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Author Topic: Full wave loop; ugly balun or ?  (Read 4219 times)
KB8VIV
Member

Posts: 47




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« on: November 03, 2012, 08:14:13 PM »

Hey gang, i was working on my full wave horizontal 80 mtr loop and decided to add either an "ugly balun," or possibly a dx engineering 1:1 balun.  The antenna is abt 275 ft long and is currently just fed directly with coax, and I use it for 10-80 operation with a tuner.

Obviously, i like the cost of building an ugly balun, but will it work as good as the 1:1 dx engineering balun?  Thanks in advance.

73's de kb8viv
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WB6BYU
Member

Posts: 13251




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« Reply #1 on: November 03, 2012, 09:15:51 PM »

A 4 : 1 balun will give a better impedance match on most bands - you may be
able to get by without the tuner on several bands.

Air core "ugly baluns" aren't very effective for wide-band use, as G3TXQ explains on his
web site here:  http://www.karinya.net/g3txq/chokes/

That's not to say you can't try one and see how it works.  I haven't noticed as many
problems with common mode currents when using loop antennas as compared to
dipoles, and it might work well enough for you.  But if it were me I'd use a 4 : 1 balun
wound on a ferrite core, even if it is a voltage balun.
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KB8VIV
Member

Posts: 47




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« Reply #2 on: November 04, 2012, 06:19:25 AM »

Thanks for the reply.  I've read lots of conflicting info regarding is matter.  Some say it's sufficient to just feed a full wave loop with coax, directly, some say use a 4:1 balun, I've seem it written a 1:1 balun is preferrable.  I guess when using this antenna on multiple bands, there are going to be compromises.

Tnx agn for the advice es 73's.

De kb8viv
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WB6BYU
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Posts: 13251




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« Reply #3 on: November 05, 2012, 09:08:34 PM »

Quote from: KB8VIV

...I've read lots of conflicting info regarding is matter.  Some say it's sufficient to just feed a full wave loop with coax, directly, some say use a 4:1 balun, I've seem it written a 1:1 balun is preferrable.  I guess when using this antenna on multiple bands, there are going to be compromises.




There are many topics that you'll find conflicting information about on the internet.
A good article shouldn't just tell you what to use, but explain why, so you can make
your own decisions whether those recommendations actually apply to your own
situation.

There are two basic approaches to feeding a loop for multiband operation:  you can
adjust it to resonance and feed it such that it has a low SWR on multiple bands, or
just feed it with a low loss coax and use a tuner in the shack regardless of the SWR.

If you make the loop resonant on the low end of 80m then the resonances will fall
in (or near) 40m, 20m, 17m, 15m, 12m, and 10m.  (The 30m resonance falls too
high, around 10.7 MHz.  The 17m resonance is actually a bit low, around 17.8 MHz.) 
Just because the antenna is resonant doesn't mean that the SWR is low, however. 
My computer model of a 280' loop up 30' suggests the follow approximate input impedances:

                          50 ohm  200 ohm
band   impedance   SWR      SWR
80m:  90 ohms       2 : 1    2 : 1
40m:  240 ohms     5 : 1    1.2 : 1
20m:  250 ohms     5 : 1    1.25 : 1
17m:  390 ohms     8 : 1    2 : 1      actual SWR will be somewhat higher
15m:  420 ohms     8 : 1    2 : 1
12m:  360 ohms     7 : 1    1.8 : 1
10m:  250 ohms     5 : 1    1.25 : 1

You can see how the SWR is lower on most bands when using a 4 : 1 balun than
with a 1 : 1 balun:  good enough that you can often operate on some bands
without needing a tuner at all!  But you may have to adjust the wire length
slightly to get the resonances to fall in the desired bands.

Even if you use a tuner, the lower SWR will mean lower losses in the coax.
While losses aren't generally too bad, if you are feeding the antenna with
75' of RG-8X coax you'd loose about half your power in 15m feeding the 420 ohm
impedance directly with a 8 : 1 SWR, compared to a only a quarter of your
power lost in the feedline with a 4 : 1 balun.  (The actual number depends
on the length and type of coax you are using.)  Not a big difference, true,
and you'll probably still make plenty of contacts either way.

So from the perspective of lower SWR, the 4 : 1 balun is preferable.  The
stress on the balun is relatively low because the SWR is pretty tame.

