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Author Topic: Dipole fed with 450 ohm ladder-line vs coax  (Read 13442 times)
N3BSZ
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Posts: 58




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« on: February 11, 2012, 10:44:19 AM »

I have a dipole cut for 80 meters fed with 450ohm ladder-line.  I want to switch to an auto-tuner so I can run digital modes remotely from inside the qth, but not in the shack.  I will VNC into the PC running HRD.

I know that I can use a 4:1 balun, but I am not sure if I need a current or voltage balun.  I also am not sure if an LDG type auto tuner will match the antenna.

If this will not work, would I be better with Hy-gain AR-620 or 640 vertical?


73, 

Tom
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WB6BYU
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Posts: 13580




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« Reply #1 on: February 11, 2012, 11:47:08 AM »

A current balun would be best in that application.  You'd have to experiment to see
whether a 1 : 1 or 4 : 1 actually works better - it depends on the feedline length and
what bands you are trying t match it on.  (If you are just using it on 80m, then using
coax all the way to the antenna, or cutting the feedline to 1/2 wavelength, would
probably give the easiest matching.)

Whether any of the tuners can match it will also depend on the feedline length, the
choice of balun, and the operating frequency.  Some combinations may be difficult
to match on some bands but not on others.  Knowing the feedline length and
the bands of interest, we could model it and see what impedances you might encounter.
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N4IAG
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Posts: 49




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« Reply #2 on: February 11, 2012, 11:58:50 AM »

Hi, many tuners have a built in 4:1 current balun for the open line feeder, my MFJ-993B does anyway. That dipole + ladder line and an auto tuner will work FB for you.
« Last Edit: February 11, 2012, 02:03:01 PM by N4IAG » Logged

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N3BSZ
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Posts: 58




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« Reply #3 on: February 11, 2012, 01:00:59 PM »

Feedline of choice would be LMR-400 or RG-213 and around 60' of cable.  The antenna is mounted at the eve of the roof with the feedline coming down the side into the basement. 

I would like to work 80-6 WARC bands are not on the list of needed bands.


I did not look at the MFJ.  I will need to read more about it.
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WA9YSD
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Posts: 138




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« Reply #4 on: February 13, 2012, 07:23:46 PM »

Is it true that if your dipole at the feed point is 50 ohms, you connect 450 ohm ladder line you have a 9:1 mismatch, and that miss match gives you around 25% loss of radiated power?

Jim K9TF
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N3BSZ
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Posts: 58




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« Reply #5 on: February 13, 2012, 08:02:05 PM »

It sounds about right, atleast the mismatch, but the antenna feedpoint is only 50 ohms at resonant frequency.  Any other frequency the impedance could be inductive or capacitive. 
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WB6BYU
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« Reply #6 on: February 13, 2012, 08:51:47 PM »

Quote from: WA9YSD
Is it true that if your dipole at the feed point is 50 ohms, you connect 450 ohm ladder line you have a 9:1 mismatch, and that miss match gives you around 25% loss of radiated power?


No.

You could create such a situation using 300 feet of Wireman 553 twinlead on 80m, and the
losses would be about 25%.  But with 100' the loss would be less than 10%.

With a short enough feedline the loss is negligible, even though there is substantial reflected
power on the twinlead.

The fallacy of this argument (though usually it would quote a higher loss) is that the reflected
power is NOT necessarily lost.  It may be reflected from the load, but then it reflects off the
tuner and back to the load.  Eventually most of it is radiated, with the exception of some that
gets lost as heat due to higher currents in the wires.

The actual losses depend on the type of twinlead, length, frequency, and especially whether
the twinlead is wet or dry.
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W5DXP
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« Reply #7 on: February 14, 2012, 05:11:23 AM »

You could create such a situation using 300 feet of Wireman 553 twinlead on 80m, and the losses would be about 25%.  But with 100' the loss would be less than 10%.

