Sunday, December 15, 2013

A Mini-Balun BLT

I have long been a fan of the balanced transmission line for efficiently transferring power from the transmitter to the antenna but that's not the only component in the system that can be lossy. There can be a tuner involved or a balun. In the transformation from balanced to unbalanced one can incur some serious losses and while we may have good intentions with baluns we must pay some attention to mismatch loss and the impedance mismatches at the input or output of the balun. If trying to match a highly reactive load a balun may introduce excess loss as I have measured 6-8 dB in very high impedance resistive loads and 8-20 dB if trying to match reactive loads. There are a few designs out there already that are specifically sized to accommodate 5-10 Watts but this design allows for 100 Watts to be applied and not go up in smoke. Plus this design has common mode rejection, something missing with all others, except ones that use a common mode choke or balun.

The Elecraft BL-2 is a switchable 1:1 / 4:1 balun that is capable of handling 250 Watts (but only when a matched load is attached) as well as small enough to be used in portable operations but again, it really needs to be matched with a non reactive load.

I have made several iterations of that design in a more compact variant called the MiniBalun and MicoBalun. The MicroBalun is the smallest of these designs and the prototype is shown next to Elecraft's BL2.

Given the need to create a reasonable match, an impedance matching device needs to be attached to the balanced side to transform whatever the antenna impedance is down to either 50 or 200 ohms balanced, hence the 4:1 / 1:1 switch that can be switched to accommodate either condition.

The goal here is to transform the load down to an acceptable match before it hits the balun, this will eliminate the mismatch loss that robs you of your 8-20dB power. After reviewing several balanced line tuners the air coil approach that Palstar incorporated in it's BT1500A looked like what I had in mind. Initially, I thought I would just make a coil attachment on the output of the balun to get the match somewhat close and the internal auto-tuner would match it the rest of the way. But that really only gets you half way. Another thing that all other commercially available BLTs lack is common mode suppression. They simply don't have it. Incorporating a Guanella style balun at the input creates enough common mode choking to keep RF off your rig. Since your antenna is rarely ever perfectly balanced, the addition of some choking reactance is a welcomed addition to this design. Touch your BLT+ or your Emtech ZM-2 with a poorly balanced load and witness the result.


I deviated from the Palstar design a bit in that my inductor is a single piece of wire wrapped on a single toroid core with two taps for the balun input 4 turns each way from the center and taps at 7, 13, and 16 turns, as well as the ends at 21 turns up from the center.


This creates an autotransformer with a 1.64μH primary and a 20μH secondary maximum inductance.

I affixed a 110pF APC style air variable capacitor and clip leads to its terminals that allowed me to attach to the taps.














I made a current probe to try to evaluate the actual amount of current being delivered to the resistive part of the load; the only thing that matters really.


However, the readings vary wildly when you move the probe closer or further away from the reactive element making repeatable measurements virtually impossible. But there is a relationship of the amount of current with high or low Z loads, where the high Z loads will produce a small current and vice versa so at least that tracks. A relative reading can be made if nothing moves.

I went back to using my IR thermometer to measure the temperature rise in the resistor. I have various carbon comp resistors in series with a 3 to 30pF air variable capacitor as well as some values in series with two air coils (10/20μH). My most difficult load to get the resistor to heat up is the 28kΩ resistor in series with 20μH at 28MHz. I can make it heat up when I halve the inductance to 10μH. The load then becomes 28,000+j1759Ω. Most loads resistors are 2 Watt resistors and can be heated to 100-150° C in 30 seconds with a 5 Watt setting on the rig. Of course, I don't know the thermal impedance nor is the heat path loss uniform for each load so this varies a bit but the fact remains, if it can't be heated, it's not receiving usable power, thus it's not an efficient transfer of power.

My application of 100 Watts gave acceptable results as the wild impedances create some hotspots on the T106-6 core wiring or if in the 4:1 position, the balun cores may get hot. Nothing above the Currie temperature though, only about half of that at worst.



The bottom line in my experimentation is that the MiniBalun and the T106-6 tapped inductor seems to match reactive loads from 2.5 to 4700+-j2000Ω on 30-15m and 27 to 2700+-j2000Ω on 40, 12, and 10 meters at the 10W level and less at the 100W level. What does this mean? I modeled some antennas and can match the NorCal doublet (60-10m), ZS6BKW (all bands it was designed for), and a 40m quad loop (60-10m) antennas.

The mismatch loss with a reactive load can reduce your transfer of power greatly. I measured 1-8dB for the "easy" loads and it only goes up from there for the "harder" loads, so with this BLT I can get common mode isolation, a decent balance, and 100-Watt performance in a 175g small package.

Myron WVØH
Printed on Recycled Data

4 comments:

  1. Hello Sir,
    Looking at building a 1:1 current balun with common mode isolation to use with a K2/10 and a remote MFJ tuner boxed near the feed of a 40m delta loop for use on 10-80m. Also want to use it on the KAT2 tuner in the k2 for a portable delta loop.

    Seems like your BLT I might do the job well.

    Mr. Robin Bayer
    KA5QQA




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  2. Hi Robin,

    Great minds think alike! That configuration is similar to what I'm trying to accomplish. I have an all band quad loop that I use when portable and really need the balanced line approach because I don't want to use transmission line in between the tuner and the antenna. I just eliminated it and feed the loop directly with the BLT.

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  3. Sounds good. The tapped toroid isn't placed withing the mini balun enclosure? I may not grasp the finished form attached to the loop.

    At Home I am using an MFJ 993b auto tuner remotely which has some type of 4:1 balance input. on ant 2 I wish to place a more efficient balun and common mode choke/match to a 40m delta loop fed with a short run of 450ohm ladder line. actually tunes 160-10 but want to decrease loss and stabilize radiation pattern.

    Thought I would start with a BL2, but may scratch build since I am ordering ferrite for other common mode suppressive devices anyway.

    I may be missing some concepts. I am not a technician so any guidance or a shove in the right direction is appreciated.

    Robin
    KA5QQA

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  4. Myron, I would like to replicate your blt and have some question on the taps and the balun design/material. Do you mind contacting me via my qrz email?

    Randall
    AF0O (i miss colorado, now in ohio)

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