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Author Topic: Baluns - good rules for use?  (Read 4036 times)
AD0AE
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Posts: 78




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« on: May 24, 2012, 08:12:00 PM »

Hi All-

I have been thinking about putting up a vertical antenna and would like to do one where I could get 40 m.  I know that it would be 32 or so feet for a 1/4 wave length.  I also have been reading a bit about these S9 antennas.  One thing that isn't clear to me are baluns.  They seem to come in 1:1 and 4:1 values.  And reading about the S9 antenna, it sounds like you can turn it into a multiple-band antenna using a 4:1 balun.  I also understand that a balance transforms a balanced antenna (like a dipole) into an unbalanced situation (like coax).  Beyond that, I don't know too much.

So my question is simple, but the answers may not be.  Are there "general rules" which dictate when to use a 4:1 balun vs a 1:1 balun?  What are the general rules regarding baluns?

Thank you all!

73s,
Steve
AD0AE
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WX7G
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« Reply #1 on: May 24, 2012, 09:13:32 PM »

For a 32' single band vertical I have never used a balun and have had no common-mode current problems.

But you're interested in multiband use. I've analyzed this and it's a toss up if the 4:1 balun is ideal. But they are not and so I would feed the antenna directly with coax - no balun - to the unbalanced output of a manual antenna tuner.
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HB9PJT
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« Reply #2 on: May 25, 2012, 04:59:31 AM »

For multiband use with a 4:1 BALUN the loss in the coax is lower and the BALUN helps a tuner to extend the tuning range. I use a 6.3 m portable vertical for 20 to 10 m with a 4:1 BALUN successfully.

73, Peter - HB9PJT
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N4CR
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Posts: 1672




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« Reply #3 on: May 25, 2012, 05:10:32 AM »

A 4:1 balun transforms the impedance in that ratio.

If your feed point is 35 ohms (a very good ground plane under a tuned 1/4 wave vertical) your balun would transform that into 35/4 or less than 10 ohms. That would cause more loss on your feed line, not less.

In order to know if a transforming balun will help, you have to know the feed point impedance at the frequency in question. Some frequencies will be easier to tune, some will be harder. The question then becomes where is your antenna tuned and which frequencies will you want to use. With that information, you can formulate answers.

A lot of hams answer this question experimentally and there's no shame in that. Empirical evidence is always reality.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W5DXP
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« Reply #4 on: May 25, 2012, 07:01:39 AM »

What are the general rules regarding baluns?

For those of us who know what impedances we are dealing with, we choose the balun that will give us minimum SWR on a 50 ohm SWR meter. Antenna analyzers are great for determining those impedances.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5FYI
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Posts: 1046




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« Reply #5 on: May 25, 2012, 07:40:48 AM »

Quote
What are the general rules regarding baluns?

The term "balun" is a shortened version of BALanced to UNbalanced, and is generally used between a balanced antenna system (e.g. a dipole), and an unbalanced transmission line feeder (such as coax). It can also be used to define the condition of a balanced feeder (twin lead) to an unbalanced antenna (off-center-fed antenna, end-fed half-wave antennas, and [usually] verticals).

An "unun" (UNbalanced-to-UNbalanced) transformer is usually used to match an unbalanced (coax) feeder to an unbalanced antenna.

Nevertheless, it's hard to answer your question with a correct answer. Your 40-meter vertical should be fed with a 1:1 unun, but for use on other bands you may need a different kind of transformation. One approach is to use a tapped transformer so that you can change the impedance ratio with a switch. You can then tune the antenna to a wide range of input impedances. You might find information on these types of ununs by searching for "ununs," "multimatch ununs," "5-ratio ununs," and "10-ratio ununs."  If you can find a book on the subject by Doug DeMaw or Jerry Sevick, all the better. GL
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KE3WD
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« Reply #6 on: May 25, 2012, 07:52:56 AM »

With a Vertical, use of the proper amount of Copper Wire Radials arrayed underneath the Vertical not only improves the performance greatly (A 1/4 wave Vertical should NOT be used without a proper Ground Plane as part of the system) -- but also will serve to do the same thing that the balun brings to the system, decoupling of RF from riding down the coax shield. 

For the record, when building Vertical 1/4 wave antenna system, I have always preferred to use BOTH.  Plenty of Copper Radials underneath the vertical, plus a simple coiled coax balun at the feedpoint.  Never had a problem with RF in the shack, nor suffering poor performance from the Vertical.  This is really a DX antenna system, often getting great signal reports from afar, but not so great when the goal is stateside net checkins, where the angle of radiation from the vertical is rather low takeoff.  If stateside contacts, net checkins or control, ragchews, etc. is the goal, consider desigining an antenna more suited to that purpose, such as the Inverted Vee, and place its heioght at apex according to need (lower height often will equal better stateside performance, within reason). 


