Tuesday, February 18, 2014

BL2 Cal Fixture for AA-600

I recently purchased a Rigexpert AA-600 and wanted to measure balanced lines, antennas, etc. The MiniBalun and the BL2 were great candidates to create an attachment that would allow a quicker method to cal out the balun while attached to the AA-600.

I used a DPDT center off switch to do just that.

The cal fixture can be used to measure just about anything balanced, including the short in my house telephone wiring with the TDR function of the analyzer! I found a short about 8 feet away from one of the phone jacks. I'll repair that later.

This is a photo showing measurement configuration to measure my 20m QRP trap. Results are within 1% of calculated!

The tan column is calculated values and the E/F columns are without calibration or fixture, just a BNC binding post to an N/M to BNC F/M adapter. The G/H columns are the measurements with the cal fixture. I measured the resonant point by a different method I will talk about later. The Z=Q*Xl method is the calculated value hanging out in the I column.

Saturday, February 15, 2014

Magnetic Loop RF Safety

While it is okay to determine the electric field strength it is not the dominant safety factor. Please use the magnetic field strength formula as this is the dominant field when this close to a magnetic loop.

Page 67 of FCC OET 65 shows the table for maximum safe RF levels for a controlled environment. These levels are from IEEE C95.1-1991.

For example, on 20 meters use 4.89/14.000 = 0.350 A/m maximum limit for a controlled environment, such as when you are pedestrian mobile with a mag loop.

I've added a row in my AA5TB loop calculator spreadsheet (C34 as shown below) to calculate the magnetic field strength. 

C34 =(C33*(E11/2)^2)/(2*((E11/2)^2+C35^2)^1.5)

My 4 foot 1/2" Heliax loop with 5 Watts input yields a 0.248A/m field strength using the loop diameter of 4 feet. So one loop diameter distance away on 20m is considered safe for a controlled environment using 5 Watts. 

This is my rule of thumb for my operations, keeping at least one loop diameter distance between me and the closest part of the loop when using 5 Watts.

Myron WVØH
Printed on Recycled Data

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

Saturday, December 14, 2013

Inverted L for 40/80m

I just got done with my 40/80 inverted L. I made a 40 meter trap out of a T80-2 toroid with 46 turns in parallel with a 39pF 500V silver mica capactitor. My method of testing the resonance point involves using a binocular core balun with 2 turns wound for the primary and 5 turns wound on the secondary. The secondary is looped through the trap core twice and loosely. I had to adjust the turns on the toroid to achieve a 7040kHz resonance. Once that was achieved I put super glue on the windings to lock them into place. The center insulator is some plastic rod that just happen to fit down the center of the finished coil. I drilled two through holes to accommodate the antenna wire to slip through both and just tied a knot in the wire.

The vertical portion of the antenna is about 30.5 feet. The trap is placed at approximately the 31.5 foot level off at a 90° angle to form the flat portion of the inverted L. My pole supports the wire close to the trap. 80 meters is brought into resonance by starting with a 25 foot piece of wire. I had to trim quite a bit off before it was dead on at 3560kHz. It seems like 2 feet came off.

Myron WVØH
Printed on Recycled Data

Monday, September 30, 2013


Ever since dropping too many dits and dahs for my liking (any). I performed the "jumper wire" mod. I have no idea what size wire I used but I soldered one end to the ground on the plug and a wire squeezed under each screw. I meandered the wire around the back to give strain relief.

Myron WVØH
Printed on Recycled Data

Wednesday, September 11, 2013

Paralleling Batteries

I've heard it said that there is a tendency for Internet experts to claim that its not a good idea to parallel batteries because they will fight each other...

I took two separate 12V 7Ah SLAs (1.9 and 4.9 Ah) recharged them, and put the parallel combination on the CBA and guess what?

Their capacities ADD. When batteries become discharged their internal resistance goes up. That resistance is higher than the load resistance or the internal resistance of the other battery so current WILL NOT flow into the discharged battery. This holds true for all types of batteries as I proved it with the CBA.

The only way this does not hold true is if one of the cells has some internal short or leakage resistance that is of lower resistance than the load or the other good battery, but you'll be able to tell this because that bad battery will have self-discharged in short order. (Ahem)

Neat huh? Bottom line, if any battery you want to parallel holds a charge for greater than a day, the parallel combination will be the two capacities added together.
Myron WVØH
Printed on Recycled Data

Myron WVØH
Printed on Recycled Data

Tuesday, September 10, 2013

Zippy Update

My 4200mAh 4S1P LiFePO4 battery died. I was testing it on the Computerized Battery Analyzer when one cell had reverse voltage applied from the other cells forcing current in the opposite direction (-0.2V). It was warm to the touch on the negative end of the battery at that time and slightly bloated. I quickly put it on the balance charger and it stopped charging about a second after initiating the charge sequence. The single negative cell had reached its maximum charge voltage and the charger stopped the charge sequence. Done.

I left it for dead in the fire safe can bound for recycling.

(Fastforward 3 weeks)
I decided to perform an autopsy. After all, there are 3 good cells in there.

I carefully removed the blue outer wrap and untaped the battery tabs and wire ends. I separated the bad cell (the neg end cell) and looked it over very carefully. It was still expanded but not tight, only slightly bulged on the broadside. So I decided that a 3S pack at 10V was better than no pack. I wired the remaining 3 cells and left the "bad" cell out, for a nice 10V pack.

Curious about the bad cell just laying there on my desk, I tested the voltage half expecting to see 0.5 V or something. No, it was more like 3 Volts. I put it on the charger and charged it at a 2A rate. I ran the CBA on it and after the third time. Hmm, "Not quite dead yet!" comes to mind. Now, no swelling of the cell, it went back to normal. I am lucky and have a useable cell that I can remake my pack.

So what's going on here. The Internet is full of advice that once a cell has experienced a reversed voltage application or heating and swelling it is damaged beyond repair. I guess one should always try to investigate and find out for themselves the real answer. The Wikipedia entry about LiFePO4 battery technology states that the Lithium-Oxygen bonds are much stronger than the bonds in LiIon or Lipo batteries, making it harder for them to combust. A safer chemistry and apparently more robust as well.

Myron WVØH
Printed on Recycled Data