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By Tom, W8JI
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General Info on Baluns
Balun is an acronym for BALanced to UNbalanced, which describes certain circuit behavior in a transmission line, source or load. Most communications applications deal with two-terminal sources, transmission lines, and loads. This includes coaxial cables, open wire lines and systems working against earth or a ground plane as the "second conductor".
Balun Fundamentals and Terms
The balun has to do a good job and be reliable. DX Engineering has the expertise to design and build a better balun that will deliver more power to the antenna, be more reliable, and in many cases cost less than products made by others.
We also realized that advertising hype over the years had confused the issue of just what type of balun was appropriate to each antenna. This article is an attempt to define in simple terms how to get the most performance from your system, both on receiving and transmitting.
The first thing to realize is that there are two types of baluns: Current Baluns and Voltage Baluns.
The balun's ratio is normally stated from balanced to unbalanced (just as the words appear in the acronym). A 4:1 balun has four times the balanced impedance as unbalanced impedance.
Balanced and Unbalanced
Balanced lines and loads, by definition, have equal voltages from each terminal to ground. Each balanced terminal or conductor must also carry precisely equal and exactly out-of-phase currents. If the feedline does not have equal voltages, equal currents, and exactly out-of-phase currents at every point, the feedline will partially act like an antenna. Current is most important to balance. Voltage is less important, although voltage can be important in specific cases.
Coaxial feedlines, like balanced lines, must also have exactly equal and opposite currents on the shield and center conductors. Equal and opposite currents can only flow inside the shield. Coaxial line shields also must have zero volts radio-frequency electrical potential to "ground" or space around the line at the operating frequency. Deviations from this ideal case will cause current flow on the outer surface of the shield. This current will cause line radiation, since it flows outside the shielding wall.
In both balanced and unbalanced lines, we call vector current difference between the two conductors "common-mode current". Reducing or eliminating common-mode current as close to the antenna as possible, and keeping it from reappearing inside the shack, can greatly improve reception and put more transmitter power in the antenna. It will also reduce RFI problems.
What many of us fail to understand is most real-world antenna systems are neither perfectly balanced nor perfectly unbalanced. Real-world antenna systems most often are somewhere between perfectly unbalanced and perfectly balanced.
In most cases, baluns are installed as close as possible to a balanced-to-unbalanced transition point.
Current baluns allow each output terminal's voltage, with respect to "ground" or chassis, to float to any value required to provide equal currents to each feedline conductor. Current baluns are universal devices that work with balanced or unbalanced loads equally well. Current baluns add common-mode isolation between systems connected at each end. While traditionally used as baluns, they work well as broadband phase-invertors or as an un-un.
Current baluns isolate or add impedance to unwanted common-mode current paths, reducing or controlling common-mode current. Current baluns are the balun of choice in all but very specialized situations, because they work better than voltage baluns in most real-world systems.
In the case of a 1:1 ratio current balun, core flux density or "magnetizing stress" on the balun core is independent of load impedance or load mismatch. Only common-mode current affects the core. This does not mean current baluns can handle infinite power or mismatch, but it does mean for equal materials and cost they handle extremes in impedance much better than baluns that operate at higher ratios.
Voltage baluns always try to force the output terminals to equal voltages. They sometimes introduce phase shift between each output terminal and "ground". If the impedance presented at each terminal is not exactly equal, feedline or load currents will not be equal and opposite. This means the feedline will radiate.
They also do not provide common-mode isolation. A voltage balun almost certainly guarantees some feedline radiation (or reception), because there are very few "perfectly balanced" loads or perfect voltage baluns.
Unlike a 1:1 ratio current balun, a voltage balun will always magnetize its core in direct proportion to load voltages. In a voltage balun, load impedance directly affects core heating and flux density.
Current baluns, rather than voltage baluns, should be used whenever possible. Current baluns provide better balance and often have lower loss. Current baluns, especially 1:1 ratio baluns, tolerate load impedance and balance variations much better than voltage baluns. Current baluns can also be used as isolators or un-un's.
