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1-3 of 3 messages
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Balum versus Unum
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by K2AAU on April 24, 2005
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To All:
There are so many products out there when it comes to a simple dipole.
Me personally, I want to keep my budget low and build my own feeding it with RG8/x. The questions that I am duanted with is to whether to use a Unum or balum[1:1 or 4:1] for the dipole.
I plan on building a dipole 70-85 feet long running coax into the shack. I am considering using a Line isolator to prevent RF coming down the coax or creating my own choke[How many turns of coax?] before it goes into the shack.
Tuner - Big question manual vs. automatic. I don't think that obstacle will be much of a problem since there are many fine review here on e-ham to help me out.
Does anyone have some idea's that they could share with me.
Thanks in advance!
73
k2aau
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RE: Balum versus Unum
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by W7DJM on April 25, 2005
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You need to go "find" some antenna theory. There is plenty of it to be found here on the internet, starting with AC6V and W4RNL.
That being said, you sound as if you are proposing to put up a "nonresonant" dipole, or doublet, which means that it will have wild excursions of impedance, depending on which bands you are going to try and use it on.
I'd like to know "where" you got that length? Is that the room in your yard? Do you have room for a resonant length on 75M? Can you put up a flatop/V of so long, and let the extra length droop towards the ground? (A resonant length on 75 is on the order of 126 feet.)
There is only one solution. That is ladder line/open line.
Coax will have very high swr and high losses. What you propose (in length) isn't even resonant on 40m.
Run the ladder line ALL THE WAY TO THE TUNER if at all possible.
If not, run the ladder line AS CLOSE to the tuner as possible, and by that I mean a few feet, put a BALUN at that point and coax the rest of the way IF YOU MUST. Far better to run the ladder line all the way to the tuner.
You want a BALUN not an UNAN
BALanced to UN balanced
UN balanced to UN balanced, just like the "names."
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RE: Balum versus Unum
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by WB6BYU on April 25, 2005
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Let's start back at the beginning.
What bands do you want to operate? What space do you have
available?
I am guessing from your description that you have chosen
a non-resonant length of wire so it won't have a very high
impedance on any band. (This is harder now with the extra
WARC HF bands.) The SWR will still be rather high
on many bands: this presents three problems. First, the
coax losses will increase. Actually, at HF with a
relatively short feedline, this probably is the least of
your worries. Yes, your signal may be weaker than it would
be without the additional loss in the coax, but if this is
the most practical antenna for you, you can live with it.
Second is the impedance seen by the tuner. A high SWR
means a wider range of impedances at the shack end of
the coax cable. Many tuner circuits are not particularly
efficient at very low impedances (and sometimes at high
ones.) I've seen commercial tuners where the coil form
melted at 100 watts in extreme cases. Since the actual
impedance seen by the tuner is a function of the coax
length, try adding an extra 10' of coax if you run into
matching problems.
The third problem is the higher voltage/current points
you may encounter. Remember that RG-8X has foam insulation
that can be compressed if step on it, drive over it,
or tie-wrap it too tightly. Compressed foam has less
voltage breakdown. Presuming you take good care of your
coax and run 100 watts, I expect it should take it.
Higher power combined with high SWR, small-radius bends,
or physical damage could cause problems.
(The use of ladder line to feed the antenna reduces
these issues.)
Baluns are an interesting question. Ideally you would
use one, since the antenna is balanced while the feedline
and tuner are not. But many of the balun designs don't
work well with loads that are far from 50 ohms, as your
antenna will be on most bands. In your situation I'd
recommend the W2DU-type, made by stringing ferrite
beads on a piece of coax, or simpling coiling some
extra coax into an 6" to 12" diameter coil. This
forms a 1 : 1 feedline choke which will tend to reduce
the currents flowing on the outside of the coax. (The
actual effectiveness will depend on a number of factors,
including the impedance of the antenna and the position
of the choke along the feedline.) Commercially you may
find the same structure sold as a BALUN (if designed to
be mounted directly at the antenna feedpoint) or a
"line isolator" or "unun" if it has coax connectors on
each end. The only difference is the type of connectors
on the antenna end: internally it does the same job.
And you may find that it works well enough without any
such balun/unun/lump in the feedline. But if you have
problems with RF in the shack, you can try such a device
either at the feedpoint or wherever else is convenient
in the feedline. The optimum location may change with
the band used, so at that point it really becomes a
matter of trying it out to see what works best for you.
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