Hi, I was just reading the user's manual for the MFJ-941E antenna
tuner,
http://www.mfjenterprises.com/produc...rodid=MFJ-941E and on page
5, there is a warning, which reads:

"WARNING: To avoid problems, a dipole antenna should be a full half-
wave on the lowest band.
On 160 meters, an 80 or 40 meter antenna fed the normal way will be
extremely reactive with only
a few Ohms of feedpoint resistance. Trying to load an 80 meter (or
higher frequency) antenna on
160 meters can be a disaster for both your signal and the tuner. The
best way to operate 160 with
an 80 or 40 meter antenna is to load either or both feedline wires (in
parallel) as a longwire. The
antenna will act like a "T" antenna worked against the station
ground."

Half wave for 160 meters is around 264ft. So, it sounds like even with
a tuner (or at least this tuner), I would need at least this much wire
to transmit on 160 meters.

My naive understanding was that I could hang up a 100ft dipole and use
a tuner to transmit on all bands. What bands can I reasonable expect
to transmit on using a 100ft dipole and a tuner?

That last part of the warning about using an 80 meter dipole as a
longwire has me totally confused. Are they saying to snip off the
coax connector and plug the ends into the longwire connector in the
back of the tuner?


Jim

P.s. I finally have the 21st edition of the ARRL Antenna book. I'm
only on chapter 2, so please feel free to point out page numbers or
chapters that would probably answer my questions.

On Mon, 26 Nov 2007 10:47:15 -0800 (PST), James barrett
wrote:

I'm
only on chapter 2, so please feel free to point out page numbers or
chapters that would probably answer my questions.

Hi Jim,

That would be too simple.

However, as to the practicality of the warning from MFJ. It is a
useful point to depart from, and it is more an issue of insurance
against their having to explain why the tuner melt down when you
didn't follow their advice. However, even as a common guideline, it
fails when compared to other points such as don't load into a full
wave antenna. It follows whatever is halfwave in one band must be
fullwave in another (or nearly so, or even twice so).

So, as undoubtedly all the correspondence that will flow from your
simple question will prove: a simple answer does not satisfy your
need.

Just sit back and follow the reactions your question will elicit, and
read other threads as well. Give this a month and you will begin to
discover the borders to the last frontier of design.

Meanwhile, put up as much wire as you can. Try to tune up. There are
no one antenna solutions, so anticipate having more than one.

73's
Richard Clark, KB7QHC

James barrett wrote:
My naive understanding was that I could hang up a 100ft dipole and use
a tuner to transmit on all bands. What bands can I reasonable expect
to transmit on using a 100ft dipole and a tuner?

Make that "all HF bands". 160m is not an HF band.
Also, the dipole must be fed with parallel-line,
ideally open-wire line, for all HF band operation.

W2DU's rule-of-thumb is that a dipole should be
at least 3/8 wavelength on the lowest frequency
of operation.

A 100 ft. dipole can usually be used on 160m,
not as a dipole, but as a Marconi-style fed
system with the transmission line conductors
shorted together and fed against a good radial
ground system.
--
73, Cecil http://www.w5dxp.com

On Nov 26, 2:21 pm, Cecil Moore wrote:
James barrett wrote:
My naive understanding was that I could hang up a 100ft dipole and use
a tuner to transmit on all bands. What bands can I reasonable expect
to transmit on using a 100ft dipole and a tuner?

Make that "all HF bands". 160m is not an HF band.
Also, the dipole must be fed with parallel-line,
ideally open-wire line, for all HF band operation.

W2DU's rule-of-thumb is that a dipole should be
at least 3/8 wavelength on the lowest frequency
of operation.