Unless otherwise noted, DXE Baluns are current-type baluns.
Systems Requiring Antenna Tuners
Antenna systems requiring antenna tuners or matching networks often have very high voltages or currents on transmission lines and baluns, even at modest power. In many cases, voltage and current are not in phase with each other. This can produce very high currents at the same place where voltages are very high, the worse of both conditions appearing at one point in the system.
In some installations, coaxial cable connects a poorly matched balanced antenna directly to a tuner. The tuner "matches" the poor antenna system impedance to station equipment. The feedline beyond the tuner still has very high voltage, current, and loss, even if tuner input has a perfect SWR. With coaxial feed, any balun would belong at the antenna, not at the tuner.
In other installations, both the antenna and antenna feedline are balanced and the tuner has an internal or external balun. Unfortunately, most internal tuner baluns are 4:1 voltage baluns, which we will see is a poor choice. In this case, the baluns should be as close as possible to the tuner.
Less often, balanced tuners are used. Such tuners come in two styles. One is a true balanced voltage network like the old E.F. Johnson Matchbox. Other better forms include link-coupled homemade tuners with fully floating tuned circuits, which behave as a more desirable floating current source.
A more recent approach is a balanced network with a balun on the input. While a balanced network with grounded center is balanced, it is a voltage-type source like the Matchbox. It needs a perfectly balanced load to function optimally. Balance is not as good as a link-coupled tuner with fully floating components.
Unbalanced networks with baluns on the input are not what we first might think. The balun has just as much core stress and flux density when placed at the input as it would have when in the traditional location, on the output. Common-mode isolation is also the same as a traditional current balun on the tuner output. Relocating the balun to the input of an unbalanced network does not help the balun do a better job and complicates tuner construction.
The ideal balanced tuner would have a link-coupled floating balanced network. Nothing else will assure optimal transmission-line balance. The output network must be ground independent. Otherwise, it is a resonant equivalent of a voltage balun.
We are often further ahead to place a good 1:1 ratio current balun at the output of a traditional "T" network tuner. In fact, even though I can build any type of tuner I want, all of my personal high power tuners are simple "T" networks with good 1:1 ratio current baluns on the output.
There are four areas of concern in tuner-matched systems:
1. In a multi-band dipole system, the antenna almost never presents a moderate impedance load to the balun over the full frequency operating range. As the operating frequency changes, balun load impedance can range from several thousand ohms to a few ohms.
2. Most antenna tuners work best into moderate to high impedances, rather than low impedances. Most baluns inside antenna tuners step the antenna impedance down. Most tuners would work better if the balun passed the line impedance through without stepping impedance down.
3. 4:1 Baluns inside antenna tuners, which are usually voltage-type baluns, are generally poor performers when presented with mismatched loads. 1:1 current baluns are generally much more efficient and have a much wider operating impedance and frequency range.
4. Voltage baluns have restricted frequency response. The "optimum performance" frequency range is much narrower in voltage baluns than in equivalent current-type baluns.
Based on the above, a 4:1 balun or any voltage-type balun is the wrong choice for use with antenna tuners in multi-band dipole systems. Most tuners use them because they are cheap, easy to build, and because almost everyone else uses them.
Special DX Engineering Tuner Baluns
For antenna tuners or systems with high SWR, we have a special balun. This balun uses high-voltage wire and has excellent performance at very high SWR. Even standard DXE baluns are better than many competing baluns, because many competing baluns use thin enamel for wire insulation. Our standard Teflon insulated wire does not fail unless voltage significantly exceeds 7,500 volts, while competing baluns using enameled wire fail at less than 25% of that voltage! That means, for the same mismatched differential load impedance, our standard balun can handle sixteen times the power of enameled wire baluns before arcing in balun windings!
Tuner baluns (denoted by "T" at the end of the balun part number) may cause a very slightly higher SWR with a perfectly matched load. Of course, this is when no tuner is required. We do NOT recommend T-suffix tuner baluns for higher frequencies (above 15 MHz) unless you are willing to tolerate a slight change in SWR. DXE 1:1 ratio tuner-baluns work equally well and handle the same power on the tuner input or output, so use them wherever most convenient for your system.
