On Nov 2, 6:12 pm, Roy Lewallen wrote:
Here's the deal. If you put the wires close together you get a lot of
interaction. The manifestation of the interaction is that the
higher-frequency dipoles end up considerably shorter than normal, and
they'll have a narrower bandwidth than an isolated dipole. The longest
one will also be affected by the others, but not nearly so much. You'll
also find that small differences in spacing can have quite an effect on
the dipole resonant frequencies, which is why a cookbook approach
usually doesn't work unless the writer is very careful to document the
antenna accurately and you're extremely careful to exactly duplicate it.

I had read the points you make above in the antenna books... but did
not realize exactly how variable the effects are especially for the
cases where the elements are physically close. The examples in the
ARRL Antenna book are particularly heinous: they show elements
separated by a fraction of an inch (e.g. the twin-lead example, the
picture that shows the wires hanging from egg insulators) and these
examples are - from my experiments - the least likely to work at all.

The interaction decreases rapidly as you spread the dipoles apart. If
you can get them around 30 degrees apart, the interaction is minimal and
you can just about treat them like separate dipoles. A lot of
installations fall between these extremes, so the dipoles have some
interaction but it's not as severe as it is when they're very closely
spaced.

This is a very fundamental piece of wisdom, and a piece that deserves
more attention in the ARRL Antenna books. The current statement - "The
separation between the dipoles for the various frequencies does not
seem to be especially critical" is incredibly wrong for the close-
spaced exampls shown in the book.

Tim.

On 5 Nov, 06:03, Tim Shoppa wrote:
On Nov 2, 6:12 pm, Roy Lewallen wrote:

Here's the deal. If you put the wires close together you get a lot of
interaction. The manifestation of the interaction is that the
higher-frequency dipoles end up considerably shorter than normal, and
they'll have a narrower bandwidth than an isolated dipole. The longest
one will also be affected by the others, but not nearly so much. You'll
also find that small differences in spacing can have quite an effect on
the dipole resonant frequencies, which is why a cookbook approach
usually doesn't work unless the writer is very careful to document the
antenna accurately and you're extremely careful to exactly duplicate it.

I had read the points you make above in the antenna books... but did
not realize exactly how variable the effects are especially for the
cases where the elements are physically close. The examples in the
ARRL Antenna book are particularly heinous: they show elements
separated by a fraction of an inch (e.g. the twin-lead example, the
picture that shows the wires hanging from egg insulators) and these
examples are - from my experiments - the least likely to work at all.

The interaction decreases rapidly as you spread the dipoles apart. If
you can get them around 30 degrees apart, the interaction is minimal and
you can just about treat them like separate dipoles. A lot of
installations fall between these extremes, so the dipoles have some
interaction but it's not as severe as it is when they're very closely
spaced.

This is a very fundamental piece of wisdom, and a piece that deserves
more attention in the ARRL Antenna books. The current statement - "The
separation between the dipoles for the various frequencies does not
seem to be especially critical" is incredibly wrong for the close-
spaced exampls shown in the book.

Tim.

After you have read the books try some thing different.
Obtain insulated wire and double it over itself to form a single
wire combination. Wind a considerable length on a former.
Using a MFJ analyser run thru the frequencies until you obtain a
resonance
at a reasonable impedance level and then scale for your desired
frequency.
Of course you must connect the MFJ to the two wire ends.
If you don't succeed first time around then short the wires in
increments
till you succeed.Wires close together can be turned into advantage if
you
go along with mother nature!
To make things easier, heat the insulation on the wire and insert
small needles
so you can hook up the MFJ at different turn lengths.
Be a leader not a follower
Art KB9MZ....XG

"art" wrote in message
oups.com...
On 5 Nov, 06:03, Tim Shoppa wrote:
On Nov 2, 6:12 pm, Roy Lewallen wrote:

Here's the deal. If you put the wires close together you get a lot of
interaction. The manifestation of the interaction is that the
higher-frequency dipoles end up considerably shorter than normal, and
they'll have a narrower bandwidth than an isolated dipole. The longest
one will also be affected by the others, but not nearly so much. You'll
also find that small differences in spacing can have quite an effect on
the dipole resonant frequencies, which is why a cookbook approach
usually doesn't work unless the writer is very careful to document the
antenna accurately and you're extremely careful to exactly duplicate
it.

I had read the points you make above in the antenna books... but did
not realize exactly how variable the effects are especially for the
cases where the elements are physically close. The examples in the
ARRL Antenna book are particularly heinous: they show elements
separated by a fraction of an inch (e.g. the twin-lead example, the
picture that shows the wires hanging from egg insulators) and these
examples are - from my experiments - the least likely to work at all.

The interaction decreases rapidly as you spread the dipoles apart. If
you can get them around 30 degrees apart, the interaction is minimal
and
you can just about treat them like separate dipoles. A lot of
installations fall between these extremes, so the dipoles have some
interaction but it's not as severe as it is when they're very closely
spaced.

This is a very fundamental piece of wisdom, and a piece that deserves
more attention in the ARRL Antenna books. The current statement - "The
separation between the dipoles for the various frequencies does not
seem to be especially critical" is incredibly wrong for the close-
spaced exampls shown in the book.

Tim.

After you have read the books try some thing different.
Obtain insulated wire and double it over itself to form a single
wire combination. Wind a considerable length on a former.
Using a MFJ analyser run thru the frequencies until you obtain a
resonance
at a reasonable impedance level and then scale for your desired
frequency.
Of course you must connect the MFJ to the two wire ends.
If you don't succeed first time around then short the wires in
increments
till you succeed.Wires close together can be turned into advantage if
you
go along with mother nature!
To make things easier, heat the insulation on the wire and insert
small needles
so you can hook up the MFJ at different turn lengths.
Be a leader not a follower
Art KB9MZ....XG

why mfj? we using another brand, can we? :-)

On Mon, 05 Nov 2007 06:03:55 -0800, Tim Shoppa
wrote:

The examples in the
ARRL Antenna book are particularly heinous: they show elements
separated by a fraction of an inch (e.g. the twin-lead example, the
picture that shows the wires hanging from egg insulators) and these
examples are - from my experiments - the least likely to work at all.

I have been complaining about that particular example in the ARRL
books for years.

I am beginning to think that you & I are the only ones to try to make
it work!

I just had a moment of inspiration...I wonder why I have not modeled
it in EZNEC? Maybe later this evening...

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"