A 100 ft. dipole can usually be used on 160m,
not as a dipole, but as a Marconi-style fed
system with the transmission line conductors
shorted together and fed against a good radial
ground system.
--
73, Cecil http://www.w5dxp.com

I keep reminding myself about the articles I've read about people
using window screens and rain gutters as antennas. ;-) I'll keep
reading and eventually I will understand Antennas. But (there's always
a but) I have to remind myself that my goal is to be able to make
contacts (CW, and eventually phone when I get my General class
license) and for now that means 10m, 15m, 40m and 80m. With that in
mind should my thinking be to put up a dipole for 80m and tune it down
to 10, 15 and 40, or should I go with some other length? Being that we
are near the bottom of the sun spot cycle, which band has the best
chance for making contacts? Maybe I should put up a dipole for that
and then tune it for the others.

If tuning for 160 meters is different than for the HF bands, then I'll
wait until I actually have my General class license before I start
thinking about that.

Jim

James barrett wrote:

I keep reminding myself about the articles I've read about people
using window screens and rain gutters as antennas. ;-) I'll keep
reading and eventually I will understand Antennas. But (there's always
a but) I have to remind myself that my goal is to be able to make
contacts (CW, and eventually phone when I get my General class
license) and for now that means 10m, 15m, 40m and 80m. With that in
mind should my thinking be to put up a dipole for 80m and tune it down
to 10, 15 and 40, or should I go with some other length? Being that we
are near the bottom of the sun spot cycle, which band has the best
chance for making contacts? Maybe I should put up a dipole for that
and then tune it for the others.

If tuning for 160 meters is different than for the HF bands, then I'll
wait until I actually have my General class license before I start
thinking about that.

Hey Jim,

Using one of those tuners is very simple. (mostly)

Assuming you are using ladder or window line,(not only a good
assumption, but a good idea) put up as much wire as high as you can.

The "mostly" part is that you don't want the wires to be 1/4 wavelength
total on any of the bands you are going to operate. I think the MFJ
manuals point out some lengths you don't want to use.

All this is to say that if you can put say 96 feet of wire in the air,
that is what you put up. Such an antenna will work a treat on 40 and up,
decently on 80 meters, and almost so-so on 160.

You are ready. Get a couple buds, your slingshot or favorite method of
launching fishing line into the air, and put up that dipole. Run the
window line to the house, avoiding running it too near to metal objects,
say keep it around 4 inches away. Connect it to the balanced line input
on the tuner, go coax to the rig, and there you have it. Make sure you
do the grounding thing correctly, but that's another subject.

- 73 de Mike N3LI -

On Mon, 26 Nov 2007 10:47:15 -0800 (PST), James barrett
wrote:

[snip]
My naive understanding was that I could hang up a 100ft dipole and use
a tuner to transmit on all bands. What bands can I reasonable expect
to transmit on using a 100ft dipole and a tuner?

That last part of the warning about using an 80 meter dipole as a
longwire has me totally confused. Are they saying to snip off the
coax connector and plug the ends into the longwire connector in the
back of the tuner?


Jim

P.s. I finally have the 21st edition of the ARRL Antenna book. I'm
only on chapter 2, so please feel free to point out page numbers or
chapters that would probably answer my questions


Jim,

Check this one out.

http://www.degendesigns.com/Downloads/TheEasyWay.PDF

Danny, K6MHE

Here's the basic problem: Somewhere along the line you've got to match
the impedance seen at the input of the feedline to the 50 ohm resistive
(50 + j0 ohm) load your transmitter needs to see. (You don't actually
have to get it exact, but reasonably close.) The impedance of a half
wave dipole is in this ballpark all by itself, but other antennas (and
even the dipole if not resonant) usually need some kind of matching. The
farther the impedance at the feedline input is from 50 + j0, the harder
the job for the matching network -- voltages across and/or currents
through the matching network components increase and can become
downright awesome, if the impedance to be matched is far from 50 + j0.

If you start shrinking a dipole to a shorter length than a half
wavelength, or a monopole to shorter than a quarter wavelength, the
resistance drops and the amount of reactance increases. See Fig. 7 on p.
2-5 of the Antenna Book, which shows that a quarter wave dipole has a
feedpoint impedance of around 15 - j1000 ohms. Shorter dipoles have even
lower resistance and larger reactance.

Matching an extreme impedance involves, as I mentioned, high voltages
and/or currents, which is why the MFJ caution. These high voltages and
currents also result in increased loss, sometimes to the point where
most of your power is going to heating the matching system components.