A resonant half-wave dipole is typically fed with coaxial feedline and tuned to a specific area of a band. Its planned use is generally within that band, although it may be useful on other bands (near odd-harmonics) where feedpoint impedance reasonably matches the coaxial feedline. The well-known length formula is L (feet) = 468/Frequency (MHz). This formula is an approximation. Antenna height, leg angles, insulation, wire diameter, and surroundings affect a dipole's resonant frequency and impedance. It is better to initially make the antenna a few percent longer than calculated and trim it back to size (higher in frequency), although dangling pigtails will work to slightly lengthen an antenna (reduce frequency) without adverse electrical or mechanical affects.
A popular misconception is because the dipole is resonant, or because the dipole feedline is small in diameter, a balun is not helpful. There are also questionable claims that "feedline radiation is good", or pattern change without a balun is insignificant. Many of these claims contradict each other. If one is true, the other claim argues against it. That is what happens when we justify a questionable practice.
Indeed there are cases where a balun is not needed at the balanced to unbalanced transition between coax and dipole, but they are very specific cases where the feedline is suspended in air from the center of the antenna straight away from the feedpoint, and is grounded ¼ wavelength away from the feedpoint.
Omitting the balun in other cases will often cause feedline length to affect SWR, increased noise in the receiver, increased RFI, or any combination of these ill effects. In unlucky cases with higher Amateur power levels permitted, omission of a balun can cause coaxial shield or connector arcing to tower legs or other metallic objects.
The best balun for this application is a 1:1 ratio current balun.
The part numbers of the correct 1:1 current baluns would be the DXE-BAL050-H05-A, DXE-BAL050-H10-A, or DXE-BAL050-H11-C, depending on power levels.
This antenna can use a coaxial cable feed and the balun is located right at the dipole element to ensure that the each side of the element receives equal currents and prevents external shield currents. The feedline should route straight away from the antenna center at right angles to the antenna conductor. This will keep the antenna's fields from introducing current on the outside of the feedline after it leaves the balun, and will keep the feedline from introducing noise onto the antenna element.
Here is an example of the balun setup that should be used with this antenna type.
The optional formed plastic piece shown is the DXE-UWA-Kit Center-T which provides the hardware required for a no-solder mounting for the antenna elements and the balun and removes the load of the balun and feedline from the element wire ends. This system will reduce the chances of the antenna wire breaking in most installations.
The top 3/8" diameter hole in the Center-T is used for a rope messenger line which is strung above the antenna wire and provides support for the balun and feedline. The line can be thin Dacron rope such as the STI-DBR-94-100 which has a breaking strength of 260 pounds. The use of the messenger line also will keep the antenna element from stretching and changing resonant frequency over time.
This is helpful when:
The connection from balun to shack is through 50-Ω coaxial cable. Use the lowest-loss coaxial cable that you can afford, with due consideration of life and mechanical properties.
Ladder Line or Open Wire fed Dipoles or Doublets
A Multi-band Dipole or Doublet antenna system is a single length wire antenna useable on virtually any band where a tuner can provide a match. Efficiency is very good when the antenna is 0.4 wavelengths long or longer. Efficiency drops rapidly with antennas shorter than 0.4 wavelengths.
This is a popular antenna system; many have been built using DX Engineering baluns. A simple multi-band dipole may be constructed by first choosing the lowest band on which operation is desired. The overall length of the multi-band dipole antenna should be shorter than one-half wavelength as shown in Table 1. For best efficiency, ladder line feed and a good antenna tuner with balanced connections are required. The ideal balun is a 1:1 ratio DX Engineering special application tuner balun. It can be connected through a short length of coaxial cable to an unbalanced tuner for tuning the different bands.
