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Author Topic: Dipole with 450 ohm ladder line ?  (Read 5580 times)
N6SBN
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« on: July 10, 2013, 05:34:18 PM »


  I have an Alpha Delta DX-EE shortened Fan Dipole.   It is well made. I tunes up on my favorite frequencies.  It is center fed with Coax.  I chose to feed the Dipole with a 75' piece of 450 ohm ladder line.  At the end of the ladder line, I have an SGC 239 AT.  From the AT is approximately 25' of coax.    The antenna tunes up well with the SGC 239.  I have no SWR or RFI down into the shack.    However, the antenna seems a bit deaf compared to other antennas.  SO the question is, should the 450 ohm ladder line be cut to a specific length.   I've seen this in other antennas.  Something to do with ? "the velocity factor" ?

  The only known hitch with the antenna is that it's at 26', which is too low.  I'll be fixing this problem down the road.

  Thanks in Advance.    N6SBN   On the beach Capitola cA
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N4JTE
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« Reply #1 on: July 10, 2013, 06:31:20 PM »

Deaf compared to what ? I'm gonna guess it heard better when fed as designed with coax, you have introduced major mismatches on every frequency and have inserted a lot feedline loss regardless of no swr/ rfi issues.
The antenna is already a compromise being shortened and fairly low, go back to the coax and get it up higher as you plan.
Bob
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N6PJB
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« Reply #2 on: July 10, 2013, 09:36:30 PM »

This should help or confuse. It took me several weeks to get it to soak in.

http://static.dxengineering.com/pdf/Choosing%20the%20Correct%20Balun.pdf
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WB6BYU
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« Reply #3 on: July 10, 2013, 10:21:26 PM »

Quote from: N6SBN

...I chose to feed the Dipole with a 75' piece of 450 ohm ladder line.  At the end of the ladder line, I have an SGC 239 AT...



How did you connect the ladder line to the tuner?  Did you use a balun, or connect the wires
directly to the tune case and the output post?  If the latter, did you add some sort of feedline
choke on the input?

If you connected one side of the feedline to the tuner case and the case is grounded, or
you don't have an effective feedline choke in the coax, then the ladder line will be
unbalanced.  That means one side of your antenna is radiating more than the other, and
the feedline will also be radiating.  What that does to your signal will depend on a lot of
factors:  usually it makes things worse, though I've seen cases where the feedline radiation
did a better job of working DX than the low dipole itself.


Otherwise, when the line is fed a proper balanced signal, the efficiency should be at
least 80% or so, so that shouldn't be the cause of your problem.  Tuner losses are
difficult to predict, but since the impedance that the tuner sees shouldn't be less
than 50 ohms I don't think they should be too bad.
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N6SBN
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« Reply #4 on: July 11, 2013, 12:35:37 AM »

   The SGC AT had two connections on one end for "Wire" antennas and coax on the other, which goes to the radio.   SWR is very low at the radio, when the AT is done with its tune cycle.   The two legs of the 450 ohm are connected to each leg of the dipole.  It looks like the Dipole should be balanced and the twin lead 450 ohm ladder line should be balanced to the the AT.

   It would appear that the SGC is designed for the two wires coming off the 450 ohm twin lead.

   MMM.  Here is an older post with a similar question:  http://www.eham.net/ehamforum/smf/index.php/topic,81176.0/prev_next,next.html#new
« Last Edit: July 11, 2013, 12:53:02 AM by N6SBN » Logged
W9IQ
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« Reply #5 on: July 11, 2013, 04:56:04 AM »

Craig,

You asked about the length of your ladder line in your original post.

If the antenna is resonant and it matches the impedance of the transmission line, then anywhere along the feedline the radio will see exactly the same impedance as the antenna. The only effect of line length is losses increase as the line gets longer. (A resonant antenna is one which has only resistance as its impedance - no capacitive or inductive reactance.)

If the antenna is not resonant, or the feedline does not match the impedance of the resonant antenna, then all along the transmission line the complex impedance will vary. Specific lengths of feedline can be used as a matching section for a given frequency. Specific lengths can also help an antenna tuner to more easily tune the antenna or prevent high voltage flashover in the tuner. As the feedline gets longer, the losses increase even more due to the additional losses caused by the SWR.

Since you are operating a multiband antenna and you are not having problems getting it to tune up, you probably need no adjustment to the length of your ladderline. If your tuner had problems tuning up on a given band, then lengthening or shortening the ladder line may help the tuner find a match.

If I were trying to optimize your setup, I would locate the tuner outside at the end of ladder line and use a balun before going into the tuner. Then I would run the coax from the remote tuner to the shack. This minimizes the losses in the antenna system.

My next best alternative would be to install a balun at the ladderline to coax transition and leave the tuner in the shack. This will help minimize common mode currents on the feedlines.

