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Author Topic: Dipole help  (Read 1075 times)
KD2BHY
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Posts: 20




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« on: July 28, 2012, 03:56:23 PM »

Hello, I need help with home brewing dipoles. A couple of weeks ago I decided I wanted to homebrew a dipole so on my summer vacation I could have hf antennas to throw up in some trees and see who I could work portable. So I built a 10 meter dipole. I cut to exact lengths and added the insulators on each ends. So I strung it up in my room and measured the swr. It was 3.1 +. I don't know why the swr is so high. I don't want to build one for 20 meters until I find out what I didb wrong with this one.

       Thanks so much and 73 s KD2BHY
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AD5ZC
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Posts: 69




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« Reply #1 on: July 28, 2012, 04:04:13 PM »

what is the swr @ 28.003?
what is the swr @ 29.697?
what formula did you use?
what kind of coax are you using?
what are you using to measure swr?
are you sure you made a good connection to the coax?

just read a little better....  could just be RF in the room with you.
take it outside and string it up somewhere where you can get a little distance and height above ground and check again.

Then:
what is swr @ 28.001?
etc. etc.
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W9GB
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Posts: 2616




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« Reply #2 on: July 28, 2012, 04:13:30 PM »

Quote
So I strung it up in my room and measured the swr. It was 3.1 +. I don't know why the swr is so high.
Not a useful testing environment.

GO OUTSIDE, to a public park -- string this dipole up and then take those measurements.

w9gb
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WB6BYU
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Posts: 13149




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« Reply #3 on: July 28, 2012, 05:00:41 PM »

I agree - you won't get a good SWR in your room.  (I've tried it.)

Put it up outside, at least 10 feet off the ground.  Toss a rope over a tree branch
for temporary use.  If you tie one end your house, use enough rope to get the end
of the wire a few feet away from the building.

Do understand that the "standard" formulas are just estimates.  The actual length
for resonance is a function of the wire diameter, insulation type, height above ground,
any wire or other conductors in the vicinity, and even how you tie the wire to the
insulators, so I always plan on some adjustment.  A common method is to cut the
antenna about 5% longer than the formula suggests and snip the end of the wire (or
fold it back on itself) a bit at a time until the SWR minimum is where you want it.
I do this by tying the insulators and support ropes a foot or two in from the end of
the wire, which allows me to trim the hanging tails without needing to retie things.
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KJ4ADN
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Posts: 39




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« Reply #4 on: July 29, 2012, 07:02:19 AM »

....
what kind of coax are you using?
what are you using to measure swr?
are you sure you made a good connection to the coax?
...

I got a 500' roll of 1/2 price coax at a HAMfest a couple of years ago, the shielding was so terrible, at 40m it was leaking RF, 6m was completely unusable.   At the higher frequencies (10m), the coax plays a bigger role than, say 75m.   This stuff wasn't 100% shield, and we absolutely could not tune a 6m mobile antenna with it.  6-8' in length!   When we switched to 100% shielded coax, the difference was immediate.

KJ4ADN - Bill
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K5KNE
Member

Posts: 65




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« Reply #5 on: August 01, 2012, 08:16:00 AM »

Look on YouTube for "dipole antennas".  There are videos that explain them.

Walter  K5KNE
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K5KNE
Member

Posts: 65




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« Reply #6 on: August 01, 2012, 08:30:50 AM »

An MFJ-259B or similar antenna analyzer is what you need in order to know what your  antennas are doing. It is much easier to use than trying different frequencies and checking SWR. If you know someone who has one maybe you could have him come over and check your antenna or loan it to you.  It is simple to operate and tells you the frequency where your antenna is resonant. Then you can shorten or lengthen the antenna to get it on the frequency you want.

Good Luck.

Walter  K5KNE
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KD2BHY
Member

Posts: 20




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« Reply #7 on: August 01, 2012, 05:39:49 PM »

Thanks everybody. I got the swr to 1:1. Having fun on 10 meters now!

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