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1-10 of 11 messages
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4:1 balun with coax & dipole
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by KG4OXP on June 25, 2002
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What would the results be if you run your dipole wires right off the terminal posts of a 4:1 balun..(mini 8/u coax fed balun)?
Just wondering if you have to run ladderline or 300 twin lead from the balun to your dipole wires??
(versus using a 1:1 matching balun)
Thanks
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RE: 4:1 balun with coax & dipole
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by N2MG on June 25, 2002
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The antenna will be ~70 ohms. A 4:1 balun is unnecessary and incorrect as your coax is ~50 ohms. Using ladder line as you describe would serve no purpose.
If you are worried about your coax radiating, you could use a 1:1 balun, or simply a choke balun of ferrite beads or even simpler, just several loops of coax.
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RE: 4:1 balun with coax & dipole
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by KC5SAO on June 25, 2002
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I agree with N2MG, but just kind of curious what frequencies were you thinking about operating
on. If you have the 4to 1 balun on hand, hold on to it you can always use it for a project that
calls for it.
Tom KC5SAO
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RE: 4:1 balun with coax & dipole
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by K3WVU on June 25, 2002
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On that subject, I've found that using the split beads in the snap-on plastic holders helped prevent the coax from radiating. On 40M, despite showing a 1:1 match at the rig and tuner, the meter was showing that my power output was around 50%. I solved that problem by installing several of the aforementioned split beads on the coax just below the feedpoint of the dipole I was using. The split beads are available in several sizes from Surplus Sales of Nebraska (www.surplussales.com).
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RE: 4:1 balun with coax & dipole
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by RobertKoernerExAE7G on June 25, 2002
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4X50=200
If your antenna is 100-400 ohms (2:1 swr range), you don’t have to worry about feed line losses due to high VSWR.
What you want to stay away from is high SWR on a medium or greater run of coax; because coax, unlike other feed lines, is lossy under those conditions.
Any ARRL Hand Book (old ones are cheap on eBay), or ON4UN’s book will give you a good back ground to understand SWR, feed line loss, and simple wire antennas. Of course, ON4UN’s book will go into greater detail about a wider variety of antennas.
Have FUN
Bob
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RE: 4:1 balun with coax & dipole
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by AC5E on June 25, 2002
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Hi: I certainly agree that an ARRL antenna handbook is a great value to every ham. But the first thing I would suggest you look at is the chart of dipole impedance vs height above ground in wavelengths.
It's worth noting for starters that's the real ground, the surface for some wet and mineralized soils, and possibly hundreds of feet down for a deep bed of dry sand.
You will see that a dipole's impedance may be as low as 15 ohms (3:1 or so SWR) for a dipole just above ground to about 120 ohms (2.5:1 SWR) and some distance above ground. That means the last thing you want to use to feed a dipole with is high impedance feedline - because that would almost certainly change a moderate SWR to high SWR. Very few modern rigs with built in tuners will work well with high SWR.
If a dipole is resonant at or near your desired operating frequency you should not need a balun of any sort. If you have RF in the shack a dozen or so bead baluns at the feedpoint will choke off the reflected RF, making the antenna much easier to tune and to live with.
For a more complete discussion of baluns, go to www.radioworks.com and read up on the subject.
73 Pete Allen AC5E
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RE: 4:1 balun with coax & dipole
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by K0BG on June 26, 2002
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Excuse me guys, but coax and twin lead (ladder line, whatever) DON'T radiate if there is a conjugate match between the load and source. This "radiation thing" is another urban myth just like lost power caused by high VSWR-It just ain't so!
Still in doubt? Download a copy of Maxwell's "Another Look at Reflections" from www.arrl.com and read for yourself.
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RE: 4:1 balun with coax & dipole
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by AC5E on June 26, 2002
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As I previously noted - ladder line will not match, conjugate or otherwise, a dipole. In fact, it will make a fair match with coax into a mismatch most "modern" rigs won't match. If the rig ain't happy ain't nobody happy!
Therefore the "great debate" about feedline radiation from matched ladder line, twin lead, or whatever, is moot.
Pete Allen AC5E
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RE: 4:1 balun with coax & dipole
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by NB6Z on June 26, 2002
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What would the results be if you run your dipole wires right off the terminal posts of a 4:1 balun..(mini 8/u coax fed balun)? If it is 1/2 wave dipole, you will get a big mismatch to your coax... You would not want to run twinlead to the balun instead of coax, because the line input to the balun is unbalanced and not suited for balanced twinlead. You would use a 1:1 balun for coax to 1/2 wave dipole, or leave out the balun and connect directly to the center of the 1/2 wave dipole. Another possibilty is to run a relatively short length of coax to the unballanced side of the 4:1 transfomer and use 300 ohm outdoor TV lead to the dipole. This is not as effecient as coax for a dipole but could give you better options for loading on other bands.
