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is a balun necessary

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mdnanwelch7(at)hotmail.co
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Posted: Fri Apr 16, 2010 5:25 am Post subject: is a balun necessary

Guys,

  

   I'm building a 1/4 wave com antenna and have most of it completed, except for adding a balun.  

   Can someone answer this question for me; do I need a balun, or more precisely, what is it about an antenna that tells you you should install one?

   If I need one, the time to add it is now!!  Easy now, not so easy later!  

  

   My antenna has (4) 1/2" wide copper foil strips, soldered to form an "X" (plus enough copper strips soldered to form a 2" x 2" sq. plate).  Mounted in the center of the X is a 1/8" steel mast (welding rod).  The coax shield solders directly to the groundplane, and the coax's center conductor is fastened under a little screw that secures the mast.

  

   Upon initial construction, the lengths of all the components are extra long.  The copper radials are 24" at the moment.  The mast will start off at 23".  Does an anyone have an opinion as to the lengths I should trim these parts down to?  

   I DO have an SWR meter that I will be using to check for standing waves.

  

   One more quick question regarding the antenna's components lengths.  Where is it that the length supposedly begins?  If you have a 2" x 2" square copper plate, and each of the 4 copper tape radials are soldered to the plate, where is the radials length considered to begin?  Is it from the very center of the 2" x 2" plate, or is it from the point where the tape leaves the edge of the plate...and actually begins being just copper foil arm?

  

   Same for the mast.  If I take a 1/8th" solid steel rod, and form a loop on one end (to be able to mount it to a small plexiglass block), where is the zero point on the rod?  Is it the point where the coax fastens to the mounting bolt, the end of the coax where it parts from the sheilding, etc?

  

   I know for a com radio we're shooting for 22"-23" roughly, for the antenna's lengths.  I'm just a little lost on where those lengths actually begin.  Plus, what if the radials are a little extra long?  Does that hurt reception?

  

 Thanks for your help,

  

 Mike Welch

 		 	   		  The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. Get busy.  [quote][b]

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sportav8r(at)gmail.com
Guest

Posted: Fri Apr 16, 2010 6:03 am Post subject: is a balun necessary

I'll take a stab at it.  Conventional wisdom is to make the radials 3-5% longer than the radiating element (mast).  For optimum impedance matching of antenna to coax, the angle between the mast and radials should be about 120 degrees.  A 90 degree angle is going to produce a lower impedance, closer to 36 ohms, and a resulting higher VSWR at resonance.  In other words, no matter how precisely you trim everything, hunting for lowest SWR, it won't reach 1:1 unless the antenna impedance at resonance is 52 ohms, just like the coax.  I don't know the exact math on this, but I'll eyeball a guess that 1.3:1 - 1.5:1 is the best you'll see with radials at right angles to the mast.  NOT likely to matter in practice, except that the air band spans a wide % frequency range, and an antenna broadband enough to cover 118-136 MHz needs to have as low as possible a resonant (center-frequency) SWR in order not to have a high SWR at the band edges.  You want less than 2:5:1 at the limits to keep the transmitter happy and prevent power output fold-back from the radio's protective circuits. On receive, you'll never hear the difference. 



A balun will make no practical difference in the operation of a quarter-wave vertical whip antenna such as you are building.  I twill add weight, cost, complexity and exact a small" efficiency surcharge."  I'd strongly recommend leaving it out.

 



Make your measurements from the point where the conductors leave the coax shield.  The radius of the copper plate counts as part of the radial length. Any center conductor beyond the coax braid counts as part of the mast length.  In practice, the dimensions matter little except as a starting point for trimming with the SWR meter.

 



Enjoy the exercise.

Bill Boyd



On Fri, Apr 16, 2010 at 9:16 AM, Mike Welch <mdnanwelch7(at)hotmail.com (mdnanwelch7(at)hotmail.com)> wrote:

 [quote]     Guys,

  

   I'm building a 1/4 wave com antenna and have most of it completed, except for adding a balun.  

