Off-center fed dipole, questions
 

On page 7-10 of the 20th edition of the ARRL Antenna Book is a description
of an off-center fed dipole. I have the necessary materials around here
so I thought I'd experiment with that a bit and see how it does.

I'm a little confused about the balun.

I don't have a 4:1 or 6:1 current balun, but I do have a 4:1 W2AU-type
balun and an MFJ-915 RF Isolator, what they call a "1:1 current balun"
(actually an unun). I figured on connecting the latter to the former with
a double male PL-259 adapter and then connecting the coax to the other end
of the unun ... rather heavy, and no way to support it at the balun, and
it'll no doubt droop like a 400-pound beer belly, but I guess I'll jump
off of that bridge when I come to it...

The instruction sheet for the MFJ-915 says I should put the unun in the
line at the transmitter end, and yet the discussion of the OCF dipole in
the Antenna Book shows the 4:1 current balun at the antenna and a length
of coax to the transmitter.

So, should I connect the unun to the balun as I have it, or should I
connect it at the transmitter end as MFJ advises?

It does seem that if the objective is to keep RF off the outside of the
coax, the unun really should go up at the antenna end.

What say you?

Off-center fed dipole, questions
 

On Mon, 20 Aug 2007 20:29:34 -0400, "Rick (W-A-one-R-K-T)"
wrote:

So, should I connect the unun to the balun as I have it, or should I
connect it at the transmitter end as MFJ advises?

Hi Rick,

Yes.

So much for the short answer. With an OCF, your line is so heavily
invested with a lack of symmetry that it will ring in the fields
surrounding the antenna. Hence it needs to be choked every quarter
wave away from the feedpoint (that is, the UnUn at the bottom of the
BalUn AND again in intervals of a quarterwave).

Now as the need for suppressing Common Mode currents (which are due
both to the off center feed AND the asymmetry). Unless you are
feeling the pain of a live chassis, or suffering from inappropriate
foldback of the transmitter (due to its circuits being confounded by
Common Mode currents); then you could live with it. Then, of course,
there is the perfectionist's point of view that this adds a vertical
radiator to an otherwise dipole design. Yes it does. How much it
contributes to the mix of fields is going to be a spin of the wheel
(just as how much CM is there going to be on the line?).

One last point. The OCF resonates at pretty much the same frequencies
as it would as a dipole, it is only the feedpoint Z that changes with
respect to placement. Some places are great for many bands, others
are abysmal, and some bring in bands that would be a nasty match for
the strictly balanced dipole. If you don't snub the feedline, you are
adding other opportunistic resonance's to the mix.

73's
Richard Clark, KB7QHC

Off-center fed dipole, questions
 

In article ,
Rick (W-A-one-R-K-T) wrote:

On page 7-10 of the 20th edition of the ARRL Antenna Book is a description
of an off-center fed dipole. I have the necessary materials around here
so I thought I'd experiment with that a bit and see how it does.

I'm a little confused about the balun.

I don't have a 4:1 or 6:1 current balun, but I do have a 4:1 W2AU-type
balun and an MFJ-915 RF Isolator, what they call a "1:1 current balun"
(actually an unun). I figured on connecting the latter to the former with
a double male PL-259 adapter and then connecting the coax to the other end
of the unun ... rather heavy, and no way to support it at the balun, and
it'll no doubt droop like a 400-pound beer belly, but I guess I'll jump
off of that bridge when I come to it...

The instruction sheet for the MFJ-915 says I should put the unun in the
line at the transmitter end, and yet the discussion of the OCF dipole in
the Antenna Book shows the 4:1 current balun at the antenna and a length
of coax to the transmitter.

So, should I connect the unun to the balun as I have it, or should I
connect it at the transmitter end as MFJ advises?

It does seem that if the objective is to keep RF off the outside of the
coax, the unun really should go up at the antenna end.

What say you?

Based on the experience of a few of my ARES/RACES cohorts with their
OCF dipoles, I'd say that "it depends".

Putting the unun/isolator up at the antenna will tend to block
*conduction* of RF down onto the feedline from the radiator.

However, because the feedline is not located symmetrically with
respect to the radiator, there will be some amount of RF induced onto
the outside of the feedline by the (unequal) RF fields from the two
sides of the radiator. This will allow some amount of RF current flow
on the feedline, and (depending on your shack grounding arrangements)
might possibly allow for some amount of "RF on the case", RF feedback
into the audio chain, and similar annoyances.

If you put the isolator down at the transmitter, you may have more RF
flowing on the outside of the feedline (as it can be induced as above,
and can also flow down via conduction) but the isolator/unun will
present a high impedance to this current flow and keep it away from
the rig.

Due to the differences in the feedline RF current flow, you may find
that the SWR as seen at the rig is better in one configuration than in
the other. Which arrangement gives the better match is likely to
depend on the length of the feedline, height of the antenna, proximity
of the antenna to buildings and trees, and so forth.

In our Field Day use of a Buckmaster 130' OCF, the guy doing the setup
ended up with the isolator located near the rig... he got a better
match to 50 ohms that way, and preferred to keep the induced RF away
from the rig, PCs, sound-card interfaces, and so forth.

I'd suggest that you try it both ways, and use whichever way that
you find you prefer.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Off-center fed dipole, questions
 

Good afternoon, Richard and Dave.

Thanks for the good information. I guess I will try as Dave suggests and
try some different configurations and see which one works best. Probably
the one I'll try first is the Carolina Windom, for which the radiating
feedline (the 22 ft. from the balun to the unun) is one of the design
objectives. Not really sure about the significance of the 22 ft section
between balun and unun ... it's not a quarter wavelength at anything of
interest... but I'll try it and see.