Now, part of the complaint about a 4 : 1 balun is because most of the
ones offered (especially older designs) are voltage baluns, rather than
current baluns.  Voltage don't necessarily do as good of a job of
equalizing current in the two wires and preventing common mode currents
on the outside of the coax, but it probably isn't any worse than not using
a balun at all, and you get a better impedance match.  There are
4 : 1 current baluns available, but they tend to be more of a speciality
item.


The other approach to feeding a loop is to put it up without worrying too
much about the wire length, and feed it with low-loss feedline, typically
twinlead or open wire line.  Actually, 300 ohm twinlead isn't a bad choice
here, either, as the SWR at resonance (at least on the bands above 80m)
is usually better than 1.5 : 1.  But if the antenna length isn't actually
resonant, the antenna will still operate, though at higher SWR.  That's
why low loss feedline is important here: the SWR can easily be
over 20 : 1 when using 50 ohm coax if the antenna isn't adjusted for
resonance, and at that point the losses in your coax become more
significant.  With the same coax arrangement I used above, now you
may be getting 30% or less of your power reaching the antenna.  Using
twinlead is more efficient in this case, probably around 90%.

If you are feeding a non-resonant loop, the SWR on your feedline
may be quite high, and that presents a problem for many baluns.
The 4 : 1 types built into many antenna tuners will step the impedance
down, and if it happens to be low anyway, it can affect the
efficiency of the tuner.  (Tuner efficiency is generally a problem mostly
for low impedance loads on the lower bands, and sometimes on 10m.)
Common tuners can match higher impedance loads more efficiently,
which is why the are often recommended for use when the impedance
may vary over a wide range.  The actual impedance will depend on both
the impedance at the antenna and the length and type of the feedline.


So if you are putting the balun directly at the feedpoint of the loop and
adjusting the loop length for minimum SWR, the 4 : 1 balun is the better
choice as it may allow you to feed the antenna without a tuner on several
bands, and it minimizes coax losses.

If you are feeding a resonant loop with 300 ohm twinlead, a 4 : 1 balun
to coax (possibly 75 ohms) isn't a bad approach, either.

If you are feeding a semi-random length loop with ladder line to a balun
at the tuner in the shack, a 1 : 1 balun may work better, as it likely will be
under greater stress / higher SWR.  If you put the balun outside the house
and run coax to the rig, keep the coax as short and low loss as possible
because you can get very high SWR:  it isn't uncommon to see losses as
high as 10dB in a 20' length of what otherwise would be reasonably
good coax in such an installation.


So there isn't a single simple answer:  in some circumstances a 4 : 1 is
better, sometimes a 1 : 1.  But if you are putting the balun at the antenna
and trimming to length, the 4 : 1 will give lower feedline losses.

Can you operate without a balun?  Obviously you can if you have been
doing it this far.  Will you see a big improvement adding a balun?  That's
hard to say, since whether or not you have common mode currents will
depend on such factors as the length of the feedline and how your rig
is grounded, making it difficult to generalize.
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KB8VIV
Member

Posts: 47




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« Reply #4 on: November 06, 2012, 04:08:17 AM »

Wow, thanks for your very detailed answer.  Sounds like I should be considering a 4:1 balun - dx engineering's offerings appear to be current baluns, so I may look a bit closer at those.  I really appreciate your advice.

73's de kb8viv.
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K9SRV
Member

Posts: 121




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« Reply #5 on: November 07, 2012, 05:47:56 AM »

it would be worth your time to call bob at balun designs before
you buy anything. great guy, will talk for 1/2 hour with you even if he
doesn't expect to make a sale. bob just plain loves the hobby!

john matson
k9srv
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AA5WG
Member

Posts: 496




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« Reply #6 on: November 07, 2012, 06:47:48 AM »

Kb8VIV:

When I was using a loop I uesd it with a link antenna coupler and open wire feeders.  There was no wound balun to worry about and the multiband setup worked.

Chuck
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KB8VIV
Member

Posts: 47




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« Reply #7 on: November 07, 2012, 06:54:34 AM »

Hi John, thanks for the advice, just e-mailed balun designs, we'll see what they say.  Hi Chuck, I wish I could use ladder line or some similar option, but my installation dictates that I go with Coax as a feedline.

