Are you saying it’s both a Monopole with a decoupling sleeve and a Dipole at the same time?
Yes 
When a ground plane is in use it produces a ‘mirror’ image of the monopole.
When the decoupling sleeve is in place it effective creates a high impedance node at the open end, which is the same as if it was a centre fed dipole.
All antennas are ‘dipoles’, then need to be in order to complete the RF ‘circuit’. It’s a bit like a lamp or any other electrical circuit It won’t light with just one wire connected, it needs two.
An antenna may not look like a dipole, as often one half is hidden, but if you search hard enough you should be able to find it
Regards,
Martin
Ah OK my mistake I looked too quickly, in which case the groundplane example should have a diameter of 1/2 wave.
The 1/4 wave dimension is measured from the antenna feed point in the centre of the can.
Regards,
Martin
Yes that’s correct
Update
Perhaps I should explain the decoupling sleeve.
Coax could be considered to have three conductors.
The inner core
The inside of the outer screen
The outside of the outer screen.
The inner core and the inside of the outer screen should form a balanced transmission line, and as such would not radiate (or receive) a signal, as the RF currents are flowing in equal and opposite directions and so should cancel each other out. If a load is connected to the far end of the coax it receives the current on one leg and returns the current on the other. Because it is connected and flows in this way, it is described as a differential current.
However if something unbalances this arrangement, such as an antenna that has one half of the dipole bigger than the other, then another current is produced which is the difference between the differential mode currents, and this is somewhat confusingly called the common mode current, and this appears on the outside of the coax screen, almost as if it was a third wire.
Because the rim of the sleeve is not connected to anything, as it is just hanging in the air, it is at a high impedance node. If it is a 1/4 wave long from the rim of the open end of the sleeve to the closed end at the antenna feed point, it must therefore be at a low impedance.
So by adding a sleeve balun we create a high impedance node on the coax outer 1/4 wavelength below the feedpoint, which is pretty much the same as the end of a dipole element.
I haven’t really explained this too well. But if you are interested, W8IJ has some useful notes on the subject and a diagram that helps explain how a colinear with sleeves can be made to work.
Interestingly he states that
“a skirt does not fully decouple the coax shield. Many collinear antennas depend on this effect to excite the collinear elements! Stopping common mode requires both a skirt and radials, or multiple large inside diameter skirts.”
You can also use a tapered cone as a decoupling sleeve, as long as the edge is 1/4 wave long.
AEA use this method, and also include multiple cones in their “Isopole” antenna designs, where one cone is mounted 1/4 wave lower than the highest one.
Regards,
Martin
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Loving this discussion/tutorial. Bring it on!
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Thanks Martin for detailed explanatiin of decoupling sleeve, and links to related web pages for further reading.
Thank you Martin! Lovely! You brought a lot of light into our CoCo darkness
Hi Martin
I first expanded the mouth of V from 55mm to 69mm (1/4 wave at 1090 MHz). This slightly improved the performance, by about say 10%.
Next I folded back the V-stub on itself, using a round pencil as former, and slide out the pencil, then watched it’s performance. It’s plane count and message rate both dropped down to about 75% of that of the unfolded V.
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It may be that by wrapping the phasing section it has slightly modified the time delay along the phasing section, due to various loading effects.
You could try shortening both ends of the phasing section slightly, perhaps say by 5% each side ?
Some experimentation is nearly always required to convert a theoretical design into one that actually works in practice.
Regards,
Martin
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Old Physics adage: In theory, theory and reality are the same. In reality, theory and reality are never the same. The assumptions and unknowns are not a part of the theory, only the reality. Theory is only a starting point to discover the assumptions and unknowns. This leads all of us to our “AH HA” moments when we learn and understand.
That is one of the things we all enjoy in our hobbies and jobs. Have fun out there.
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Thank you Martin for the advise. I will give it a try.