"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

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.
Hi Roy,
I have never heard of *fractional* bandwidth. Not unusual that I have never
heard of stuff. If one has a band that is 100khz wide and the 2:1 swr
bandwidth is say 20khz, what is the fractional bandwidth, and the bandwidth?
If you scale the antenna to double the frequency (neglecting losses, or
assuming negligible) and scale the dimensions, what are the 2 bandwidths?
I hope you enjoy Dayton, first time in 5 years I won't be there. It rains
too much when I am up the-(


73 Gary N4AST

JGBOYLES wrote:
I have never heard of *fractional* bandwidth.

Fractional bandwidth is bandwidth in Hz divided by
the center frequency in Hz. Multiplying by 100 gives
a percentage bandwidth. Both are normalized bandwidth.
--
73, Cecil http://www.qsl.com/w5dxp



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By fractional bandwidth, I mean the fraction of the operating frequency
that the bandwidth is; or in other words the ratio of the bandwidth to
the operating frequency. For example, if the operating frequency is 10
MHz and the bandwidth is 1 MHz, the fractional bandwidth is 0.1 (1 MHz /
10 MHz). If you scale the antenna to 20 MHz, the bandwidth of the scaled
antenna is 2 MHz. The fractional bandwidth is 2 MHz / 20 MHz = 0.1, the
same as before it was scaled.

I don't have enough information to answer your question, since you
didn't give the operating frequency. I hope the example I gave will
clarify what I meant.

I always enjoy Dayton. It's a pleasure to meet EZNEC users, potential
EZNEC users, and some of the many people who read this newsgroup. And if
rain bothered me, I sho' 'nuff wouldn't live here in Oregon!

Roy Lewallen, W7EL

JGBOYLES wrote:
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.
Hi Roy,
I have never heard of *fractional* bandwidth. Not unusual that I have never
heard of stuff. If one has a band that is 100khz wide and the 2:1 swr
bandwidth is say 20khz, what is the fractional bandwidth, and the bandwidth?
If you scale the antenna to double the frequency (neglecting losses, or
assuming negligible) and scale the dimensions, what are the 2 bandwidths?
I hope you enjoy Dayton, first time in 5 years I won't be there. It rains
too much when I am up the-(


73 Gary N4AST

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