J-pole discrepancy

[cassidy1190]

I am still pretty new to antenna building. I want to try out a j-pole antenna for the first time, and while doing some online research I have noticed one bit of information that varies from site to site. Some pages instruct you to attach the center conductor to the ¼ wave element and others to the ¾ wave element. Everything else stays the same except for this slight discrepancy. Which one is right…and why?

 

[LtDoc]

It really doesn't make any difference. So which would be easier, more convenient for you to do?
Why is that? Because 'RF' is alternating current, it switches every half cycle. What used to be the "hot" element a half cycle ago then becomes the "cold" element. Or, it it makes more sense to you, exchange that "hot/cold" to "+/-".
- 'Doc

 

[cassidy1190]

Ah now that makes sense. Thanks for clearing it up for me. From what else I understand, this should be quite an improvement from my current discone antenna. I am hoping to design the j-pole for VHF airband, and maybe getting it up on the roof, in order to pick up some ground stations.

 

[majoco]

I've never been happy with the designs of some J-pole where the long leg is extended down and becomes the mounting. I believe that the original J-Pole was mounted on a bit of PVC pipe so that the mounting was not part of the antenna - after all the real 'ground' is the braid of the coax, yet here you are also grounding at the end of the extended 'leg'

Here is a design for a "Slim-Jim" where the long vertical is folded over at the half-wave point. I made a similar dessign from aluminium rod on the ouside of some 1.5" PVC pipe -works well at it's design frequency.

2 Meter Slim Jim Copper Tubing Antenna by KE5FXU

 

[cassidy1190]

Well I went ahead and constructed my j-pole for 127.5 MHz and to be honest, I am not too impressed. Even standing on top of my roof (about 40ft up) holding the antenna as high as I can, I was still not bringing in much more than my attic discone. I am thinking of scrapping the whole idea and putting a nice sturdy diamond d-130j discone on the roof. If I'm gonna do it, I might as well do it right. My ultimate goal is to bring in towers at a distance of 30 miles. Call me overly optimistic, but I feel that will a high roof mount, flat terrain, and a relatively short cable run (about 50 ft) I can do it. I’ll save the questionable homebrews for the attic.

Here are a few pics of my j-pole, perhaps you can spot some issues that I cannot…

 

[majoco]

Yup, like I said in the earlier post, the lower part of the long vertical should not be there1.

See the original article...

~ J-POLE DOUBLE HEADER ~

and also the Wiki.

 

[pointman1177]

And j poles aren't ideal for airband because of it's pattern. Think about it airplanes are for the most part or at least I hope overhead discones Are best for air band because it's omni directional in not only 360 degree around it but also above and at angles being able to pick up those planes in various altitudes and directions. You can still build a cool airband antenna just try and look for a discone setup. Most of us do for airband recieve which uses the so239 chAsis mount and wires coming off it cut to specific lengths.

 

[WA1ATA]

Cassidy1190, you may have already gone to other antenna designs, but a simple thing you can try is to saw off the extra leg on your antenna, as Mojoco recommended. Then temporarily tie it to some PCV pipe or a broomstick and go check out the performance again.

You might also quickly throw together a twinlead version of the J-pole to do a comparison with your discone.

I use a twinlead J-pole for airband with good results. The low elevation lobe works well for picking up the weak signals ...... the tower and control centers. High elevation signals come from airplanes that are nearby and are strong signals.

You might also consider making a J-pole out of twinlead as a starter. They are a lot easier to trim and tune, and if you decide to mount it permanently, just slide it inside a PVC pipe (although you will have to shorten it up a few percent as putting it inside the PVC lowers the resonant frequency by about 5%).

I found that I could roughly tune up an airband twinlead J-pole without instruments by adding and removing some length, while checking to see how various ATIS signals and VOR signals changed in strength. This worked well for me because at my location those signals ranged from 110.35 MHz up through 135.275.

