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Author Topic: How to build a J-Pole antenna ?  (Read 18392 times)
pinko
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« on: February 12, 2006, 10:12:13 PM »

Hi!

I need a J-Pole antenna to use with a Uniden 105XLT airband scanner. Where can I find a guide with the information to make one ?
I know there are a lot of guides but I would like to get one optimized for airband.

Thanks in advance for any help.  Cheesy
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Jason
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« Reply #1 on: February 12, 2006, 10:38:12 PM »

Quote from: pinko
Hi!

I need a J-Pole antenna to use with a Uniden 105XLT airband scanner. Where can I find a guide with the information to make one ?
I know there are a lot of guides but I would like to get one optimized for airband.

Thanks in advance for any help.  Cheesy


This is the link I used to build my copper J-pole:

http://larc.hamgate.net/J_Pole_2_meters.htm

I tuned it to these specifications:
Copper J-Pole

Frequency {MHz}         125
Total Length {in.}    70.810
To Notch {in.}           23.652
Stub {in.}               3.504
Spacing {in.}            3.504
Diameter {in}           0.584

Results on this page: http://www.liveatc.net/forum/viewtopic.php?t=651

Regards,
Jason
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Lezam
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« Reply #2 on: February 13, 2006, 10:10:36 AM »

Howd you mount it onto the roof?
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Jason
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« Reply #3 on: February 13, 2006, 03:20:03 PM »

Quote from: Lezam
Howd you mount it onto the roof?


Simple radioshack mounting brackets.

I think I have the 8" ones, but I can't remember.  I think they also come in 6" and 12."

Jason
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pinko
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« Reply #4 on: February 13, 2006, 09:14:52 PM »

Quote from: Jason


This is the link I used to build my copper J-pole:

http://larc.hamgate.net/J_Pole_2_meters.htm



Thanks a lot for your help Jason!

I'm from Europe, so I don't understand some technical terms, so can you please tell me what's the meaning of:

RTV ?
Braid ?
FLUX ?
Feed Point ? VSWR ?

BTW, which type of coax cable should I use ?

Thanks again!  Cheesy
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Jason
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« Reply #5 on: February 13, 2006, 09:44:54 PM »

Quote from: pinko
Quote from: Jason


This is the link I used to build my copper J-pole:

http://larc.hamgate.net/J_Pole_2_meters.htm



Thanks a lot for your help Jason!

I'm from Europe, so I don't understand some technical terms, so can you please tell me what's the meaning of:

RTV ?
Braid ?
FLUX ?
Feed Point ? VSWR ?

BTW, which type of coax cable should I use ?

Thanks again!  Cheesy


RTV - Silicon sealer to protect/seal connections between coax and antenna.

Braid - refers to the item used to connect the coax lead and shield to the J-pole.  Also see that a copper strap can be used.

Flux - flux paste is a material used to ensure good solder joints/connections.  It's usually in a vaseline consistancy

VSWR -  Voltage standing wave ratio (VSWR) in this case is most likely referring to the/a  VSWR meter which measures the standing wave ratio in a transmission line. This is a item of radio equipment used to check the goodness of the match between the antenna and the transmitter.  My Uniden BC780XLT has a built in VSWR meter (I think...Dave: feel free to correct me) which shows up as signal strength.  Only if you're *really* desperate to get a little better signal than you already are with the J-pole, you can tweak this until you become frustrated.  I adjusted mine very quickly and didn't find a major difference between small differences in locations.

Regarding coax - RG8/U is great coax, and what I use.  I haven't had much luck with RG58/U since the shielding isn't always great and varies from coax cable to coax cable.  Belden 9913 is also regarded as one of the best coax cables out there for the intended purpose.

Hope this helps,

Jason

P.S. I wouldn't totally recommend attempting this project unless you have/know of the materials to use and what is required to sucessfully build it (especially w/o hurting yourself in some shape, way, or form).
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sean
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« Reply #6 on: February 13, 2006, 10:01:47 PM »

Quote from: Jason

VSWR -  Voltage standing wave ratio (VSWR) in this case is most likely referring tothe/a  VSWR meter which measures the standing wave ratio in a transmission line. This is a item of radio equipment used to check the goodness of the match between the antenna and the transmitter.  My Uniden BC780XLT has a built in VSWR meter (I think...Dave: feel free to correct me) which shows up as signal strength.  Only if your *really* desperate to get a little better signal than you already are with the J-pole, you can tweak this until you become frustrated.  I adjusted mine very quickly and didn't find a major difference between different locations.


[Technical Drivel ON - Feel free to skip]

VSWR is essentially a measure of the ratio of power applied to the coax versus the amount of power returning from the antenna due to an imperfect match.  It's a rough measure of the quality of the match, but in commercial applications and strictly technical discussions is rarely used.  Return loss, which is measured by frequency domain reflectometry, is the more common measure, and is quantified in decibels.  FDR essentially transmits a very low power signal along a band of frequencies specified by  the operator, and then measures the level of the signal that returns.  By using an FDR you can see the performance of an antenna across a huge bandwidth, and never have to connect a transmitter to it.  You wouldn't want to connect a transmitter up and put 1000 watts into an antenna that had a VSWR of 3 or 4 to 1.  Ouch.  VSWR will also only measure at one frequency at a time - so if you want to check your antenna, you would have to transmit on every frequency throughout its tuned range, and look at the VSWR.  With FDR, you can sweep 200MHz in seconds, and see exactly where the antenna will perform poorly, or where it has problems.  Good FDRs are priceless.  You can see bad connectors, kinks in cable, water infiltration, you name it.  And you can tell how far up the cable it is.  Indespensible.

[Technical Drivel OFF]

As for the meter on scanners, it's just an S-Meter, or Signal Meter.  It shows a relative signal strength, adjusted to some arbitrary max value that varies between models and even different units.  That is to say my 780 next to Jasons may show 4 bars, and his may show 5.  It just gives you a rough indication of how strong a signal is, but I wouldn't rely on it for more than curiousity reasons.  It's not an indication of the quality of your antenna.  A really strong signal can conceivably peg an S-Meter, even though your antenna is crap.  

Sean
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pinko
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« Reply #7 on: February 14, 2006, 08:53:47 AM »

Thanks again Jason.
Just another question, you said that the total lenght is 70.81. Is it the lenght of the copper pipe from the copper T to the top or all the copper pipe, from the bottom to the top ?
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Jason
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« Reply #8 on: February 14, 2006, 04:08:10 PM »

Quote from: pinko
Thanks again Jason.
Just another question, you said that the total lenght is 70.81. Is it the lenght of the copper pipe from the copper T to the top or all the copper pipe, from the bottom to the top ?


That total length refers to the middle of the copper T to the top.  The 23.652 refers to the bottom of the copper L to the top of the plastic cap on the "J" portion of the antenna.

Below the middle of the copper T can be any length (allows you to build in a long mast, but keep in mind stability.  The longer the antenna mast, the more support you'll need).

Jason

P.S. Attached is a .jpg file of the document I used as reference when I built my J-pole.  It may be of some interest to you.
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Jason
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« Reply #9 on: February 14, 2006, 09:08:54 PM »

Here are the demensions for the 125MHz frequency range for both roll-up and copper J-poles

Jason
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Fryy
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« Reply #10 on: February 17, 2006, 01:33:22 AM »

Any good links for building your own discone? Would you suggest a discone over a j-pole?
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