Although it may not seem logical, for 160 through 10-meter operation, a dipole around 220 feet long may actually help antenna tuner and balun performance, especially on lower frequencies. This is because standing waves on the transmission line transform or change reactance and resistance presented to the balun and antenna tuner. The coaxial cable from the DX Engineering 1:1 Tuner Balun to the tuner should be kept short; 10 feet or less is best. The recommended 300-Ω ladder line provides better overall impedances at the tuner and balun, as opposed to typical 450-Ω ladder line. Conductor resistance dominates transmission line losses below VHF, so choose the largest diameter conductor you can for a given transmission line size and impedance. Do not substitute smaller conductor television-style feedline to save money. Losses will increase.
The DXE-LL300 300-Ω ladder feedline for a multi-band dipole must be in odd multiple lengths of 1/8 wavelength on the lowest operating frequency. This helps optimize impedance presented to the balun and tuner over the frequency range of the antenna. This length can be calculated using the following formula or use Table 1. The DX Engineering 300 Ω ladder feedline has a VF (Velocity Factor) of approximately 0.88.
Where: Freq (MHz) = Frequency in Megahertz
0.88 = Velocity Factor of DXE-LL300 300 Ω Ladder Feedline
Multiply the result times the odd multiple (1, 3, 5, 7, etc) to get the correct length closest to your required feedline length.
Note: When using an external balun, the feedline length should be calculated to the balun.
If you need to splice ladder line together for longer lengths, use the DXE-LLC-1P Ladder Line Coupler.
If you have excess ladder line, it can be zigzagged while suspended in air, but it can't be closer than a few conductor widths to metallic objects and cannot be coiled on itself or laid on the ground. If it is necessary to pass closely to a metallic object, twist the line to partially balance the effect on both sides of the feedline. Ideally, the feedline should be brought away from the antenna at right angles.
250-350 ohm impedance feedlines result in less extreme impedance changes from band-to-band. They are a good compromise between impedance extremes and feedline losses. For instance, 600-Ω feedlines tend to present wider load impedance variations at the tuner in multi-band applications than lower impedance feedlines. In addition to better impedance performance, the 300-Ω line has less wind drag than 450-Ω window line.
Coaxial cable has too low of an impedance, higher initial matched loss, and significantly higher SWR on bands where the antenna feedpoint impedance is high.
True open wire feedlines are much more difficult to build and maintain. Their 500-Ω to 800-Ω impedance allows low-loss multi-band operation, but band-to-band impedance variations at the balun and tuner are greater. 300-Ω transmitting feedline is a better choice, since it moderates multi-band impedance extremes and still offers significantly less loss than coaxial cable in this application.
Why you don't want to use coaxial cable when the SWR is high. For each 100 feet of coaxial cable, you lose half your power at an SWR of 10:1. At frequencies higher than 14 MHz, it's worse. For higher loss coaxial cables, like RG-58 or RG-8X, have even more loss. Plus, the SWR shown here is measured at the antenna, not at the radio. At the radio, SWR would measure significantly lower because the lossy feedline absorbs reflected power.
Additional Info on Feedline Length with Multi-band Dipoles: Feedline length is critical to antenna tuning and performance. Always choose a feedline (connects the antenna to the balun, in this instance) that is 1/8th wavelength or some odd-multiple of 1/8th wavelength long on the lowest band. The table above shows the correct dimensions for the antenna and feedline for your Multi-band Dipole Antenna when using DX Engineering Ladder Line. Make the feedline any ODD multiple of the lengths shown.
The best balun for this application is a 1:1 ratio current balun.
A 1:1 balun has the widest operating frequency range, lowest core stress, and provides the best overall balance of any balun for given cost, size, and weight.
The DX Engineering part number of the correct unit would be the DXE-BAL050-H10-AT Current Balun or the DXE-BAL050-H11-CT Current Balun depending on power levels.
Do not match the balun impedance to the transmission line impedance. The transmission line is grossly mismatched. This means the impedance at the balun and tuner varies greatly from band-to-band. Tuners have an easier time with modest to high impedances. They don't work well into very low impedances. A balun with a ratio of 4:1 or more will transform the already low impedances appearing on some bands to even lower values. This will greatly reduce system efficiency and reduce tuner power ratings. The 1:1 ratio balun will just pass the low impedance through. In addition, higher ratio baluns will not handle differential impedance extremes nearly as well as 1:1 current baluns.
