J-pole counterpose

[KC9QQM]

Per the wonderful help from other hams on the web, I built a dual band (2m and 440) J-pole.

I would like to use some PVC piping I have around to get it above the ground as kind of a portable base antenna that I can disassemble and throw in my truck. Does the J-pole type antenna need a ground or counterpose? Can I, right under the J part of the assembly, put some ground radials or something of the like? Or should I just buy some pieces of mast pipe to use?

Thanks for the input! 73

Jeff
KC9QQM

[N4CR]

A ground is not required. You will probably benefit from a choke at the feedpoint.

Either that or build one of these: http://www.hamuniverse.com/2metergp.html

Easier to build, performs more or less identically and has less bad habits. Also weighs less and has less wind load.

A J-Pole is an end fed half wave. Because it is end fed, it is fed at a high impedance point. This can make it couple into nearby metal objects including your coax shield. A quarter wave ground plane has none of these problems.

[WB2WIK]

A ground is not required. You will probably benefit from a choke at the feedpoint.

Either that or build one of these: http://www.hamuniverse.com/2metergp.html

Easier to build, performs more or less identically and has less bad habits. Also weighs less and has less wind load.

A J-Pole is an end fed half wave. Because it is end fed, it is fed at a high impedance point.

Except that by design, it's tapped at a low impedance point.  Still it has common mode problems and an RF choke balun can help.

This is a very silly antenna design, I have no clue why they became popular.  Its performance isn't any better than a quarter-wave ground plane antenna, which is smaller and easier to build.

One of those old "folklore" antennas, I guess.

[W7ETA]

You might find this article "Verticals without Vertigo" on W4RNL's site.  It will take you through counterpoise, top hats, radials and more.

73
Bob

[KC9QQM]

Thanks for the replies guys!

Jeff
KC9QQM

[KE3WD]

One big difference between the J-Pole configuration and the common homebrew groundplane antenna is that the J-Pole is at DC ground potential. 

if you like the first stage FET in your receiver, this is indeed a big difference. 

[N4CR]

One big difference between the J-Pole configuration and the common homebrew groundplane antenna is that the J-Pole is at DC ground potential. 

No, it is not necessarily at DC ground. The difference between the center conductor and the shield may be very low resistance, but that does not mean it is grounded. It could very easily be floating and infinite to ground.

[WB2WIK]

One big difference between the J-Pole configuration and the common homebrew groundplane antenna is that the J-Pole is at DC ground potential. 

if you like the first stage FET in your receiver, this is indeed a big difference. 

I use low noise GaAsFETs in all my VHF-UHF receivers and never lost one yet.  A "DC grounded" antenna actually doesn't make any difference with regard to this, unless you charge up your antenna with a battery.

At higher frequencies, the "ground" isn't really there because the series impedance is so high.

[KE3WD]

No, it is not necessarily at DC ground. The difference between the center conductor and the shield may be very low resistance, but that does not mean it is grounded. It could very easily be floating and infinite to ground.

Bottom of the j-pole, whether made of copper tubing or twinlead, is connected.  If built properly.  That's DC ground potential, always was and always will be. 

[KE3WD]

I use low noise GaAsFETs in all my VHF-UHF receivers and never lost one yet.  A "DC grounded" antenna actually doesn't make any difference with regard to this, unless you charge up your antenna with a battery.

At higher frequencies, the "ground" isn't really there because the series impedance is so high.

It is electrostatic buildup that punches holes in FETs under this scenario. 

They don't blow all at once either, rather the failure mode is that each successive electrostatic discharge punches one more hole in the substrate.  This means that failure is exhibited in a gradual decrease in receiver sensitivity. 

Not just talking lightning, either, although lightning quite obviously is not a good thing here, even the electrostatic charge that can build up on an antenna element from windborn dust can take its toll. 

I've replaced quite a few first stage RF FETs in receivers that have crossed my bench.  It is a rather common thing. 

There are surely other factors involved, of course, among them being location, climate in that location, height above ground, and likely a few more I can't think of or don't know about. 

