As a rule, more wire is "said" to be better. Theoretically, you can make a
different case but just try transmitting at 160m with a tuner and 80m
dipole. At that MF frequency, your tuner will try to reflect so much energy
that you will probably hear the crackling and may see a corona, no matter if
it is rated at 3KW and your are transmitting at 100W.
snip
======================================
It all depends what feeder you use.
When using twin feeder and a matching unit to suit both such a feeder
and the traditional asymmetric 50 Ohms output of any
transmitter/receiver , an 80m dipole,or a dipole of any length for that
matter, can be readily used on 160 m provided that half the dipole + the
length of the feeder is roughly an odd number of quarter wavelengths at
the operating frequency (assuming the velocity factor of the feeder is 1)
In this situation the current in the twin feeder at the matching unit is
relatively high ,hence the impedance is low ,because the voltage is
relatively low (hence no crackling/corona problems.

Cecil Moore's web site shows how he adds lengths of twin feeder to suit
the different bands.


Frank GM0CSZ / KN6WH

Highland Ham wrote:
Cecil Moore's web site shows how he adds lengths of twin feeder to suit
the different bands.

If the SWR on 450 ohm ladder-line is between 4.5:1 and
18:1, the impedance at a current maximum point will be
resistive between 25 ohms and 100 ohms for an SWR on
50 ohm coax of 2:1 or less with no tuner. See the Smith
Chart at: http://www.w5dxp.com/smith.htm
--
73, Cecil http://www.w5dxp.com

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Cecil Moore wrote:
Highland Ham wrote:
Cecil Moore's web site shows how he adds lengths of twin feeder to
suit the different bands.

If the SWR on 450 ohm ladder-line is between 4.5:1 and
18:1, the impedance at a current maximum point will be
resistive between 25 ohms and 100 ohms for an SWR on
50 ohm coax of 2:1 or less with no tuner. See the Smith
Chart at: http://www.w5dxp.com/smith.htm

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?




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James Barrett wrote:

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

You're not alone. SWR and transmission lines are widely misunderstood,
even, I'm sad to say, among many engineers working with RF. Because of
the widely held misconceptions about these topics, writers of some of
the tutorials and discussions which have made it to print have also
fallen victim to mistaken concepts. I'd be especially leery of
explanations found on the web. While surely there are some which are
entirely correct and very well done, I'm certain there are many others
which aren't.

I recommend the _ARRL Antenna Book_ as a good place to start.

Roy Lewallen, W7EL

James Barrett wrote in
:

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I
wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish.
Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

Hi James,

Little wonder.

You asked a few questions:

1. Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner.

2. Well, If I put up a wire dipole, and then use a tuner, what is the
best length of wire to use?

3. If I use an 80 meter dipole with a tuner, is that better than using a
10 meter dipole with a tuner?

My offering is:

1. That is a very simple statement, and for instance does not address
efficiency or a host of other issues (eg EMR safety). It is a restatement
of the popular ham maxim that "anything works" or the "any antenna is
better than no antenna".

2. The elements of an antenna system have a complex interaction, and
system performance can only be understood when the entire system is
analysed as a system. That means you have to start at the element level
and gain an understanding of those and then how they interact in a
system. Another popular ham maxim is "bigger is always better", it is
easy to say, but is doesn't apply in practice and is usually stated to
mask a lack of fundamental understanding.

3. You are a bit more specific, but not specific enough to answer
definitively. A half wave dipole fed with a balun and a reasonable length
of appropriate coax is an antenna system that takes only moderate
knowledge to design, fabricate, install and set to work with a high level
of confidence that it is working reasonably efficiently. You may even
wish to use an ATU (which is essentially an impedance transforming
network) for small optimisation of the load impedance seen by the
transmitter.

Whilst the temptation to use the antenna system described at 3 on
multiple bands may be great, and it is done, the outcome is often very
poor. For example, such an antenna designed for 80m (system efficiency
should be greater than 80%) is likely to be well less than 10% efficient
on 40m.

Be ware of simple Rules Of Thumb, there are often a plethora of unstated
assumptions, which when considered make them ROT.

Owen

James Barrett wrote:
This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

"The ARRL Antenna Book" is a pretty good start.
Here's a matched antenna system with an SWR of 9:1
on the ladder-line. Understanding this system will
be a step in the right direction.

50 ohm
XMTR----------1/2WL 450 ohm feedline-------50 ohm antenna
--
73, Cecil http://www.w5dxp.com

"James Barrett" wrote in message
news:WN6dnT9x6ZU6I7HanZ2dnUVZ_v-

snip

I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case.

You were mostly right; this is the theoretical ideal, but reality forces
compromises on all of us.

To put it simply, yes, you want the most power to "jump off the antenna"
into space. Whatever doesn't jump off is dissipated (wasted) as heat
somewhere in the system. If too much is reflected back from the antenna and
dissipated within in your transmitter, the transmitter overheats ($$$) or it
reduces power to protect itself and nobody hears you.

A simple, inexpensive antenna that comes close to the ideal will probably
work on only one band. For multiband operation you'd need several of them
($$$) and maybe need multiple poles. ($$$) ... or you could buy a
multiband, combination antenna. ($$$)

The tuner is trickery to deal with the power that didn't jump off. It
allows a compromise antenna -- one not perfectly suited for the intended
purpose -- to be used without overheating the transmitter. (The transmitter
must see a 50-ohm load to stay happy.) Changing the length of the feedline
is another form of trickery to keep the transmitter happy, but it doesn't
improve the antenna, either.

Tuners are very useful and not terribly expensive, which is what makes them
so common in the shack.

I need to learn all
about SWR and impedance in regards to Antennas, from start to finish.

Maybe, but only if you want to know the _why_ of antennas. You can buy and
use lots of great ready-made ham things without understanding exactly how
they work. [Example: My car has fuel injection and electronic ignition; I
only sort of understand how they work and could NOT fix them if I had to. I
don't need to.]

Is there an easy-to-read tutorial out there for a beginner like me?

Somebody said The ARRL Antenna Manual. I agree. At first, you can pick and
choose what to read. As you read more of it, a coherent picture should
emerge. I hope you didn't ditch too much HS math.

"Sal"
(KD6VKW)

"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Owen

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Roy Lewallen, W7EL

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

If he said "sometimes overheats", he would be correct.
An SWR of 10:1 certainly *can* cause an over-current condition
in an unprotected transmitter assuming the reflected current
is in phase with the forward current at the transmitter.

However, just as likely is that the reflected voltage is in
phase with the forward voltage at the transmitter and an
over-voltage condition *can* result in punch-through of the
final transistor.

If over-current and over-voltage were not a problem caused
by reflected waves, protection of the finals would not be
necessary.

Note that the impedance seen by the transmitter above is a
*virtual* impedance, not an impedor. Virtual impedances are
only a *result* and not the cause of anything. Virtual impedances
are not the *cause* of over-current or over-voltage conditions.

Anyone who scoffs at virtual opens and virtual shorts being
the *cause* of the re-reflection of reflected energy cannot,
without contradicting himself, turn around and argue that
the virtual impedance seen by a transmitter is the *cause* of
the mismatch. One cannot have it both ways.
--
73, Cecil http://www.w5dxp.com