Ian Jackson wrote:
In message ,
writes
Ralph Mowery wrote:

wrote in message
...
I've done a lot of modeling with V's and inverted V's. Except for a slight
diffenence due to ground effects at low heights, there is no difference
between them.

As the angle goes from 180 degrees, i.e. a dipole, the impedance goes
down and and the pattern spreads out.


I have not played with the programs , but often wondered what the effect is
on antennas that are suported on the ends and the middle is dropped in a U
or V shape and not the inverted V shape. Many antenna books and articals
seem to show the flat top and inverted V paterns, but I have not seen any
with the actual V type.

Like I said, there is no difference in free space between a V and an
inverted V.

There are slight differences when close to ground.

Assuming it's a halfwave dipole, I would have thought it was better to
concentrate on getting the centre (where the current is) as high as
possible, rather than the ends. [Obviously this depends on whether you
have a suitable sky-hook available for and at the centre.] However, I
believe that there is at least one commercial loaded dipole (essentially
for portable use, I think) that has a short mast, and the ends of the V
are up in the air.

Model it let me know what you find.

Also, regarding the radiation pattern, as the ends are dropped, doesn't
the signal that starts coming off the ends have an increasingly
vertically-polarised component?

As I recall, yes, the vertical component increases with decreasing angle.


--
Jim Pennino

wrote in :

I hope you have the equations for the sag and tension on the wire at hand
and run through them first.



Not until I try it. First attempt was only 50 metres, and it was so
straight I can't extropolate. While modelling can avoid mistakes, I think in
this case an empirical test will eliminate a lot of modelling of situations I
can never acheive anyway. I'm thinking the half-mile is doable, but I may
find that's just because it's a relatively easy length on a habitual run
during a 6.5 mile course. I'm pretty certain that any wind will wreak a lot
more havoc that gravity will, too.

"Ian Jackson" wrote in message
...
In message ,
writes
Like I said, there is no difference in free space between a V and an
inverted V.

There are slight differences when close to ground.

Assuming it's a halfwave dipole, I would have thought it was better to
concentrate on getting the centre (where the current is) as high as
possible, rather than the ends. [Obviously this depends on whether you
have a suitable sky-hook available for and at the centre.] However, I
believe that there is at least one commercial loaded dipole (essentially
for portable use, I think) that has a short mast, and the ends of the V
are up in the air.



In free space I would not think there would be much of any differance.

Near the ground where most of us are is what we have to live with. In my
back yard I have enough room to string wire dipoles for 80 meters and
shorter. I don't have a way to support the antennas in the middle. I can
go up about 60 feet or so at the ends.

One of my antennas is sort of flat as it is a windom and the long leg runs
by my tower and was drooping to about 40 feet, so I put a piece of PVC pipe
at the 50 foot level to help support it.

The OCF antenna is suported so the ends are on some pulleys and a cinder
block on the ground with a rope so that as trees swing the block will
sometimes raise off the ground with high wind. I have put a spectrum
analizer/tracking generator and return loss bridge on it a few times and
watched the SWR/RL go up and down as the wind blew. Really interisting as
the RL did not change much in amplitude, but the frequency shifted.




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wrote in :

If nothing else, the stainless could be used as a support for the copper.


Yes. I thought of that one just after posting the previous message.. Given
the length of copper, and my past experiments with stretching it, I was
already prepared for thsi idea. I'm not sure how the support should work,
ideally a simple very loose twist ought to do it. To be honest, I'll be far
likelier just settle for as much distance as I cam get in practise, whatever
I try.

Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
In message ,
writes
Like I said, there is no difference in free space between a V and an
inverted V.

There are slight differences when close to ground.

Assuming it's a halfwave dipole, I would have thought it was better to
concentrate on getting the centre (where the current is) as high as
possible, rather than the ends. [Obviously this depends on whether you
have a suitable sky-hook available for and at the centre.] However, I
believe that there is at least one commercial loaded dipole (essentially
for portable use, I think) that has a short mast, and the ends of the V
are up in the air.



In free space I would not think there would be much of any differance.

Near the ground where most of us are is what we have to live with. In my
back yard I have enough room to string wire dipoles for 80 meters and
shorter. I don't have a way to support the antennas in the middle. I can
go up about 60 feet or so at the ends.

60 feet is good for 30M, marginal for 40M, and crap for 80M and below
for a dipole.

It is also too high for 15M and up.

For best results, a diple should be at .5 labda.

Then again, a marginal 40M dipole is better than no antenna at all.



--
Jim Pennino

wrote in :

These guys did an actual study.


Too late for me to read much now (or post more for some time). Severe
eyestrain, hence the typos that creep in...

What I did see straight away was the mention of ferromagnetism, and I knew
that this was public enemy number one in a antenna wire. One thing I know
about stainless steel: if is isn't very austenitic, a bit of bending or
stretching will make it more magnetic fairly easily, but the bit I tested so
far will not cause a gently suspecned NdFeB magnet to deviate at all from
position so I think it will withstand a further test or two yet before I
assume it's a dud.

wrote in message
...
the ground where most of us are is what we have to live with. In my
back yard I have enough room to string wire dipoles for 80 meters and
shorter. I don't have a way to support the antennas in the middle. I
can
go up about 60 feet or so at the ends.