This is one reason why almost all, if not all, commercially made groundplanes are indeed placed at DC potential.  The problem for the homebrewer is not unsolvable, I've seen plans that include the matching network at the bottom where the coax connects, this can be as simple as a tapped coil that passes RF at the target frequencies yet allows DC from coax center to shield.  That is enough to keep the static discharged.  In some instances, it may also provide quieter receive, too. 

That said, I've used both types without losing that FET also.  Kinda like those guys who don't ground their antenna or station, they just haven't gotten a strike.  Yet. 

[KE3WD]

FYI the J-Pole design can be spotted at many airports around the country, tuned for the Aircraft band and turning in yeoman's duty day in and day out, stainless steel J-poles. 

[N4CR]

No, it is not necessarily at DC ground. The difference between the center conductor and the shield may be very low resistance, but that does not mean it is grounded. It could very easily be floating and infinite to ground.

Bottom of the j-pole, whether made of copper tubing or twinlead, is connected.  If built properly.  That's DC ground potential, always was and always will be. 

So, what you are saying here is if I connect two pieces of metal together, they are a ground. That's news to me.

Say I have a battery powered transceiver sitting on a plastic table with a j-pole c-clamped to the table and coax to the radio.

Tell me where the ground is. Common I can see. DC low impedance at the feed point I can see. Ground I cannot see.

[W8JI]

I cannot recall ever seeing a J-pole in commercial service, although with the dumbing down of the radio service industry I suppose some might start to creep in.

The "dc ground" has virtually no effect on front end device life. If a radio is designed so poorly that 19 inches of conductor charge enough to blow an FET gate, it's time to buy a different model radio.

The J-pole is nothing but an end-fed 1/2 wave vertical with a very poor feed system and no groundplane. It has an UNbalanced antenna fed by a BALanced stub, fed by an UNbalanced coaxial line. That is two poorly engineered balanced to unbalanced transitions in one antenna. What a bargain!

As WB2WIK says, it works no better than a 1/4 wave groundplane at best. It is also not fixable with a coax balun or string of beads because it has two areas of poor balance transition, not one.


http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

73 Tom

[W8JI]

Per the wonderful help from other hams on the web, I built a dual band (2m and 440) J-pole.

I would like to use some PVC piping I have around to get it above the ground as kind of a portable base antenna that I can disassemble and throw in my truck. Does the J-pole type antenna need a ground or counterpose? Can I, right under the J part of the assembly, put some ground radials or something of the like? Or should I just buy some pieces of mast pipe to use?

Thanks for the input! 73

Jeff
KC9QQM

Jeff,

To answer your question there is very little you can do to change the J-pole. It is an easy antenna to build and will work OK. Although it is not a great antenna and really is not any better than a 1/4 wave groundplane, it will serve your use and it will work.

It is easy to build, and that is enough.

If you wanted to add a groundplane, the groundplane would attach at the junction of the coax shield to the antenna. The ground plane would attach to the shield. I needs to attach right at the point where the shield meets the antenna, no where else.

I do not think you would notice much difference, except the SWR will be more stable and the antenna pattern a little cleaner.

It isn't a bad antenna, it is just not anything special or good either. The important things are it will work and you built it. That is all you need to worry about. :-)

73 Tom

[KE3WD]

http://www.dfrsolutions.com/uploads/publications/2003_Mmic.pdf

/http://www.patentstorm.us/patents/6542036/description.html

Never seen a commercial J-pole antenna? 

Here's one, as mentioned, tuned for the 118 – 128 MHz aircraft band:

http://www.jpole-antenna.com/category/aviation/

I used to run one of another brand that was taken down from a closed airport, it was all stainless steel and built like a tank, I trimmed it down to work on the 2-meter band and it served as a repeater antenna for years.  We finally replaced it with a Stationmaster, only for reasons of better coverage and a bit of downtilt to help the signal get down into the Western Pennsylvania valleys from the top of the hill location. 

But you guys are the experts, no question. 

I never worked for MFJ.