And the matched line loss in 100' of RG-58 would be almost 20% at 4 MHz.
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KF7CG
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« Reply #8 on: February 14, 2012, 10:04:20 AM »

If you could convert the 80 meter dipole to a folded dipole for the same frequencies you would now have approximately 4 times the feedpoint impedance and a broader bandwidth at resonance.  Folded dipoles when they can be erected are often an overlooked friend.

KF7CG
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WB6BYU
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« Reply #9 on: February 14, 2012, 12:18:15 PM »

But folded dipoles aren't as good for multi-band - they are particularly bad on even harmonics (like
40m and 20m.)

If you are using ladder line to allow the use of the antenna on multiple bands, a folded dipole is NOT
a good solution.

If you are using it simply to decrease the losses, it won't really make much difference compared to
coax unless the line is very long.
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K0ZN
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Posts: 1563




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« Reply #10 on: February 14, 2012, 09:44:00 PM »


If you want to use an 80 M dipole on all bands, you want to MINIMIZE or eliminate the coax portion of that system. The losses, for practical purposes
will be very minor in the ladder line, even with 10:1 SWR. SWR is an electrical CONDITION that exists on the transmission line.....it does not CAUSE "loss".
The loss is caused by the type of transmission line (i.e. the ohmic loss in the conductors and the loss in the insulation material). With coax, the loss would be quite high; probably very high on the upper bands and intolerable on 6 M. With 450 ohm ladderline the loss would be minor....with true 600 ohm open wire line the loss, would be negligible.

One of the unending myths in ham radio is that SWR causes loss. This is because so many hams don't really understand SWR and transmission line theory and they also use coax....and then, yes, the losses get bad quickly. Open wire line is, by far, the lowest loss transmission line typically used at HF. A 200 ft. run of 600 ohm open wire line with # 12 wire at 10:1 SWR would have very small loss on 15 M where as 100 ft. of RG-8 coax at 2:1 would have significant loss at 21 Mhz. "it is all relative...."

In terms of loss, the best situation is to run ladderline from the antenna feedpoint into the shack to a balanced tuner or a tuner with a GOOD low loss balun and not
use any coax. Next best thing would be a SHORT run of coax from an UNbalanced tuner to a good quality 1:1 current/choking type balun. With a high SWR, even
20 ft. of coax is significant on the upper bands. A 4:1 balun is typically a voltage balun and this is not the best type. The 4:1 ratio looks like it would be good, but in reality, the balun will NEVER see the correct termination impedances in that type of system so all kinds of "weird stuff" will be going on in the balun at high SWR. Baluns really only work in their designed mode/ratio when they are terminated in the impedance they are designed for. A 1:1 current balun has better choking properties, so this is what
you want.

My "go to" antenna is a 128 ft. Center Fed Doublet/Zepp fed with ladderline clear into the shack to a Johnson KW Matchbox. It works beautifully on all bands. The only
problem you will run into is that we have so many HF ham bands now that the odds almost guarantee that on or possibly two bands, the length of the transmission line
will present an "impossible" load to the tuner and it may be necessary to change the line length to get a match on that band. (I have that problem on 30 M....I have
to either add/splice in a length of ladderline that will present a "friendly" impedance to the tuner or shunt the tuner terminals with an inductor. I use the inductor, simply
because it is easier and it works quite well.

FYI: an auto tuner is not a "magic" box. It is just a transmatch like any other and it will have upper and lower limits of what it can match. Usually, if the tuner can't find a match, adding 1/8th to 1/4 wave of transmission line will change the impedance enough that the tuner will be able to find a match. I had to do a fair amount
of cut and try to find a length of ladderline that would present a match that was acceptable on MOST ( Not all !!) bands. Also, the addition of a balun is NOT a
"magic solution" either....putting a balun in the system can help or hurt in terms of the impedance presented to the tuner. Again, all things being equal, you are
better off without a balun, but if you "must" have it, they can be made to work pretty well. The balun will add to the system losses, however. BE SURE to get a balun that is specifically designed for high SWR service because the voltages can become truly extreme with high SWR and a common, cheap balun will not have high quality insulation and will quickly fry or short out at high SWR unless you are running QRP !

