Hi:
There is a large amount of interaction between the dipoles as they are
tightly coupled to each other.
The shorter or higher frequency are affected more by changes to the lower
frequency i.e. longer dipoles than the lower frequency dipoles are by
changes to the higher frequency or shorter dipoles.
So to keep hair pulling to a minimum start with the lowest frequency dipole,
tune that for the lowest SWR or for resonance and then do the next lower
one. If you do it in any other order you will spend your life going back and
forth retuning dipoles until you give up and buy a G5RV kit and join the
dark side.


--
John Passaneau, W3JXP
Penn State University


".J.S..." wrote in message
. ..

"W9DMK (Robert Lay)" skrev i en meddelelse
...
On Tue, 3 May 2005 18:41:17 +0200, ".J.S..."
wrote:

Building something a bit like this :
http://www.hamuniverse.com/multidipole.html

He suggests setting up one wire at a time starting with the longest, is
that
the easy way ?

I would think adding all wires, a bit too long , and then starting the
adjustment with the shortest would be better ?
All the other wires overlap the shortest 100%, so I would imagine later
trimming of longer wires i parallell would not affect the short wire
much.

You are placing far too much importance on your objective of having
each dipole resonant. It matters oh so very little from any point of
view.


What would the purpose of building a multidipole be if not to get a
resonance on each band ?

My goal is to run psk31 on several bands without a tuner.




/JS

"John Passaneau" skrev i en meddelelse
...
Hi:
There is a large amount of interaction between the dipoles as they are
tightly coupled to each other.
The shorter or higher frequency are affected more by changes to the lower
frequency i.e. longer dipoles than the lower frequency dipoles are by
changes to the higher frequency or shorter dipoles.

Strange..
I can understand that hanging a long wire next to a short will affect more
than the other way round.

But if they are both up there, I would have thought trimming the long wire
would not change its affect on the short wire much.

So to keep hair pulling to a minimum start with the lowest frequency
dipole,
tune that for the lowest SWR or for resonance and then do the next lower
one.

Ok, thats the order of trimming, but do you install one wire at a time or
all of them at once ?




/JS

".J.S..." wrote in message
. ..

"John Passaneau" skrev i en meddelelse
...
Hi:
There is a large amount of interaction between the dipoles as they are
tightly coupled to each other.
The shorter or higher frequency are affected more by changes to the lower
frequency i.e. longer dipoles than the lower frequency dipoles are by
changes to the higher frequency or shorter dipoles.

Strange..
I can understand that hanging a long wire next to a short will affect more
than the other way round.

But if they are both up there, I would have thought trimming the long wire
would not change its affect on the short wire much.

So to keep hair pulling to a minimum start with the lowest frequency
dipole,
tune that for the lowest SWR or for resonance and then do the next lower
one.

Ok, thats the order of trimming, but do you install one wire at a time or
all of them at once ?




/JS

Yes put up all the wires at the same time. There will be an effect on the
lower frequency antennas by the higher frequency ones and putting up all the
wire will get you closer on the first try than putting them up one at time
and tuning them will. I've used this type of antenna for years and they work
quite well. The interaction between the dipoles can be minimized by putting
the dipoles at right angles to each other. That is say, run the 80m dipole
north/south and the 40m dipole east/west. That minimizes the coupling and
the dipoles act almost like single band antennas. In my system I have a
160/80/40/30 meter dipole antenna. It has the 80/40 wires running parallel,
spaced about 12" apart and at right angles to that is a loaded (shortened)
dipole for 160m and the loading coils in the 160m antenna act like traps and
make a 30m dipole. It's all feed with one coax.
The 80m dipole acts just about the same as if it was the only dipole up
there, but the 40m dipole is strongly affected by the 80m wire. The most
noticeable effect is the SWR bandwidth is a bit smaller than I would expect
from a single 40m dipole and the tuning is a bit more sensitive.By that I
mean it takes a smaller change in length of the 40m wire to move the
resonate point some KHz's than it would with a single band dipole. All that
means is you have to be careful with how much you cut off and take it in
baby steps, not big chunks. I first up the 80/40 antennas and a year later
I decided to get on 160 so I added the 160 wires. When I added the 160/30m
wires at right angles to the 80/40m ones there was almost no noticeable
effect on the 80/40m dipoles. You can download the EZNEC demo antenna
modeling program and see how the antennas interact your self. One thing you
will notice is that when your operating on 40m the 80m is also working a
little bit too. Anyway I have DXCC on 80 and 40 and working on it on 160 and
30m which are the last bands I need DXCC on.