   Can someone answer this question for me; do I need a balun, or more precisely, what is it about an antenna that tells you you should install one?

   If I need one, the time to add it is now!!  Easy now, not so easy later!  

  

   My antenna has (4) 1/2" wide copper foil strips, soldered to form an "X" (plus enough copper strips soldered to form a 2" x 2" sq. plate).  Mounted in the center of the X is a 1/8" steel mast (welding rod).  The coax shield solders directly to the groundplane, and the coax's center conductor is fastened under a little screw that secures the mast.

   

   Upon initial construction, the lengths of all the components are extra long.  The copper radials are 24" at the moment.  The mast will start off at 23".  Does an anyone have an opinion as to the lengths I should trim these parts down to?  

    I DO have an SWR meter that I will be using to check for standing waves.

  

   One more quick question regarding the antenna's components lengths.  Where is it that the length supposedly begins?  If you have a 2" x 2" square copper plate, and each of the 4 copper tape radials are soldered to the plate, where is the radials length considered to begin?  Is it from the very center of the 2" x 2" plate, or is it from the point where the tape leaves the edge of the plate...and actually begins being just copper foil arm?

   

   Same for the mast.  If I take a 1/8th" solid steel rod, and form a loop on one end (to be able to mount it to a small plexiglass block), where is the zero point on the rod?  Is it the point where the coax fastens to the mounting bolt, the end of the coax where it parts from the sheilding, etc?

   

   I know for a com radio we're shooting for 22"-23" roughly, for the antenna's lengths.  I'm just a little lost on where those lengths actually begin.  Plus, what if the radials are a little extra long?  Does that hurt reception?

   

 Thanks for your help,

  

 Mike Welch

 		 	   		  

The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. Get busy.   	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution



	


[b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Apr 16, 2010 6:17 am Post subject: is a balun necessary

At 08:16 AM 4/16/2010, you wrote:

 Guys,

  

 I'm building a 1/4 wave com antenna and have most of it completed, except for adding a balun.

 Can someone answer this question for me; do I need a balun, or more precisely, what is it about an antenna that tells you you should install one?



    The chapter on antennas and feedlines in the 'Connection

    explains this. A 1/4-wave, bottom feed whip antenna

    is an UNbalanced antenna and does not need a balun

    to accept connection to a coaxial feed line.



 My antenna has (4) 1/2" wide copper foil strips, soldered to form an "X" (plus enough copper strips soldered to form a 2" x 2" sq. plate).  Mounted in the center of the X is a 1/8" steel mast (welding rod).  The coax shield solders directly to the groundplane, and the coax's center conductor is fastened under a little screw that secures the mast.



    How did you insulate the mast from the ground plate?

    How is the mast attached to the rest of the assembly?

    

 Upon initial construction, the lengths of all the components are extra long.  The copper radials are 24" at the moment.  The mast will start off at 23".  Does an anyone have an opinion as to the lengths I should trim these parts down to?  

 I DO have an SWR meter that I will be using to check for standing waves.



    Suggest you cut the radials and antenna to 22".  Use

    the SWR meter at the transceiver to assure that SWR

    is 3:1 or less over 118 to 135 mHz.

  

 One more quick question regarding the antenna's components lengths.  Where is it that the length supposedly begins?  If you have a 2" x 2" square copper plate, and each of the 4 copper tape radials are soldered to the plate, where is the radials length considered to begin?  Is it from the very center of the 2" x 2" plate, or is it from the point where the tape leaves the edge of the plate...and actually begins being just copper foil arm?



   At the center.

  

 Same for the mast.  If I take a 1/8th" solid steel rod, and form a loop on one end (to be able to mount it to a small plexiglass block),



   Hmmmm . . . What kind of airplane are we talking about here?

   Plexiglas is not particularly "structural" and a single

   fastener through a loop on the end of the antenna

   rod is not terribly resistant to rotation and loosening.



   Antennas tend to be ignored until AFTER the radio quits

   working. The design you describe is not particularly robust.