Ladder line fed antennas should be constructed according to the chart. The balun should be located near the tuner, keeping the coaxial cable between it and the tuner as short as possible. Avoid routing the line parallel and close to other conductors or structures for any significant distance. Keep feedline length inside a dwelling as short as possible to reduce chances of RF feedback. Coupling directly from the line to sensitive wiring can cause distorted transmitted audio, often described as "RF in the audio" or CW keyer malfunctions. In severe cases, there may be enough RF present on the microphone, key, or other equipment to cause an RF burn.
Even when properly done, this arrangement will subject the coaxial line between the tuner and balun to very high standing waves and high voltage and/or current. You should use good low-loss coaxial line and keep the coaxial line as short as possible. RG8/X and smaller will not do a proper job. Belden RG-213 or equivalent is the minimum size that handles the higher voltages and currents properly.
DX Engineering baluns have significantly higher common-mode impedance and larger effective core area than other similar designs. They are much more effective antenna tuner baluns than standard bead or air-core baluns.
Conventional or True Windom Antennas
The True or Conventional Windom antenna, shown below, is fed with a single-wire line, and fed as an unbalanced system against a reasonable RF ground or counterpoise. The feed is similar to a long-wire antenna, except the horizontal wire is fed with a few percent offset from the center.
Single Wire Windom Feed.
Red "D" in DX indicates same phase (positive phase) output terminal on that side.
When you use a single wire feed, ground the unused balanced terminal to the counterpoise or radial system. DO NOT connect that system to the station ground. Isolating the station ground from the antenna ground will keep unwanted RF off station equipment, and reduce potential problems with unwanted RF in the house.
Balanced Feed Windom Antennas Off-Center Fed Dipoles
Another, more popular version of a Windom antenna, shown below, is fed with open wire or ladder line. It is sometimes called a "balanced feed" Windom, even though it is actually an "Off-Center Fed" dipole.
Properly installed Windom balanced feed or off-center fed antennas have impedances in the 200-400 Ω range at resonant frequencies. Depending on the installation, a Windom antenna may have reasonable impedances at several harmonically related frequencies.
The best balun for both antennas, assuming they operated where standing waves on the feed system are low, are 4:1 baluns.
Unless otherwise labeled, DX Engineering 4:1 ratio baluns have the advantage of being current baluns. Current baluns, as mentioned earlier, can be used to feed unbalanced loads or balanced loads.
When using a balanced feed system the length of the feedline is the same as shown in the table for the Multi-band Dipoles above, an odd-eighth-wave depending on the lowest frequency used.
The best balun for the Windom or Off-Center Fed Dipole is a 4:1 ratio current balun.
The DX Engineering part number of the correct balun would be the DXE-BAL200-H10-A
or the DXE-BAL200H11-C.
Off-Center fed antennas have a large amount of feedline when compared to a conventional dipole. This means they are more sensitive to their surroundings than a center fed dipole. It isn't unusual to have to take additional steps to decouple the feedline when using antennas that are not fed in the center.
End or Non-symmetrically Fed Antennas
Antennas that are end-fed or asymmetrically fed almost always cause unwanted current on feedline shields.
Examples would be a "dipole" or vertical, either symmetrically or asymmetrically constructed, where the feed cable leaves the antenna near a high voltage point. This can be because of a marginal counterpoise, because the coaxial cable itself is the counterpoise, or because the feedline routes along or through the antenna.
Commercial examples of this are Gap, MFJ, HyGain, and Cushcraft "no radial" verticals, as well as Force 12 vertical dipoles. In these cases, a DX Engineering DXE-FCC050-H05-A Feedline Current Choke placed no more than five feet away from the antenna feedpoint will greatly reduce feedline currents.