73
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WB6BYU
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Posts: 13331




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« Reply #7 on: May 25, 2012, 08:39:43 AM »

A balun is used to couple a BALanced antenna or feedline to an UNbalanced one.  Hence
the name.

The reason you use one is to keep the antenna balanced.  Without one, connecting the
coax shield to one side of the antenna allows current to flow down the outside of the
coax, causing the antenna to be unbalanced.  In practice, reducing common mode current
on the coax is the same as maintaining a balanced antenna:  anything that does one will
do the other.  (And if it doesn't do one, it won't do the other.)

Baluns come in two flavors:  voltage baluns (that force equal voltages on each side of the
antenna) and current baluns (that force equal currents on each side.)  Both work about the
same when the antenna is balanced (that is, when you don't need a balun) but the current
balun is the preferred type when you do need one.



Guidelines for use:
1) If you are using a balanced antenna (such as a dipole) with unbalanced feedline (such
as coax), choose the impedance ratio that gives the best match for that antenna.

2) If you are feeding an unbalanced antenna with unbalanced line, do NOT use a balun:  use
an UN-UN (Unbalanced to Unbalanced).  This was a common problem when the 43' verticals
were first introduced using a 4 : 1 voltage balun:  it actually FORCES currents to flow on
the outside of the coax.

3) If you are matching a wide range of unknown impedances at the output to a tuner, a
1 : 1 balun is probably better than a 4 : 1 balun because tuners tend to be more efficient
matching higher impedance loads than low ones.

4) If you are matching a wide range of impedances through a long length of coax, then which
impedance is best depends on the impedance of the antenna on each band and the relative
losses in the coax.  Typically for verticals used in this manner the length is chosen as a
compromise keep the SWR reasonably low on most bands.  One or the other might be better
on any given band.  Note that a 40m 1/4 wave vertical is NOT a good choice for this, as the
input impedance is very high on 20m.  I think the S9 verticals are 29 feet tall rather than a
full quarter wave, to avoid this.


Adding a balun (or, better yet, an UN-UN) to the base of a vertical does NOT automatically
make it a multi-band vertical any more than it was already.  What it does is to try to lower
the SWR somewhat on some bands to reduce losses, making it a bit more efficient to use
with a tuner in the shack.
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G8JNJ
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« Reply #8 on: May 25, 2012, 11:49:03 AM »

Hi,

You can find some very good notes relating to this subject on Steve's G3TXQ and Owen's VK1OD web sites. I suggest reading them in this order.

http://www.karinya.net/g3txq/unun/

http://vk1od.net/antenna/multibandunloadedvertical/index.htm

http://vk1od.net/antenna/multibandunloadedvertical/FP-40C.htm

http://www.karinya.net/g3txq/tuner_balun/

Regards,

Martin - G8JNJ

www.g8jnj.webs.com

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K0ZN
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Posts: 1553




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« Reply #9 on: May 25, 2012, 07:32:41 PM »


Hi.

The most current issue of QST has a fairly detailed article about the pro's & con's and "issues" surrounding the now-fairly-common 43 ft. verticals.
WELL worth your time/money to pick up a copy if you don't take QST. Multi band operation involves COMPROMISES.

Short version is that with virtually ANY single element vertical whether it is a 1/4 wave, 1/2 wave or 5/8ths wave radiator, it will be a good match to coax
ONLY on ONE band. All the other bands will have between 5:1 and maybe 12:1 SWR. A balun will NOT reduce SWR in a situation where there is a gross mismatch.
This means that unless you use a very high quality, LOW LOSS coaxial cable, for the transmission line, you will lose considerable power in line losses, especially
if your transmission line is long. At high power, you might have arcing problems in the coax. The only real solution is to put a tuning network (aka "Tuner") at the base of the antenna. ....and don't forget, that if the tuner is trying to match two widely separated impedances (i.e. the antenna feed point Z to the coax Z) you likely will
have moderate losses in the tuner itself.  There is no free lunch!

Bottomline: with the exception of 40 and 15 M, due to the harmonic relationship, there is no simple way to make a single monopole radiator work EFFECTIVELY on all bands
without either spending a lot of money on super low loss coaxial cable or by putting a Tuner at the base of the antenna.

You just can't get something for nothing in the antenna game.