Off-center fed dipole, questions
 

"Rick (W-A-one-R-K-T)" wrote in
:


Good afternoon, Richard and Dave.

Thanks for the good information. I guess I will try as Dave suggests
and
try some different configurations and see which one works best.
Probably
the one I'll try first is the Carolina Windom, for which the radiating
feedline (the 22 ft. from the balun to the unun) is one of the design
objectives.

Or a marketing solution for what many might consider an unavoidable
problem. Problem becomes feature, problem solved, product hype enhanced,
everone wins!

Not really sure about the significance of the 22 ft section
between balun and unun ... it's not a quarter wavelength at anything of
interest... but I'll try it and see.

It is another proprietary antenna with secret components that prevent
reliable independent exploration of the design.

I am not saying it doesn't 'work' (whatever that term means), just that
it isn't capable of independent explanation.

Owen

Off-center fed dipole, questions
 

Owen Duffy wrote:
"Rick (W-A-one-R-K-T)" wrote in
:


Good afternoon, Richard and Dave.

Thanks for the good information. I guess I will try as Dave suggests

and

try some different configurations and see which one works best.

Probably

the one I'll try first is the Carolina Windom, for which the radiating
feedline (the 22 ft. from the balun to the unun) is one of the design
objectives.


Or a marketing solution for what many might consider an unavoidable
problem. Problem becomes feature, problem solved, product hype enhanced,
everone wins!

Not really sure about the significance of the 22 ft section

between balun and unun ... it's not a quarter wavelength at anything of
interest... but I'll try it and see.


It is another proprietary antenna with secret components that prevent
reliable independent exploration of the design.

It's probably because they got a good deal on some premade 25 foot
cables which were inadvertently too short, or the first one built
happened to be 22 feet off the ground, or something like that. Maybe
they had a source for 22 foot scraps?

I doubt there's anything special about the 22 ft. As many have pointed
out, it's a deliberately unbalanced antenna which radiates from both
vertical and horizontal parts (and influenced by the towers, trees, and
buildings nearby), with a nice choke at some point to keep RF from
coming back in the shack on the outside of the coax.



I am not saying it doesn't 'work' (whatever that term means), just that
it isn't capable of independent explanation.

Owen

Off-center fed dipole, questions
 

Jim Lux wrote:
I doubt there's anything special about the 22 ft.

I've been told that 22 ft. is special, i.e.
virtually everyone can get his vertical
section 22 feet off the ground. (The original
Windom had a vertical radiating section.)
--
73, Cecil http://www.w5dxp.com

Off-center fed dipole, questions
 

"Rick (W-A-one-R-K-T)" wrote in message
.. .

On page 7-10 of the 20th edition of the ARRL Antenna Book is a description
of an off-center fed dipole. I have the necessary materials around here
so I thought I'd experiment with that a bit and see how it does.

I'm a little confused about the balun.

I don't have a 4:1 or 6:1 current balun, but I do have a 4:1 W2AU-type
balun and an MFJ-915 RF Isolator, what they call a "1:1 current balun"
(actually an unun). I figured on connecting the latter to the former with
a double male PL-259 adapter and then connecting the coax to the other end
of the unun ... rather heavy, and no way to support it at the balun, and
it'll no doubt droop like a 400-pound beer belly, but I guess I'll jump
off of that bridge when I come to it...

The instruction sheet for the MFJ-915 says I should put the unun in the
line at the transmitter end, and yet the discussion of the OCF dipole in
the Antenna Book shows the 4:1 current balun at the antenna and a length
of coax to the transmitter.

So, should I connect the unun to the balun as I have it, or should I
connect it at the transmitter end as MFJ advises?

It does seem that if the objective is to keep RF off the outside of the
coax, the unun really should go up at the antenna end.

What say you?


I would not worry too much about it, Just stick up something and see what
hapens. Wire is not that expensive. I have an OCF up about 45 feet and it
seems to work ok. Usually beter on 80 meters than an 80 meter dipole at 20
feet that is at right angles to it. The OCF is around 125 feet long and has
a 4:1 balun of somekind about 1/3 of the way on the horizontal wire, then
about 40 feet drooping rg8x to about 10 turns of coax on a piece of pvc pipe
and then into the shack. Seems to work ok on 80 and 40 meters. I have a
beam up for 20/15/10 so don't care how it works there, but sometimes it does
not do too much less thant he beam in some directions. The beam is usually
much beter most of the time as it is at 60 feet.

Off-center fed dipole, questions
 

Cecil Moore wrote:
Jim Lux wrote:

I doubt there's anything special about the 22 ft.


I've been told that 22 ft. is special, i.e.
virtually everyone can get his vertical
section 22 feet off the ground. (The original
Windom had a vertical radiating section.)

As opposed to 20 ft or 21 ft or 25 ft?

I find it hard to believe that there's an electrical reason for 22ft.
Mechanical or convenience I can believe.

Maybe it fits well with the length of their isolators, etc. and matches
a convenient pole height (e.g. isolator plus sag plus 22ft plus whatever
exactly matches 26 ft 3" or whatever the pole is)

Off-center fed dipole, questions
 

"Jim Lux" wrote in message
...
I doubt there's anything special about the 22 ft. As many have pointed
out, it's a deliberately unbalanced antenna which radiates from both
vertical and horizontal parts (and influenced by the towers, trees, and
buildings nearby), with a nice choke at some point to keep RF from coming
back in the shack on the outside of the coax.
...
If the choke is effective in removing the common mode current, then the
feedline will not radiate very much.

73,
Bob AD3K