Perplexing Arrow J-Pole tuning question...?

[Mikejo]

I built and arrow J-Pole using plans I found online....(you know the one, 3/8 aluminum rod, a 1" x 1" x 5 inch piece of aluminum angle etc.)

I shortened the elements for the Railroad frequencies (160.000 MHZ) range.

Connected up with 10 feet of RG58/U with SO239 Connector at the antenna, a BNC connector for my scanner.

(also made an in line Balun, about 1 foot back from the antenna connection, of about four turns around a 4 inch diameter object).

Ran it all through my antenna analyzer, and depending on how I moved it, I got readings for the best SWR of about 1:2 to 1 anywhere between 159.300mhz and 162.000 mhz

As I was trying to fine tune it to get a better swr, I began checking everything and discovered that my piece of coax had a little bit of the "black death" in it, so I decided to replace it.

I replaced it with an 18 foot piece of RG8x, it had two S0239, connectors, so I put a gender changer on the scanner end (SO 239 to BNC) also made an in line Balun (same as above).

So now, I run the whole mess through my analyzer, and I'm getting in SWR of 1:16 to 1 at around 132.300 MHZ. Wow!


Here is where all the problems began, no matter what I do, it stays between 132.300 MHZ and 137.000 MHZ. I have the long shaft made up of two pieces which I can separate, so I took one 2 foot piece off… Same results! I replace that, and remove the short piece (the 17.5" stub) completely and get roughly the same thing? I removed the short piece completely, and still get roughly the same thing? In other words, no matter what I do, I get readings within that same range.

I tested everything, looking for shorts in the new piece of RG8x, no shorts, it is good.

I connected to 50 ohm dummy load, and I get 50 - 52 ohms.

I checked the SO239 to BNC adapter for shorts, it's good.

Made sure all the connections are tight and making good contact... good!

I advanced the frequency into the 160.000MHZ range on the analyzer, and I get roughly 1:2 to 1.

So I'm perplexed, no matter what adjustment I make I can't get that antenna to show resonance anything above 137.000mhz on my analyzer.

Analyzer is a SureCom SA-250.

What could be the problem?

(Please don't just tell me that 1:2 to one is Great, I know it's ok,....I want it better...!)

I want to know also, why am I getting these weird results with the analyzer... (It works fine with other antenna's).

 

[belvdr]

Have you tried a different piece of coax or a different antenna?

 

[ko6jw_2]

First of all the Arrow design is direct fed. It does not require a balun. You can connect coax directly to the antenna.

Next question. Have you tested your coax? Are the connectors properly attached? It sounds like the antenna was more or less OK until you changed the cable.

Final point. SWR is a measure of how much power is reflected back from the antenna to the transmitter. A 1:1 SWR means (in theory) that all the power is being radiated from the antenna. Your receiver does not care. You are not transmitting (we hope). 1.2:1 is just fine. You will not notice any difference in reception. Really, you won't.

 

[SpugEddy]

First of all the Arrow design is direct fed. It does not require a balun. You can connect coax directly to the antenna.

Exactly what I was thinking. Not really sure if that would create
the results you are getting. I recently built the same antenna for
GMRS frequencies. All I have inline is a ferrite about 12"-16" below
the point of connection on the antenna. My SWR on 467.600 is just
about 1.1 - 1.2:1

 

[AK9R]

He's not referring to a balun in the traditional sense (converting a balanced transmission line to a unbalanced transmission line). He's talking about something more along the lines of a choke in the feedline. His words "also made an in line Balun, about 1 foot back from the antenna connection, of about four turns around a 4 inch diameter object".

 

[Ubbe]

....Ran it all through my antenna analyzer, and depending on how I moved it, I got readings for the best SWR of about 1:2 to 1 anywhere between 159.300mhz and 162.000 mhz....

Was it the analyzer with the coax that you moved? Seems like the coax are a major part of the antenna and it will then be very critical to how you route the coax. The design are probably not the best one and most experts agree that a standard j-pole design must be kept away from any nearby metal objects, including the bottom part of the antenna, and from a metal mast it is attached to. The Arrow design are probably a hit or miss affair depending of a lucky strike with the grounding and coax.

