WB6ZQZ Triple Ratio Balun

Testing the TRB (Triple Ratio Balun) with the Autek RX Vector Analyst

Document Ver 1.38 1/20/2005 Kits available now from our Online Order Form


Have you had an antenna that, no matter how you adjusted or trimmed it, would not get below 1.5:1 (or perhaps even 2:1 or 3:1) Standing Wave Ratio (SWR)? The SWR would drop tantalizingly down and then rise back up as it was adjusted?? There are a number of possible causes for this, and the major one is IMPEDANCE MISMATCH. Even though the antenna is RESONANT, they often present an impedance other than 50 ohms to the feedline.

Portable Antennas need to be small and light. They are often loaded with inductors to make up for less than full length elements, and may be operated close to the ground. This usually results in low feedpoint impedances. Even fullsize dipoles close to the ground have low feedpoint impedances. Due to varying conditions and configurations the impedance may vary considerably and more than one impedance ratio may be necessary to obtain an adequately low standing wave ratio (SWR) to the transceiver.

In addition, it is desirable to choke off any imbalance current and avoid RF on the chassis of the transceiver which results in many problems. This can be especially troublesome in portable work where the coax is very short and the antenna very close to the transceiver.

This design works on both balanced antennas (such as the excellent Buddipole (tm) portable lightweight dipole, or low wire NVIS dipoles) as well as verticals (such as the Buddistick (tm) configuration of the Buddipole (tm) Antenna System). Even a full size quarterwave vertical near the ground will have a feedpoint impedance that is a bit low for good direct matching with 50 ohm coax. Most high efficiency mobile HF antennas also present impedances lower than 50 ohms at the feedpoint on some bands. This balun brings these antenna impedances within range of a transceiver without the need for an antenna tuner.

TRB Design Requirements

The TRB Design

I went to my copy of "Understanding, Building and Using Baluns and Ununs" by Jerry Sevick, W2FMI. This excellent book has many balun designs and measurements of their losses. If you are designing baluns you should have this book.

After reviewing pretty nearly every balun in Jerry's book I chose to combine two of his toroidal designs into my final design to meet the TRB requirements.

In the photo above you can see the two Balun cores. The one on the left is the 1:1 balun which serves to choke off the common mode currents to perform the Balanced to Unbalanced conversion and isolation functions. This consists of 8 to 12 bifilar turns of #16 thermaleze wire on a ferrite core.

The #16 Thermaleze was chosen because when taped tightly into a pair it provides very close to 50 ohms impedance. Additionally it has very low loss and more than adequate power handling capability. It is also easier to form than heavier wire. Thermaleze is a physically very tough, high temperature (200C) and high voltage resistant (2000V) multi-layered enamel coated wire. Other enameled wire may be substituted with potentially some loss in performance.

The second core is a multi-ratio UnUn providing the 2:1 and 4:1 impedance transformation options. This is 4 turns of quadrifilar (four taped together) #16 thermaleze wire connected in the configuration shown on the construction page. (The precise ratios are 1.78:1 and 4:1, or 28 and 12.5 ohms).

Input is via the SO239 connector to the 1:1 Balun, this feeds into the UnUn (the two are in series) and the various outputs are connected to the Banana jacks. Common is on one jack, only one of the other three is used at one time for 1:1, 2:1 and 4:1 ratios. Changing ratios is as simple as moving one banana plug. The TRB housing is constructed from low cost high availability schedule 40 PVC plastic pipe components. Serviceability is maintained by not glueing every joint and using a sheet metal screw to hold the unit together. Water repellency for the connectors is provided by hanging the unit upside down with the connectors pointed down. A planned drip-ring around the outside edge is not shown in the photos above, this would conduct water away from the connectors. Sealant can also be applied around the connectors.

For ultra lightweight portable operation it may be desirable to use a less weighty housing. The PVC is easy and low cost and very tough, but it does add a bit of weight. I may try mounting my next version in a water bottle with the connections recessed up inside allowing water to drip by without getting inside.


If the antenna is resonant, the above chart shows the relationship between the antenna impedance, the tap settings and the SWR. Through the 8 to 18 ohm range, the 12 ohm tap provides 1.5:1 or better, from 19 to 42 ohms the 25 ohm tap is 1.5:1 or better, and from 34 to 75 ohms the 50 ohm tap provides 1.5:1 or better SWR. Relaxing the SWR requirements at the edges of the range to 2:1 allows impedance coverage from 6 to 100 ohms.

TRB Parts List

The parts are listed along with a source. Parts cost is approximately $25-30, and build time is approximately one to two hours.

TRB Construction

Construction details and photographs are on The Balun Construction Page.

TRB Testing

I used the Autek VA1 Vector Analyst to test the completed TRB. This instrument allows measurement of complex impedances (both reactive and resistive components, including phase angle) at frequencies from 0.5 to 32 megahertz. I used a 50 ohm load on the antenna side and measured impedances on the radio side. This confirmed that the balun was wired correctly and working - producing the correct impedance transformation ratios. I measured the isolation impedance at various frequencies. I found I could only measure this impedance at low frequencies - anything approaching a few megahertz was offscale - the impedance was too high to measure with the Autek.

The real test is with some real antennas. For this Budd, W3FF and I tested with a number of configurations of his Buddipole Antenna in different configurations, at different heights, on different frequencies on a variety of bands. We used Budd's MFJ Antenna Analyzer. We were always able to find a very good SWR and a clean resonance, and there was no problem with 'wandering SWR' when touching the MFJ analyzer's ground connection - indicating that the balun isolation was in fact working well. Since there were the various impedance ratios available to select from it was not necessary to use asymmetrical coil and whip settings to get a good match. All in all, it appeared to meet the design performance requirements with flying colors. The 2:1 25 ohm ratio produced the best SWR most of the time, but there were occasions that 1:1 or 4:1 produced the best results. All three ratios were useful. Being able to change quickly between the ratios really helped optimize the system quickly when making changes in the frequency or configuration of the antenna system.


