Antenna bandwidth is usually expressed in terms of the limits of SWR such as
1.5 to 1, or 2.0 to 1.

It can be calculated, or otherwise derived, on the assumption that at some
frequency in ONE band the SWR is EXACTLY 1 to 1.

But it never is exactly 1 to 1. The transmission line impedance Zo is never
exactly equal to the resistive component of antenna input resistance at
resonance. It is usually well away from it. So in practice the actual SWR
bandwidth is always broader than the predicted, or stated, or claimed as the
best possible value.

Purchasers should be wary of curves provided by antenna manufacturers (or in
published articles) purporting to show SWR versus frequency which have a
perfect match to the line at some central point along a curve.

Although it should be stated there is much more importance attached to SWR
at HF than is warranted by the practical effects of SWR on system
peration - particularly in these days of almost universal use of antenna
tuners.

Perhaps undue importance arises because the SWR meter is the very last
indicating instrument to be found associated with transmitters. It is quite
reassuring to see a needle jumping about as one gabbles into the mike.

But will the ubiquitous SWR meter eventually disappear as the automatic
tuner becomes the vogue? That will be a sad day.

Incidentally, sorry to be so disappointing, the SWR meter does NOT indicate
SWR on the transmission line anyway.

I'm on Spanish red tonight.
----
Reg, G4FGQ

"Reg Edwards" wrote in message
...
Antenna bandwidth is usually expressed in terms of the limits of SWR such
as
1.5 to 1, or 2.0 to 1.

It can be calculated, or otherwise derived, on the assumption that at some
frequency in ONE band the SWR is EXACTLY 1 to 1.

But it never is exactly 1 to 1. The transmission line impedance Zo is
never
exactly equal to the resistive component of antenna input resistance at
resonance. It is usually well away from it. So in practice the actual SWR
bandwidth is always broader than the predicted, or stated, or claimed as
the
best possible value.

Purchasers should be wary of curves provided by antenna manufacturers (or
in
published articles) purporting to show SWR versus frequency which have a
perfect match to the line at some central point along a curve.

Although it should be stated there is much more importance attached to SWR
at HF than is warranted by the practical effects of SWR on system
peration - particularly in these days of almost universal use of antenna
tuners.

Perhaps undue importance arises because the SWR meter is the very last
indicating instrument to be found associated with transmitters. It is
quite
reassuring to see a needle jumping about as one gabbles into the mike.

But will the ubiquitous SWR meter eventually disappear as the automatic
tuner becomes the vogue? That will be a sad day.

Incidentally, sorry to be so disappointing, the SWR meter does NOT
indicate
SWR on the transmission line anyway.

I'm on Spanish red tonight.
----
Reg, G4FGQ

Hi Reg,

That's good to remember, thanks. When setting up a specific frequency dipole
(intending no tuner required for two frequencies, the main dipole and a
shorter dipole "fan" under it), the only logical place to test with the
bird-meter seemed to be at the transmitter output. Certainly that swr
changes as it gets to the radiator, as you say. But once the antenna was
trimmed to as near perfect swr as possible, what else would you do? Where
else would it be meaningful to measure the swr and best power output, and
then make wire length changes accordingly? It seems to work wonderfully as
trimmed to 1:1 (best you can read on the tuner anyway) from the tx output.
Both the bird and the MFJ-962D and tx meters all seemed to agree they liked
that setting.

73's

Jack
Virginia Beach Va

Jack, I think you know what you are about. But you may be doing the right
things for the wrong reasons.

The ONE and ONLY purpose of the SWR meter is to facilitate the
antenna-plus-line (plus tuner if you use one) to be adjusted such that the
transmitter is loaded with its design impedance of 50 ohms, plus or minus 20
percent or thereabouts.

What the SWR on the line may be is not relevant. It can be anything you
like! It doesn't matter insofar as the adjusting process itself is
concerned. And it is not of great consequence anyway.

The meter, Bird or MFJ or otherwise, does NOT indicate SWR on the feedline.
This is a popular misconception - a very old Old Wives' tale. It merely
indicates whether or not the impedance looking into the transmitter end of
the line is 50 ohms or is not 50 ohms. If it is not 50 ohms it won't even
tell you what it actually is.

So after adjusting line input impedance to be 50 ohms by some means or
other, you may be quite happy to retire under the impression the SWR is a
nice 1-to-1 and the antenna input impedance is what you think it is. Whereas
it very likely isn't.

But you will have achieved the true objective - the transmitter will be
correctly loaded with 50 ohms within plus or minus 20 percent even after
taking so-called SWR meter uncertainty into account.
----
Happy adjusting, Reg, G4FGQ

Thanks Reg. I only presumed that if my standing wave back at the transmitter
was a good balance therefore best output from the antenna was possible. I
did think that implies that even considering the somewhat long run to the
balun, that reflected power was at a mimimum possible level because a
resonant antenna condition (or as close as I was able to estimate) existed.
Each time that meter readings were taken was after the whole antenna(s) were
raised back up in the air, then lower to trim, raise and measure, etc.
Starting with about 10% beyond formula length, it noetheless had to have
some material added to one antenna and trimmed from another. From what I
have learned from you guys, I attribute this to the varying and unique
ground absorbtion variables of my particular installation. The help provided
on this list is invaluable. Many thanks.

73's

Jack
Virginia Beach


"Reg Edwards" wrote in message
...
Jack, I think you know what you are about. But you may be doing the right
things for the wrong reasons.

The ONE and ONLY purpose of the SWR meter is to facilitate the
antenna-plus-line (plus tuner if you use one) to be adjusted such that the
transmitter is loaded with its design impedance of 50 ohms, plus or minus
20
percent or thereabouts.

