J-pole, done, mounted & working.

[Pro-95]

So here it is. The completion of the project for the most part. 3 J-poles one 155Mhz on top, 460Mhz in the middle and 860Mhz on the bottom.

I do however have a few minor details left to finish as I was in a hurry to just get it up.

First I need to replace the section between the 860Mhz and the 460Mhz as there isn't enough pole (distance) for good vertical seperation and interference can occur. I will remove each 3/4" section and put a 3/4" wood dowel inside to stiffen the antenna. Finish cleaning the copper and paint with non-metalic paint. On the connection end I need a triplexer to combine the signals down to one RG-8. Replace the feedline from RG59 to RG-8.

I am very much happy with this antenna arrangement and with mounting and adding a ground improved the signals even more. I am still running a "T" and only have the 155 and 860 connected however there is significant improvement in reach and quality in both 155Mhz and 860Mhz bands. Once I add the triplexer I'm sure I'll see even more improvement.

To give you an idea of the improvement, I am having to lock out the 5watt Carson Tahoe Hospital maintenenance frequencies (155Mhz) as they are being picked up for the first time at this location. The hospital is 30 miles away. :shock:

I am also picking up the REMSA Ambulances even when they are 40+ miles north of here. I wasn't picking up the ambulances except occasionally and scratchy with the dispatch being loud and clear on the same 460Mhz frequencies.

Happy Scanning.

 

[Voyager]

So here it is. The completion of the project for the most part. 3 J-poles one 155Mhz on top, 460Mhz in the middle and 860Mhz on the bottom.

How did you isolate the antennas from each other?

Joe M.

 

[Pro-95]

How did you isolate the antennas from each other?

Vertical seperation keeps each antennas electromagnetic field away from the other. The 5 loops of coax acts as a decoupler to isolate and seperate the ground. (Thanks for the help in understanding this N-Jay. "If it is truly for decoupling, then you are building a high impeadance path to prevent RF from traveling down the sheild of the cable.")

The elongated ("scrunched donut") field is greater due to gain in horizontal distance than vertical as the vertical is ~= 1/2 wave on either side of the driven element. Horizontal is 1.5 times (3db gain) a full wavelength.

There would be even better performance with significant horizontal seperation of each antenna but the distances needed to acheive horizontal seperation are beyond the capabilities of anything less than a 4-posted tower.

I am however considering just placing a series of antennas down the ridge line as this exercise was more of a test than to acheive a final project.

 

[Pro-95]

Also some notes I've been told regarding the J-poles.

The J-pole will do a good job for you on a scanner if you are only interested in a small bandwidth. The J-pole typically has a band width of 4.0 MHz from the center point the antenna was cut for and the 2:1 SWR point. This means if you cut (build) your J-pole for 155.00 MHz you should get good results from about 153.00 MHz to 157.00 MHz.

The J-Poles has low angle of radiation making it an excellent long distance antenna! It's about as low as you can get from a VHF homebrew antenna! Second is the db gain which is quite respectable, at least a "decibel" over a ground plane in terms of efficiency. This is the same on receive or transmit so it will work well in a scanner application.

The antenna may be fastened to any supporting structure including grounded metal. Ideally, the antenna should be mounted at least a quarter-wavelength above any metal structures. The J-Pole likes to couple into things nearby, including trees! Try to keep it in the clear as much as possible. This may even be more important than the height. (but height is important so get as high as you can)

As far as separation, Horizontal spacing distances may exceed practical tower applications. The best orientation is vertical separation when tower space permits it. A supporting pipe, tower structure and transmission line in the aperture of the antenna will impact the antenna pattern and coverage in the direction of the obstruction. The bottom line is to get the J-Pole as far away from the other antennas and the tower structure as you can. At very minimum 1/2 wave separation should be kept. At 155 MHz that would be 36.23 inches.

Also a note (thinking out loud) on common groundings, what would be the difference between two band specific antennas mounted on the same car using mag mount to acheive ground?

 

[Voyager]

How did you isolate the antennas from each other?

Vertical seperation keeps each antennas electromagnetic field away from the other. The 5 loops of coax acts as a decoupler to isolate and seperate the ground. (Thanks for the help in understanding this N-Jay. "If it is truly for decoupling, then you are building a high impeadance path to prevent RF from traveling down the sheild of the cable.")

No, I mean how do you physically separate the pieces of copper pipe from each other? No matter what you do with the coax, you have to keep the actual antennas electrically separate from each other? Did you have them separated with a wood rod or something? I can't quote tell from the pic.

Joe M.

 

[Voyager]

Also some notes I've been told regarding the J-poles.

