of Q for HF tuning coils are
70 to 400. The physically large coils have the higher Q values. HF
capacitors can have Q values as high as several thousands.

In the present context a radio antenna, having distributed inductance and
capacitance, behaves as a tuned resonant circuit and has a low value of Q.
The loss resistance is the radiation resistance.

A 160 meter dipole of thin wire will have a value of Q around 14.

A 2 meter dipole made of aluminium tubing will have a Q around 5.

The working bandwidth of an antenna can be defined in different ways. But
it is always inversely proportional to Q. High selectivity = narrow
bandwidth. Low selectivity = broad bandwidth, a desirable property of an
antenna.
----
Reg, G4FGQ

"Reg Edwards" wrote
In any case an antenna can present a match to a line at only one frequency
in the band. The transmission line transforms the mismatch at the antenna
to something else at the tuner and something yet again at the transmitter.
__________________

Correction: some FM broadcast transmit antennas have an input VSWR less than
1.15:1 from 88 to 108 MHz (50 ohm coaxial environment). There IS no
significant mismatch at the antenna input requiring the use of a matching
network there.

It depends on the application as to what VSWR bandwidth is necessary, but
certainly it is not difficult or expensive in many antenna designs to span
several hundred kilohertz with a good match to the transmission line.

RF

Visit http://rfry.org for FM broadcast RF system papers.

"Reg Edwards" wrote in message ...
Knowledge of antenna bandwidth is not of great practical use at HF
because
the Q of the feedline plus tuner has at least the same effect on the
overall
system bandwidth as the antenna.

I'm sorry, you lost me there Reg. How does the Q of a modern
broadband amplifier feeding a nom. 75 ohm feedline contribute more to
system Q than an 80 meter 1/2 wave dipole made from 2mm wire -- or for
that matter even one that's a half-meter diameter cage (though by then
it doesn't much matter for ham use)?

Cheers,
Tom
================================
Tom, who said anything about 75-ohm lines? And there's still a tuner, with
FIXED settings, to contend with.

I did, and in the scenario I proposed, there's a broadband amplifier,
but there IS no tuner (or if there is, it's set to a very low Q
anyway; the Q of an L network to match 75 to 50 ohms is far lower than
that of a single-wire antenna). There's no need for one if the
antenna is sufficiently broadband.

I thank you for the antenna-Q equation. But I disagree that "the Q of
the feedline plus tuner" always, or even very often, contributes more
to limiting the useful (single-setting) bandwidth of a nom. 75 ohm
half-wave dipole driven by a broadband amplifier than does the antenna
itself.

Cheers,
Tom

Correction: some FM broadcast transmit antennas have an input VSWR less than
1.15:1 from 88 to 108 MHz (50 ohm coaxial environment).

I don't think a correction is necessary, comparing apples and oranges.

It depends on the application as to what VSWR bandwidth is necessary, but
certainly it is not difficult or expensive in many antenna designs to span
several hundred kilohertz with a good match to the transmission line.
A 100mhz antenna has several hundred khz bandwidth using #8 wire, but try a
dipole on 1.8 mhz using #8 and you have 10-20 khz.
You are apparently an expert on VHF antennas. I am an Amateur. The antennas
discussed on this group are often 1.8-30 MHZ. HF and VHF antennas are just
alike, but they are different. Got any idea about 1.8 mhz bandwidth?













gto

73 Gary N4AST

Further to antenna bandwidth.

The concept of Q = Inductive reactance / loss resistance applies equally to
coil plus capacitor tuned circuits and to antennas.

Because capacitance loss is negligible, antenna Q depends only on loss
resistance of the wire inductance. Loss resistance = uniformly distributed
radiation resistance + wire resistance. Uniformly distributed radiation
resistance for a half-wave dipole is exactly twice the centre-fed value,
ie., approx 140 ohms.

Formula for the inductance of a straight length of wire can be found in
Terman and many other places. It is then a simple matter to calculate
dipole Q = inductive reactance / loss resistance.

Bandwidth can be described in terms of the 3dB points or in terms of the SWR
= X points.

But, as stated earlier, what matters is radiating system bandwidth, antenna
+ transmission line + tuner. And doubling antenna wire diameter has no
noticeable effect on operating bandwidth. Increasing wire diameter by 100
times may double bandwidth but is not worth the trouble and expense for only
one band.

Discussion of antenna bandwidth at HF, always in non-numerical terms, is an
overated topic.
----
Reg, G4FGQ