- Glenn W9IQ

« Last Edit: July 11, 2013, 05:01:14 AM by W9IQ » Logged
W9IQ
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« Reply #6 on: July 11, 2013, 05:31:32 AM »

Craig,

I neglected to answer your question about the velocity factor of the transmission line.

When we are trying to create a given length of transmission line, such as a 1/4 wave section, this is referring to the electrical length of the line. Since current takes longer to travel in a transmission line than in a vacuum, we need to adjust for this when determining the physical length of the 1/4 wave section. To convert the electrical length to the physical length, you multiply by the velocity factor of the transmission line you are using.

For example, a 1/4 wave length section of transmission line at 3.6 MHz would be calculated as 245.9/3.6 MHz or 68.31 feet. But this is the electrical length. If we were using a piece of transmission line with a 0.67 velocity factor, we would then calculate the physical length as 68.31 feet * 0.67 giving the length to measure and cut as 45.76 feet.

It is interesting to note that when a manufacturer states the velocity factor of a line, this should be used only as an approximation. It can vary from spool to spool from the same manufacturer. So when trying to cut a line to a specific electrical length, it is always better to cut long and then use instrument measurement techniques to trim it to the exact length or to measure the velocity factor of the line in question before beginning the calculations.

- Glenn W9IQ


« Last Edit: July 11, 2013, 05:49:25 AM by W9IQ » Logged
W5DXP
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« Reply #7 on: July 11, 2013, 08:22:12 AM »

The DX-EE is designed to be fed with coax and work on 40m, 20m, 15m, and 10m. Feeding it with ladder-line will transform the impedance seen by the autotuner. If you want the autotuner to see the same impedance as the feedpoint impedance, then make the ladder-line a multiple of 1/2WL long. For a 0.9 velocity factor on the ladder-line, 1/2WL is 61.5 feet on 40m. That length will be about 1 WL on 20m, 1.5WL on 15m, and 2WL on 10m so it should work well on those bands. 75 ft. is getting close to a multiple of 1/4WL on 20m and may give the tuner some problems.

My SG-230 manual says not to connect anything except the antenna to the output terminals and that includes any kind of transmission line. Have they changed their recommendation? When I asked them about that recommendation, they said the SG-230 was capable of causing transmission lines to arc.

In any case, the output of those SGC tuners is unbalanced and ladder-line is balanced so there exists the possibility of some unbalanced mischief. If I were using your configuration, I would be tempted to try installing a balun on the tuner output. But note that if a transmission line can arc so can a balun.
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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.
N6SBN
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« Reply #8 on: July 11, 2013, 04:08:59 PM »

Thanks all.  Good info
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KA4NMA
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« Reply #9 on: July 11, 2013, 05:18:21 PM »

The SGC tuner should have 2 screws, one is ground the other is hot (or antenna). One side of the ladder line is connected to hot, and the other to ground.

Randy ka4nma
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N4JTE
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« Reply #10 on: July 11, 2013, 06:32:48 PM »

I gotta stick to my first response, why all the extra stuff, ladderline, tuners, 1/4 wl matchers, etc, stick with the coax and raise it higher and eliminate all this matching stuff and resultant loss.
Am I missing something?
Bob
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N6SBN
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« Reply #11 on: July 12, 2013, 12:03:01 AM »

  I'm beginning to agree with you.   I had the ladder line and not the coax.   I have had good luck with ladder line dipoles in the past.  It's clear this one is designed for coax.
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N8CBX
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« Reply #12 on: July 13, 2013, 11:00:40 AM »

This should help or confuse. It took me several weeks to get it to soak in.

http://static.dxengineering.com/pdf/Choosing%20the%20Correct%20Balun.pdf

"The recommended 300-Ω ladder line provides better overall impedances at the tuner and balun, as opposed to typical 450-Ω ladder line."
I never heard that before, thats going to take some time for me to understand why 300 ohm is better than 450 ohm.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
W5DXP
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« Reply #13 on: July 13, 2013, 02:41:22 PM »

I never heard that before, thats going to take some time for me to understand why 300 ohm is better than 450 ohm.

The statement may not be true in general but may be true for the stated special conditions, one of which is limiting the feedline length to an odd multiple of 1/8 wavelength.
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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.
K8JD
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« Reply #14 on: July 15, 2013, 03:32:52 PM »

If the antenna is a trap or fan dipole (my first guess for this one You better forget about ladder line!
First you are mismatching a low Z antenna feedpoint into high Z feedline by almost 10:1 !
Next you have to use a tuner to get what ever comes out on the transmitter end rematched to a 50 ohm radio, and that will give you MORE losses.
The only place I could see this being worth while is if your antenna was something like 500+ ft from the rig !
Get the coax and use it, and save the 450 line for a non resonant antenna someday.
I gotta stick to my first response, why all the extra stuff, ladderline, tuners, 1/4 wl matchers, etc, stick with the coax and raise it higher and eliminate all this matching stuff and resultant loss.
Am I missing something?
Bob
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73...John
SKCC 1395T, FISTS 3853
Official US Taxpayer
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