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Author Topic: Fan dipole question. (to balun or not to balun)  (Read 3498 times)
W6NZX
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Posts: 451




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« on: August 10, 2007, 11:01:01 PM »

I'm planing on a 10-20-40-80 M fan dipole, and I'm wondering what would be better to use at the center feed point, 50 ohm coax or 300 twin lead.

I'm leaning towards 50 ohm twin lead as that is what I will need to feed it into the tuner.

Question, and this is one that always throws me, will I need a balun or not?  My G5RV does just fine without one,  though 30ft of it *is* fed by a balanced line.
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NA0AA
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« Reply #1 on: August 10, 2007, 11:39:28 PM »

I find it's best to try it out first.  My two band parallel dipole [40 and 80 meters] is cut resonant and loads up just fine with a direct coax feed and no balun.

why not put it up, run the numbers then see how it looks?  If you don't like what you get, try a balun and see if it is happier that way.

I suggest you will be happier with a co-ax feed line provided you are decently close to resonance on each band - which I suspect you will be.
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W6NZX
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« Reply #2 on: August 10, 2007, 11:51:44 PM »

Meant to say leaning towards 50 ohm COAX.



-R
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KB9CRY
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« Reply #3 on: August 11, 2007, 01:18:24 AM »

50 ohm coax and a 1:1 balun.
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W6NZX
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« Reply #4 on: August 11, 2007, 03:58:01 AM »

A 1:1 balun would be aprox 5 feet of coax looped and zip tied, or wraped around a PVC pipe correct?
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NG0K
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Posts: 334




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« Reply #5 on: August 11, 2007, 06:06:47 AM »

I'd make a 10 or 12 turn selenoid balun on a 4 inch form using some RG8X.  That would be around 12 feet of coax.   I don't think a 5 turn balun will be good enough.  It might be just as easy to get a Van Gorden 1:1 balun for around $15.  Cheap and makes a nicer installation.

Doug
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73, Doug - NG0K
W5RB
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Posts: 564




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« Reply #6 on: August 11, 2007, 06:09:58 AM »

My ARRL Handbook suggests 10 feet formed into 7 turns for a multiband 3.5 - 30 MHz antenna .
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N3JBH
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Posts: 2358




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« Reply #7 on: August 11, 2007, 06:11:58 AM »

"A 1:1 balun would be aprox 5 feet of coax looped and zip tied, or wraped around a PVC pipe correct?"

That would be more of a choke. and 5 feet of coax wound around anything would not be much of a choke.

10 turns of coax in a 6 to 8 inch circle would be closer to what you want. but i suggest you google 1:1 curent balun. that be much more effective to your need's

Beside the recomended reading of the various ARRL publications. may i suggest you go to this link and read please: http://www.dxengineering.com/techarticlepopup.asp?ID={3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

Also Tom W8JI has some good reading on the subject on his own web site. allthough Tom's site some times goes a tad deep in explaing things for us laymen types. it does do a great job of putting the proof in them.

Lastly i but you will get response's  both for and agianst baluns. it seem's the pro and anti balun crowd's are even on the subject.

personally i say use a balun or choke depending on the type antenna all the time. but then that is simply my take.

 
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KE3WD
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« Reply #8 on: August 11, 2007, 08:34:51 AM »

The properly constructed Air Choke balun will keep the RF from radiating back down your coax shield, which can distort your antenna pattern and also bring RF into the shack.  

Go with the ARRL formula.  


.
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K1BXI
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Posts: 812




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« Reply #9 on: August 11, 2007, 08:38:11 AM »

If you mean 4 separate 1/2 wave dipoles all connected at the center, yes, feed it with coax. Using a choke on the outside of the coax at the feed point is not a bad idea, but it will work fine without one. Been there....done that and it works well, but you may need to tweak the lengths to get all 4 bands where you want them. Also you will find the 40 Mtr dipole will work fairly well on 15 Mtrs as a 3 half wave center fed dipole. Like your G5RV works on 20 Mtrs.

John......K1BXI


 
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WB6BYU
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Posts: 13486




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« Reply #10 on: August 11, 2007, 09:53:52 AM »

The reason for putting dipoles for different bands on a common feedpoint
is so that you don't need a tuner in the shack - they will match 50 ohm
coax directly.   The lengths of the wires have to be adjusted to resonance
on each band.

On the other hand you can put up a SINGLE dipole fed with 300 ohm
twinlead to a tuner and use it on multiple bands, without having to
adjust the length of the wire.

Both approaches work.  If I'm putting up a relatively permanent antenna
I use the multiple dipoles approach so I don't have to bother with a
tuner when changing bands.  For a temporary installation the single
wire and tuner is faster to set up and works well.  For portable work
I have a set of dipole wires so I can choose any combination of bands
and/or antenna length to fit each situation:  for backpacking this avoids
having to carry a tuner along.  Just a matter of preference.

