- READ FIRST!
How Should I Implement My New True Ladder Line Antenna System?
600 ohm ladder line is extremely low loss feed line which lends itself well to operating under high swr conditions. By using a single wire dipole antenna for multiple bands, the swr on the feedline will be high. With extremely low loss feed line such as True Ladder Line, these swr losses are negligible. The net result is a highly efficient antenna system. We need to think of our antenna as a system, not just an antenna. So the question is: how do I get as much RF power to the antenna as possible through my antenna system? The answer is low loss feedline combined with a tuner that can resolve the impedances presented to it by the feed line over a wide range of frequencies (bands).Here are the choices we have (most efficient to least efficient):
A) Ladder Line from antenna feedpoint to the Balanced ATU at the transmitter location (in the shack).
B) Ladder Line from antenna feedpoint to the Balanced ATU at the base of the antenna support structure and fed with low loss coax into the shack. This affords a 1:1 match for the coax and minimal loss! The remotely located ATU may be remotely controlled via stepper motors and a steppertune controller (see at www.steppertune.com).
C) Ladder Line from antenna feedpoint to the Unbalanced ATU (with 4:1 balun at output of ATU) at the transmitter location (in the shack).
D) Ladder Line from antenna feedpoint to a 4:1 balun located outside the shack and fed with coax (short as possible and low loss) in to the unbalanced ATU in the shack.
Because the feed line transforms the feedpoint impedance, the length variation will change the impedance presented to the ATU. Therefore, it is a good idea to use the "taboo length" table (below) if one is using a typical commercially available ATU. If one is building his/her own ATU, then it is possible to design an ATU that can resolve the wide range of impedances presented by the feed line. Keep in mind that system efficiency is a function of keeping a low swr (1:1 match) on any coax used in the system. If that is not possible, then keep the coax run (with high swr) as short as possible and use extremely low loss ladder line as much as possible. This will make the system losses as low as possible and thus a bigger signal!
I look forward to having a QSO with you on the air! Best 73's, Gary K7EMF
Matching Problems using a tuner and high impedance feedline
Most matching problems occur when the antenna system presents an extremely high impedance to the tuner. When the antenna impedance is much lower than the feedline impedance, an odd quarter-wavelength feedline converts the low antenna impedance to a very high impedance at the tuner.
A similar problem occurs if the antenna has an extremely high impedance and the transmission line is a multiple of a half-wavelength.The half-wavelength line repeats the very high antenna impedance at the tuner. Incorrect feedline and antenna lengths can make an antenna system very difficult or impossible for the ATU to resolve the impedance presented to it.
This problem often occurs on 80 meters if an odd quarter-wave (60 to 70 foot) open wire line is used to feed a half-wave (100 to 140 foot) dipole. The odd quarter-wave line transforms the dipole's low impedance to over three thousand ohms at the tuner. The feedl line inverts the antenna impedance.
A problem also occurs on 40 meters with this 80 meter antenna example above. The feedline is now a multiple of a half-wave (60 to 70 foot) and connects to a full-wave high impedance antenna (100 to 140 foot). The half-wave line repeats the high antenna impedance at the tuner. The antenna system looks like several thousand ohms at the tuner on 40 meters.
The following suggestions will reduce the difficulty in matching a feedline with a tuner:
1. Never center feed a half-wave multi-band antenna with any feedline that is close to an odd multiple of a quarter-wave long.
2. Never center feed a full-wave antenna with any feedline close to a multiple of a halfwave long.
3. If a tuner will not tune a multi-band antenna, add or subtract 1/8 wave of feedline (for the band that won't tune) and try again.
4. Most hams encounter problems resolving a match when operating on an antenna that is less than a half wavelength. As an example, trying to tune the ATU for 160 meters with an antenna a half wavelength long on 80 meters. The reason is that the ATU cannot resolve the impedance presented to the ATU by the feedline. The feedpoint impedance of a 80 meter half wave operated on 160 meters is in the order of 15 to 25 ohms resistive and very capacitivly reactive. If the feedline is 1/4 wavelength or a odd multiple of 1/4 wavelength, the impedance will be inverted to the ATU and can be very high impedance.
To avoid problems matching or feeding any dipole antenna with high impedance lines, keep the lines around the length in the green area of the chart below.
Suggested lengths for high impedance feedline on dipole type antennas
Good lengths are green shaded area in the chart below.
|160 meter dipole||35-60, 170-195 or 210-235 feet||(Avoid 130, 260 ft)|
|80 meter dipole||34-40, 90-102 or 160-172 feet||(Avoid 66, 135, 190 ft)|
|40 meter dipole||42-52, 73-83, 112-123 or 145-155 feet||(Avoid 32, 64, 96, 128 ft)|
The worst possible line lengths are shown in the red shaded area.
Some trimming or adding of line may be necessary to accommodate higher bands.
