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Author Topic: dipoles  (Read 1863 times)
WA9LEO
Member

Posts: 6




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« on: July 28, 2012, 07:17:14 PM »

i have 2 different dipoles that i use. one is a windom  6-40 dipole. last week i took a close fhit and my 6-40, puked and i also lost my ldg tuner.  the dipole operates on 20 very nicely with a swr of 1.3 and about 80 watts output on ssb.  today i made a new 20 meter dipole, hung it, swr  is 1.4 across the main operating range, however i only have about 30 watts output on ssb, this is ruining my thinking pattern andf irritating me. can somebody explain what i did wrong? wa9leo
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KJ4ADN
Member

Posts: 39




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« Reply #1 on: July 29, 2012, 06:52:25 AM »

last week i took a close fhit and my 6-40, puked and i also lost my ldg tuner.

If you meant LIGHTNING - look at the coax.
Put a known good 50ohm resistor on the end of the coax and check it.  (or you can buy a very expensive antenna analyzer and find the exact spot it's bad.)  If it's something other than 1:1, replace it...  don't cut out the bad spot, replace it.

One of the girls in our club took a near strike on her Ringo-Ranger, it shortened the antenna a few inches and blew a tiny hole in the coax.  We metered the coax, found the hole and the insulation had what looked like the center conductor had nearly vaporized in one spot.  It looked OK with a VOM, but the antenna analyzer said it was bad.  It was not hooked up to the radio at the time of the strike, she had the end in a glass jar...?

We replaced the coax, cleaned up & retuned the antenna, she's back on the air.

KJ4ADN
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K7KBN
Member

Posts: 2766




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« Reply #2 on: July 29, 2012, 10:23:22 AM »

last week i took a close fhit and my 6-40, puked and i also lost my ldg tuner.

If you meant LIGHTNING - look at the coax.
Put a known good 50ohm resistor on the end of the coax and check it.  (or you can buy a very expensive antenna analyzer and find the exact spot it's bad.)  If it's something other than 1:1, replace it...  don't cut out the bad spot, replace it.

One of the girls in our club took a near strike on her Ringo-Ranger, it shortened the antenna a few inches and blew a tiny hole in the coax.  We metered the coax, found the hole and the insulation had what looked like the center conductor had nearly vaporized in one spot.  It looked OK with a VOM, but the antenna analyzer said it was bad.  It was not hooked up to the radio at the time of the strike, she had the end in a glass jar...?

We replaced the coax, cleaned up & retuned the antenna, she's back on the air.

KJ4ADN

She's lucky.  The lightning had just traveled hundreds, maybe thousands, of feet through air.  What makes her or anyone else think that maybe 1/4 inch of glass is going to stop anything.  Fortunately, the charge found an easier way to ground, through the spot in the coax.
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73
Pat K7KBN
CWO4 USNR Ret.
AE5JU
Member

Posts: 223




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« Reply #3 on: July 29, 2012, 09:24:55 PM »

"what makes her or anyone else think that maybe 1/4 inch of glass is going to stop anything."

No, but I've seen an unplugged coax sparking across the PL-259.  Sticking it in a glass jar will hopefully stop a spark jumping anywhere else and setting your house on fire.

Check this out!

http://www.youtube.com/watch?v=hlSpZ-ffacA

This from "snow static."

http://www.youtube.com/watch?v=_7HSTPl6e-I

Where's that mason jar?

73,
Paul - AE5JU
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W5DXP
Member

Posts: 3546


WWW

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« Reply #4 on: July 30, 2012, 03:16:43 AM »

I've seen an unplugged coax sparking across the PL-259.

So have I - although it was under dust-storm conditions in the AZ desert and it sounded like a machine gun. Some hams claim that precipitation static doesn't exist but the US government recognizes it:

http://www.its.bldrdoc.gov/fs-1037/dir-028/_4096.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
STAYVERTICAL
Member

Posts: 854




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« Reply #5 on: July 30, 2012, 04:26:40 AM »

When I was younger, a lightning storm was happening with cloud to cloud discharges above us.
I had unplugged the coax and noticed a clicking sound periodically.
It turned out that my high dipole was acting as the secondary of a transformer with the primary in the clouds, with discharges across the PL259.

I threw the coax out the window where it swung up and rested about 2 feet off the ground.
The curious dog decided to use her nose to see what this new thing hanging in the yard was.

The dog survived.

73 - Rob
 
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K7KBN
Member

Posts: 2766




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« Reply #6 on: July 30, 2012, 03:21:32 PM »

"what makes her or anyone else think that maybe 1/4 inch of glass is going to stop anything."

No, but I've seen an unplugged coax sparking across the PL-259.  Sticking it in a glass jar will hopefully stop a spark jumping anywhere else and setting your house on fire.

Check this out!

http://www.youtube.com/watch?v=hlSpZ-ffacA

This from "snow static."

http://www.youtube.com/watch?v=_7HSTPl6e-I

Where's that mason jar?

73,
Paul - AE5JU

I've seen the results of what was certainly a closer strike than what the young lady experienced.  The Mason jar exploded and there were little slivers of glass evenly distributed throughout the room.  Disconnect the antenna OUTSIDE the house to keep the lightning out there.

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73
Pat K7KBN
CWO4 USNR Ret.
WX7G
Member

Posts: 5920




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« Reply #7 on: July 31, 2012, 06:51:35 AM »

Nearby lightning strike destroys antenna.
Radio now outputs only 30 watts.

Radio was damaged by the nearby lightning strike.
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WA9LEO
Member

Posts: 6




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« Reply #8 on: August 01, 2012, 06:53:02 PM »

i did check out the coax, and found a open spot, where the outer jacket and shield, were damaged, so, now its time to replace the coax and antenna, the 20 meter dipole has been redone, i found that i had mismeasured the wire, one was 6 inches longer then the other leg.   Angry ::)apparently reading a tape measure is not in my field of experience.  thanks for all the help  wa9leo
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KB5UBI
Member

Posts: 97




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« Reply #9 on: August 03, 2012, 04:59:36 PM »

Years ago, I had a center fed vertical dipole and I always disconnected the rig when I was not on the radio. The combination of low humidity and a mild wind on clear days was enough to draw an arc across the PL-259 every ten seconds or so.
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