The J-pole will do a good job for you on a scanner if you are only interested in a small bandwidth. The J-pole typically has a band width of 4.0 MHz from the center point the antenna was cut for and the 2:1 SWR point. This means if you cut (build) your J-pole for 155.00 MHz you should get good results from about 153.00 MHz to 157.00 MHz.

While I would agree with that for TRANSMITTING, for receiving you should see a lot wider bandwidth without degradation in performance. The main difference being the SWR factor. On RX, it doesn't matter nearly as much.

The J-Poles has low angle of radiation making it an excellent long distance antenna! It's about as low as you can get from a VHF homebrew antenna! Second is the db gain which is quite respectable, at least a "decibel" over a ground plane in terms of efficiency. This is the same on receive or transmit so it will work well in a scanner application.

At least a decibel is respectable? It takes THREE dB to notice any change by the human ear. One dB isn't going to mean squat.

The antenna may be fastened to any supporting structure including grounded metal. Ideally, the antenna should be mounted at least a quarter-wavelength above any metal structures.

This is what I'm talking about - how did you isolate the antennas from each other? I somewhat question the statement that the antennas can be mounted to metal, as that would affect the matching of the antenna.

Joe M.

 

[Pro-95]

This is what I'm talking about - how did you isolate the antennas from each other? I somewhat question the statement that the antennas can be mounted to metal, as that would affect the matching of the antenna.

They are not isolated from each other except in vertical distance.

Since I am really new to all this and my little knowledge in the subject I have to make assumptions and deductions from what is presented and with what little I know.

First I found the design then google'd on J-poles, read many different presentations and forum styled threads on J-poles and this is what I gathered and deduced. Some of which could be totally wrong, which is why I built this antenna to begin with to help me understand antennas and electromagnetic interactions and prove or disprove my understanding.

Here is what I came up with:

The J-pole design provides gain. Although as stated above "a respectable +1", In just building the J-pole by itself I heard significant improvement both in signal gathering and distance of signals received. So I proceeded on building the other 2 frequencies.

All designs for J-poles: Indicated that the copper below the "J" was of little consequence to the J-pole itself and had "virtually" no effect on the antenna itself. In fact most designs suggested a piece of copper, of any length, to be used in mounting the J-pole to the masting. Nowhere did anyone give any indication of isolation being required. I made the deduction here that I could stack antennas because there was no consequence to the "stuff" above or below the "J". I also considered vehicle mounted antennas and those you see with an array of antennas on their vehicle. Since this are obviously using the same ground, ground has little consequence after the field is created and stabilized.[note]Corrections accepted[/note]

My limited understanding of an electromagnetic field and how a J-pole creates one: Two wires, one the center conductor and the other the braid are excited by DC emissions from the receiver. Electricity always follows the path of least resistance. Charging an element with positive then another with negative creates the effect of attraction or rejection. The energy of ground attracts the positive energy over. However since the closest and least resitive path is via jumping the elements this spreads the energy across the elements which in turn creates the field or bubble of energy. So the elements are used to create and stabilize the field. The creation and stabilization of the field are directly related to the "tuning" connections of the coax in relation to the position on the wave tuned elements. Once the field is created it attracts and gathers signals then drives them down the center of the field into the center conductor of the coax. [note:] I'd really appreciate any corrections in my understanding of the above.[/note]

So therefore my deduction was that only seperation of the fields is important not isolation of ground. So I built a stacked set of J-poles to accomplish two things. First help me understand how an antenna works and to get me an inexpensive and better antenna for listening.

I have accomplished the better listening and now I'm awaiting my antennas book to help in the overall understanding of the antenna and the elctromagnetic field. So far though it seems as though isolation of ground has little effect on the antennas itself and if things are kept out of the field for the most part your good. This of course is providing that the attractions nearby are not more attractive than where you want the attraction to take place. This could cause the field to become severly distorted or collapse alltogether.

I'll end this with, yes I am just trying to understand this science and I learn by reading but far more by doing/experimenting. If you have corrections I am standing by with open ears.

 

[N_Jay]

Now you have done it!

I have been avoiding studying up on the J-Pole ever since a ham friend built one and was raving about it.

I took a quick look and it does not look like the theory is that complicated, but I want to do some reading up before make any comments. (don't want to get pounced on for incorrect info!)

 

[Pro-95]

Re: Now you have done it!

I have been avoiding studying up on the J-Pole ever since a ham friend built one and was raving about it.

:?: You've confused me on this one. If it's being raved about and is soooo simple to make, why not. :?:

I took a quick look and it does not look like the theory is that complicated, but I want to do some reading up before make any comments. (don't want to get pounced on for incorrect info!)

I'm already way out on that limb! Thanks I'd really appreciate the input.

 

[N_Jay]

Re: Now you have done it!

I have been avoiding studying up on the J-Pole ever since a ham friend built one and was raving about it.

