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Author Topic: To balun or not to balun? That is my question  (Read 1263 times)
VE3VVF
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Posts: 46




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« on: February 05, 2017, 08:21:04 AM »

Hi all,
I have a couple tuned fan dipoles in inverted vee configuration on two supports at the top of the 40' tower outside my second story window. One (for 30M and 17M) has a commercially made coaxial fed 1:1 current balun feedpoint while the other (for 20M and 40M), also fed with coax, does not have a balun.  My question is, with the short distance, would I really benefit from ladder line at 2W with a balun just inside my window and if not should I bother with a balun on the 20/40 antenna at its feedpoint?

Being a QRP minimalist I tend to follow the simplest path but with such crappy conditions these days I'm wondering if I would benefit from a little tweaking.

many thanks
Scott
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K8AXW
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Posts: 5818




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« Reply #1 on: February 05, 2017, 09:58:26 PM »

I had been into ham radio for about 45 years before I bothered to look up the word "balun."  Never made or bought of of the things.....with the exception of the one provided with my Hy-Gain beam.

My suggestion is simple.  Buy or make one.....as something to play with.  How you will tell the difference in performance is anyone's guess.
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OZ8AGB
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Posts: 230




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« Reply #2 on: February 06, 2017, 01:57:20 AM »

You can try to calculate the losses in your coax vs loss in e.g. a 450 ohm ladder line.
http://www.ve3sqb.com/hamaerials/ocarc/
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VE3VVF
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Posts: 46




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« Reply #3 on: February 06, 2017, 07:57:00 AM »

You can try to calculate the losses in your coax vs loss in e.g. a 450 ohm ladder line.
http://www.ve3sqb.com/hamaerials/ocarc/

That's a great calculator.  Very cool.  Thanks
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KU3X
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Posts: 336




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« Reply #4 on: February 06, 2017, 08:36:22 AM »

If your antennas are resonant and well matched, with a short run of coax, I don't see where changing to parallel feeders will make that
much difference. Installing a 1 to 1 current balun on the second antenna will help prevent the outside of the shield of the coax from
radiating, if there is an issue?

But, with parallel feeder cut to the proper length and a 4 to 1 guanella current balun outside of your window, you can use one dipole and
operate on numerous bands with minimal loss.   DO NOT use this array on any frequency lower than the design frequency of the
antenna. In other words, don't use a 40 meter dipole, fed with parallel feeders on 60, 80 or 160 meters.

Barry
www.ku3x.net
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G4AON
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Posts: 908




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« Reply #5 on: February 06, 2017, 11:00:27 AM »

Feeder radiation at QRP power levels isn't usually an issue, however the reverse can be a problem with noise pickup on a poorly terminated feeder.

Using a well made balun at the feed point of a coax fed dipole has no downside, except weight perhaps. Not using one results in currents flowing on the outer of the coax, which can have strange effects such as an SWR meter reading differently depending on feeder length. Is that why the transceiver SWR meter often reads differently to an external one? I was once told to adjust the feeder length to improve the SWR!

Years ago the use of a 4:1 Voltage balun on open wire or ribbon fed doublets was pretty much the way to use an ordinary ATU. The only advantage was the centre tap of the balun was grounded to avoid static build up. There were two disadvantages, a 4:1 Voltage balun doesn't balance currents and often results in feeder radiation and noise pickup on the feeder. The other disadvantage is the reduction in feed impedance increases matching difficulties on the lower frequency range of the doublet, typically the CW end of 80m on a 100' doublet (or 40m with one half the length). The lower feed impedance often results in increased loss in an ATU too.

These days a 1:1 current balun is usually considered the better option to use on the rear of an ATU to feed a doublet.

Baluns are easy to make, but aren't particularly cheap to make compared to commercial ones such as those by Balun Designs. If you make your own use ferrite cores that are beefy enough for higher power levels than you expect to use.

73 Dave
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WB6BYU
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Posts: 16287




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« Reply #6 on: February 06, 2017, 11:03:38 AM »

Quote from: VE3VVF

...My question is, with the short distance, would I really benefit from ladder line at 2W with a balun just inside my window...



It is unlikely to make any significant difference.

The losses through 50' of RG-58 coax are less than 1dB.  While the losses using open wire line may be
less than that, you'll also need to add a balun and a tuner, both of which can add losses that, while
small, may eat up much of the potential savings.

But changing to open wire line would also require that you use a tuner, compared to the simplicity of
direct coax feed without a tuner for the existing installation.  Simplicity is good!

The situation would be different if you were using a single doublet on multiple bands, however.




Quote

... and if not should I bother with a balun on the 20/40 antenna at its feedpoint?



A balun might not make any difference.  Or sometimes it may.

What a balun does is make antenna performance more stable and predictable by reducing
the interaction between the antenna and the length of the coax (and what is connected to it.)

Sure, I've put up a lot of dipoles without using a balun, and most of them seem to work fine.
My guess is that 75% - 80% of the time a ham won't notice any problems that they attribute
to not using a balun.  But I've also seen a lot of quirky problems, some of which hams don't
realize are due to lack of a balun:  resonant frequencies that don't change then the wire length
is modified; extra noise picked up from electronics in the shack due to the coax acting as part
of the antenna; changes in SWR due to "minor" changes in the shack, such as changing the
cabling or plugging in headphones; RF shocks off exposed metal parts of the rig, mic or key;
different SWR when measured by the rig vs. an analyzer; etc.  In a couple cases adding a balun
has decreased DX performance by reducing vertically polarized radiation from the feedline
of a low dipole.

So should you bother adding a balun to your existing antenna?  The choice is up to you.
If it seems to work well enough, and your receive isn't limited by noise pickup from the shack,
then there isn't much reason to.  If you often change your shack around and find that the
SWR on the antenna changes when you do so, or experience other quirks, then it may help.

My backpacking dipoles use RG-174 coax with no balun.  Most of my other portable dipole kits
don't use them either.  But in one case we were able to reduce the noise level from S9 to
S4 by adding a balun, and in other cases it has eliminated problems from "RF-in-the-shack".
Sometimes it makes a difference, other times it doesn't.
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VE3VVF
Member

Posts: 46




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« Reply #7 on: February 06, 2017, 05:26:56 PM »

Thanks so much for all the input.  The commercially bought balun already in use is definitely heavy and avoiding that would be a plus.  My antenna that has no balun is fed with RG-174 and was a backpack/camping antenna that uses a plastic T from the HoPo that is usually used for underground water sprinklers.  I need to beef it up with more robust antenna wire but I think I'll forego the balun.  It hasn't given me grief so far so it's working well enough.  Thanks again for all the suggestions and information.  I read a joke somewhere where it said "ask 10 people in <insert forum here> and you'll get 11 opinions".  This place is great for informed opinions and I appreciate it.

cheers de Scott
ve3vvf 
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