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Author Topic: When to use a 4:1 or 1:1 balun?  (Read 6854 times)
KJ4RQV
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Posts: 130




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« on: February 05, 2013, 11:04:00 AM »

I have put up several dipoles and followed someone else's plan and they work well. Mine are fed with 450 ohm ladder line to a 4:1 current balun and the signal is carried to the shack over RG 58.
As I wrote, this works.
I want to understand why that balun is the one to use and why not a 1:1 or a 9:1or something else.

I am hoping someone will explain this to me. I like to know the why of things, not just the how. I think it will help me to be a better antenna builder.
Don
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WB6BYU
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Posts: 13574




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« Reply #1 on: February 05, 2013, 12:38:50 PM »

Are your dipoles 1/2 wavelength long, or some other length?

Are you using a specific length of twinlead to match them, or a random length?

What is the SWR on your coax if you don't use a tuner?

All of these affect the answer.



Some of the basic principles you need to understand the various facets of what
is happening:

1)  Baluns are designed to operate at a specific impedance level over a given
frequency range.  When used outside that range they will still do something, but
might not work as you would expect.  Most antennas do NOT match the impedance
of the balun, especially when feed through a length of ladder line.

2)  Feedlines act as an impedance transformer when they are not perfectly matched.
This is usually demonstrated using resistive loads and quarter wave feedlines:  for
example, with a 100 ohm load connected through 1/4 of 50 ohm coax it will actually
look like (50 * 50 / 100 = ) 25 ohms at the rig end.  Using a half wavelength of coax
brings it back up to 100 ohms, and so it goes down the feedline.  In between these
points the resistance will take on an intermediate value, along with some reactance
so that, in this case, the SWR will be (100 / 50 = ) 2 : 1 at every point along the
coax, even though the impedance is changing.

3)  All feedlines have loss.  One possible definition of an antenna "working well" is
that most of the power generated by the transmitter is radiated by the antenna,
and to do that we need to keep the losses low in the feedline.  Generally losses
increase with SWR (though there are some exceptions for very short lengths.)  Many
hams can have high feedline losses and not notice it.


So now lets put these together and see if we can give you some examples.

a)  a full wave dipole (2 half waves in phase) has a high feedpoint impedance, let's
say 4000 ohms.  (In practice it depends on the diameter of the wire, etc.)  If we
connect 1/2 wavelength of ladder line between this and the balun, the balun will
also see 4000 ohms.  A 4 : 1 balun would, in theory, step this down to 1000 ohms,
but in practice it will depend on balun construction.  Connecting RG-58 to the
1000 ohm load gives us an SWR of 20 : 1, and if you use 50' of it on 20m then
about 2/3 of your transmitter power goes to heating the coax, and 1/3 gets
radiated.

I make liberal use of VK1OD's transmission line loss calculator for such problems:
http://vk1od.net/calc/tl/tllc.php

b)  Same situation, but we'll use 1/4 wavelength of ladder line.  Now there will
be a transformation of impedance and the balun will see (450 * 450 / 4000 = )
50.6 ohms!  We'd now choose a 1 : 1 balun and have a very low SWR on the coax.
now only 17% of your power is dissipated in the RG-58.

See how a small change in feedline length makes a big difference in efficiency?

c)  What if we used a half wave dipole instead.  Let's say that the impedance is
50 ohms (it will depend on height above ground, among other things.)  With a
half wave of ladder line we'll again see 50 ohms at the balun, so would use a
1 : 1 balun and have a low SWR on the coax.  But with a quarter wavelength
line we're back with ( 450 * 450 / 50 = ) 4050 ohms at the balun, and even
a 4 : 1 balun will still give us a 20 : 1 SWR on the coax, with high losses.

d)  Now you can get creative.  If you choose an antenna with a feedpoint
impedance that gives a 9 : 1 SWR on 450 line, at some point the impedance
along that line will be ( 450 / 9 = ) 50 ohms.  Similarly if you find a point
where the SWR is 2.25 : 1, then there will be points where the impedance is
( 450 / 2.25 = ) 200 ohms, and a 4 : 1 balun should give you a good match.

But in either case, changing the length of either the antenna or the feedline
will result in higher SWR.


e)  Another common approach is to use, say, an 80m dipole fed with ladder
line as an all-band antenna with a tuner in the shack.  In this case the
antenna impedance varies widely from band to band, as does the length of
the ladder line in wavelengths.  (The physical length stays the same, but the
electrical length varies because the wavelength changes with frequency.)
So we end up with a relatively random impedance at the balun.  You can use
either a 4 : 1 or 1 : 1 here - the 4 : 1 voltage balun was commonly used,
and while a modern 1 : 1 current balun is often a better choice, the difference
will depend on the actual impedance you have to match on each band.
That presents a relatively random impedance to the coax, so it may operate
with a high SWR on some bands, but reasonable on others.  My advice in
such cases is to keep the coax as short as possible (inches are better than
feet) to keep efficiency high.


One or more of these might apply to your specific antennas, but at least
it will give you a start on the underlying principles.  If you can give some
specific examples we can work them through.
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N4JTE
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Posts: 1159




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« Reply #2 on: February 05, 2013, 02:54:30 PM »

W6BYU; That was the BEST, well written, understandable answer I have ever read on a complex subject.
Thank you for sharing your extensive knowledge here on eham.
Bob
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AF5C
Member

Posts: 123




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« Reply #3 on: February 06, 2013, 10:19:05 AM »

I was getting ready to ask a question similar to this.  I have a G5RV type antenna I homebrewed. It is around 100 feet long, fed in the center with around 25 feet Radio Shack twin lead (300ohm).  At the end of the twin lead is currently a HI-Q brand 4:1 voltage balun, and then a run of 75ohm coax to the shack.  I want to replace the balun with something that will keep RF off of the coax shield-I understand I need a current balun for this.  What are the best brands and models to use for this?

