Friday, February 5, 2016

The Park Portable Doublet System Revisited (Part 4)

"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 multi-turn pot adjusted to 765 ohms. One of those blue 10-turn jobbies while using the Sark-110 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.

Sark-110 screenshot of an HF sweep, Rs left axis and Xs right axis

I attached each BLT to the open-wire line, one at each end. The MiniBalun BLT was at the rig end and the Elecraft BL2-BLT was at the far end with the DL1 attached to the voltmeter. I used 5 Watts for the test.

MiniBalun BLT with AA-600

I set up 36 feet of open-wire 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 BL2-BLT with a double BNC make adapter then the DC output is attached to my Fluke 89 set to auto-hold 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 re-measure. 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 three-quarters 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 open-wire 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 T106-6 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 4154-j2314 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 muli-band 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 sub-atomic 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 HP-432A 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 open-wire 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 AA-600 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 AA-600 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

Saturday, January 30, 2016

The Park Portable Doublet System Revisited (Part 3)

"Yeah, But That Wide-Spaced Transmission Line Radiates!"

No. Balanced transmission lines don't radiate enough to contribute to the far field pattern. True, they do radiate a little and in practice at levels way down from anything destructive to the pattern or constructive to the far-field radiation. At most, they can upset the pattern due to coupling of the TL wire adjacent to the antenna wire thus skewing the pattern but they don't contribute to the total radiation.

Background

Electromagnetic radiation results from a charge acceleration (AC current) imposed on a conductor. A balanced transmission line (TL) does not radiate because of the equal an opposite current that exists along the TL. It only radiates if the load impedance at the other end is not balanced creating an imbalance of current along the TL. This imbalance is not cancelled out at any point along the TL where is produces a net charge acceleration and it radiates.

Modeling Difficulties

I ran an EZNEC model of the Park Portable Doublet antenna's TL, 33 feet, 825 ohm, #26 copper wire, balanced transmission line. The results show that the radiation from the structure is a result of the small pieces of horizontal wires used to anchor each end of the TL, these little horizontal wires radiate. So, EZNEC may not be the best way to show what could happen. I just showed the results for grins below. The worst case radiation occurred at 6m. (-37dBi at the green dot)


Spreadsheet

I found this spreadsheet on the internet that has a formula in it to show the amount of radiated power you get from an open wire line written by KM9O, Dennis and published in QST. I am not quite sure the origin of the fancy formula used to calculate the power radiated by the TL, but I'll take it.

P(rad)=160*(((SQRT(E4/(276*(LOG10(2*E6/E7))))^2)*((3.14*((E6/12)/(968/E5))))^2))

The radiated signal on 6m is 26dB down from the main antenna radiation and that's 6m. On 20m your mainline radiation will be at least 35dB down. In any case, hardly a contributor to the far field radiation pattern.


The bottom line about open wire transmission lines is this: They don't radiate if you keep the currents balanced and that is not hard to do if you pay reasonable attention to the deployment of the antenna and design/construction of the matching circuit.

On-Air Testing

I intended to perform a field test using the Reverse Beacon Network to establish a baseline SNR with the PPD. Then switch quickly to another setup of just the transmission line without the antenna attached, to see if I could still radiate a signal with just the balanced open-wire feeder.

I have to work out the matching scheme to do a quick A/B switchover while still presenting the same power level to each setup. I did find out that my 100-Watt Kenwood TS-2000 will put out 5 Watts (when set to 5W) at any load impedance, (except into a short or an open) so I may not have to mess with retuning for the experiment.

Software Model Verification

No analysis would be complete without looking at where the errors can occur in a modeled system, after all, garbage in garbage out.

One way to know if an EZNEC model is accurate is to do a convergence test. Set all losses equal to zero and look at the "Average Gain" at the bottom of the main window. Cebik outlined this (and other methods) in his multi-article series in QST while back. If there a greater than 0.1dB discrepancy, set the "Ground Type" to "Free Space" and the "Wire Loss" to "Zero" then adjust the wire segmentation until it converges on a zero dB gain. You have to re-run the FF Plot each time you adjust the segment length to re-compute the result.


Here the average gain is displayed in both linear and decibel units. You would ideally like it to be zero but in practice there is a trade between the number of segments and processing speed, you have to settle on something that is close enough and go with it. Since I try to stay within 0.1dB, my modeled results come within 5-10% of my measured results out in the field.

825 Ohm Transmission Line Using #26 Wire

Establishing transmission line loss is tricky, but with the right tools you can obtain reasonably accurate results using any Rigexpert AA-54 or AA-600 series or Sark-110 analyzer to measure and store the open and short circuit impedances. But first you have to obtain a reasonably long run of TL, around 100 feet using HF frequencies. For me that meant I needed two runs of 100 feet #26 wire spaced 8.25 inches apart. I say 8.25 inches apart because that is the spacing of the PVC spreaders I cut to hold the TL in the back yard on some photographic light-stands.

