- READ FIRST!!!!
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. An antenna system should be considered everything from the tuner to the tip of the antenna. High impedance feedline is usually considered that which has over 300 ohms impedance and are typically are 300 ohms, 450 ohms and 600 ohms in impedance. 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 to tune.
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. This is because the mismatched feedline is an odd multiple of 1/4 wavelength long. The 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 an antenna with a tuner:
1. Never center feed a half-wave multi-band antenna with a high impedance 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. Never try to load a G5RV or center fed dipole on a band below the half-wave design frequency. If you want to operate an 80 meter antenna on 160 meters, feed either or both conductors as a longwire against the station ground.
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 450 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 both your signal and the tuner. The best way to operate 160 with an 80 or 40 meter antenna is to load either or both feedline wires (in parallel) as a longwire. The antenna will act like a "T" antenna worked against the station ground.
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.