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Author Topic: Frequency range for a Balun  (Read 1934 times)
VY1MR
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Posts: 8




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« on: December 02, 2012, 02:06:19 AM »

I wish to build a 1:1 ferrite rod balun like the one in the link provided at the end of this post.
On that page it states that the design is good through HF.
My problem is that I do not understand what characteristic sets the workable frequency range for a balun.
Is it the diameter of the conductor or, preserving the ratio, the number of wire turns?  
I will need the balun to work for recieve on VHF, specificly, the FM BC band.

Thanks in advance for your help on this


   --Matt

http://www.m0ukd.com/1to1_HF_Balun_for_dipole/index.php
« Last Edit: December 02, 2012, 02:10:59 AM by VY1MR » Logged
G3RZP
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Posts: 4391




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« Reply #1 on: December 02, 2012, 03:57:32 AM »

Stray capacity and inductance will have an effect, likewise the ferrite - the trifilar balun works on transformer action. Above 30 MHz, especially with those lead lengths, it's unlikely to work too well.

Note, by the way, the solder lug connection to the SO239 socket.   Not a good way to do it, because over time, the plastic can 'creep' and then the connection comes loose.

You would be better off with a transmission line balun, with some ferrite beads (you won't need many at VHF, but I can't tell you what mix - someone else may be able to) slipped over the coax cable up at the antenna.
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WB6BYU
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Posts: 13038




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« Reply #2 on: December 03, 2012, 09:17:43 AM »

The ferrite beads designed for filtering computer leads are often optimized
for the 50 - 150 MHz range.  A couple of those slipped over the coax
should work well enough for your needs.

The frequency range of a balun depends on the stray reactances of the
windings and the core material.  G3TXQ gives a number of measurements
 of choke baluns on his site:  http://www.karinya.net/g3txq/chokes/

The design you linked to is a 1 : 1 voltage balun.  A better approach
is a current balun, commonly made either by slipping ferrite beads
over the outside of the coax or winding the feedline into a choke coil.
Using coax + ferrite give a particularly wide operating bandwidth - even
at frequencies where the balun isn't effective, at least you are still
feeding the antenna directly through coax, so it will have less effect on
the SWR than a transformer arrangement.

Meawhile there are lots of inexpensive 4 : 1 TV baluns (75 ohms to 300
ohms) around, and you may be able to saw open the case and rewind
the balun core to give a 1 : 1 ratio instead.
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KH2G
Member

Posts: 239




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« Reply #3 on: December 31, 2012, 01:23:18 PM »

Easiest thing for receive is to find an old TV and on the back where the 300 ohm antenna hooked up there is a nice little balun that connects the balanced antenna to the unbalanced 75 ohm tuner input. The bonus is that the TV band is vhf/uhf 8-)
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