If you work with resonant antennas that are small and/or near the ground then this Triple Ratio Balun can effectively provide the impedance transformation to achieve good matching over the wide range of 9 to 75 ohms with 1.5:1 SWR or better, or even 6 to 100 ohms with less than 2:1 SWR. At the same time it isolates against unwanted RF current on the outside of the coax and attendant problems. It separates the resonance and impedance matching functions, making the antenna faster and easier to tune, and reducing loss in the system. Note that this is not the same as an antenna tuner - the antenna should be resonant or near resonance for best performance with this balun. If the antenna is not resonant the balun will see increased voltages and currents that are somewhat out of phase. Under these conditions the balun will still work, however if the currents and voltages are extreme enough (the mismatch is very large) the efficiency will be reduced (losses will increase). For example, frequency excursions within a band may be fine, but changing bands without re-resonating the antenna may produce notable losses in the balun. Test it and let us know what you find! In normal use this balun is efficient enough that it should not heat up appreciably from RF at the 100-200 watt level.


The production version of the TRB is now complete and kits are available. It includes a high quality printed circuit board and parts. See the web pages at PCB TRB for further information.

PCB Balun Testing

I have done some testing on the PCB version of the TRB-1. First results - resistive impedance measurements in the lab - are good, showing that the balun is producing the design transfomation ratios. Now I need to do some testing with a real antenna...

PCB Balun Progress

4/11/2004: The second test was with my Buddipole on 40 meters. I experimented using the standard Buddipole configuration and settings, and then added a 22 inch arm to both sides between the coils and the center tee. As you would expect this changed the resonance only slightly, but the impedance substantially. The Buddipole was 14 feet above the ground for these tests. The results of SWR measurements at resonance were excellent, reaching less than 1.2:1 SWR in both configurations. The interesting thing was that with the smaller Buddipole configuration using single arms, the 12 ohm tap produced the best SWR. With the longer Buddipole using double arms, the 25 ohm tap produced the best SWR. Using the 4:1 ratio 12 ohm tap with the longer arms resulted in a 2:1 SWR, which is still quite acceptable.

The third test was 20 meters. Still using the extended poles (two 22" poles on each side of the tee before the coils), I reset the Buddipole for the standard 20 meter settings (full 66" whips, 10 Turns on the red side and 14 turns on the black side coils. The resonance was a bit high at 15 mhz. Reset coils to 13 taps both sides and adjusted whips in 1.5" from full length on both sides. Selecting the 25 ohm balun tap, resonance now about 14.2 mhz and SWR 1.2:1. Very nice.

For the fourth test I moved up to 17 meters using Stock settings - 10 turns on the red side, 5 on the black, full whips, but still using 2 arms per side from tee to coil (44"). Set the balun to 25 ohms, shortened the whips about 4" each side (from full), now swr is 1.02:1 at 17.09 mhz. Shorten another 1/2" both sides, move it up to about 17.157 mhz, now 1.1:1 SWR.

Basic Adjustment Procedure - adjust the coils for coarse and the whips for fine resonance, then adjust the balun tap to get the SWR below 1.5:1.

Having the extra taps often makes the difference between an SWR at or somewhat above 2:1, and an SWR of 1.5:1 or less. While this will not make a noticeable difference in your signal strength, it does make the transmitter a bit happier. If I can improve the SWR from 2.5:1 to 1.5:1 (or 2.0:1 to 1.2:1) by moving a banana plug from one socket to another, I'll probably do it. There also may be a somewhat larger benefit in the efficiency bandwidth around the resonant frequency for those using antenna tuners at the transmitter to move around the band - the bandwidth may be a bit wider for the same system efficiency.

4/15/2004: The next version of the PCB TRB (#3) has slightly smaller and significantly ligher weight cores. The lightweight prototype TRB is now built and bench tested. It works well on the bench with the Autek VA1 and fixed resistor loads.

Dayton 2004

The new PCBoards did not arrive quite in time for Dayton.. I put a comment regarding the triple ratio balun on my scrolling sign-badge but did not receive any comments about the balun. I did get lots of comments about the badge, perhaps I should build those instead...

6/13/2004: This update is overdue, lots going on. I got busy after Dayton with work and other distractions and this project has slowed but it is still moving. The boards came in, and they are very pretty with plated through holes, reflowed tin plating, solder mask and silkscreen. I built one up and have been testing it. It weighs about 3.5 ounces. I have parts on order for the first kits, and pictures for the assembly instructions. The new layout is 1.3 by 5.3 inches. It can be housed in 1.5" PVC if you want a sturdy weatherproof housing, or bubble wrap if you want a lighweight setup. The coax comes off one end, and there are anchor holes for zip ties to support it -- OR there is a provision to attach a PCB mounted BNC jack instead of the direct coax connection, using the same PCB. The antenna end has Anderson Powerpole connectors for the common and the three impedance levels and a hole to attach a cord for supporting it below the antenna.

Coming Soon

The kits are available now for the PC Board version of this balun. The Production TRB web pages are now up, for further info on kits please visit them.


Thanks to Budd W3FF and Chris W6HFP for encouragement and help with testing on the TRB project, and to Jerry W2FMI for his excellent book on Baluns. And thanks to Ken WB6MLC for field testing! Also thanks to the folks at www.qsl.net (Allan Waller) for the website!


Triple Ratio Balun Construction Page

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