Testing the TRB (Triple Ratio Balun) with the Autek RX Vector Analyst
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
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.
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.
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.
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.
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.
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You may email author Alan Biocca via wb6zqz at arrl.net