Fan Dipole insight
 

On Nov 3, 5:25 pm, "Stefan Wolfe" wrote:


I guess in my case, the G5RV is the best I can do. I live in a restricted
PUD but I have a 'relatively' large attic. I could fit a bent 80m dipole up
there or a bent G5RV that is just a few feet shorter. But with the G5RV, I
do get some good performance on several ham bands and I think it is more
efficient than the 80m dipole because the ladder line also functions as part
of the antenna (as you know, according to G5RV theory).

It's not more efficient, but there are cases where the feed radiation
could help if there is a need for some vertical polarization.
But... thats just another of what I consider a problem.
I don't want any feedline radiation. If I want vertical, I'll
run a vertical.
But I realize you are limited on room, so not a perfect world.. :/
If you are using the usual G5RV setup with the tuner to coax to
choke to twin lead, you would actually be better off running the same
antenna, but using only the tuner to twin lead all the way to the
antenna.
In cases of limited room, I usually prefer "Z" dipoles fed with coax
if for single band use. Here at home, I can't run a full length 160m
dipole on this lot. So I used a "Z" dipole fed in parallel with my
other usual dipoles.
I did tests comparing the Z dipole with loaded dipoles, and the Z
dipole always won. This was even with using heavy fat wire coils,
and optimum coil locations on the elements.
If I used a Z dipole for all band use, I would feed it with ladder
line
the whole way.. Or twin lead.. Twin lead is fairly decent indoors as
it never gets wet.
I have a 40m dipole in my attic for emergency use when it's real
stormy. I feed it with twin lead so I can run 40-10. Being it's half
size on 80m, and fed with a T-net tuner, the efficiency is poor on
80m. Lots of tuner loss..

But I also must defer to yourself and others
because this is a technical group where we get into more than "how stuff
works.com"...I participate if I think I can contribute in a certain area.
Thanks for your comment.

Ya gotta use what you can fit in.. I have no problems with anyone
using any type of antenna. If it works well enough for you, that's all
that really matters.
I'm just warning of losses you may not notice if you don't have a
coax fed dipole to A/B test them to compare..
BTW, I don't just pick on the G5RV's.. :/ I also am not crazy about
most windoms either if they are fed using the tuner to coax to choke
to wire scenario..
I did a A/B test against one of those one time on 40m.
The windom seemed to be working just great. They could hear
stations, and make contacts, and all was well in the world.
But then I hooked up a coax fed 40m dipole, and used an
antenna switch to be able to quickly A/B.
The coax fed dipole brought "everything" up 2 S units.
Noise floor, signals, the whole ball of wax.
I kid you not. The windom guy almost fell over..
He had no idea he was losing that much.
Some may say, well 2 S units ain't much..
But when you consider that the usual change of
running 100w vs 600w usually amounts to about 2 S units
on most run of the mill S meters, thats quite a
bit of loss.. :(
It was like I was getting the effects of a free 500-600 watt
amp vs the windom..
But to each his own, or to his room.. I'm not trying to
be an antenna snob, I just jibber jabber about them..
MPG will vary..
MK

Fan Dipole insight
 

Stefan Wolfe wrote:

I guess in my case, the G5RV is the best I can do. I live in a restricted
PUD but I have a 'relatively' large attic. I could fit a bent 80m dipole up
there or a bent G5RV that is just a few feet shorter. But with the G5RV, I
do get some good performance on several ham bands and I think it is more
efficient than the 80m dipole because the ladder line also functions as part
of the antenna (as you know, according to G5RV theory). . .

There's no theory I know of which causes a G5RV feedline to radiate more
or less than any other dipole. The amount of feedline radiation depends
on the amount of common mode current it carries, and its length. The
amount of common mode current depends on a number of factors, including
how the transmission line is fed, how it's oriented, its length, and the
common mode impedance of the antenna. This is discussed in some detail
at http://eznec.com/Amateur/Articles/Baluns.pdf.

Increasing the amount of vertical radiation and/or having the feedline
radiate doesn't improve the efficiency of an antenna. If you prevent the
feedline from radiating, the power it would have radiated ends up being
radiated by the antenna instead.

Roy Lewallen, W7EL

Fan Dipole insight
 

Stefan Wolfe wrote:

I stated it improperly. I agree that total radiation efficieny in all
directions does not increase but I believe efficiency as a vertical radiator
(ie in the horizontal plane) does increase with G5RV at some frequencies due
to the ladder line. See http://www.roadkill.com/~unwin/G5RV.html

Oh, no, you've been reading Art's postings! He uses the term
"efficiency" in creative and inscrutable ways.

When used with respect to antennas (and a lot of other things),
"efficiency" has a universally understood, precise definition. The
efficiency of an antenna is the ratio of the power radiated to the power
applied. What you meant was effectiveness, not efficiency. If you do
find it necessary to use "efficiency" to mean anything other than what
it's universally understood to mean, you'll need to carefully point this
out at the time, or people are bound to misinterpret what you've said --
as I did.

If you prevent the
feedline from radiating, the power it would have radiated ends up being
radiated by the antenna instead.

Of course, this is correct. However, when you're cramped for space and are
trying to get as much RF as possible in the horizontal plane (rather than
warmimg clouds directly overhead :-)), it is advantageous to intentionally
use the vertical ladder line as a radiator when possible. At least, that is
the theory of one who has always had to use attic antennas. Fortunately I
will retire soon and will be moving to a place where I can put up a 160m
horizontal dipole if I wish.

So what you're doing is intentionally making or allowing the feedline to
radiate in order to get some vertically polarized radiation so you can
get more radiation at lower angles from a low antenna. That's a method
some of us would avoid, but in circumstances like yours it's very likely
one of the best solutions available.

Roy Lewallen, W7EL

Fan Dipole insight
 

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.

Fan Dipole insight
 

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

Fan Dipole insight
 

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"

Fan Dipole insight
 

"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? :-)