What the SWR on the line may be is not relevant. It can be anything you
like! It doesn't matter insofar as the adjusting process itself is
concerned. And it is not of great consequence anyway.

The meter, Bird or MFJ or otherwise, does NOT indicate SWR on the
feedline.
This is a popular misconception - a very old Old Wives' tale. It merely
indicates whether or not the impedance looking into the transmitter end of
the line is 50 ohms or is not 50 ohms. If it is not 50 ohms it won't even
tell you what it actually is.

So after adjusting line input impedance to be 50 ohms by some means or
other, you may be quite happy to retire under the impression the SWR is a
nice 1-to-1 and the antenna input impedance is what you think it is.
Whereas
it very likely isn't.

But you will have achieved the true objective - the transmitter will be
correctly loaded with 50 ohms within plus or minus 20 percent even after
taking so-called SWR meter uncertainty into account.
----
Happy adjusting, Reg, G4FGQ

Antenna bandwidth (without the use of a "tuner":
A reminder that the BW of an antenna is not always determined by SWR
limits. Practical antennas exist where the bandwidth is more reasonably
limited by degradation of the antenna's pattern than by degradation of
SWR. Antennas also exist where the BW might be specified in terms of
limits on efficiency.
The 40 foot high TCI dipole (fat, resistively loaded dipole) might
have its BW specified by SWR, pattern, or efficiency.
Mac N8TT

--
J. Mc Laughlin - Michigan USA
Home:

Jack Painter wrote:
That's good to remember, thanks. When setting up a specific frequency dipole
(intending no tuner required for two frequencies, the main dipole and a
shorter dipole "fan" under it), the only logical place to test with the
bird-meter seemed to be at the transmitter output. Certainly that swr
changes as it gets to the radiator, as you say. But once the antenna was
trimmed to as near perfect swr as possible, what else would you do?

It depends on whether you are trying to characterize the antenna or
the antenna system. Characterizing the antenna system is useful
for an individual ham but is not very useful, for instance, for an
antenna manufacturer. Knowing the antenna system parameters and
the individual component specifications allows one to indirectly
calculate the antenna parameters to some degree of accuracy.

Seems to me it is reasonable to quote a 3:1 bandwidth even though
the SWR at the center frequency never goes to 1:1.
--
73, Cecil http://www.qsl.net/w5dxp



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(sent last night, didn't show up on my reflector, so,,,)

Thanks Reg. I only presumed that if my standing wave back at the transmitter
was a good balance therefore best output from the antenna was possible. I
did think that implies that even considering the somewhat long run to the
balun, that reflected power was at a mimimum possible level because a
resonant antenna condition (or as close as I was able to estimate) existed.
Each time that meter readings were taken was after the whole antenna(s) were
raised back up in the air, then lower to trim, raise and measure, etc.
Starting with about 10% beyond formula length, it noetheless had to have
some material added to one antenna and trimmed from another. From what I
have learned from you guys, I attribute this to the varying and unique
ground absorbtion variables of my particular installation. The help provided
on this list is invaluable. Many thanks.

73's

Jack
Virginia Beach

"Reg Edwards" wrote in message
...
Jack, I think you know what you are about. But you may be doing the right
things for the wrong reasons.

The ONE and ONLY purpose of the SWR meter is to facilitate the
antenna-plus-line (plus tuner if you use one) to be adjusted such that the
transmitter is loaded with its design impedance of 50 ohms, plus or minus
20
percent or thereabouts.

What the SWR on the line may be is not relevant. It can be anything you
like! It doesn't matter insofar as the adjusting process itself is
concerned. And it is not of great consequence anyway.

The meter, Bird or MFJ or otherwise, does NOT indicate SWR on the
feedline.
This is a popular misconception - a very old Old Wives' tale. It merely
indicates whether or not the impedance looking into the transmitter end of
the line is 50 ohms or is not 50 ohms. If it is not 50 ohms it won't even
tell you what it actually is.

So after adjusting line input impedance to be 50 ohms by some means or
other, you may be quite happy to retire under the impression the SWR is a
nice 1-to-1 and the antenna input impedance is what you think it is.
Whereas
it very likely isn't.

But you will have achieved the true objective - the transmitter will be
correctly loaded with 50 ohms within plus or minus 20 percent even after
taking so-called SWR meter uncertainty into account.
----
Happy adjusting, Reg, G4FGQ

Jack, I got your message first time.

Juggling with antenna height, lengths and angles, perhaps even varying line
length, does indeed eventually obtain a 50-ohm load for the transmitter.
But the ONLY way of finding what the antenna impedance actually is is to get
up there and measure it in situ - out of idle curiosity. ;o)

Incidentally, the forward power indicated by the Bird is correct regardless
of SWR on the line simply because the impedance looking into the line is
near enough to 50 ohms. In this respect the meter behaves as intended.

And practically all of the power entering the line will eventually be
radiated because, although the SWR is not known, by virtue of the
near-to-resonance antenna(s) it cannot possibly be high enough to worry
about.

Fascinating subject! I've always been hooked on it. There's something
about transmission lines. Yet, believe it or not, I've never used a Smith
Chart in 60 years. Too unecessarily complicated.

Cecil, must pour myself a small nightcap of 2002 Merlot.
----
Reg, G4FGQ

Reg Edwards wrote:
Cecil, must pour myself a small nightcap of 2002 Merlot.

Wish I could do that. Acid reflux has caused me to quit
drinking around 6pm. That means I have to get an early
start. :-) Love Merlot.
--
73, Cecil http://www.qsl.net/w5dxp



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