:?: You've confused me on this one. If it's being raved about and is soooo simple to make, why not. :?:

Cause I (believe it or not from the other thread) realy hate trying to figure out what each component is doing in an RF network. Especially in an antenna where the fuctional components are not always clearly separtated.

[ quote="N_Jay"]I took a quick look and it does not look like the theory is that complicated, but I want to do some reading up before make any comments. (don't want to get pounced on for incorrect info!)

I'm already way out on that limb! Thanks I'd really appreciate the input.[/quote]

OK, I have a little write up.
It is not as neet as I would like, but don't know if I will have time to clean it up.

 

[N_Jay]

J-Pole Theory Explained - (Poorly)

A simplistic view of the theory behind the J-Pole.

First a few points to keep in mind.
1) An antenna works best when it is resonant at the desired frequency
2) An antenna works best when it has an impeadance match to the transmission line
3) The frequency where it is resonant may not be the frequency where it is best matched
4) Many antenna designs have a natural characteristic impedance other than the desired 50 ohm
5) There are several ways to match an antenna to a transmission line.
6) A section of “Transmission Line” can be used to create an RF transformer

When the resonant frequency provides a close match it is often possible to tune the antenna for a best match by operating it slightly off resonance. This is why many antenna designs call for a radiator length slightly off the calculated value.

The two other ways to provide an impedance match at or near resonance are;
1) To use a loading coil or shunt capacitor to shift the resonant frequency of the antenna. This works well, but narrows the bandwidth of the match.
2) To use a transformer of some type to match the natural impedance of the antenna at resonance to the desired impedance of the transmission line.

The second method is what is done with the J-Pole.

-----------------------
Legend
"Transmission Line" (with quotes); The two pieces of pipe paralel to each other at the bopttom of the J-Pole

transmission line (no quotes); 50-Ohm coax cable
------------------------------


The “Antenna portion” of the J-Pole is a simple end-fed ½ wave antenna. This is the taller of the two vertical sections measured from the top of the shorter section to the top. The issue with the end-fed ½ wave antenna is that it is naturally very high impedance at resonance. To match this “antenna portion” to the 50-Ohm transmission line, a transformer is used. The transformer is made of a short section of “Transmission Line”. This is the part of the J-Pole made up of the two parallel vertical tubes. In order to take this “transmission line” section and use it as a transformer one end is shorted together (the bottom of the “J”), and the other end it left open. Since this section is approximately 1/4 wavelength long, as you move from the shorted end (0 Ohms) towards the open end (High Ohms) there is a natural 50-Ohm point. The 50-Ohm transmission line (Coax cable) is connected to the “transmission Line” section of the J-Pole at this point near the shorted end. The “antenna portion” is connected to the other end of the transmission line where it is a good match.

I hope this explanation helps people. It may be a bit technical for some, and is probably a bit simplistic for others.

A few extra notes:

Since the bottom of the "J" is nothing but a short across the two legs of the "Transmission Line" it is a null point and that is why it can be grounded of connected to additiona conductors without effecting the antenna.

Since the coax is feeding a ballenced (open) transmission system (the two sides of teh "transmission line") the sheild of the coax is not at relitive ground. That is why it needs to be decoupled. The better way for be to feed it with a balun (ballanced/Unbalanced transformer), but that would take away from the simplicity of the antenna design.

 

[Voyager]

Re: J-Pole Theory Explained - (Poorly)

Since the bottom of the "J" is nothing but a short across the two legs of the "Transmission Line" it is a null point and that is why it can be grounded of connected to additiona conductors without effecting the antenna.

But what about connecting a short to the top of the 1/2 wave?
(AKA the 'next' antenna up)

Joe M.

 

[Pro-95]

Thanks N-Jay!

I have a question and a confirmation though. I'll add my drawing file to help understand the relationships and locations:

Section "D": Tuning section. So the location of "D" the connection across the two bottom 1/4 waves (transformer) is the location in which 50 Ohms is acheived on the driven side of the antenna? Any other location would alter the match between the coax and the antenna?

Again Section "D": Would placing a 300(assumption) to 50(or 75) transformer at location "D" alleviate the inclusion of the decoupling hoops. And also reduce the design to a simple 1/2 wave end fed pole?

Also the formula of 22 * 12 / Mhz, with 22*12 being the conversion to inches, what is 22? Where does this number come from? And then also 21 on the "gap" or "C" between poles. I mean it was easy enough to calculate the 2 waves but the gaps and tuning elude me.

And the final thought on "D"; If the connection between the radio and antenna was 300 ohm twin lead, would any transformer be needed?

Hmm now to go do some reading on baluns.

THANKS!

 

[N_Jay]

Re: J-Pole Theory Explained - (Poorly)

Since the bottom of the "J" is nothing but a short across the two legs of the "Transmission Line" it is a null point and that is why it can be grounded of connected to additiona conductors without effecting the antenna.

