1. “So . . how does this J-Pole thingamabob work?”
Guided by Dave Wickert, AE7TD (with YouTube assistance from Dave Casler, KEØOG)

2. Classic Dipole

5. ½ 

6. the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of resistance and reactance.
Resistance has two components: Radiation and Ohmic (or Loss)
Reactance has two components: Capacitive and Inductive

7. Capacitor: Current i “leads” Voltage v
Inductor: Voltage v “leads” Current i i v
To DC: Looks like its open
To DC: Looks like a short

8. Antenna “efficiency” =
At “resonance”, the phase shifts cancel each other out, i.e. XL = XC
Antenna “efficiency” =

9. Single band dipole = ~ 93% efficiency
Single band vertical = ~ 5% efficiency
(coil loaded at bottom)
(mobile HF adds loading inductor/coil for HF bands, e.g. for the 40m band, where λ/4 is 33 feet!)

10. Power is distributed uniformly across the element.
MHz
Power is distributed uniformly across the element.
At the end, the current is 0. Why? What does this mean that the voltage is?
What is the impedance at the end of the dipole?
What is the impedance at the middle of the dipole?
Where is the current flow maximum?
Notice that at λ/4, whatever you measure, i.e. Voltage, Current, or Impedance, its values are opposite on the other side.

11. What is a J-Pole? λ/2 A crazy looking antenna
Because its overall length is ¾ λ, it is normally used for VHF and UHF, or ~3.5 feet (for VHF)
Some kind of weird matching technique being done at the end.
λ/4

13. Let’s take a look at how it works . . .
“Ask Dave #36”, part of Dave Casler’s “Ask Dave” series on YouTube

25. Open Stub J-Pole (sold by Arrow Antenna and K9JEB.com)
Another common variation:
Open Stub J-Pole (sold by Arrow Antenna and K9JEB.com)
Overall design is very similar
Top λ/2 section is the radiator
Bottom λ/4 parallel sections are the matching unit
However, instead of the cross connected stub, this design uses a second parallel stub to match at 50 Ω
λ/2
λ/4
Tune for best SWR

26. Build your own? Commercial versions:
($35)
($30 open stub j-pole, locally sourced)
From Arrow Antennas: (parts list)
Article on building your own:
Videos for your own j-pole from copper piping:
Variants:
(Super-J variant w/ calculator)
(also a Super-J but curved matching)
(open stub j-pole variant – uses a short matching stub instead of elevating the feedpoint)
(details on another OSJ design)

27. Yet another variation on a theme
J Pole Handbook, but Glynn “Buck” Rodgers, K7ABT
Talks about how a j-pole is just a variant of a Windom HF antenna, but adapted for VHF/UHF (see for extensive details on a Windom HF antenna)
The center line of the feedline connects to the same long element point, but the shield of the feedline connects at the bottom of the short element.

28. Open Stub j-pole
From Reply #7
“This design also has the feedpoint at the end of the so called "stub" which makes it not act like a stub but a feedline. Think 1/4 transformer here as that is what it appears to be to me. The two elements essentially become a high impedance ladder line, and over 1/4 electrical wavelength transform the low 50 ohm source to a very high impedance, which can then directly feed the end fed half wavelength antenna above.
Yes there will be some imbalance, but I have yet to encounter a significant amount of common mode currents from one of these antennas yet...”