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|Balanced and Unbalanced
Many articles and technical sources on baluns or balun design ignore voltage, focusing only on current. Focusing only on current is a serious oversight that can result in defective system design.
Equal currents alone do not assure balance, and SWR is not a confirmation of proper system operation.
Properly working systems, both balanced and unbalanced, have exactly equal and opposite currents in each conductor. VOLTAGE distribution determines if a system is balanced or unbalanced.
Current balun balance quality can be evaluated by moving a ground (common with signal source) to points A, B, and C while watching voltages or currents in the load. As a current balun becomes better, it will show less input SWR change or less current change through R1 and R2 as the test ground is moved.
The lack of system perturbations with grounds at any point are good current baluns function also perfectly as un un's!
The sum of R1 and R2 are selected to equal system load impedance. In this case the test is for a 4:1 current balun with 50 ohm unbalanced and 200 ohm balanced ports.
Note: A 50 ohm balun can be tested using a pair of 25 ohm resistors in place of the 100 ohm resistors used for 4:1. In similar fashion, a 9:1 would use 225 ohm resistors for R1 and R2.
The same test above can be used, at high power with suitable resistors, to roughly evaluate balun power handling in ideal systems. The balun should not overheat at full power in the experimental worse case jumper position. Heating limits in an HF balun, regardless of load impedance, is almost exclusively due to losses in the core. This is true for any type of balun in the real world. Do not confuse heating with flux-saturation of magnetic materials. Flux saturation does not necessarily cause heating, it simply means the core cannot carry any more flux and any additional current causes a reduction in inductance. Virtually all HF cores heat from the loss tangent of the core. The loss tangent causes the core impedance to appear as a complex combination of resistance and reactance. The resistive part represents the dissipative characteristics, while reactance is lossless.
All baluns, even transmission line baluns, will have significant flux in the core with real-world loads. This flux density is the primary loss or heating mechanism in a balun.
See 4:1 balun page for an analysis of winding common mode currents, which can be represented by voltages across the winding.
This page shows measurements of various baluns and how various baluns compare.
This data shows the common mode impedance of the balun. In general, the highest impedance at the operating frequency or over the operating frequency range is desirable. This impedance isolates the antenna from undesired signals on the feed line shield, and prevents antenna terminal voltage from exciting the feed line with unwanted currents. Common mode impedance is directly related to the care in design and construction.
Pay particular attention to the impedance peak in air-core baluns. For narrow-band applications they make excellent baluns if load common mode impedance is not capacitive.
Unfortunately common-mode impedance is all over the place, as this Smith Chart plot shows:
The air-core balun is good only for a three or four-to-one frequency range, unless you pick a winding style and size that places unwanted series resonances outside desired bands, and is only good where common mode impedance is inductive or the same sign as the balun's common mode impedance. If we are not careful with system design, an air-core balun can make system balance worse!
In contrast a good core-type balun looks like this:
The lowest SWR is desirable, although any mismatch can often be compensated by adjustment of antenna dimensions. This SWR mainly comes from incorrect wire impedance inside the balun. It may be caused by excessive length of internal leads, or incorrect cable or winding impedance inside the balun. It generally is a construction related problem.
The following data is measured using a currently certified network analyzer with low capacitance test fixture:
|Centaur||DXE||W2DU(1)||W2DU(2)||Force 12||Scramble||Solenoid||W2AU volt|
|Choke Impedance||Note 2|
|R+X@1.8||84 129j||554 1.4k j||378 617j||230 325j||169 286j||1.67 245j||1 +1.18k j||.488 5j|
|R+X@15||3.76 2.7k j||835 -1.84k j||727 -611j||761 -10j||883 -105j||11.97 -850j||62 -895j||1.36 42j|
|R+X@30||143 -729j||153 -893j||284 -440j||610 -296j||538 -381j||73 162j||68 -168j||8.2 68j|
|Max Z@F||17 MHz||6.65 MHz||7.16 MHz||15.3 MHz||13.24 MHz||6.42MHz||4.25 MHz||60 MHz|
|R+X at max Z||5.87k -943j||4.5K -340j||1.3K -13j||770 -20j||914 2.25j||42.7k 0j||34K 37K j||75 286j|
|R+X at min Z||10 -2j||198 -252j|
|F SWR=1.25||6.8 MHz||65 MHz||21.15 MHz||20.2 MHz||note 1||note 1||20.9 MHz|
note 1: SWR not measured because construction and cable type affects SWR
note 2: This is a W2AU voltage balun. It is only shown as a example of poor shield isolation offered by voltage baluns if the antenna is not perfectly matched to the balun with the feed line exiting the balun at right angles. This type of balun is unsuitable for non-symmetrical systems such as off-center-fed antennas, verticals, or antennas with the feed line paralleling the antenna (even at a fairly large distance).