50 Ω Broad Band Antennas
One manufacturer of log-periodic antennas suggests running the feedline along a boom that is "hot" with RF, which means the shield of the coaxial cable is coupled directly to one conductor of a "hot" transmission line! In this case, the DXE-FCC050-H05-A Feedline Current Choke should be placed at the point where the feedline leaves the antenna boom, but before it reaches the tower.
Feedline Current Choke: Use with any vertical or horizontal antenna that is coaxially fed. The RF isolated SO-239 at the top of the DXE-FCC050-H05-AFeedline Current Choke provides a high common-mode impedance from 1.8 to 60 MHz.
Examples where this may be necessary are small dipole antennas such as Force 12 vertical dipoles, shortened or loaded antennas using the coaxial cable as a counterpoise, verticals with few or shortened radials, full-size dipoles using the feedline shield as the "other leg" of a dipole, so-called 'end-fed' dipoles which use the feedline as the other half of the antenna, and even regular dipoles if the feedline parallels the antenna element for any appreciable distance. In all of these cases, a DXE-FCC050-H05-A Feedline Current Choke will greatly reduce unwanted or harmful feedline radiation or reception. The DXE-FCC050-H05-A Feedline Current Choke should be installed several feet away from the radiating antenna. In all cases, it must be installed before the feedline is routed against other cables, a
metallic mast, or a tower.
What are the benefits of using a Feedline Current Choke?
The above antenna examples usually have very high common-mode feedline currents which
often lead to:
While most common advice is to improve the station's RF ground, the root of the problem is in the poor isolation of the feedline from antenna currents. If you wish to reduce feedline radiation and improve reception, a Feedline Current Choke is a good idea. In these examples adding a DX Engineering DXE-FCC050-H05-A Feedline Current Choke at the point where the feedline exits the area of the antenna
will substantially reduce unwanted feedline radiation or reception. This is something a station ground cannot do, no matter how good it is. The feedline current choke is not recommended for use on high SWR systems.
Quarter-Wave Vertical Antennas
Any antenna directly fed with a coaxial line against a ground system, like a quarter-wave vertical, depends on the ground system being at zero volts with respect to earth. Unless the ground connection point is held solidly at zero volts, current will be introduced onto the feedline shield. Vertical
antennas with less than perfect grounds will have.
When the vertical uses elevated radials, it is difficult to keep current from traveling back to the operating position via the feedline unless a feedline current choke is used. Not only does this unwanted current cause RFI problems, it also reduces antenna system efficiency.
With a ground-mounted vertical using a smaller ground system, and especially with poor radial systems, there is the need for a feedline choke or current balun system to keep unwanted currents off the outside of the feedline shield.
The correct location for a choke system is at the base of the vertical portion of the antenna. For raised quarter-wave verticals with elevated radials, the correct item to use is the Feedline Current Choke. This should be positioned UNDER the radials at the feedpoint of the raised vertical. The coax should not parallel a radial, but should exit midway between radials.
The DX Engineering Feedline Current Choke is model DXE-FCC050-H05-A.
For ground mounted quarter-wave verticals, the best device for this application is a 1:1 ratio current choke.
The DX Engineering part number of the correct Vertical Feedline Current Choke would be the DXE-VFCC-H05-A or the higher power DXE-VFCC-H10-A.
Below is a picture of the DX Engineering DXE-VFCC-H05-A Vertical Feedline Current Choke (VFCC) used with the Hustler BTV family of vertical antennas. Notice that the VFCC housing is insulated from the radial system.
Insulated Choke System for use with Hustler Antenna and Radial Plate.
The braid is sized for use with the Tilt-Base and Radial Plate.
When installing a Hustler antenna the best way to do it is with a Radial Plate and a minimum of 16 radials that are ¼-wavelength at the lowest frequency of operation. The installation of 30 radials is highly recommended. The DX Engineering DXE-VFCC-H05-A Vertical Feedline Current Choke is mounted on the DXE-VFCC-BRKT Insulated Mounting Bracket which is attached to the antenna support post, and is connected to the antenna feedpoint and directly to the DXE-RADP-1P Radial Plate.
