Straum balún 1:1
Requiring a balun to feed a balanced feed line with an un-balanced T-Match tuner, a 1:1 Guanella Current balun design using a L15 ferrite toroid core was selected among others. A current balun may be for most situations the most suitable balun however due the extremely low and high impedances often encounter with multi-band balanced antenna system a balun may be required to sep up or down the feed impedance presented at the T-Match tuner, it is for this reason that I chose to not include the balun as an integral feature of the T-Match tuner, opting for the flexibility of an outboard balun and the ability to trial various baluns subject to the antenna system and impedances presented.
The Guanella Current balun is a low loss, broadband balun that will ideally choke off common mode currents entering the radio room and importantly provide a transition from the un-balanced output of the T-Match tuner to the balanced antenna system feed line.
While using the balun to choke off common mode currents is best achieved at the antenna end of the feed line, this is not a practical arrangement for a balanced feed line system.
The Guanella Current balun will have application on other antenna systems such as coaxial feed dipoles and sloper antenna to mitigate the problem of common mode currents on the feed line.
The double bifilar winding of 10 turns are wound evenly spaced around the L15 ferrite toroid core with the two individual windings wound close together with a crossover half way through the winding so that balanced side terminates on the opposite side of the balun from the un-balanced termination. The toroidal core was rapped in an overlapping layer pink heavy duty Teflon plumbers tapeto protect the enamelled copper wire from insulation puncture from abrasion with the toroid core.
Figure 1 Schematic of the 1:1 Guanella Current balun
Figure 2 Wiring of the 1:1 Guanella Current balun.
L15 ferrite toroid core. Jaycar Cat. No. LO-1238
Pink heavy duty Teflon plumbers tape.
About 2 x 600mm of 1.25mm Enamelled copper wire.
Two Gold Banana Socket Binding Post - Black. Jaycar Cat. No. PT-0431
SO-239 UHF chassis mount connector
Sealed Polycarbonate Enclosures 82 x 80 x 55mm from Jaycar Cat. No. HB-6230. See Fig 3 for details
Figure 3 Sealed Polycarbonate Enclosures 82 x 80 x 55mm details. Designed to IP65 of IEC 529 and NEMA 4
Photo 1 1:1 Guanella current balun assembled.
The evaluation of the efficiency of the balun over the desired bandwidth (1.8 - 30MHz) was carried out by testing the impedance that could be seen from unbalanced side to a resistive load applied to the balanced side using an antenna analyser. The efficiency is shown to be relatively flat from below 1.8MHz to above 30MHz. The below antenna analyser plot viewing a 50ohm resistive load attached to the balanced side of the balun and measured at a nominal impedance of 50ohms presented as anticipated an approximate 50ohm load to the analyser and ideally produced about a 1:1 SWR. Despite not having carried out this test previously the results are more or less what was expected and demonstrates that the balun's 1:1 current transformation occurs efficiently from 1.8 to well above 30MHz
Figure 4 AIM 4170C antenna analyser plot viewing a 50ohm resistive load through the Guanella current balun. Note the 50ohm resistor appears as 50ohms due to the 1:1 balun ratio resulting in an ideal SWR of 1:1. This plot shows an SWR of almost exactly 1:1 with no reactance at a frequency of 500kHz with consistent rise in SWR throughout the HF spectrum to an SWR of approximately 1.8:1 and some inductive reactance at 30MHz.
Figure Figure 5 AIM 4170C antenna analyser plot viewing a 100ohm resistive load through the Guanella current balun. Note the 100ohm resistor appears as 100ohms due to the 1:1 balun ratio resulting in an ideal SWR of 2:1. This plot shows an SWR of approximately 2:1 from 500kHz through to 30MHz and with modest capacitive reactance towards the upper frequencies.
Figure 6 AIM 4170C antenna analyser plot viewing a 200ohm resistive load through the Guanella current balun. Note the 200ohm resistor appears as 200ohms due to the 1:1 balun ratio resulting in an ideal SWR of 4:1. This plot shows an SWR of approximately 4:1 at 500kHz dropping to approximately 2:1 at 30MHz and with significant capacitive reactance towards the mid and upper frequencies.
AIM 4170C antenna analyser explanation;
Standing Wave Ratio.
Resistive component of the total impedance
Phase angle between voltage and current. + indicates inductive reactance while - indicates capacitive reactance.
1:1 Ruthroff voltage balun, 1:1 Ruthroff voltage balun (1.8 - 30MHz).
4:1 Ruthroff voltage balun 4:1 Ruthroff voltage balun (1.8 - 30MHz).
1:9 voltage unun 9:1 voltage unun (1.8 - 30MHz).
Búðu til þinn eigin balún
Build your own HF balun
A balun is a MUST for dipoles or similar antennas when they are feed with coaxial cables. Many hams connect the center conductor of the coaxial cable to one side of the dipole, and the shield to the other. Wrong!
From the RF point of view, the shield can be modeled as two conductors, the internal shield (the real shield, this is, ground) and the external shield, who is really far to be ground. In this way, your dipole has 3 arms, the two from the dipole and the coaxial cable shield (external face).
Many times the coaxial is too long that represents a high impedance and a low effect on the dipole, but others, it presents a low impedance and radiates. This can cause serious RF feedback in the shack, TVI problems, among really poor lobules and directivity in the dipole pattern.
To prevent all these effects, a balun is needed. A balun is only a RF transformer. You can build your own balun, is quite easy. Use a ferrite bar from a AM receiver or better, a toroid. Turn all the coils at the same time and connect them properly as you can see in the above diagram. The most common balun for a dipole is the 1:1 ( the one at the left ), but a 4:1 balun can be useful also for wire antennas.
Make six turns of wire around the toroid and keep all wires parallel between them. It's an easy process that can be done in a few minutes. Use a wire according with the power you will use. Pay attention to the toroid you use. Use only a good RF toroid. Many surplus toroids are from switching power supplies or telephony equipment and are not usable in the HF range. In case of doubt, test the toroid. Use a dummy load (50 ohm for 1:1 and 200 ohm for 4:1 balun) and connect it to the output.
1:1 balun wiring diagram
Connect your transceiver at the input with a SWR meter and check the balun on the bands you will use it for. You should see a clear 1:1 SWR on all bands. If you don't see 1:1, is possible the toroid is not suitable for HF use, or you has made wrong connections between coils. The asterisks at the drawing represent the start of the coil. They all must be turned in this precise direction or the balun will not work.
4:1 balun wiring diagram
I can't assure you the balun will work with all the power you will use. For up to 100 watts, there must no be too much problem with any good made balun, but higher power will require another kind of toroids or ferrite bars. If you made your balun with a ferrite bar, remember all coils are turned on the same bar! A stripped ribbon cable can be quite useful for QRP, and cooper insulated wire for higher power levels.
Playing with my very first balun I learned a HF dipole NEED a balun. If you never used a balun, build one and play with it, you will be as surprised as I was!