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

"AM200" wrote in message ...

What does "Q" actually stand for as i can't find anyone that can explain
this.

Q is generally taken to "stand for" quality factor. The definition I
like to go back to when all else fails is "energy stored divided by
energy dissipated per radian." Q really only makes sense when you are
talking about single resonators. This definition of Q applies to LC
tanks, coaxial cavities, hollow cavities, acoustic resonators,
pendulums, or any other singly resonant structure.

High Q resonators "ring" for a long time, a lot of cycles. Low Q
resonators dissipate energy rapidly and the ringing fades quickly.

Cheers,
Tom

"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.

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

(JGBOYLES) wrote in message ...
....
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.

1.8MHz dipole with 8AWG wi just use three parallel pieces, spaced
about a foot apart...or make a "bowtie" out of the 8AWG. Or use a
steel cable center core and a cage of cheap 14AWG wires around it.
There ARE cures for narrow bandwidth in wire antennas. Of course, it
may be simpler to just use a matching network ("tuner"), since the
efficiency and pattern of even the simple single thin wire dipole
doesn't depend much of frequency within an HF band.

You have choices here, and I don't much care for generalities that
unnecessarily limit the choices. Just because one person wants to use
a tuner, and retune after any QSY greater than 20kHz doesn't mean
another who wants to use a broadband system should be discouraged from
doing so.

Cheers,
Tom

Tom's comment below -

For an example of the VSWR bandwidth achievable in an HF design,
check the antenna described at this link
http://www.tcibr.com/PDFs/613tfwebs.pdf. Nominal input VSWR is 2:1 from
2-30MHz . No input tuner is used.

Good VSWR bandwidth in an antenna is not limited to VHF and above.

RF

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

"Tom Bruhns" wrote
Just because one person wants to use a tuner, and
retune after any QSY greater than 20kHz doesn't mean
another who wants to use a broadband system should
be discouraged from doing so.

You have choices here, and I don't much care for generalities that
unnecessarily limit the choices. Just because one person wants to use
a tuner, and retune after any QSY greater than 20kHz doesn't mean
another who wants to use a broadband system should be discouraged from
doing so.
Hi Tom,
The thread was "wire thickness versus bandwidth" As you know, to scale an
antenna from one frequency to another, you need to scale the wire diameter as
well. This will keep the bandwidth pretty much the same.
I thought I was commenting on the bandwidth as one changes the frequency and
length of an antenna, but keep the effective wire diameter constant. Of course
if you vary the effective diameter, 3 parallel pieces of #8 a foot apart on 1.8
mhz, you change the effective bandwidth.

you have choices here, and I don't much care for generalities that
unnecessarily limit the choices.
I did not mean to be general and limit choices, sorry.

73 Gary N4AST

JGBOYLES wrote:
The thread was "wire thickness versus bandwidth" As you know, to scale an
antenna from one frequency to another, you need to scale the wire diameter as
well. This will keep the bandwidth pretty much the same. . .

When you scale an antenna's dimensions, including the wire diameter, the
*fractional* bandwidth remains the same. So if you scale it for twice
the frequency, the bandwidth doubles.

This is assuming that loss is negligible. If loss is appreciable, it
becomes a factor in determining the bandwidth. And in order to preserve
the loss characteristics when scaling, you've also got to scale the
conductivity (as the square root of frequency). This is generally
impossible or at best highly impractical, so the bandwidth of a lossy
antenna won't scale like it will for a low-loss one.

Roy Lewallen, W7EL

At HF, increasing wire diameter has an absolute negligible effect on antenna
bandwidth. Multiply effective conductor diameter by 100 or 1000 and you are
getting somewhere on one band only.

But it can be completely spoiled by use of a tuner.
----
Reg, G4FGQ