"Yeah, But How Do You Know The Tuners Are Efficient?"
Or better yet, "How do you know the whole system is efficient?"
Do a back to back test.
Whenever I encounter a complex matching network or system, I at least consider the possibility of connecting them back to back and measure the power difference between the source and when the devices are inline. Both the MiniBalun and the Elecraft BL2 baluns are dual core Guanella types with switchable impedance ratios, either 1:1 or 4:1.
But first, the BLTs are adjusted to a resistor that has the characteristic impedance of the transmission line. Then they connected to the TL strung up in the air.
Balanced Line Tuners (BLTs)
I adjusted the BLTs separately to give less than a 1.3:1 reading on each attached to the 765 ohm load.
Where do you get a 765 ohm load?
The 765 ohm load is actually a 1000 ohm multiturn pot adjusted to 765 ohms. One of those blue 10turn jobbies while using the Sark110 to tell me where 765 ohms was at 14 MHz. There was a very small capacitive reactance component to it but it doesn't matter. Good enough for this experiment anyway.

Sark110 screenshot of an HF sweep, Rs left axis and Xs right axis 
I attached each BLT to the openwire line, one at each end. The MiniBalun BLT was at the rig end and the Elecraft BL2BLT was at the far end with the DL1 attached to the voltmeter. I used 5 Watts for the test.

MiniBalun BLT with AA600 
I set up 36 feet of openwire line using the #26 wire I use in the field. I stretched it between the deck pole and the fence in the back yard, up about 4 feet off the frozen Colorado ground. It is January after all.
This 36 feet of wire is the wire I had left over from cutting off 66 feet from the 102 foot spools ordered from Amazon. I guess 102 feet is the 'bakers dozen' of 100 foot wire spools.

BL2 BLT setup at far end with DL1 and voltmeter 
The above photo is my setup at the far end with the power detector, the Elecraft DL1 attached to the unbalanced end of the BL2BLT with a double BNC make adapter then the DC output is attached to my Fluke 89 set to autohold so when I key the transmitter I don't have to be at both ends at the same time to take a reading.
The wire is white. The string is white and in the picture you can't see where one ends and the other begins but suffice it to say I clipped the clip leads to the wire ends. I used the spreader and anchor as if I were out in the field to give the most representative example I could.

DL1 attached to unbalanced side of BLT 
I rechecked the SWR and all is good. To establish the reference I attached the DL1 directly to the KX3 set at 5 Watts, the input level if you will. I obtained 11.367V which equates to 5.389 Watts at the rig end. (TUNE PWR menu set to 5W).
The other thing I noticed while performing this test is the lack of receive signals heard on the band. another indication that there is good balance not allowing common mode signals to enter the receiver. If it was horrible and the line radiated then I would have been able to hear many signals and quite possible work somebody. That wasn't the case, the band was really quiet giving me some indication that the TL was working as designed not allowing signals to leave or enter the transmission line.

Reference power level setup with rig into DL1 
Now we relocate the DL1/voltmeter down to the other end and remeasure. Key the transmitter and wait for the beep of the voltmeter. Run down there and see what we have. (Later I got my binoculars out).