60 feet is good for 30M, marginal for 40M, and crap for 80M and below
for a dipole.

It is also too high for 15M and up.

For best results, a diple should be at .5 labda.

Then again, a marginal 40M dipole is better than no antenna at all.


I have been putting out a lot of furtlizer around the trees, but they do not
seem to be growning much..

Most ideal antennas for most hams are just not practictal or cost too much.
I will agree with what you are saying, especially about they should be up .5
wavelength if possiable. Just difficult to get an 80 meter dipole up 130
feet or so for most hams.

The 50 to 60 feet of my 80 meter antenna seems to work well for what I do.
Mostly talk to a couple of friends within 100 miles from my house.
It should send most of the signal up at a high angle for local coverage.

I stopped at 60 feet for my triband beam antenna as I did not want to climb
any higher. Not afraid of heigths,but not physically able to do much more.
Also the spacing of the guy wires worked out just right for that height.

Like you say, an antenna at 10 or 20 feet is still beter than no antenna.



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"Lostgallifreyan" wrote in message
. ..
Lostgallifreyan wrote in
:

Actually to be fair, with ,y longwire tests I'll likely be using very thin
strong stainless wire and pulling it tight enough to reduce my need to
worry
about it much. STill interesting though, I've seen heavy-looking HF
antennas
strung over the apex of a roof in a valley near here, and that had a
pronounced sag that may or may not have been bothersome to whoever owned
it.

Looking at a chart in an old ARRL antenna handbook gives a rough estiment of
a length of 500 feet and a tension of 400 pounds a wire of around 12 to 14
gauge will drop about 10 feet if Idid it right.



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On Sun, 12 Oct 2014 18:13:39 -0000, wrote:

There are lots of plans out there for J-poles made of pipe where one
of the legs is extended at the bottom to provide the mounting, typically
by bolting it to a mast. The assumption is there is no current in that
bottom leg.

If you model that you find that there really is current in that bottom
leg and it can REALLY screw up the antenna characteristics.

I don't like J-Poles. Various reasons, some of what are
non-technical. I'll spare you the details.

Our local radio club had a meeting where everyone was suppose to build
a twinlead J-Pole or a Slim Jim and have it tested by the experts and
their MFJ antenna analyzers. It wasn't too difficult for me to
predict that things were not going to work according to the science
fiction found in the collection of construction articles, so I elected
to just attend and watch. I arrived late and found about 12 J-Poles
in various stages of construction and testing in groups of 2 to 4
members. Many J-Poles were not even close to tuning onto the target
frequencies. All of them changed tuning when the antenna was moved.
Measurements were stable unless something moved (including people),
where everything changed. Someone had the bright idea of hanging the
antennas from the metal framework supporting the acoustic tile
ceiling, thus adding the framework to the antenna. There were no
baluns in sight, so when I recommended adding some additional coax
cable just to see what happens, the tuning again changed radically.
Running my hand up and down the coax cable did much the same thing,
demonstrating that a balun was probably a good idea. Minimum VSWR
improved when I convinced two builders to add series trimmer caps to
tune out the inductance of the feed tap wire, but usable bandwidth
decreased. In the past, someone would usually bring a copper pipe
J-pole with the coax feed and ground reversed, and wonder why it
wouldn't work. Not this time.

I tried to drag out my laptop and show some interested builders an
NEC2 model of their J-Pole, but couldn't figure out how to deal with
the velocity factor of elements made from twinlead. The IS card in
NEC4 handles it easily, but there's no equivalent in NEC2.

Incidentally, I use an insulated PVC pipe for the "handle", large
diameter elements, a balun, and a series tuning cap (Gamma match
style), when building a J-Pole.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Ralph Mowery wrote:

wrote in message
...
the ground where most of us are is what we have to live with. In my
back yard I have enough room to string wire dipoles for 80 meters and
shorter. I don't have a way to support the antennas in the middle. I
can
go up about 60 feet or so at the ends.

60 feet is good for 30M, marginal for 40M, and crap for 80M and below
for a dipole.

It is also too high for 15M and up.

For best results, a diple should be at .5 labda.

Then again, a marginal 40M dipole is better than no antenna at all.


I have been putting out a lot of furtlizer around the trees, but they do not
seem to be growning much..

Most ideal antennas for most hams are just not practictal or cost too much.
I will agree with what you are saying, especially about they should be up .5
wavelength if possiable. Just difficult to get an 80 meter dipole up 130
feet or so for most hams.

The 50 to 60 feet of my 80 meter antenna seems to work well for what I do.
Mostly talk to a couple of friends within 100 miles from my house.
It should send most of the signal up at a high angle for local coverage.

Yes, it does.

There is a discussion here worth reading:

http://en.wikipedia.org/wiki/Near_ve...idence_skywave

I stopped at 60 feet for my triband beam antenna as I did not want to climb
any higher. Not afraid of heigths,but not physically able to do much more.
Also the spacing of the guy wires worked out just right for that height.

Like you say, an antenna at 10 or 20 feet is still beter than no antenna.

The highest I can get at my QTH is about 25 feet and for years I wondered
why I could never work anyone other than locals with my dipoles on the
lower bands.

Then I did some study on the effects of ground and switched to verticals.

That was when I made my very first contact on 160 after many years of
being licenced.


--
Jim Pennino