For various reasons, people sometimes install the Hustler antennas with no radials or with an inadequate number of radials. This is not recommended, but it happens. As a result, the antenna uses the coaxial cable as a radial and by doing so, introduces current to the braid of the feedline, which can travel to the operating position with the results mentioned above.
In order to reduce the current on the feedline in these situations it is still possible to use the DXE-VFCC-H05-A Vertical Feedline Current Choke. In this case, attach the terminal that would normally go to the Radial Plate to the frame of the antenna support or mounting pipe.
Sometimes, as the coaxial cable feedline travels through the near-field of the antenna, the current can be re-introduced to the feedline after the balun. In that case, a DXE-FCC050-H05-A Feedline Current Choke may be inserted into the feedline at the radio shack end of the feedline.
Loop Antennas - Horizontal and Vertical
Closed-loop antennas have moderately low impedances if they are about 1 wavelength in circumference. They present low feedpoint impedances at all multiples or harmonics of the initial resonant frequency, as opposed to dipole antennas that have low feedpoint impedances only at ODD multiples of the initial resonant frequency. Loops make better multiband antennas.
There are three common operating conditions for a loop antenna:
Loop Antenna #1 - Operation near fundamental resonance - A Single Band Loop:
When a Horizontal Full-Wave Loop is operated near resonance on the full-wavelength band a 4:1 balun works very well. Using good-quality 50 Ω coaxial cable to the shack from the balun, SWR at resonance will normally be below 1.5:1. An external antenna tuner will generally not be required; the radio's internal tuner can be used if needed.
Typical SWR Plot of full-wave horizontal loop at approximately 60 feet above average ground using DX Engineering 4:1 balun. Operation is safe for balun and coaxial feedline over the entire band.
Horizontal Full-Wave Loop: The best balun for operation at resonance is a 4:1 ratio current balun.
The part number of the correct balun would be the DXE-BAL200-H10-A.
Vertical Full-Wave Loop: The best balun for operation at resonance is a 2:1 ratio current balun.
The DX Engineering part number of the correct balun would be the DXE-BAL100-H11-C for any power levels and operating environment where the resonant impedance of the system is around 100 Ω.
Loop Antenna # 2 - Operation moderately far from resonance or on harmonics:
When operating a Vertical or Horizontal Loop moderately far from resonance or on harmonics, average SWR will be lowest using a 4:1 balun. An example of this is operating a loop cut for 3.7 MHz near the bottom or top of the band, or using a 3.6 MHz loop on 7.2 MHz. When operating moderately close (within 200 kHz) of resonance, losses will be reasonable. Good quality coaxial cable should be installed between the balun and the loop. An external tuner can be used at the operating position, be sure SWR without the tuner is under 5:1.
On the left is an SWR Plot of full-wave horizontal loop at 60 feet, resonant at 3.54 MHz while using a DX Engineering 4:1 balun. On the right is the 40 meter SWR plot.
The plots above show with a DX Engineering 4:1 balun and reasonably good coaxial cable, you can operate a loop over most of 80 and all of 40 meters. This system is generally good on 80, 40, 20, 15 and 12 meters.
Vertical or Horizontal Loop: The best balun for operation moderately far from resonance or on harmonics is a 4:1 ratio current Tuner Balun.
The DX Engineering part number of the correct balun would be the DXE-BAL200-H10-AT or the DXE-BAL200-H11-CT depending on power levels and operating environment.
Loop Antenna # 3 - Operation far from resonance: When operating the loop on random unplanned frequencies far from resonance, use the balanced feedline with the lowest impedance and loss available (DXE-LL300-1C 300 §Ù ladder-line is best). Place the balun as close to the tuner as possible. Use a 1:1 Tuner Balun. Use as short a coaxial cable as in a multi-band dipole system.
Vertical or Horizontal Loop: The best balun for operation far from resonance is a 1:1 ratio current Tuner Balun.
A 1:1 balun has the widest operating frequency range, lowest core stress, and provides the best overall balance of any balun for given cost, size, and weight.