The W2DU baluns were manufactured by Unidilla. (1) is a Maxi balun and 2 is a 10-40 meter model.
The DX Engineering balun is the dipole balun type DXE-BAL050-H05-P
The scramble-wound choke was about 20 feet of RG8X in a six-inch diameter "bundle".
The solenoid balun was about 60 feet of RG-8X on a 4" PVC thin wall drain pipe coated with rubberized roofing tar.
Note: This section revised 1/2/2003 to correct model error. Please report any other errors to me!
Balun power dissipation is estimated using Eznec to simulate a perfectly balanced dipole.
Here is a copy of the model used:
160m balun EZ
Please be aware I made no special effort to create a "bad antenna" other than I intuitively understand what the worse case condition of feed line length would normally be and I selected that length. I dropped the wire representing the feed line vertically from the center of a perfectly balanced dipole, and made that wire 1/4 wl long.
Here is a view of the model with no balun:
SWR is 1.46:1 power is 1500 watts
Currents at 1500 watts are approximately:
5.65 amperes into wire 1
2.63 amperes into wire 2
3.73 amperes into wire 3 (coaxial cable shield)
Using this model (a 135 foot high 160-meter dipole) we can add each of the balun impedances in the coaxial cable shield and estimate feed line current and power dissipated in the balun:
|0.90 amp||0.12 amp||.25 amp||.47 amps||.57 amps||.87 amps||.16 amp||.4 amps|
|69 watts||8.5 watts||25 watts||51 watts||55 watts||1.3 watts
(26w TL loss)
|.03 watt (74w TL loss)||34 watts|
From this we can see the following:
Think about the heating this way. Imagine you had a 60-watt light bulb. Nearly all of the applied power is turned to heat, and the surface area of the bulb and conduction through the base radiates that heat. Would you hold a 60 watt light bulb?
Now picture a balun core with a surface area a fraction of the size of the light bulb. This core area is enclosed in a case that often has poor thermal conductivity.
Every ampere of current not going onto the cable shield goes to the dipole's shield-fed leg! The total is not the exact sum, because of phase differences. Here is a view of currents in the dipole with the DXE balun:
Wire 1 4.52 amperes
Wire 2 4.52 amperes
Wire 3 .12 ampere
Remember power radiated increases by the square of current. The feed line shield current is now .12A compared to 3.73A without a balun! The feed line radiates about .1% of the power it radiated without a balun. While pattern distortion on transmit may not hurt, the feed line radiation probably aggravates RFI and allows noise to couple into the antenna when receiving.
Remember this model is worse case in the NO BALUN condition. This does not mean every system or most systems will be this bad. This example was only intended to show how bad balance can be and how much power baluns (even with a matched load) can dissipate!
Ladder Line Voltage and Current tests
The data below was taken with a half-wave 80 meter doublet about 120 feet or so above ground. This doublet is suspended between two towers and is suspended by rope for at least 30 feet from each tower. Nothing is within 100 feet of the doublet. feed line is a vertical drop of Wireman "heavy duty" ladder line, twisted to maintain electrical balance and reduce wind oscillation.
In the table below, "D C" is a standard dual-core two-transmission line balun. It has two independent magnetic paths for each balun transmission line.
Look at how terrible balance is on a single-core 4:1 "current balun" below. A 4:1 single-core "current balun", as presented in "Baluns and Unun's" and used in some commercial designs, forces a system into gross voltage unbalance. It is not a balun. This is what happens when we offer a balun design without understanding or measuring balance.