73,  K0ZN
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K2OWK
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Posts: 1065




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« Reply #10 on: May 25, 2012, 09:10:48 PM »

I thought a 43 foot vertical was able to operate on multi-bands (80 thru 6) if you use a 4 to1 UN-UN and a tuner. Am I wrong?

73s

K2OWK
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WB6BYU
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Posts: 13331




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« Reply #11 on: May 25, 2012, 09:57:16 PM »

Quote from: K2OWK

I thought a 43 foot vertical was able to operate on multi-bands (80 thru 6) if you use a 4 to1 UN-UN and a tuner. Am I wrong?



Any antenna can be advertised as operating on any band - it all depends on your criteria.

There are a number of constraints, depending on your definition of the term "operate".  Yes, you can
match it with a tuner and get a low SWR.  (In that respect it works from 160m through UHF.)  But
you also have to consider the losses in the feedline, the radiation pattern, and the antenna
efficiency.  A useful antenna is a mixture of compromises among these three on each band, in
such a way that fits your operating needs.

For example, you can load such an antenna on 160m with a wide-range tuner.  The SWR on the
coax will be high, so losses will be significant.  Ground losses may be high because the antenna
has a low radiation resistance.  And the voltages at the feedpoint may be beyond what the
mounting insulator could handle if you really had an efficient antenna, but fortunately you
don't, so you might get a few percent of your power radiated.

As you go higher in frequency and the antenna is longer in terms of wavelength, the radiation
resistance increases and the efficiency improves.  (But the losses in the feedline will also
increase.)  When the antenna exceeds about 5/8 wavelength (20m for a 43' antenna) then
the radiation pattern breaks up in to multiple lobes, and much of your power can be wasted
at high angles where it doesn't get reflected back to Earth.  So performance drops off on the
higher bands, but probably still useful on 15m and 10m.  You might make a few contacts on
6m, but a standard quarter wave ground plane will work much better.

The SWR varies from one band to another:  in some cases it is better with a 4 : 1 un-un,
while other bands it may be better with a 1 : 1.  VK1OD's article that G8JNJ linked to shows the
overall feedline losses on each band:  some people may be content with a 3dB loss (half their
transmitter power) in the feedline, others wouldn't consider such an antenna as "operating"
on that band because of it.  Others might only accept an antenna that had an SWR below
2 : 1 without needing a tuner, in which case this antenna would not fit their criterion on
any band.  But with low-loss coax it is capable of reasonably efficient operation across the
HF ham bands using a tuner in the shack, so many hams may find it an acceptable
compromise in some circumstances.

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W5WSS
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Posts: 1742




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« Reply #12 on: May 26, 2012, 12:22:45 PM »

I am using a single 90 ft. length of #14 thhn copper insulated wire that is directly connected to an LDG 200 pro auto tuner. The losses are tolerable via the tuner but there is no feedline. The line connects directly to the tuners center hole of the so 239 with a crimp and solder banana plug. The tuner uses common shield to the ground lug without going thru the tuner network allowing radials to be added directly from the tuner to a not grounded condition elevated counterpoise for the vertical to push against directly underneath the radiating element. This helps to calm the displacement currents trying to find a way back to the operating position. The tuner finds a match for the rig that is only 2ft coaxial pigtail away. 100 watts enf fed multiband end fed zep vertically oriented quiet and having a ball working dx. the tip at the top end is high up about 85f tuse it on 6,10,12,15,17,20,30,40,60 and 80 no balun no common mode choke just brought/routed the wires physically parallel to form a roughly 300 ohm transmission line and connected them to the tuner.73
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K0ZN
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Posts: 1553




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« Reply #13 on: May 26, 2012, 10:27:39 PM »

 To respond to K2OWK.

 ANY antenna will "operate" (i.e. radiate energy) to a greater or lesser extent, as WB6BYU points out.... the main issue is: will the antenna be EFFECTIVE and EFFICIENT !!

 YES!  You *can* feed power to a 43 ft. vertical with a 4:1 balun at the base.... is that the BEST or most efficient way to transfer energy from a 50 ohm coaxial
 cable to the antenna??   NO.  The 'best' way would be to have a dedicated matching network (aka: "Antenna Tuner ) at the base of the antenna. This would allow
for a 1:1 swr and Low loss in the transmission line between the antenna and the transmitter.

You *could* also feed it with a 1:1 balun or NO balun! ...for that matter you *could* feed it with 450 ohm ladderline or even Home Depot AC lamp cord!
Ladderline in particular, would have pros and cons but certainly would *work*...and maybe better than coax if the line run was very long because
the high SWR that would occur due to the mismatch between the transmission line would cause much higher losses in coax than ladderline. That said,
feeding an UNbalanced antenna with a balanced line is not the best way to go about it, but it WOULD "work". ( I have seen commercial/military vertical
antennas fed with open wire line; it CAN and has been done!)