Lastly, remember that you can have very high voltage on ladderline; DANGEROUSLY high voltages on ladderline at legal limit power and the points of high voltage will change dramatically depending upon the frequency. You don't want people/kids/pets touching ladderline while you are transmitting, especially if you run high power. ....and ladderline must be cut to length or any extra handled in a neat, clean manner. You cannot treat ladderline like coax; it must be installed with some care and TLC and kept away from
metal objects. Remember, that with ladderline or any parallel conductor transmission line, the energy is AROUND the line. With coax the energy is INSIDE the line. BIG difference.

73,  K0ZN
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W5DXP
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« Reply #11 on: February 15, 2012, 04:33:03 AM »

FYI: an auto tuner is not a "magic" box.

However, an autotuner at the antenna feedpoint can achieve what a tuner at the transmitter in the real world cannot do - a conjugate match at the antenna feedpoint plus minimum (Z0-matched line) losses in the coax.
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KI8BL
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Posts: 22




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« Reply #12 on: February 15, 2012, 05:29:07 AM »

use coax off of the ladder line,come up two rows off of the bottom of your ladder line.connect a so-239. then you can also use the bottom as a tunning stub!
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WA9YSD
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Posts: 138




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« Reply #13 on: February 15, 2012, 07:49:27 AM »

Hi Jon

I am just getting confused with my old age.  Mind not working as fast as it use to. For many years I have been told just what you have just said.

If you find your balun heating up or you burn it up, thats telling you you need more inductance and the coupling is too tight. All you need to do is add more inductance or more open line.

I built a balanced tuner that had a roller inductor in series with each side of my 600 ohm open feed line. Tuned great. Later on in years I built a 40m folded dipole inverted vee and built my 200 ohm home brew ladder line. Best antenna I ever had. I learned that if you tweak the antenna for one band it just does not get any better. Your right about multi banding cause thats just the way it is.

You brought up an interesting point.

TUNERS
If you take an tuner like the MFJ or even a Johnson match box and tune it into a pure resistive load, 50 or 300 ohms, the tuner will tune to a 1:1 SWR at various settings.

After messing with a tuner for a while you eventually stumble across a sweet spot where you can switch to another band and you will have to do very little tuning or just adjust one knob a little.

With this in mind, when you take an AUTO tuner it will tune to a 1:1 SWR only at one location.  Of course you can over ride the auto feature and it will tune to various settings as well, like the others. Put it in the AUTO mode and the tuner goes back to that one location. Signals and out seams to just play better at that one point.

So my question is with an AUTO tuner  what is it looking for so it can tune to that sweet spot?

If you have a manual tuner what do you look for to find that sweet spot with out the trial and error routine of finding it?

Jim K9TF
« Last Edit: February 15, 2012, 08:52:06 AM by WA9YSD » Logged
K0ZN
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Posts: 1563




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« Reply #14 on: February 15, 2012, 07:47:48 PM »


Hi, Jim.

I am not well versed on the algorithms used to find a match with auto tuners. I may be wrong, but my understanding is that there are a couple of
ways that this can be accomplished (parameters that can be used/measured to achieve a match) so it is kind of up to how the design engineer wants to
solve it.  Hopefully, someone can provide more detail on this.

Regarding manual tuning:  I suppose you could crunch some numbers, based on line length, Zo, antenna feedpoint impedance, etc. and come up with the value of
component reactance needed to achieve a match....but that seems like a lot of work to me, especially for multiple frequencies. I have made matching Stubs by
calculation, but those have been single frequency (+/-) matching devices. I just use the "tune and try" method to find the best match then log the dial numbers.
(This is one place the Johnson MatchBox really shines: it excellent in its ability to be reset to a previously logged match.)

Usually, you will have the lowest tuner loss where the match is made with the least amount of inductance because the losses in the coil are typically
higher than in a good capacitor. If one understands a little about what the individual components in the tuner are doing, the Tuner will kind of tell you
what kind of load it is seeing and, if need be, you can compensate in some manner (line length or addition of external L or C) to give a match.

73,  K0ZN
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