Building a J-Pole

[midwestsw]

I've had a couple of different variations of manufactured antennas for my scanners. My main preference is monitoring the air bands. So, I finally decided
to construct my own, specific for the band, getting the recipe for a J- Pole off the web. My question that is tripping me up is why-how does the antenna function
if there is no "insulator" of some sort between the long vertical and the short vertical ? Seems to me there should be some separation as it appears
the whole thing is connected to each other. I did a search but haven't found anything conclusive. Not super interested in theory, just a simple explanation for my simple mind...Thanks

 

[nd5y]

High frequency AC (RF) doesn't behave the same as DC. You can have a direct short at DC and it can appear open or at least very high impedance at RF. You can also have a capacitor that appears to be a direct short at RF but an open circuit at DC (this doesn't apply to J-poles because they don't have capacitors).
The whole antenna can be one piece of DC grounded metal.
If you have not studied basic AC/DC elelctronics or antenna theory then you won't understand.

 

[prcguy]

The J-pole long element is 3/4 wavelength long and the first 1/4 wavelength of that is a matching circuit to feed the very high impedance of an end fed half wave. The bottom pipe between the 1/4 wave stub and the 3/4 wave element is ground but the coax attaches at a point above that where the match is around 50 ohms. It is a direct short at DC but not at the operational frequency of the antenna.

I'm curious why you settled on the J-pole for VHF air as it only covers maybe 5MHz of band width at VHF and the VHF air band is about 20MHz wide. The J-pole will not cove the entire band without loosing some performance at the edges.

 

[midwestsw]

Ah, thanks, great explanation. I'm well aware of the band width, just settled for approximately the middle of it @ 123.000. I guess
I hadn't considered there would be that much of an appreciable loss. What would be Your suggestion ?
Again, thank You.

 

[prcguy]

A really fat ground plane would work fine and its fairly standard at many airports. Make the vertical element about 2" dia and the radials can be thinner like 1/4" thick or even thin whips. You could use parts of an SO-239 to 3/8" CB stud mount to hold a 2" or so copper pipe cap then a resonant length of copper pipe and another cap on top for the vertical element. You would also want an antenna analyzer to get everything tuned and cut to the right length.

I used to have a garage full of surplus airport antennas, not sure whats inside the radome but they were full band and about 4ft tall. They can be had fairly cheap if you look around.

Ah, thanks, great explanation. I'm well aware of the band width, just settled for approximately the middle of it @ 123.000. I guess
I hadn't considered there would be that much of an appreciable loss. What would be Your suggestion ?
Again, thank You.

 

[cmdrwill]

As Mr PRC Guy says, the simple groundplane antenna will work very well for VHF Airband.

BUILD A GROUND PLANE ANTENNA FOR 2 METERS AND MORE - SIMPLE AND EASY!

1/4 Wave Ground Plane Antenna Calculator

 

[midwestsw]

Thanks Guys, easy enough. I incorrectly typed 123.000 above for the middle of the band. The dimensions I was going
for was based on 127.000 for a nice middle compromise.

 

[W5lz]

That section below the actual connection points of the Feed line make up an impedance matching network. That network gets the impedance close to 50 ohms which a transmitter likes to see. For receiving only, that network isn't as important (receivers arent as "picky" as transmitters.