Steve
kb8viv
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KB8VIV
Member

Posts: 47




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« Reply #8 on: November 07, 2012, 07:43:12 AM »

Ok, I guess like others have recommended, a 4:1 balun is the way to go.  Thanks again for all the advice.

kb8viv
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AK4SK
Member

Posts: 150




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« Reply #9 on: December 07, 2012, 07:29:51 AM »

I'm considering a full wave loop for 80 meters also. I've seen some designs that suggest a section of 75 ohm "TV" coax as a "Q match" or something like that. If that Q match section is included is a 4:1 balun still necessary?
73,
Chris
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WV4L
Member

Posts: 138


WWW

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« Reply #10 on: December 07, 2012, 08:29:16 AM »

WB6BYU is dead on in his info. Thanks.

My Horizontal Delta loop is fed with 600 ohm ladder line down to a 4:1 DX Engineering current Balun attached to the outside basement wall with about 20 ft. of Davis RF Bury Flex into my shack.  This loop is somewhere around 225 ft.  I didn't measure for a specific band, I just strung it up based on the points I was going to raise it up with. The tower, and two tall trees on the back side of my property. Sometimes you have to put up antennas based on your given parameters and deal with any compromises dealt to you.

73

Wayne C.
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WB6BYU
Member

Posts: 13251




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« Reply #11 on: December 07, 2012, 11:27:17 AM »

Quote from: AK4SK

I'm considering a full wave loop for 80 meters also. I've seen some designs that suggest a section of 75 ohm "TV" coax as a "Q match" or something like that. If that Q match section is included is a 4:1 balun still necessary?



The full wave loop has an impedance around 100 ohms.  As such, either a 1 : 1 or a 4 : 1
balun will give about a 2 : 1 SWR.  (The actual impedance depends on height above ground.
In my previous model it came to 90 ohms.)  So on the fundamental, either type of balun
works about as well - it is only for the higher bands that the SWR is better with the 4 :1.

A quarter wave of 75 ohm cable transforms 50 ohms to 112 ohms, so gives a pretty good
match to a full wave loop.  In the 90 ohm case, the impedance is transformed to 62.5 ohms,
for an SWR of 1.25 : 1 at resonance.  In that case a 1 : 1 balun is better for 80m.  The SWR
will be higher on the upper bands, however:  on 40m the transformer is now 1/2 wave long
and won't provide any impedance transformation, so the SWR will still be about 5 : 1.
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AK4SK
Member

Posts: 150




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« Reply #12 on: December 07, 2012, 11:35:41 AM »

Quote from: AK4SK

I'm considering a full wave loop for 80 meters also. I've seen some designs that suggest a section of 75 ohm "TV" coax as a "Q match" or something like that. If that Q match section is included is a 4:1 balun still necessary?



The full wave loop has an impedance around 100 ohms.  As such, either a 1 : 1 or a 4 : 1
balun will give about a 2 : 1 SWR.  (The actual impedance depends on height above ground.
In my previous model it came to 90 ohms.)  So on the fundamental, either type of balun
works about as well - it is only for the higher bands that the SWR is better with the 4 :1.

A quarter wave of 75 ohm cable transforms 50 ohms to 112 ohms, so gives a pretty good
match to a full wave loop.  In the 90 ohm case, the impedance is transformed to 62.5 ohms,
for an SWR of 1.25 : 1 at resonance.  In that case a 1 : 1 balun is better for 80m.  The SWR
will be higher on the upper bands, however:  on 40m the transformer is now 1/2 wave long
and won't provide any impedance transformation, so the SWR will still be about 5 : 1.

Ok, so a 1/4 wave 75 ohm coax section is just matching the full wave loop only for the frequency that it is cut for at one wave length and it gets around the need for a balun at the full wave length frequency only. It sounds like the best option is the 4:1 current balun.

73,
Chris
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WB6BYU
Member

Posts: 13251




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« Reply #13 on: December 07, 2012, 12:45:09 PM »


... it gets around the need for a balun at the full wave length frequency only...



Not exactly...


There are TWO SEPARATE functions involved:

1) matching the loop feedpoint impedance to the feedline

2) providing a balanced-to-unbalanced (balun) transformation to reduce common
mode currents on the feedline.

While some baluns may also provide impedance transformation, you have to
consider the functions separately.


A quarter wave of 75 ohm coax is only a quarter wavelength on one frequency.
On even harmonics (such as 40m or 20m) it doesn't provide any impedance
transformation.  To see how it does on other frequencies, you can plug the
length and coax type into VK1OD's handy transmission line loss calculator
( http://vk1od.net/calc/tl/tllc.php ) along with the impedances that I gave
earlier for each band (as "Zload".)  That will tell you the impedance at the
end of the 75 ohm coax.