Balun on a dipole?  Perhaps if I have one handy, but I don't let the lack
of one stop me from getting on the air.
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W7AIT
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Posts: 491




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« Reply #11 on: August 11, 2007, 02:11:15 PM »

4/15/2007
Should I use a balun on my antenna?  QUESTION AND ANSWER

I DO recommend a balun be used on any antenna HF, VHF, for the following reasons:

1.   Balances the antenna currents and voltages.  
2.   Makes antenna pattern uniform.
3.   Provides impedance match.
4.   Reduces & eliminates RF radiation from coax feedlines and helps reduce TVI, RFI.
5.   Provides balanced to un balanced function.  Most rigs are coax connector unbalanced outputs.  Most wire antennas including dipoles are balanced antennas.  A balun is required to match unbalanced transmitter circuit to balanced antenna circuit.  Gets all the power into the antenna evenly.
6.   Helps match impedances.  Baluns are wonderful impedance transformation devices and by doing this, gets all the power from the transmitter into the antenna.
7.   Conversely, direct feed – taking a unbalanced coax cable and attaching to a dipole – is awful because:
a.   Currents and voltages are uneven
b.   Pattern is all screwed up
c.   Impedances are mis matched and power is lost
d.   RF on the coax shield, increases TVI and RFI
e.   Rig case can be RF hot, RF burns to operator
8.   Many types of baluns.  Can be ferrite beads, coiled up coax around a coke bottle, a W2AU balun, etc.
9.   Come in different matching ratios.  1:1, 4:1, 12:1 etc.  Pick the proper one.
10.   Most handle full KW.
11.   VHF / UHF – still use balun & probably more important than ever at these frequencies.  Baluns for VHF/ UHF / Microwave are different than HF baluns.
12.   Not using a balun is one of the biggest reasons hams either get poor performance or no performance out of antennas (my antenna doesn’t work!).  If your antenna isn’t working, use a balun! (Also make sure the counterpoise is there too! See separate write up)
13.   Repeat after me “USE A BALUN!”.  
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KE3WD
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Posts: 5689




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« Reply #12 on: August 11, 2007, 05:45:39 PM »

FWIW I'm agreed with W7AIT on this subject.  


Those RF burns can hurt.  



.
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N6AJR
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Posts: 9921




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« Reply #13 on: August 11, 2007, 06:54:42 PM »

I could not pass this one up Smiley

Bye the way, none of this is mine, Its just stuff I pulled from here and there. thanks to all the folks with the great sites. this is My take on the multi-band, single feed wire dipole ( but fan dipole is easier to type.  a true fan dipole is also called similar to a cage dipole and has 3 or 4 wires of different lengths on the same band , like a 64 foot , 66 foot and 68 foot  double leg wires on the same feed to give wider bandwidth, but bottom line, they are cheap, easy to build, and they work..
 
 
http://www.ku4ay.net/dipole.html
http://www.qsl.net/kd7rem/antdipole.htm
http://www.hamuniverse.com/multidipole.html
http://www.angelfire.com/nb/ni4l/ni4ldipole.html
http://www.arrl.org/tis/info/pdf/9611073.pdf
http://www.geocities.com/n2uhc/2banddipole.html
http://www.qsl.net/na4it/fandipole.html

 
 
 
by N3JBH on October 12, 2006    
FAN DIPOLE OR MULTIPLE BAND DIPOLE SPECS:
(Each leg is shown in length so you will need two legs.)

10 METERS = 8'4"
12 METERS = 9'5"
15 METERS = 11'1"
17 METERS = 12'10"
20 METERS = 16'8"
30 METERS = 23'2"
40 METERS = 32'9"
60 METERS = 43'7"
75 METERS = 60'9"
80 METERS = 65'6"
160 METERS = 123'5"
137 KHZ = 1708'1"  
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KB3MDT
Member

Posts: 205




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« Reply #14 on: August 11, 2007, 07:31:57 PM »

Hi,
   I have a 80-40-20 "Inverted V" Fan Dipole.  Each wire points to a different direction on the compass. The peak is at about 32 feet and the ends are anywhere from 10 to 20 feet above the ground.   I tried adding 10 meters and could never get the 10 meter wire to be resonant - possibly because I ran out of good "end points" for the wires.  

   Anyway, I'm using about 100 foot total of 50 ohm coax to a MFJ 949E tuner.  I rarely use the tuner as I'm usually mid band and have a decent SWR.   I'm not using a balun of any kind.   I don't have any RF problems in the shack that I know of.  (I run at 100 watts max).  My transceiver has a dedicated ground of about 10 foot of heavy gauge wire to a 8 foot grounding rod.  I can't comment on the technical pros and cons of balans for a dipole, but I can share my experience.

    Let us know if you "try it both ways" and if you see any difference.

73
Ken
KB3MDT
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