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Author Topic: Antenna Feed  (Read 222 times)
KC0TAS
Member

Posts: 14




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« on: March 11, 2008, 05:57:56 PM »

Heloo all,

I have a question about feeding a dipole/inverted vee.  I have lots of RG-58 (with a foil sheet between the outer casing and the sheild braid).  A friend of mine thought that he saw an article many yaers ago aobut using coax to feed the antenna.  Basically cutting two lengths exactly the same and using them similar to open wire to feed the antenna.

Does anyone know anything about this?  Is there a "magical" length or a "don't use" length?  And exactly how would I connect them to an automatic tuner with a hot and ground side.

Thanks for the help and or directions.

Also my call is NJ0E, but the site managers haven't changed it yet.

Joe Carney
NJ0E
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W5FYI
Member

Posts: 1044




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« Reply #1 on: March 11, 2008, 06:32:09 PM »

Shielded parallel lines have been used from time to time, primarily where nearby metallic structures could unbalance ordinary unshielded lines. Typically the shields are connected together at both ends, but grounded only at the transmitter end. Impedance is twice the single coax impedance (e.g. 100 ohms for your paired RG58), and losses are said to be higher than unshielded parallel lines, but noise pick-up is greatly reduced. If length is critical, e.g. for making a quarter-wave matching sections, you'd have to test for velocity factor, but that is relatively easy. Since the line is balanced, you might want to use a balanced tuner or a balun.

FYI, Stew
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W8JI
Member

Posts: 9304


WWW

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« Reply #2 on: March 12, 2008, 12:08:08 AM »

Joe,

While that forms a balanced line of sorts, it is just as subject to common mode coupling as any other line.

Also it has the same loss as any coaxial line. It does NOT have lower loss, or replace ladder lines.

It is simply good for passing a SHORT length of line through a wall or other tight spot.

Keep that all in mind!!!

It is a solution for getting through a tight or difficult area with a balanced line, nothing more.

73 Tom
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WA3SKN
Member

Posts: 5458




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« Reply #3 on: March 12, 2008, 05:00:55 AM »

Well Joe, you can do it... but there will be coax type losses.  Why not just use the 450 Ohm open-wire line?  What band(s) are you trying to operate on?

-Mike.
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W3LK
Member

Posts: 5644




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« Reply #4 on: March 12, 2008, 10:46:02 AM »

<< Also my call is NJ0E, but the site managers haven't changed it yet. >>

It's your responsibility to change it, not the forum managers.

Go to the left side of the page to SITE INFO and click on EHAM HELP (FAQ).

Click on the first selection under MY EHAM,

then click on "I have a new callsign. How do I change it on eHam.net?"

The call sign change takes effect immediately after you log in under the new call sign.

73,

Lon - W3LK
Naugatuck, Connecticut
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