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Author Topic: Changed Coax - Antenna Changed  (Read 1430 times)
KJ6TSX
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Posts: 116




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« on: April 07, 2012, 04:15:27 PM »

I upgraded my coax from RG-8 to RG-213 and the antenna became resident at 3.775MH instead of the previous 3.900MH??
The only other difference is that the first coax was 110 feet long the the new coax is 125 feet long. Just trying to decide what this is telling me?
Did the coax length change the residence or the coax type? Is this a improvement or not?
Of course the easy fix is to shorten the dipole to get it into range??

Any thoughts on this would be greatly appreciated
Thanks
George
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WB6BYU
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Posts: 13112




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« Reply #1 on: April 07, 2012, 06:34:37 PM »

This is a very common occurrence if you are not using an effective balun.

In that case the coax shield is part of the antenna in addition to the two dipole wires
(along with everything connected to it in the shack.)  So in changing the length of the
coax you changed the antenna.
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KJ6TSX
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Posts: 116




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« Reply #2 on: April 07, 2012, 06:37:56 PM »

I am using a 1:1 balun. what would be the best solution?
also this antenna is 8 feet off the ground
George
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WB6BYU
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Posts: 13112




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« Reply #3 on: April 07, 2012, 07:37:59 PM »

How are you measuring where the antenna is resonant?

Did the SWR change also?
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KJ6TSX
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Posts: 116




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« Reply #4 on: April 07, 2012, 08:43:01 PM »

With transmitter output at about 5watts changing frequency in till 1:1 SWR, using MFJ antenna tuner (in bypass mode)
Thanks
George
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WB6BYU
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Posts: 13112




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« Reply #5 on: April 07, 2012, 09:35:10 PM »

OK, that's the best way to do it.  You don't always get a 1 : 1 SWR, but at least you can
find the point where the SWR is minimum.

It might be that your balun doesn't have enough choking inductance on 80m.  Or perhaps
the coax is running close to one of the elements  on its way to your shack?  Either one
of those would allow common mode currents on the outside of the coax in spite of the
use of the balun.
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KJ6TSX
Member

Posts: 116




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« Reply #6 on: April 07, 2012, 09:56:25 PM »

WOW!
I think you nailed it, This is a dipole without the poles <GRIN> the wire is nailed to the fence and the coax runs about 6 feet below one of the elements. Now that we know whats happening whats the best way to fix this? Shorten the elements??
Thanks
George
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W8GP
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Posts: 196




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« Reply #7 on: April 08, 2012, 08:25:15 AM »

The real issue here is the height of the antenna.At that height, moving the antenna up or down even a few feet will have a profound effect on SWR.Changing the positioning of the feedline could have similar effect. With that in mind you can try raising the antenna and track the effect on SWR. If you must keep the antenna at it's present height, then changing (shortening in your case)the length of the wires is the way to go.
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KJ6TSX
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Posts: 116




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« Reply #8 on: April 08, 2012, 09:18:04 AM »

Thanks Everyone for the help, looks like I got a lesson in inductance <GRIN>
Thanks
George
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WB6BYU
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« Reply #9 on: April 08, 2012, 09:42:46 AM »

Quote from: KJ6TSX

... whats the best way to fix this? Shorten the elements??



Fold back the ends of the wires.  You might try just adjusting the leg with the coax
running under it.  Or revert to your old piece of coax.

A balun reducing the common mode currents due to the shield being connected to one
side of the dipole, but doesn't help prevent coupling between two wires running
parallel to each other.  For example, with a 2-element yagi there is enough coupling
between the elements to create the pattern, and they are much further apart (in
wavelengths) than your coax is from one leg of your dipole.

This is why it is recommended to run the coax off at right angles to the antenna
elements for a quarter wavelength or so, to minimize the coupling between them.

But we don't always live in a perfect world, and we sometimes have to improvise to
fit our situation rather than making the situation match the text books.  Just retune
your antenna and get on with making contacts.
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