Try adding a foot or two of coax and see what happens.

/Ubbe

 

[Mikejo]

@Ubbe

To answer the question "Was it the analyzer with the coax that you moved?" the answer is yes!

I added 1 foot of coax, and my lowest swr went from 132.4100Mhz to 137.3000Mhz. What does that indicate?

Mike

 

[jackj]

It means that you've got an impedance glitch somewhere in the antenna system.

 

[nd5y]

It means you learned the hard way why many people don't like j-poles or slim-jims. The coax is part of the antenna. Your 4-turn choke probably isn't effective, especially since you said it was a foot away from the feedpoint.

 

[Mikejo]

....Your 4-turn choke probably isn't effective, especially since you said it was a foot away from the feedpoint.

And so, perhaps moving it closer to the feed point will help?

 

[zz0468]

I added 1 foot of coax, and my lowest swr went from 132.4100Mhz to 137.3000Mhz. What does that indicate?

It means the antenna has a mismatch problem.

Is the antenna connectorized? In other words, can you hook the analyzer up direct to the antenna? That is always the best place to put the instrument when measuring an antenna.

Barring the ability to do that, fabricate a cable that is a multiple of an electrical half-wavelength and use that to tune the antenna.

If the antenna has a problem, the 10 and 18 foot pieces of coax will alter the reading.

 

[chief21]

Also... remember that j-pole antennas, in general, are very sensitive to nearly anything in the near field of the antenna. How does the antenna behave if you mount it to a mast and take a reading from a fair distance away?

John AC4JK

 

[Ubbe]

...Barring the ability to do that, fabricate a cable that is a multiple of an electrical half-wavelength and use that to tune the antenna.....

That won't help one bit. It has to be a lenght that are between 0-1/4 wave to adjust for a mismatch. I have a 3 short lenghts of ruffly 1/16, a 1/8 and a 1/4 coax lenghts (@160MHz) that I use to check if they will change anything by adding them separately or in a combination.

/Ubbe

 

[Mikejo]

Well gentlemen, I decided to manufacture myself a dipole, connected up that same length of RG8x and got the same results! It does say it's 50ohm cable, there's got to be something else going on, perhaps I just got a bad piece of RG8x. I'm going to switch back to the old RG58u ( The one that's experiencing a little bit of the black death) , and perhaps I'll buy a new piece of that when I get a chance.

 

[majoco]

That so-called "J-pole" is not a J-pole at all, it's three quarter wave rods that get excited from the only one that's fed by the coax. How it ever works is a mystery to me.

 

[zz0468]

That won't help one bit. It has to be a lenght that are between 0-1/4 wave to adjust for a mismatch. I have a 3 short lenghts of ruffly 1/16, a 1/8 and a 1/4 coax lenghts (@160MHz) that I use to check if they will change anything by adding them separately or in a combination.

/Ubbe

The impedance of the load end is not altered at the source end of a half wave transmission line. If there is a .mismatch at the load, a random length of coax will transform the load impedance to something else, and render measurements meaningless.

 

[prcguy]

An antenna who's VSWR and resonant frequency are altered by connecting different lengths of coax is something to avoid. A stock store bought Arrow J pole also fits this description. I would make a different kind of antenna, something that can be easily duplicated with proven performance.

The impedance of the load end is not altered at the source end of a half wave transmission line. If there is a .mismatch at the load, a random length of coax will transform the load impedance to something else, and render measurements meaningless.

 

[zz0468]

I agree. I am merely suggesting how he can make a semi-accurate measurement, not mask an antenna problem.

An antenna who's VSWR and resonant frequency are altered by connecting different lengths of coax is something to avoid. A stock store bought Arrow J pole also fits this description. I would make a different kind of antenna, something that can be easily duplicated with proven performance.

 

[majoco]

Hence the old myth that you can tune a CB antenna by chopping bits off the coax feedline until you get down to the required VSWR......

 

[zz0468]

Hence the old myth that you can tune a CB antenna by chopping bits off the coax feedline until you get down to the required VSWR......

Yep. And in that case, the "antenna" starts at the source end of the coax, not the load end.