Again, keep in mind the ONLY band that will show a good match (the antenna feedpoint impedance = the coaxial cable impedance) is 60 M. On  ALL  the other bands, with
a 43 ft. vertical there will be a BAD mismatch between the antenna feedpoint impedance and the coax resulting in 'moderate' to very high SWR. Coax is not "happy"
and is quite lossy at high SWR. The use of a balun does NOT change or reduce the mismatch problem very much and CAN actually make it WORSE on some bands.

A balun is NOT a "magic box" that makes mismatch problems just disappear ! In SOME impedance mismatch situations, a balun will help a good deal...in others
it won't because it cannot operate "properly". A balun is a specific type of TRANSFORMER and has certain purposes and CONDITIONS in which it works best and most efficiently. It is a "Tool". So is a "Crescent wrench"!.... you *CAN* drive nails with a crescent wrench, but it is not the best tool for that...same
with a balun. There are function where a balun works properly and best and others with which it is like a crescent wrench driving nails! You can drive the nails
(put a balun at the base of a vertical) but a hammer would drive nails better (use a proper matching network). It is all relative.......

A "balun" at the base of a 43 ft. vertical is NOT operating "properly". It *may* choke some RF off the coax shield and it *may* help the
impedance mismatch on SOME bands.....  To wit: a BALUN is intended and works PROPERLY when it is connected to a BALANCED and minimally reactive load.....
and the base of that vertical is an UN-balanced, and on some bands, HIGHLY reactive load. This is not a condition where a balun is optimal....but it does
perform/provide some function in this situation.

Again....what will "work" and what will work the best are not necessarily the same thing. Even if you have a "messed up situation" that loses 50% of the energy put into
it, that is only a 3 db loss.... i.e. your 100 watts out of the transmitter will only have 50 watts radiated, (-3db) but you can work a  TON  of stuff on 50 watts and *think*
your antenna is working just fine. (but it would work even better if you could get all 100 watts to radiate...although the additional 3 db is NOT really a big deal to the
guy hearing your signal 1,000 miles away...minimal difference on his S meter.)

The other issue with a 43 ft. vertical is that on the upper HF bands, they radiate MOST of the energy at a very high angle which is *usually* not very effective.
...i.e. On the bands above 20 M: 17, 15, 12 and 10, most of the energy is at a 45 degree or higher angle, which is NOT effective for those bands. Now, that said, SOME of the energy will be radiated at a lower angle and probably "useable", but certainly FAR from optimum.  Note that NONE of the radiation angle issues have anything to do
with how the antenna is fed!!  ....it is just a function of the length of the vertical radiator.

All of the above is covered in much more detail and with graphics in the ARRL Antenna Book.  Understanding what REALLY happens with ANY antenna will go a long
way towards giving you a bigger signal and more fun and less frustration in this hobby. There is a LOT of difference between "myths and rumors", advertising
hype and the FACTS of antenna matching and radiation patterns. Some time studying the ARRL Antenna Book will sort and expose these things.

BOTTOMLINE:  Yes. You can feed a 43 ft. vertical on all bands with a 4:1 balun** at the base. Just be sure that you fully understand that the SWR will be HIGH on ALL
                     bands in this situation. On some bands VERY high. Such being the case, unless you are willing to tolerate the loss of a significant amount of your
                     transmitter output power in the coax, it is necessary to use a VERY high quality coax to prevent high losses from the high SWR. One then MUST
                     have a good low loss Tuner in the shack to create a match that the rig will work into.  Using a large diameter, low loss line like LMR-400 or BIGGER
                     or Hard Line probably would work pretty well because the losses induced by the high SWR would be minimal. Using a small diameter or cheap coax
                     would be very unwise in this situation. Placing an Auto Tuner (or other Tuner or matching network) at the base would be the best way to go.
                     But, again, understand the reduced losses, etc. has NOTHING to do with a poor angle of radiation on the upper bands. That is a separate issue.

            73,  K0ZN

  **  Note that you MUST have a 4:1 balun that is designed and intended to work at a high SWR....which also means high voltages in the balun. Not all 4:1
        balun will stand up to that kind of service.   i.e. One basically needs a "tuner" type balun that has more insulation in the design; these are commercially
        available from several sources.
« Last Edit: May 27, 2012, 05:10:25 PM by K0ZN » Logged
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