Here are 2 examples:
1. You have a dipole and you want to make it into a multibander using a tuner.
You calculate that it is about 135 feet long for 80 meters...
You would use either, 34-40, 90-102 or 160-172 feet for the feedline going to your tuner or balun such as models 1171, 4114, 4115, or 4116.
2. Your dipole is cut for 40 meters or about 66 feet total length and you feed it with ladder line to a tuner to make it a multibander.
You would use either, 42-52, 73-83, 112-123 or 145-155 feet according to the chart above.
WARNING: To avoid problems, a dipole antenna should be a full half-wave on the lowest band. On 160 meters, an 80 or 40 meter antenna fed the normal way will be extremely reactive with only a few ohms of feedpoint resistance. Trying to load an 80 meter (or higher frequency) antenna on 160 meters can be a disaster for the tuner.
So if you're having trouble matching your antenna system on a particular band using high impedance feedline with your tuner, add or subtract the appropriate amount of feedline according to the chart above and try again.
Here's a post from Bob Rumsey of Balun Designs to address the infamous "12:1" Balun Myth!
Posted by Bob, KZ5R on 20th Nov 2014
At least once a week we receive a request a for high ratio balun (6:1, 9:1, 12:1) to manage the transition from high impedance ladder line / open wire feedline to coax. This is a common misconception and when using a loop, doublet or double extended Zep (and several others) for multiband operation will result in "operational frustration". This is because any type of open wire (including ladder line and twinlead) will present nearly the same complex impedance of the antenna feedpoint to the other end of the open wire and can result in many, if not all bands, being difficult or impossible to match.
An example would be the primary band a full size loop is cut for. Typically this will have a 100-125 ohm feedpoint impedance and when divided by the ratio of the balun, i.e. 12:1 (if trying to match 600 ohm open wire) the resulting 8-10 ohms is impossible for all but the absolute best tuner to match. In addition, when a low impedance match is created, the losses in the tuner are higher than a high impedance match.
This problem can be even worse for doublets. If the doublet is cut for resonance on the primary or lowest band, the feedpoint impedance will be around 60-70 ohms and the resulting impedance, after a high ratio balun, will be even lower than a similar loop antenna. This is why doublets work better when sized smaller (read shorter) than a standard resonant dipole of the same primary band.
The solution is using a balun with a much lower ratio such as a 1:1 or a 4:1 which will transform the balanced line to the unbalanced coax. Matching the resulting high impedance is far easier for a tuner and losses within the tuner are also minimized. Which ratio to use is the other frequently asked question and the answer is not as black and white as many would have you believe.
Our position, derived from experience rather than mathematics, is you have two choices based on your tuner. If your tuner has the ability to match a wide impedance range, then our line of ATU Baluns are an excellent choice and designed just for this type of application. Keep in mind this is a specialized 1:1 balun and even though it provides the highest efficiency, it also places all the work load for the match on your tuner. Please be careful when determining whether or not your tuner fits this description as many of the compact size auto tuners claim to have a wide range but will struggle to match a very high or very low impedance.
If you want to try using your internal tuner or want to provide some additional margin to your external tuner, a 4:1 current balun such as our 4113T, 4114T or the Hybrid 4116T may be the best choice. These models also provide a broader bandwidth for each tune and thus require less frequent retuning as you move within a band. As a side comment, for some reason most LDG and MFJ auto tuners prefer a 4:1, but definitely use a current balun rather than a voltage wind.
Brian, WB2JIX, finds that in most situations, the low power LDG tuners are just not up to the task of matching such a wide range of impedance found with true open wire fed antennas. As a side note, the LDG baluns are also problematic. Swap it out for a Balun Designs model of the same ratio, and the problem is solved.
One last note of caution, whenever possible your ladder line should be terminated at the balun mounted outside your operating position and coax run to your tuner. This will prevent RFI (radiated from the ladder line) from entering your equipment and allow the balun to stop any common mode current from coming back on the shield of your coax.
All of the baluns we recommend provide excellent transformation from balanced to unbalanced feedline, high level choking and isolation, very low insertion loss and overall best in class performance.
Fabricating A Balanced ATU by Gary Baker, K7EMF
There are three major reasons to utilize a “balanced” ATU:
1) The balun is placed in a 50 ohm “matched” (low swr) position in the circuit. This allows the balun to do it’s job, namely to convert the RF energy from unbalanced (Coax) into a true 180 deg out of phase (one lead referenced to the other lead of the ladder line) and exactly the same amplitude. By doing this, the balanced line (TrueLadderLine) is operating in a “symmetrical mode” and thus minimal feedline radiation and minimal extraneous noise pickup in receive.
2) Low SWR (typically 1:1 match) on the balun eliminates core heating when operated within the power handling specifications of the balun.
3) Eliminates core saturation which also eliminates harmonic generation due to no wave distortion.
