Author Topic: Man my J pole mojo is very low. Two fails.  (Read 1041 times)

Offline kid_couteau

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Man my J pole mojo is very low. Two fails.
« on: January 16, 2018, 07:58:14 AM »
Hi All

I tried to use some 450 ohm ladder line to make a J pole antenna.

First one I had the SWR down to a 1:1.1 or so on low power and 1.75 on high. BEAUTIFUL.
The second I soldered the connections the SWR went up the flue.

Any ideas guys?
Kid

Offline Smurf Hunter

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Re: Man my J pole mojo is very low. Two fails.
« Reply #1 on: January 16, 2018, 10:24:18 AM »
Hi All

I tried to use some 450 ohm ladder line to make a J pole antenna.

First one I had the SWR down to a 1:1.1 or so on low power and 1.75 on high. BEAUTIFUL.
The second I soldered the connections the SWR went up the flue.

Any ideas guys?
Kid

Photos or it didn't happen ;)

Also, your SWR should not change with added power.  Are there nearby objects that are affecting it?

Lastly, when the SWR went up, was everything else exactly the same, or did you also move the antenna, change radios, your body, etc?
Stuff happens.  I can move a magnet mount to 4 places on my car roof and SWR will for from 1:1 to 1:2.  Nothing to cry over, but I try to make it as good as is feasible.

Offline kid_couteau

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Re: Man my J pole mojo is very low. Two fails.
« Reply #2 on: January 16, 2018, 11:55:04 AM »
Only things to change

After checking SWR and getting good results I soldered the feedline to the wires in the same spot. They did not move.

Then I put on heat shrink tubing and added a piece of paracord to hang it with. I punched 3 holes in the ladder line to put the rope through.

Only thing I can figure is I did have quite a bit of solder on the joint.

Offline Carl

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Re: Man my J pole mojo is very low. Two fails.
« Reply #3 on: January 16, 2018, 12:36:20 PM »
What are the long side and short side measurements (length) and how far up from the bottom shunt is the coax connected?

If similar to my big stick antenna the long to short ratio is EXACTLY 3 to 1 with a length from the point the coax is split from center to shield conductor length (coax and shunt ) 4 1/4 to 4 1/2 inches.

https://drive.google.com/open?id=0B5ZIZLZV4AwIRHFZbm1CZHh3Mkk

Offline kid_couteau

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Re: Man my J pole mojo is very low. Two fails.
« Reply #4 on: January 16, 2018, 12:51:38 PM »
http://www.w4ava.org/2mjpole.pdf

I used these instructions

After initial cut to 39.5 inches I probably cut off about an inch altogether to tune it to 146.00

Offline Carl

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Re: Man my J pole mojo is very low. Two fails.
« Reply #5 on: January 16, 2018, 02:26:31 PM »
  The longest side is connected to the coax center and about 59 inches long (NOT 39)
The short side is connected to the coax shield and is 19 1/2 inches or so long.
Hang it in the clear ,at least 3 feet from any metal (to the sides) and it should be OK.
Possibly you shorted the coax by melting the center of a 'hair' of shield is contacting the center of coax.
These are fairly foolproof. I would also make sure the bottom shunt is well soldered and making contact.

Offline kid_couteau

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Re: Man my J pole mojo is very low. Two fails.
« Reply #6 on: January 16, 2018, 02:38:10 PM »
Hi
I meant to say the upper part above the short part was 39
Overall it was 59 until I started trimming it.

Im not sure but I might have had the coax center on the short lead. I thought it was like a dipole and didnt matter.
So much to learn.

Ill try again.

Offline Smurf Hunter

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Re: Man my J pole mojo is very low. Two fails.
« Reply #7 on: January 16, 2018, 03:17:48 PM »
Hi
I meant to say the upper part above the short part was 39
Overall it was 59 until I started trimming it.

Im not sure but I might have had the coax center on the short lead. I thought it was like a dipole and didnt matter.
So much to learn.

Ill try again.

It ain't a di-pole, it's a j-pole ;)

Offline Carl

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Re: Man my J pole mojo is very low. Two fails.
« Reply #8 on: January 16, 2018, 05:55:38 PM »
  Have you a 'system' of SWR testing antennas ? As in test SWR at 145 MHZ and 147 MHZ ,if SWR is higher at 145,then antenna is too short, If higher at 147 the antenna is too long...When you cut to length..you should cut 1/8" from the short side and 3/8" from the long side...the antenna should stay very close to a 3 to 1 ratio in length from measuring full length of long vs short elements from the shunt at bottom of antenna.
« Last Edit: January 16, 2018, 06:01:04 PM by Carl »

Offline kid_couteau

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Re: Man my J pole mojo is very low. Two fails.
« Reply #9 on: January 16, 2018, 07:31:07 PM »
Funny you should mention that

I first tuned my new one tonight at 144.00
Got it close to that and slowly crept up to my goal of 146.00 with a close to perfect at 146.730 [our repeater]

This new one SWR is less than 1.5 and close to 1:1 at the repeater freq.

Oddly enough I think it was the heat shrink tubing I was using cause every time I put some on the SWR rose. Is that plausible????????

Offline Carl

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Re: Man my J pole mojo is very low. Two fails.
« Reply #10 on: January 16, 2018, 08:01:09 PM »
 It can happen as the Schedule 40 PVC I use for the Big Stick does conduct and causes the 59" length to be 53 3/8"...
and the 18  1/2" to be 17  3/8"