It's entirely possible to use electrically small antennas, but there are
tradeoffs involved. Here are some ways you can do it:

1. Split the matching chore between the tuner and other external
components, such as a loading inductor to reduce the amount of reactance
the tuner has to deal with.
2. Use transmission lines to accomplish some or all of the matching.
This isn't usually the most efficient possible way (contrary to
folklore) but it distributes the heat and voltage gradient. Often you
can use the transmission line to transform an extreme impedance to
another impedance that might also be extreme but within the range a
tuner can more comfortably handle.
3. Use relatively lossy components, even possibly an intentional
resistor, as part of the matching network. This reduces voltages and
currents and increases bandwidth at the expense of some reduction in
radiated power.
4. Reduce power if necessary to keep your tuner from self destructing.

If you make an efficient matching network for an extreme impedance
transformation, it will be very narrow banded, so will require frequent
retuning as you QSY. Lossier systems have broader bandwidth (in
general). Trading loss for bandwidth might be worthwhile depending on
your circumstances. B & W has sold an antenna for decades which
incorporates a resistor, and it's widely used. Browse through the QRP
sites and you'll see that large numbers of QSOs are routinely had by
people running a watt or less. Countless others are undoubtedly made by
people with 100 watt transmitters who are radiating 10 watts without
realizing it. So don't stay off the air just because you can't make an
efficient antenna system. Any radiated power is better than none.

If you have a decent ground system, connecting the two transmission line
conductors of a short dipole together and feeding it against ground
(that is, connect the shorted dipole to the "hot" side of the tuner
output and the ground to the "cold" side) is a good suggestion. What you
have then is a top-loaded vertical, with the radiating vertical being
the feedline and the dipole being the largely nonradiating top hat. But
I wouldn't consider this unless you can bury at least a few radial wires
or lay them on the surface of the ground.

Roy Lewallen, W7EL

"James barrett" wrote in message
...

I keep reminding myself about the articles I've read about people
using window screens and rain gutters as antennas. ;-) I'll keep
reading and eventually I will understand Antennas.

Take comfort from the knowledge that sub-optimal antennas, including rain
gutters, are still OK. Until I get a tower up, I am contenting myself with
sub-optimal. Heck, I loaded up an aluminum extension ladder and had a QSO
with Hawaii from San Diego.

This NG had a recent thread on multiband fan dipoles. I made one and it
works fine. I started with a 10m copper pipe dipole a few feet over my
garage roof, coax-fed through a balun. It is parallel to the peak of the
roof.

I added two 20m elements of #16 insulated stranded wire (lamp cord,
actually) and ran them downslope toward opposite corners of the garage roof.
They make about a 45-degree angle with the 10m antenna. Construction
articles encourage separating elements of different bands. The first
measured 20m elements were too long and the SWR dipped at 12-something MHz.
I trimmed them to 14.1 MHz, SWR = 1.1:1 at the radio. The only change to
the 10m performance was a very slight increase in SWR (1.5:1 vs. 1.7:1 per
MFJ-269).

I output only 100 watts and have logged 20m QSO's to Canada, Alaska, Hawaii
and the US East Coast on those hunks of lamp cord laying on my garage roof.
I plan to add some 40m elements and see what happens. ("Let's see what
happens if ... " is one of the very best things about ham radio.)

"Sal"
(KD6VKW)

My naive understanding was that I could hang up a 100ft dipole and use
a tuner to transmit on all bands.

I find that set up extremely lossy on 10GHz

Jeff

If tuning for 160 meters is different than for the HF bands, then I'll
wait until I actually have my General class license before I start
thinking about that.

Tuning is no different on 160 compared to anywhere else. It is just that the
very long wavelength makes short antennas difficult to match, and be outside
of the range of that particular tuner.

I have about 45' of end-fed wire that I can match on 160m to a low swr with
the auto atu that I have. Of course that does not mean that it makes an
effective antenna, but I can work people on at a push.

73
Jeff