Commercially built ATU’s typically can accommodate a moderate impedance range where as home brew ATU’s can be designed to accommodate a wide range of impedances which will probably be encountered with 600 ohm ladder line feeding a non-resonant multiband center fed dipole. I personally prefer to fabricate my ATU so it is able to resolve these wide range of impedances and therefore do not need to respect the “feedline taboo lengths” that are published.
Designing A Balanced ATU:
In order to design the ATU, I first model the antenna I wish to erect using EzNec. Within EzNec, I perform a SWR Sweep which gives me the feed point impedance of the antenna at a given frequency. Once armed with this info, I then enter this impedance into TLD (Transmission Line Details).With the load impedance, feedline type (generic 600 ohm ladder line) and feedline length loaded into TLD, several bits of data are available (see example below):
1) Impedance at the far end of the feedline (impedance presented to the ATU).
2) Transmission system efficiency (ATU to Antenna--this is an indicator of how your antenna will function).
Now that I have the impedance presented to the ATU for each band I wish to operate this antenna on, I use another freeware called “SimSmith” (see example below). I choose my circuit elements by dragging and dropping into the circuit. Typically for this application, I us a unbalanced “L” network. The “generator” side of the circuit is 50 ohms 0J Ohms. The “load” side of the circuit is set to the impedance provided by TLD. Then I use the right mouse button in a “drag” mode to move the plot to the 50 ohm, non-reactive location on the Smith Chart. Final step—pull the values off the display for the inductor and capacitor design for the ATU.
By attacking this issue in this manner and using vacuum relays in lieu of AC Contactors, I am able to see return loss numbers in the order of 70 db! It has not been necessary to adjust feedline length or antenna length to modify the impedances presented for 160M through 40M.
Since I cannot bring the TrueLadderLine directly into the shack, I terminate the ladder line feed into the ATU which is located at the base of my antenna support structure (Tower Base). From there I use high quality coax like Andrew LDF4-50A (LMR400 is next best) to the radio room. By remotely locating my ATU, I eliminate standing waves on the coax and thus am looking at system efficiency numbers better than 90% from 160M through 40M (5% LadderLine Loss & 5% Coax Loss)!
Because the ATU is remotely located, I use a StepperTune to control the two stepper motors within the ATU. StepperTune has 8 memories which I use to “preset” the ATU to memorized operating frequencies. I only need to “touch up” the SWR if I QSY significantly from the memorized settings for each of the bands of operation. StepperTune is described at www.steppertune.com and is sold on the W7FG TrueLadderLine and Antennas store.
EzNec Setup-215ft Dipole Center Fed
EzNec SWR Sweep For 215ft Center Fed Dipole (3.9Mhz)
Impedance Derived: 1099 Ohms Resistive, +J2371 Ohms (Loaded Into TLD Below)
Efficiency Model (TLD) For 215ft Dipole and 50ft of TrueLadderLine (98.6% !)
(Input Power 100 Watts--98.6 Watts To The Antenna)
SimSmith Used For “L Network” For 75 Meters ATU (215 Ft Dipole-50 Ft TrueLadderLine)
Resultant ATU Design Model-Inductors: 9.35/2=4.7uh each Capacitor: 27pf
StepperTune-Two Stepper Motor Controller
--Designed To Control Two Stepper Motors For Controlling A Remotely Located Antenna Tuner.
--May Remotely Control Tuning Capacitor For Magnetic Loop.
--Control Cable May Be CAT5 Cable Up To 200 ft while Using A 12 Volt Power Supply.
--Cable Lengths Greater Than 200 ft May Be Realized Using A Supply Voltage Up To 24 Volts DC.
--Eight Memory Locations For Both Steppers.
--Auto Band Memories For Auto-Band Change From Icom Transceivers.
--Automatic Antenna Tuning Capable With External Specified Directional Coupler Connected.
--Stepper Motor Current Control and Stepper Motor Current Displayed. Capable Of Up To 2 Amps of Stepper Drive Current.
--Stepper-Over Current/Over Temp Shutdown Protection.
--“Jog Mode” To Easily Fine Tune Either Stepper Motor.
--Stepper Motor Speed Easily Programmed.
--Limits Set In Software To Limit Travel
This Controller Is Capable Of Driving Two Stepper Motors Over 8 Conductor Cable (typically CAT5 cable). Current Control Is Necessary To Protect The Stepper Motors. The Display Shows Stepper Motor Position In Terms Of Step Number And Current Used For The Latest Stepper Motor Movement. This Unit Is Extremely Accurate And Re-Settable. It Utilizes Replaceable H-Driver Modules Which Are Current And Temp Protected. I Have Been Using This Controller For Four Years With 100% Functionality at the EMF Ranch. See full Description and Manual download at steppertune.com Gary, K7EMF
Balanced ATU-Two Stepper Motor Control
Remote ATU located at the K7EMF Ranch
Balanced ATU Feeding TrueLadderLine at K7EMF QTH