But what about connecting a short to the top of the 1/2 wave?
(AKA the 'next' antenna up)

Joe M.

I don't think anyone has suggested using the top of the 1/2 wave section as a mount for another antenna.

As designed the top of the 1/2 wave section is not attached to anything.

 

[Voyager]

Re: J-Pole Theory Explained - (Poorly)

Since the bottom of the "J" is nothing but a short across the two legs of the "Transmission Line" it is a null point and that is why it can be grounded of connected to additiona conductors without effecting the antenna.

But what about connecting a short to the top of the 1/2 wave?
(AKA the 'next' antenna up)

Joe M.

I don't think anyone has suggested using the top of the 1/2 wave section as a mount for another antenna.

As designed the top of the 1/2 wave section is not attached to anything.

OK. I see how he has them mounted now. I thought each 1/2 wave was supporting the antenna above it. That's not the case. My bad.

Joe M.

 

[N_Jay]

Section "D": Tuning section. So the location of "D" the connection across the two bottom 1/4 waves (transformer) is the location in which 50 Ohms is acheived on the driven side of the antenna? Any other location would alter the match between the coax and the antenna?
THANKS!

Yes, higher on the section would give a higher impeadance and lower would give a lower, but it is not linear.

Again Section "D": Would placing a 300(assumption) to 50(or 75) transformer at location "D" alleviate the inclusion of the decoupling hoops. And also reduce the design to a simple 1/2 wave end fed pole?
THANKS!

If you used a typical 300 to 50 Ohm transformer, it would need to be attached at the 300 Ohm point (higher), and if it were a true Balun then it would remove the need to decouple.

You still need the lower section to transform from the 300 Ohm to the 4000 ohm (or so I have read) that the end of the 1/2 wave end-fed anttena is.

Also the formula of 22 * 12 / Mhz, with 22*12 being the conversion to inches, what is 22? Where does this number come from? And then also 21 on the "gap" or "C" between poles. I mean it was easy enough to calculate the 2 waves but the gaps and tuning elude me.
THANKS!

I would guess that 22*12/MHz is length is inches where the 50 Ohm point is found. Why the numebr 22? Sh-t if I know.

Like I said, I hate reverse engineering antannas. Heck, I hate engineering antennas at all.

22 * 12/MHz = 264/MHz

Wavelength is inches is 39.36 * 300 / MHz = 11808/MHz

(264/MHz) / (11808/MHz) = 0.02236 wavelength. I guerss that is the 50 Ohm point. :roll:

As for the gap, it is not truly a tuning gap. It is actually the spacing between two conductors of a section of transmissing line. It is determined by the desired impeadance of the line, the diameter of the conductors and teh dielectric constant of the insulator.

If you were to dig up the formula for a open transmission line you could enter all the factors and get the chariteristic impeadance the designer was shooting for.

And the final thought on "D"; If the connection between the radio and antenna was 300 ohm twin lead, would any transformer be needed?

THANKS!

Yes as stated above. The antenna (1/2 wave end-fed) is still way abouve 300 Ohms

Hmm now to go do some reading on baluns.

THANKS!

Next semester dude. :wink:

 

[Pro-95]

Thanks N-Jay!

I final (really, well for now) question. So one could take an Ohm meter and read the resistance with one lead where SO-239 is and the other moving down the driven side?

Thanks again!

 

[N_Jay]

Thanks N-Jay!

I final (really, well for now) question. So one could take an Ohm meter and read the resistance with one lead where SO-239 is and the other moving down the driven side?

Thanks again!

No.

Impeadance, while measured in Ohms is significanly different than resistance.

Impeadance must be measured at a particular frequency or over a frequency range.

The impeadance at any point could (and most likly, would) be affected by any conductive (or dielectric) material in the near field. This would include any test probes.

 

[scanfan03]

what is the tunning connection? Is that how long you strip the wire and do you drill a hole in the 1/2 wave pole so you can soldier it to the pole or do you just soldier it to the outside?

 

[Pro-95]

what is the tunning connection? Is that how long you strip the wire and do you drill a hole in the 1/2 wave pole so you can soldier it to the pole or do you just soldier it to the outside?

The tuning connection ("D" on my illustration), is the point where 50 Ohms/ matching resistance is on the bottom of the driven element. I have seen several designs where they used stainless hose clamps to attach the center wire of the coax. I drilled a hole in the element and soldered the wire in place via a So-239 connector. It is very important that the center conductor not touch the shorter element as this will create a short and your antenna will not work. The length of the wire isn't important however the placement of the connection on the long element is. There are close up pictures in the other "Copper Cactus"/J-pole threads as well as a J-pole calculator on my web page.