The DX Engineering part number of the correct balun would be the DXE-BAL050-H10-AT or the DXE-BAL050-H11-CT depending on power levels and operating environment.
Some Notes on Feedline Length When Using a Loop Far From Resonance: Feedline length is critical to antenna performance. Try to choose a feedline that is a multiple of 1/2 wavelength long on the lowest band, and make the loop antenna for the lowest band on which you will operate.
Long wire antennas are typically horizontal antennas, fed at one end, and well over 1/2-wavelength long at the lowest operating frequency. The impedance of a long wire antenna varies as the frequency is changed, but the normally accepted values are from a few hundred to a few thousand ohms depending on length, height, ground conditions and frequency. The only way to know the impedance is to measure it or at least model it with antenna software.
A 4:1 balun using good coaxial cable leading to an antenna tuner will provide a relatively well-behaved installation. The output terminal closest to the red "D" in the DX Engineering label connects to the antenna. The other terminal connects to the antenna's ground or counterpoise. The ground ideally would be a modest system of radials. At the very least, an elevated counterpoise several feet above ground and parallel to the antenna is used. The counterpoise length should be ¼-wave on the lowest frequency planned. Multiple counterpoise wires will improve performance. The case of the balun should be attached to a separate ground rod.
Long-wire Feed using a 4:1 Balun. The antenna ground side should be attached to a radial system as with a vertical antenna.
Like any other antenna system that involves high SWR, use the shortest length and best quality coaxial cable possible.
Longwire Antenna: The suggested balun is a 4:1 ratio current choke Tuner Balun.
The DX Engineering part number of the correct balun is DXE-BAL200-H10-AT or the DXE-BAL200-H11-CT depending on power levels and operating environment.
Rhombic, V-Beam, and Other Broadband Antennas
Some antennas have very little impedance change with frequency. Log Periodic, Terminated Inverted V's, Terminated V-beam antennas, and Terminated Rhombic antennas are examples of antennas that have stable feedpoint impedances over very wide frequency ranges. Select a balun closest to the antenna feedpoint impedance (you'll have to get this from textbooks or modeling), and use it at the feedpoint or where the feedline from the antenna ends. Be sure the feedline between the balun and antenna feedpoint matches the antenna impedance as closely as possible.
The balun for a terminated Rhombic, V-Beams and Other high impedance broadband antennas is the 12:1 model DXE-BAL600-H10A.
An Explanation of Our Ratings
When terminated in the designed output impedance, DX Engineering Baluns have lower SWR over much wider frequency ranges than competing baluns. Typical SWR curves of DX Engineering 1:1 and 4:1 baluns remain well under 1.3:1 beyond the stated range of operation. Even our least expensive baluns have lower SWR over much wider frequency ranges than other baluns. The extremely wide frequency range where SWR is low shows how we carefully select materials, layout, and the construction process.
DX Engineering Baluns are conservatively rated. Materials are selected to provide substantial headroom. Baluns are tested to work beyond specified worst case conditions. We specify the power rating into a 3:1 SWR resistive load in normal ICAS duty. Abnormalities in the system, such as extremely high common-mode currents, can adversely affect power ratings.
DX Engineering Baluns, unless specified otherwise, are current baluns. Current baluns try to force equal and opposite currents, regardless of load balance conditions. Equal antenna currents mean minimum feedline radiation near the balun.
High common-mode impedance is a very desirable trait in most baluns, since high common-mode impedance assures the best balance under widely varying load conditions. DX Engineering baluns have significantly higher common-mode impedance than popular competing baluns, often being several times better. More important, DX Engineering baluns maintain the high impedance over wider frequency ranges. Our baluns do a better job of reducing feedline radiation and RF-in-the-shack.
DX Engineering Baluns have such low loss they handle very high power without heating. Losses in our baluns, when properly terminated, are nearly immeasurable on typical equipment like power meters. Even greatly mismatched, our baluns' losses remain lower than competing baluns. The extra power goes to your antenna, rather than overheating and possibly damaging the balun.