The other adjustment is the location of the connection of the coax feed on the tuning stub. Right at the short, the impedance is 0 ohms. Near the top of the stub the impedance is around 3000 to 5000 ohms. One quick check on whether the coax is connected to high or too low is to short the top wires of the twinlead to make the antenna into a slim jim. That raises the impedance seen by the matching section by a factor 4, thereby increasing the impedance at the coax connection.

 

[nanZor]

Attaching the center conductor to the shorter leg of the stub results in a higher angle of elevation as opposed to the usual method of attaching to the "long" side. About half-way down this page is a nice plot of the radiation angles comparing the two connections:

End-fed Vertical and J-pole

Ideally the mast should be "cold" at a zero reference point. That is, it shouldn't have a lot of common-mode current on it - which end-fed antennas are very susceptible to. In many cases, this is why you see current-baluns (coax coil or ferrite) very near the feedpoint to help preserve the desired pattern. Without it, the antenna will work, it just may be "looking" in strange directions.

 

[prcguy]

A Discone has a similar low angle radiation pattern over most of its range just like a J-pole or dipole or ground plane, etc.

You want a low angle antenna for aircraft reception not high angle because at any angle above your local surroundings you can pick up the aircraft with a paper clip due to the clear line of sight. When the aircraft is at extreme distance its low on the horizon and that's where you want the best antenna performance.
prcguy

And j poles aren't ideal for airband because of it's pattern. Think about it airplanes are for the most part or at least I hope overhead discones Are best for air band because it's omni directional in not only 360 degree around it but also above and at angles being able to pick up those planes in various altitudes and directions. You can still build a cool airband antenna just try and look for a discone setup. Most of us do for airband recieve which uses the so239 chAsis mount and wires coming off it cut to specific lengths.

 

[LtDoc]

cassidy1190,
While you will see/hear some differences between that 'J'-pole and a comparable disk-cone antenna, don't expect a huge difference. The difference you reported is probably about average for an inside/outside antenna unless one of those antennas isn't very 'comparable' to the other (as in gobs of gain).
As for hearing the towers at 30 miles, I'm afraid you may be expecting just a bit 'much'. That's with absolutely flat terrain, and from roof top levels, and even with that 'd-130j' antenna. I honestly think that something like 10 miles would be doing very well with almost any antenna at that frequency range. Those towers don't usually have very high antennas, so that 'line of sight' thingy will greatly reduce the effective range on the ground.
(Hope I'm wrong about that.)
- 'Doc

 

[cassidy1190]

Actually, I have been making slight improvements to my system and I have to say reception continues to get better. Sometimes under certain weather conditions I am able to receive McGuire and Lakehurst Towers/approach, JFK tower, ground, clearance...you name it, and also CAMRN and ROBER sectors of NY TRACON very well!! When conditions are unfavorable I can still here them if I turn down the squelch all the way. I can always receive a nearby RCAG in Colts Neck, NJ and some more distant ones in Ship Bottom, NJ.

With an Antronix amplifier I can receive the Jersey City TRS to the north and as far as Bucks County, PA TRS to the west (Not quite as well as JC). The amplifier shows excellent improvement all over UHF but unfortunately, until I get my PAR filter for 152 MHz in the mail, these nearby pagers make the amp practically useless on VHF.

The only filter I have now is one for FM broadcasts. I will probably end up finding that I need a filter for 162 MHz. I have two very strong NOAA stations that may be de-sensing my front end as well. Luckily I had the amp already from a previous cable TV setup. I was also able to find the source of some electrical noise on VHF. It turned out to be the ballast of a fluorescent light fixture on top of my fish tank, which was radiating RF from the power cord. An RFI choke on the light fixture power cord took care of that.

To anyone who thinks that a discone will solve all their problems, you are somewhat right, but be warned! You may need to spend an additional 100-200 dollars depending on your location and the amount of interference you get from surrounding sources. I know I will be pleased with the end result, and in a way I am glad that I had no idea about all this before I put it up. Knowing me, I probably wouldn't have bothered at all, and never would have learned so much more about the hobby.