73 John AF5CC
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N3JBH
Member

Posts: 2358




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« Reply #4 on: February 06, 2013, 12:13:06 PM »

John i will say a quality 1:1 current balun is what you need not a 4:1. Also while i wont point out and brand names i suggest you look at ones from Balun Designs, D Engineering, Array solutions, Or just slide a bunch of 43 mix torriods of the existing coax and seal them up some how. My only other comment i make here is like anything else if the product is cheap there a reason for it ! Stay with some thing that is quality if if it cost a little bit more.
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WD4ELG
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Posts: 880




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« Reply #5 on: February 06, 2013, 12:23:41 PM »

WB6BYU, how about submitting that writeup to QST?  Well done.
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NK7Z
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Posts: 880


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« Reply #6 on: February 06, 2013, 12:41:10 PM »

WB6BYU:  Thank you very interesting and very well spoken!
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
WD4ELG
Member

Posts: 880




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« Reply #7 on: February 06, 2013, 12:41:19 PM »

Let's also discuss the intent of a 1:1 current balun.  With "balanced" antennas like half wave dipoles (and even "unbalanced" antennas like a vertical), we want to prevent common mode currents (current running back along the outside/shield of the coax).  The 1:1 current balun does this for us, and is useful for all antennas.  Common mode currents are wasted energy and undesired RF radiation from the coax, plus possibly RF in the shack. We want the RF to maximized at the radiator.

A good article from W8JI - even "balanced" antennas like dipoles should consider common mode issues.

http://www.w8ji.com/common_mode_current.htm

Another consideration is coax losses.  6:1 SWR on coax is a LOT of heat and wasted RF for a length of something like 100 feet of commercial grade cable.  6:1 on ladder line?  WAY LESS LOSS.  So we can live with a mismatch with the ladder line.  (For the history buffs, ladder line was used by ALL hams way back in the day.  It was only after WW2 when we had surplus of coax hit the market that we started using it).

Another personal note is that I never really worried about common mode RF with my dipoles back in the day with my tube rigs, until I got "nipped" when I touched the transmitter while running it into a dipole that was not cut for the specific band.  I then built one of those cheap radio shack RF field meters from a kit, and started sniffing around the shack.  There was a LOT of undesired RF in the shack!  But I was learning.  This was back when I was 13.  Maybe that's why I am the way I am today?

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KJ4RQV
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Posts: 130




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« Reply #8 on: February 06, 2013, 01:31:53 PM »

Thanks for all the help!

I have a much better understanding of the subject now and being old school, I have printed all of this off so that I can REALLY study it at my leisure.
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AA4HA
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Posts: 1640




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« Reply #9 on: February 06, 2013, 03:01:05 PM »

WB6BYU; It is seldom that I see someone who has such a good grasp of the fundamentals. Excellent write-up.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W4RS
Member

Posts: 66




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« Reply #10 on: February 22, 2013, 11:34:14 AM »

thanks for such a wonderful article abut balun's.
still i always buy the best coax i can, as 90 percent of your signal will radiate from it!
jim, w4rs
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W5DXP
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« Reply #11 on: February 22, 2013, 01:22:19 PM »

I have a G5RV type antenna I homebrewed. It is around 100 feet long, fed in the center with around 25 feet Radio Shack twin lead (300ohm).  At the end of the twin lead is currently a HI-Q brand 4:1 voltage balun,

Any "G5RV-type" antenna that is close to the dimensions of a Standard 102', 30' of twinlead G5RV should use a 1:1 current-choke-balun, not a 4:1.
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KF5PGT
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Posts: 38




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« Reply #12 on: February 23, 2013, 06:08:21 AM »

Excellent explanation! But I have a question.

How do baluns effect receiving? I see a lot of info about maximizing transmission but it's hard to find info on maximizing reception.
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W4OP
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« Reply #13 on: February 23, 2013, 06:42:32 AM »

The balun issue for receiving is  a bit different. For HF receive, our noise floor is not limited by the receiver front end noise as it is on VHF and up. At HF, the limiting factor is :
1.Man made noise
2. Galactic Noise
3. Atmospheric noise

You can easily prove this by tuning your receiver to a dead portion of the lower HF band and removing your antenna- the noise floor will drop.

With that said then, antenna efficiency means little for HF receiving. Loss because of lossy coax, reflection loss etc does not change the signal to noise ratio at the receiver because signal and noise are both attenuated equally.

So does a balun help? The answer is yes, because without it, the outside shield of your feedline becomes part of your antenna. It will pick up noise from all the noise sources in your home, transfer that noise on the outside of the shield (remember, because of skin depth, the outside  surface of the shield is electrically separate from the inside) to the antenna where that noise is picked up and brought back to your receiver on the inside of your coax (differential mode).

So, you may have worked very hard to locate your antenna away from your house, only to have that low noise location compromised by common mode noise.

The other issue of course is that because the coax w/o a balun is now part of your antenna, it will alter the pattern of your dipole, beam etc.

Dale W4OP
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KF5PGT
Member

Posts: 38




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« Reply #14 on: February 23, 2013, 08:09:06 AM »

Ok, so if a 1:1 balun connected between the feed line and the antenna enhances transmission, would a 1:1 balun connected between the transmitter and the feed line enhance reception?
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