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Author Topic: building an antenna  (Read 373 times)
KK7GB
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Posts: 32




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« on: January 16, 2009, 07:13:10 PM »

I need to build an antenna for 159.090 Mhz. I think a J-Pole would best suit my needs as it needs to be transportable in a footlocker of sorts. A ground plane would be more suspectible to breakage or bending. It also needs to be rapidly deployable. I plan on securing it to a pushup mast when I need it.

Can anyone suggest the equation for determining the lengths of the tubes?

Thx

Glenn KE7JHH
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WB2WIK
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Posts: 20666




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« Reply #1 on: January 16, 2009, 07:20:05 PM »

Sure.  4'5" for the longer one, 1'5-1/2" for the shorter one.

J-poles are not very good antennas.   This is a "ham fiasco" that you almost never see in commercial service because they simply don't work as well as most other designs occupying similar or less space.

They also waste vertical space that does not contribute to radiation, which is probably why you don't see them used in commercial service.

WB2WIK/6
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N5LRZ
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Posts: 0




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« Reply #2 on: January 17, 2009, 03:55:01 AM »

But back to the original post  WIK is absolutely correct though it is indirect.  The J pole is not the antenna for your situation for the simple reason that it is a 3/4 lengh long or about 60 inches long.  You would have to figure out means to take the antenna appart for travel.  You could make yourself one of those 300 ohm twin lead J poles.  I have tried them before but I have had no luck in getting the SWR down low enough for my comfort level.  PERHAPS the twin lead J Pole will work for you where it did not for me.  

A two element yagi type beam for VHF is easy to build.  AND IF you use a construction method that allows you to screw and unscrew the elements you can quickly take appart a simple 2 element beam that will as a bonus give you a utility gain as well as directivity.

An alternative antenna is the simple quarter wave ground plane anenna that disassembles for ease of transport.  The advantage of the ground plane is that it takes up far less space in transport--no boom to worry about.

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PULLRAFT
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Posts: 1




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« Reply #3 on: January 17, 2009, 05:13:23 AM »

What is going on, on 159.090 megahertz?

And why the need for "rapid deployment" from a footlocker?
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VR2AX
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Posts: 595




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« Reply #4 on: January 17, 2009, 08:49:52 AM »

What about one of the hand held 2 metres 2-ele  You just scale the elements down 144/159 approximately.

They are pretty robust. I have a Negara Denshi DO-11, 2/70cm, it can be deployed in a couple of minutes.
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KD4LLA
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Posts: 465




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« Reply #5 on: January 17, 2009, 09:31:26 AM »

Radio Shack used to sell a ground plane scanner antenna that also is good for VHF transmitting, cost, about $20.  The one I am using now is over fifteen years old.  It is smaller and lighter than a J-pole.  MFJ also has a 2 meter portable beam that might work for you (yes, I have one of those too).  The MFJ beam has some adjustment to it.

Mike
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KK7GB
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Posts: 32




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« Reply #6 on: January 17, 2009, 04:49:20 PM »

Thanks to everybody who reaponded. It appears that a J-Pole is not the best solution for my needs.

How would I go about finding out the bandwidth of either the yagi or the Radio Shack ground plane.

Thank you again for all the suggestions!

Glenn KE7JHH
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WB6BYU
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Posts: 13574




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« Reply #7 on: January 18, 2009, 07:40:44 PM »

Bandwidth for most ground planes will be sufficient for your needs.
Trim them down for minimum SWR at your operating frequency.

I make my ground planes using #12 copper wire for the radials and
 brass brazing rod for the vertical whip.  One possible set of dimensions
(as developed years ago by W6BCX) would be 17" for the vertical whip
and 21" for the radials.  Slope the radials down about 45 degrees and
adjust for best SWR.  The SWR bandwidth is about 17 MHz, so it will
tolerate a lot of detuning and still work OK.   Since the copper wire
radials are flexible I just fold them straight for storage, and you can
fold them up around the vertical part if you need to fit it in a small
space.  Performance is virtually the same as a J-pole in theory, and
often better in practice because it is more immune to construction problems.
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