Good luck


--
John Passaneau
State College Pa.

"John Passaneau" skrev i en meddelelse
...
So to keep hair pulling to a minimum start with the lowest frequency
dipole,
tune that for the lowest SWR or for resonance and then do the next lower
one.

Ok, thats the order of trimming, but do you install one wire at a time or
all of them at once ?



Yes put up all the wires at the same time. There will be an effect on the
lower frequency antennas by the higher frequency ones and putting up all
the wire will get you closer on the first try than putting them up one at
time and tuning them will. I've used this type of antenna for years and
they work quite well. The interaction between the dipoles can be minimized
by putting the dipoles at right angles to each other. That is say, run the
80m dipole north/south and the 40m dipole east/west. That minimizes the
coupling and the dipoles act almost like single band antennas.

In my system I have a 160/80/40/30 meter dipole antenna.

Thats exactly what I am trying make on top of a flat-roof building.
The building is 56x12 m so I cant quite get anything at a right angle but I
will try to get as much angle as possible. Also it is not long enough for
the 160m but the idea is to go diagonal and then across at the ends like a
'Z'.

It has the 80/40 wires running parallel,
spaced about 12" apart and at right angles to that is a loaded (shortened)
dipole for 160m and the loading coils in the 160m antenna act like traps
and make a 30m dipole. It's all feed with one coax.
The 80m dipole acts just about the same as if it was the only dipole up
there, but the 40m dipole is strongly affected by the 80m wire. The most
noticeable effect is the SWR bandwidth is a bit smaller than I would
expect from a single 40m dipole and the tuning is a bit more sensitive.By
that I mean it takes a smaller change in length of the 40m wire to move
the resonate point some KHz's than it would with a single band dipole. All
that means is you have to be careful with how much you cut off and take it
in baby steps, not big chunks. I first up the 80/40 antennas and a year
later I decided to get on 160 so I added the 160 wires. When I added the
160/30m wires at right angles to the 80/40m ones there was almost no
noticeable effect on the 80/40m dipoles. You can download the EZNEC demo
antenna modeling program and see how the antennas interact your self. One
thing you will notice is that when your operating on 40m the 80m is also
working a little bit too. Anyway I have DXCC on 80 and 40 and working on
it on 160 and 30m which are the last bands I need DXCC on.

Good luck

Thx and thx for the info



/JS

There will be an effect on the
lower frequency antennas by the higher frequency ones and putting up
all the
wire will get you closer on the first try than putting them up one at
time
and tuning them will. ..................................

Myself, I think starting with the low band, and building up, is the
best practical method. You can still do them all at once, but I'd
tune each in the ballpark first, starting low,before I moved to the
next
higher band. Note your later statement...
" The 80m dipole acts just about the same as if it was the only dipole
up
there, but the 40m dipole is strongly affected by the 80m wire."

It's almost always the higher band that will have problems, as this
example shows. It will be very rare for say a 40 to mess up a 80 ant...
But quite common the other way around.. So being thats the case,
I always start low, and work up through the bands...You can always
fine tune after it's all done...I usually do in fact....
Like you say, if the dipoles are at right angles, there is almost no
interaction...In fact, I've had a leg fall down before, and not effect
the match of the other bands. I always spread them apart with the
widest spacing I can...IE: right angles if two bands...
Stacking them like that example on the web page is the worst way
to go about it as far as coupling problems. But....Once you get one
tuned, they will work...I just prefer spread apart...


Also it is not long enough for
the 160m but the idea is to go diagonal and then across at the ends
like a
'Z'. .................................................. ...

That's what I'm using here. I'm on a city lot, so 80m is the largest
dipole I can put up in a straight line. I have a 160m "Z" dipole. It's
a
better compromise compared a loaded dipole to fit 120 ft. I have
the 160, 80 and 40 on one feedline. I have extra insulators to
turn the 160 into 20m if I want....I do that sometimes in the summer...
MK