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Author Topic: Help with a balun design  (Read 10329 times)
KA4NMA
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Posts: 336




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« on: July 01, 2014, 06:40:13 PM »

I have a ft-817 and a LDG 817 tuner as a portable system. I am  planning on using this system with a doublet (about 40 ft of wire that could  be configured as a doublet, vertical with 1 radial, inverted L, etc).  The wire comes from what I have in my junk box.  I am also thinking of getting one of the MFJ telescoping whip with a ground spike and using the 2 40ft wires as radials.  Bands would b e 75m-6m. I am looking for a balun or uunun for my system. What would you suggest? I am disabled so building things are difficult.  Can somebody help me out?

Randy ka4nma
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KB3ONA
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Posts: 84




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« Reply #1 on: July 02, 2014, 03:18:56 PM »

I'm fairly new to building baluns and ununs but I learned a lot from reading J Sevick's two books on the subject, Transmission Line Transformers and Building and Using Baluns and Ununs. If you could provide more info on your setup it would be helpful. Such as what type of feedline you're using, the approximate feed point height above ground, etc. Also, you stated you want it for 75-6m which can be done but is a bit more difficult if you're looking to build a common mode choke, since there are tradeoff's in the choking impedance and losses depending on frequency. So anyway, if you could provide more details it'll help.

Gene
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KA4NMA
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Posts: 336




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« Reply #2 on: July 09, 2014, 06:27:59 PM »

Gene,

The purpose is to help transition to the antenna tuner.

Randy
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KA3NXN
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Posts: 108


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« Reply #3 on: July 13, 2014, 03:46:26 PM »

I highly recommend this little gem. It's easy to build and will match just about anything

http://www.ad5x.com/images/Articles/MultitapRevA.pdf

Jaime-KA3NXN
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KA4NMA
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Posts: 336




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« Reply #4 on: July 13, 2014, 07:33:41 PM »

Looks interesting, but I am not sure I could build it with bad eyes and shaky hands.
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WB6BYU
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Posts: 13239




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« Reply #5 on: July 13, 2014, 08:36:32 PM »

The first step is to figure out exactly what you need for the antennas you want to put up.

If you are trying to match a doublet fed with twinlead using a tuner designed for coax, then
you would be looking for a balun to do that.  Probably the simplest type is a length of coax
wound through a ferrite core to make a 1 : 1 current balun.  (While you could use a 4 : 1
balun, the general consensus is that the 1 : 1 balun is usually a better choice if you don't
know the specific impedances you have to match.)

G3TXQ's page gives lots of options:  http://www.karinya.net/g3txq/chokes/


For a vertical or end-fed wire, just attach it directly to the tuner.  (You can get adaptors from
a BNC connector to a pair of binding posts that will make it easier to connect the wire and
radials if your tuner just has a coaxial output jack.)

You don't need an un-un:  just connect the tuner directly to the wire.  Same with the
telescoping whip, though you may find that several shorter wires are more efficient
as radials than just the two longer ones.

If your tuner can't match the whip on 40m or 80m, then you may have to make a loading
coil for the base to improve efficiency.  Generally, however, you'd be better off using one
of the 40' wires tossed over a tree branch for a radiator rather than the much shorter whip.

AD5X's matching transformer is for low impedance mobile antennas fed with coax:  as long
as you can mount the tuner right at the antenna, the tuner should have sufficient matching
range without it.
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KA4NMA
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Posts: 336




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« Reply #6 on: July 14, 2014, 06:54:32 PM »

What is an easy to build 1:1 QRP balun for 75m-6m? Core? Air core? Bolt or wood dowl? Wire? windings?

Randy ka4nma
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WB6BYU
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Posts: 13239




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« Reply #7 on: July 14, 2014, 08:21:48 PM »

Extrapolating from G3TXQ's data (which may or may not be accurate), it looks like
10 - 12 turns of RG-174 or RG-58 on a FT-240-43 or -31 core, or 4 turns of RG-174
on a pair of FB-31-1020 beads configured as a binocular core, would be a reasonable
starting point (though his data stop at 30 MHz.)

I have some smaller cores that I'm going to try winding with RG-174 at some point,
but Steve doesn't list very many such combinations.
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