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RE: 4:1 balun with coax & dipole
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by WB6BYU on June 26, 2002
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Let's see if we can get back to the original question...
Baluns are often installed at the feedpoint of a dipole. If it is a
half-wave dipole, a 1 : 1 balun will give a lower SWR on common
types of coax cable than a 4 : 1 balun would.
If you are only working one band with a half-wave dipole, it would
be easier to connect the coax directly to the antenna wires without
a balun than using a 4 : 1 balun. I know, this may not seem like
the ideal approach, but thousands of hams have done it this way
for over 50 years, and it actually works fairly well (in most cases.)
However, if you want to use a single dipole on several bands, the
impedance will vary widely from band to band. This means that the
SWR will be very high on some bands, and coax cable can be very
lossy at high SWR. This is where many hams use twinlead or open
wire line ("ladder line"). These have much lower losses when run
at high SWR. This approach requires a good tuner in the shack to
match the impedance at the end of the feedline. Overall, losses are
lower than when coax is used.
Now, I will guess that your question came up because some folks
recommend running the twinlead from the antenna to a 4 : 1 balun
at the point where the feedline enters the house, with coax from
there to a tuner in the shack. This has the advantage that coax is
easier to run through a building than twinlead or open wire line.
(Most antenna tuners with balanced output have a built-in 4 : 1
balun, so the only difference is the length of coax connecting the
balun to the tuner.) Since this coax will be operating at a high
SWR, efficiency will suffer if it is too long, but if kept reasonably
short the system will certainly work. So the reason that hams use
twinlead between the 4 : 1 balun and the antenna is to reduce the
feedline losses when the system is operating at a high SWR.
Now, one other thing you need to learn about transmission lines is
that they will change or transform the impedance when they are not
perfectly matched: the impedance seen by the rig at one end of a
transmission line is a function of the load connected to the other
end, the length of the transmission line (in wavelengths at the
operating frequency), and the impedance of the transmission line.
Here is an example: say we have a 40m dipole feed with 30' of
twinlead to a 4 : 1 balun, then 25' of coax to the rig. If the dipole
has an impedance of 50 ohms and the twinlead is 300 ohms, then
the impedance at the bottom of the twinlead will be 300 * 300 / 50
= 1800 ohms. (I picked lengths that made it easy to calculate
because they are about a quarter wavelength. Other lengths will
give different values.) If we assume the 4 : 1 balun works at this
high impedance, it would step the impedance down to 450 ohms.
The 50 ohm coax will transform this to 50 * 50 / 450 = 5.56 ohms
at the tuner. (The SWR on the coax is 450/50 = 9 : 1.) While this
impedance can be matched my many tuners, tuner efficiency often
suffers at low impedances.
If we extended the coax and put the balun at the antenna feedpoint
(50 ohms), it would present a load of 12.5 ohms to the coax. The
swr would be 4 : 1 on the cable, and the tuner would see the same
12.5 ohms (because the cable is now half a wavelength long.) So
in this case, the antenna would be easier to match - and possibly
more efficient - with the balun at the feedpoint. (Of course, if the
coax were connected directly to the dipole, the SWR would be
1 : 1 and no tuner would be needed.)
Now let's see what happens when we try tuning this antenna up on
20m. The dipole is now a full wavelength, and the impedance is
around 2000 ohms. Each piece of feedline is a multiple of half a
wavelength, so there will be no impedance transformation. The
balun in both cases sees 2000 ohms and presents a load of 500
ohms to the coax (SWR is 10 : 1 in both cases). The tuner also
sees 2000 ohms, which, though high, will usually be matchable
without too much loss. The difference between the two approaches
is that in one case we have 25 feet of coax with a 10 : 1 SWR, and
in the second case we have 50 feet with the same SWR, so the
loses in the coax will be double. (In the first case, there is also
a section of twinlead operating at an SWR of about 7 : 1, but this
won't account for much additional loss.)
(Of course it isn't always this bad, depending on the lengths of the
antenna and the feedline sections. For example, if the antenna had
an impedance closer to 1000 ohms on 20m, and it used 15' of 450
ohm twinlead, then the balun would see very close to 200 ohms,
and the coax would see a good match to 50 ohms. Then the losses
would be relatively low, and you wouldn't need the tuner. But for
multiband operation we rarely have a combination that works well
on all bands. Sometimes you get lucky, sometimes you don't.)
So that is why the general recommendation is to use twinlead or
ladder line to replace as much coax as possible when the system
is operated at a high SWR, as is often the case when a single
dipole is operated on different bands. But it doesn't mean it won't
still work if you do something different.
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