   Have you considered using a CB antenna adapter like I linked in a

   posting earlier this week? 



  http://tinyurl.com/y3a4lpf 



   You can drill a hole in a 3/8-24 bolt to accept your antenna rod

   and silver-solder the rod to the bolt. The "booger" the treads of

   the bolt and lube it with grease before you screw it into

   the top fitting of the adapter. This makes a nice, gas-tight

   electrical and robust mechanical fit between the adapter and

   the antenna rod.



   The other end is fitted with a common coax connector which

   is easy to adapt to your feedline.

   

 . . .  where is the zero point on the rod?  Is it the point where the coax fastens to the mounting bolt,



   Yes.



 or the end of the coax where it parts from the sheilding, etc?



   Exposed center conductor ADDS to overall length

   and should be minimized. But this is pretty easy

   to keep at or below 1" and is not significant in

   terms of antenna performance.

  

 I know for a com radio we're shooting for 22"-23" roughly, for the antenna's lengths.  I'm just a little lost on where those lengths actually begin.  Plus, what if the radials are a little extra long?  Does that hurt reception?



   No, the radials are not critical. If you were cutting

   your copper foil strips from a large sheet, I would have

   made them wider for robustness . . . 1-2" would have

   been nice. But if you're working with 1/2" wide tape,

   then what you have is what you have and will work fine.

   Make periodic inspections of the joints where the

   tape comes onto the grounding plate.



 

   Bob . . .    [quote][b]

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mdnanwelch7(at)hotmail.co
Guest

Posted: Fri Apr 16, 2010 7:03 am Post subject: is a balun necessary

Bill & Bob,

 

  Thanks for your help.  I think I have information at the point to complete the antenna and generate some results.  I'll plot the SWR graph, and get back to you.

 

  Re: the plexiglass.  No, Bob, none of the airplane uses it for, frankly, anything!!  Without you seeing what I'm doing, I know it's hard to understand what I'm actually building.

  The plexiglass I referred to in only a 2" x 2", 3/8" thick block, that the antenna is built from.  It allows for securing the copper tapes on the bottom, and a way to bolt the steel mast through it.  

 

  I'm sending a photo of essentially what it looks like.  I know this photo is of a halfwave dipole.  My plexiglass block is serving a similar function.....just a way to make everything secure.  See photo #10 for my similar mounting method.

  

 http://forums.matronics.com//files/antenna_design_118.pdf

  

 Mike Welch

  

 

 



 



 	  Quote: 	
		    	
 		 	   		  The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. Get busy.  [quote][b]

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AV8ORJWC

Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"

Posted: Fri Apr 16, 2010 7:49 am Post subject: is a balun necessary

Mike, can you expound on the process to plot the SWR graph on your project?  Dean Scott's pdf referenced the EZNEC 4.0 plot which was fascinating.  

   

John Cox  

       

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Mike Welch

 Sent: Friday, April 16, 2010 7:51 AM

 To: aeroelectric-list(at)matronics.com

 Subject: RE: is a balun necessary  

  

  

   

Bill & Bob,

  

   Thanks for your help.  I think I have information at the point to complete the antenna and generate some results.  I'll plot the SWR graph, and get back to you.

  

   Re: the plexiglass.  No, Bob, none of the airplane uses it for, frankly, anything!!  Without you seeing what I'm doing, I know it's hard to understand what I'm actually building.

   The plexiglass I referred to in only a 2" x 2", 3/8" thick block, that the antenna is built from.  It allows for securing the copper tapes on the bottom, and a way to bolt the steel mast through it.  

  

   I'm sending a photo of essentially what it looks like.  I know this photo is of a halfwave dipole.  My plexiglass block is serving a similar function.....just a way to make everything secure.  See photo #10 for my similar mounting method.

  

 http://forums.matronics.com//files/antenna_design_118.pdf

  

 Mike Welch

  

 

  

 

     	  Quote: 	
		   	
  

       

  

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