Hit Tune, read Voltage after beep 
Above is the photo of the far end measurement, 10.547V which equates to 4.654 Watts!
"What? A loss of only threequarters of a Watt?
Running the numbers of the ratio of voltages instead of converting to power and trying to decipher the diode loss and square root 2 factors, I will just use the ratio of the two voltages and use the 20*LOG(V1/V2) formula.
End to End Loss = 20*LOG(11.367/10.547) = 0.65 dB
How can that be?
That transmission line will radiate?
It will be lossy.
It's not supposed to work!
This is fun. What this tells us is that the aggregate matched system loss including the two BLTs and my lossy openwire transmission line has only 0.7dB matched loss.
Doing a little check to see where the loss may exist let's take a look at more measured data from an earlier post. I attached two Elecraft BL2s back to back and ran 100 Watts through them, measured the loss and this is what I came up with. The columns at the right are divided by 2 to obtain the loss of one balun.
So for 20 meters, in the 1:1 position, there is approximately 0.14 dB loss for one BL2.
So we have 0.65 dB to account for: (at 14MHz)
Here are the results of the insertion loss test of two back to back MiniBaluns with the KX3 set to 5W and the DL1 doing our power measurement.
The loss formula in the spreadsheet below is =20*LOG10(D17/C17), 20 instead of 10 because we have voltage and not power to ratio.
There is 0.12dB loss in the MiniBalun alone and 0.14dB loss in the Elecraft BL2 leaving 0.246dB lost in the TL and reactive components of the BLTs, A & B.
0.12 + A + 0.144 + B + 0.14 = 0.65 dB
A + B = 0.246 dB
(the loss of the L & C components of the BLTs)
Assuming a 70/30 split because of high Q air coil versus T1066 wound coil, we can ratio the remaining 0.246dB loss.
A = 0.7(0.246) = 0.172 dB
B = 0.3(0.246) = 0.074 dB
So, this leaves a total matched loss of the MiniBalun BLT and the transmission line of 0.436dB on 20 meters.
Using the 20m broadside gain 7.54dBi max gain and the EZNEC impedance of 4154j2314 ohms I put that into SimSmith for a resulting 0.56dB mismatched loss.
Adjustments to the model
Notice there is no power lost in the C block (the Minibalun) in the SimSmith model below. We know it has 0.12dB matched loss so we can add it to the total.
0.559 + 0.12 = 0.68dB
Since the balun is not seeing a load impedance away from its design impedance
there will be no mismatch loss. Restated, anytime you attach a muliband antenna with open wire/balanced transmission line directly to the output of a balun you introduce loss beyond the rated insertion loss rating. This is an important distinction between the end fed halfwave system using a 9:1 unun over a broad frequency range hooked up to either a bit of coax or an internal antenna tuner such as the internal antenna tuner in the KX3. You would be better off to ditch the balun (any ferrite device) and attach the wire directly to the output of the tuner. The tuner has iron powder cores in a tuned circuit whereas baluns and ununs use ferrite material in broadband circuit.
It is very difficult to analyze the balun mismatch loss due to the complex load impedance interaction between the antenna and the unun and the subatomic nature of the ferrimagnetic material. You can come close with reflective open/short methods to characterize the unun but cannot determine how it interacts with the load (at least I haven't come up with any way to do this).
Error Analysis
So none of this would be complete unless you step back from the numbers and took a look at the source of errors. The power is measured across the lower 25 ohms of the DL1 with a 1N5711 diode and a smoothing capacitor. Some error is removed from the process by not converting to and from power but using the voltage readings directly to obtain the dB ratios from input to output. That eliminates guessing what the diode loss will be for a given power level. It is convenient that the readings around the 5 Watt power level are almost exactly in step with the dBm readings, meaning 5.0 Watts = 36.99 dBm and removing 0.1 dB from that power level leaves you with 36.89 dBm = 4.886 Watts. A delta of 0.11 Watts. So to have accurate readings within 0.1 Watt or 0.1 dB seems like a tall order for the class of test equipment that I have.
Power Measurement
To help in the verification process I do have an HP432A with a 478A thermistor mount to do some comparisons. My DL1 and HP power meter are pretty close but are limited by the needles widths, parallax (although it does have a mirrored scale), and resolving to tenths of dB is difficult at best, whereas the Fluke 89 resolves to 3 decimal places. But again, I am stretching the limits of accuracy here and have come to realize that I have done the best I can to characterize the situation given the test equipment constraints at my disposal.
In this post, part 4 highlights the efficiency of the PPD's openwire transmission line in a matched condition. It is important to have measured results align with predicted results because that correlation is needed to gain confidence when we analyze the mismatched case where I cannot measure the complete system efficiency directly and have to rely on predicted results.
Other Thoughts
Through the course of power measurement and insertion loss characterization of the back to back baluns I discovered that since the AA600 generates a square wave signal at its output it produces inaccurate power readings at the other end of what every you are measuring. For instance, when measuring the 2 MiniBaluns back to back, the power loss was measured to be 0.8dB. This for a while was my insertion loss baseline. It wasn't until I characterized the loss using my KX3 as a source and I couldn't reconcile the results between the MiniBalun and the BL2.
After running the AA600 through my 20 meter bandpass filter, all readings fell into the same range as the BL2 power test. Both MiniBalun and BL2 baluns are about dead equal.
Coming up:
The Park Portable Doublet System Revisited (Part 5)
Using all we know to predict efficiency when an antenna is attached
72/73
WV0H Myron
Printed on Recycled Data