There are other ways to match a loop, including a tuner in the shack.  It
all depends on your specific installation and how much loss you are willing
to tolerate on each band.  With a fairly short length of low-loss coax, even
a 5 : 1 or 8 : 1 SWR at the antenna isn't going to cause much loss, so
tuning in the shack is a reasonable alternative.  If you are using 100' of
RG-58, it might not be as attractive.  That's a decision you have to make.

If the losses are acceptable to you on the higher bands and you want the
convenience of not needing a tuner on 80m, then the quarter wave section
may be a good approach for you, whether or not you use a balun with it.
(The quarter wave section does not provide the balun function, though
you could probably make it into a sleeve balun if you wanted to.)


Similarly, it is possible to feed a loop directly with coax without a balun.
I've had fewer common mode issues with full wave loops than with other
types of wire antennas when I have fed them that way, but that doesn't
mean that you won't have common mode currents.  Again, that is your
choice.


So whether or not you use a balun is independent of whether or not you
use a quarter wave matching section or other impedance matching on
one or more bands.


But here is a possible combination:  you sometimes find dual 75 ohm coax,
or can make your own by running two pieces of coax in parallel.  If you
use the center conductors as a balanced line the impedance is 150 ohms.
A quarter wavelength of that would transform the impedance up to 250
ohms on 80m, which would give a good match with a 4 : 1 balun.  On 20m
and 40m it would provide no transformation, so it is still around 240 - 250
ohms.  You've used a quarter wave transformer to improve the match to
a 4 : 1 balun on 80m without degrading the match on 40m and 20m.

You'd have to check the numbers for each band to see whether the SWR
is higher or lower with this approach, but it is an example that there are
many other approaches that can be used.
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AK4SK
Member

Posts: 150




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« Reply #14 on: December 07, 2012, 01:13:31 PM »


... it gets around the need for a balun at the full wave length frequency only...



Not exactly...


There are TWO SEPARATE functions involved:

1) matching the loop feedpoint impedance to the feedline

2) providing a balanced-to-unbalanced (balun) transformation to reduce common
mode currents on the feedline.

While some baluns may also provide impedance transformation, you have to
consider the functions separately.


A quarter wave of 75 ohm coax is only a quarter wavelength on one frequency.
On even harmonics (such as 40m or 20m) it doesn't provide any impedance
transformation.  To see how it does on other frequencies, you can plug the
length and coax type into VK1OD's handy transmission line loss calculator
( http://vk1od.net/calc/tl/tllc.php ) along with the impedances that I gave
earlier for each band (as "Zload".)  That will tell you the impedance at the
end of the 75 ohm coax.

There are other ways to match a loop, including a tuner in the shack.  It
all depends on your specific installation and how much loss you are willing
to tolerate on each band.  With a fairly short length of low-loss coax, even
a 5 : 1 or 8 : 1 SWR at the antenna isn't going to cause much loss, so
tuning in the shack is a reasonable alternative.  If you are using 100' of
RG-58, it might not be as attractive.  That's a decision you have to make.

If the losses are acceptable to you on the higher bands and you want the
convenience of not needing a tuner on 80m, then the quarter wave section
may be a good approach for you, whether or not you use a balun with it.
(The quarter wave section does not provide the balun function, though
you could probably make it into a sleeve balun if you wanted to.)


Similarly, it is possible to feed a loop directly with coax without a balun.
I've had fewer common mode issues with full wave loops than with other
types of wire antennas when I have fed them that way, but that doesn't
mean that you won't have common mode currents.  Again, that is your
choice.


So whether or not you use a balun is independent of whether or not you
use a quarter wave matching section or other impedance matching on
one or more bands.


But here is a possible combination:  you sometimes find dual 75 ohm coax,
or can make your own by running two pieces of coax in parallel.  If you
use the center conductors as a balanced line the impedance is 150 ohms.
A quarter wavelength of that would transform the impedance up to 250
ohms on 80m, which would give a good match with a 4 : 1 balun.  On 20m
and 40m it would provide no transformation, so it is still around 240 - 250
ohms.  You've used a quarter wave transformer to improve the match to
a 4 : 1 balun on 80m without degrading the match on 40m and 20m.

You'd have to check the numbers for each band to see whether the SWR
is higher or lower with this approach, but it is an example that there are
many other approaches that can be used.

Thanks for the clarification. I think the easiest way for me would be to use a 4:1 current balun and a tuner.
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