805 Ohms Maybe?

I wondered about the origin of my "825 ohm TL" and remembered using Sevick's Two-Wire graph in his hardcover balun book (and quite possibly the textbook formula for balanced line) and thought about rerunning the numbers using KM9O's Two-Wire calculator spreadsheet. I got answers closer to 800 ohms than 825 as the the spacing and wire diameter just won't support a higher Z line result. I'll have to dig up the Zplots file to double check, but it seemed to me that 800 sounded about right now thinking back on this. I guess I got the spacing numbers confused with the impedance of the line, and things like that seem to stick in my brain.




The wire I use has more strands then the "normal" 7 stands of #34 wire, I use (19/38), 19 strands of #38 wire I got on Amazon in two 100 foot spools for $26. The increased stranding makes it more resistant to wear out failures due to repeated stressing and flexing and the insulation of PVC seems to hold up when reeled and unreeled from the chalk line reels. Since air fills the gap between the wires, there is no need for Teflon or fancy low loss wire insulation.

The secret sauce in the original spreadsheet results cell (not pictured) is Zo=276*(LOG10(2*F32/F33)) where, F32=8.25 and F33=0.02, which is the text book formula for characteristic impedance of two wire line with a dielectric constant of 1. To account for velocity factor of 0.95 of the PPD TL, we need to modify the formula where the vf = 1/ sqrt(dk) is fit into the formula, =F34*276*(LOG10(2*F32/F33)) where, F34 is the vf = 0.95.



So there it is, 765 ohms characteristic impedance. I've been using 825 and I will need to adjust my future calculations to reflect this change. It may not matter much but we'll see.

Okay, Now Breakout SimSmith

In order to correlate measured results with modeled results we need to establish a baseline about the matched loss of the TL. This line has matched line loss of around 0.12dB for 31 feet at 14MHz as shown in the screenshot below.


Since TL is slightly reactive, an adjustment of the X in the load will tweak the SWR a bit but it's hardly worth the trouble. So, for completeness I adjusted the reactance of the load to -j2 ohms. The SWR (blue line) came down just a tad.


Coming up:

The Park Portable Doublet System Revisited (Part 4)

"Yeah, But How Do You Know The Tuners Are Efficient."

72/73
WV0H Myron
Printed On Recycled Data

Friday, January 22, 2016

The Park Portable Doublet System Revisited (Part 2)

But Really? How Good Is It?

Part of...no, all of the reason for going with a doublet antenna in the first place is the extra 7dB of gain you get in ditching the end fed halfwave or vertical wire over our less than perfect Colorado soil.

The whole PPD premise started on wanting to work stateside with as much gain as possible, turns out, there is enough gain that it will even outperform a vertical for DX over this blasted soil. I don't want to say, "I worked Germany, 5 watts" because that is a reckless statement but I have been called a liar by one guy in Sweden when I said I was QRP one day on 17m. Well, he didn't call me a liar but did say I had an amazing signal for 5 Watts. It didn't hurt that he had a 3 element yagi up 25m.

Let's take a look at what the maximum radiation you get when you have a vertical (any vertical) out set up in less than excellent, salt-water dirt.

Not bad you say? Yes, looks great. Until you read the fine print about the gain of such an antenna.


Yes, that is right. Pretty much zero dBi gain.

Now compare it to the Park Portable Doublet. 

Oh boy, that poor end fed is just not a viable option for me anymore. This brings up another topic I can hit on later. Because gain is a function of wire length, what is the shortest doublet length that I can run and match the gain of the vertical...hmm.

Okay back on topic...
What an advantage the doublet has over the end fed anything at any radiation angle. Except for maybe off the ends of the wires, end-fire.

End-fire response is shown below.


So, okay there may be stations off the ends of the doublet you want to work and for that having the ends forming an inverted Vee gives rise to radiation off the ends. Of course, that's we've all been told since we were little hams. Well sure, but not much. Have a look at the 3D wire frame model of the pattern on 20m, below.




So pretty much looks like a jelly bean. Nicely filled with no suck-outs or deformities. The doublet is positioned such that the wires exist in the Y-plane and radiation is broadside towards the X-axis direction.

On 40m you pretty much have a blob pattern with the PPD as it is after all close to the ground. Let's compare what the doublet does compared to a 1/4-wave 40m vertical with 4 radials laid out on the ground.

Again, the fine print puts the gain at about 0dBi where the green dot is on the screenshot above. (The outer grid at 0-dB is 5.64dBi). So I'd rather have this antenna out when I'm operating 40m. Day or night. Another reason to not get worried about low angle of radiation, if the sun is up, your 20 or 40m signal is coming from the sky and not the horizon making the vertical less effective.

Off the ends we have a squishing of the pattern a bit. Not as drastic but still there.


Below, we have a comparison of end-fire radiation off the doublet and the vertical.

Looking at the EF pattern reveals a focusing of the RF energy not inline with the ends. But, yeah, I still want the doublet because I will favor my target operating areas before I set up the antenna.

But Wait, You're Using #26 Wire. That's Really Lossy.

No. It isn't.

Below is a plot of the same antenna using #26 Copper wire versus #14 Copper wire, a 0.2dB difference! If this were a short vertical, it's current density would be much higher and the dependence on conductivity orders of magnitude greater, it would exhibit more of a difference. Here, we have a doublet that is not shortened and the current density is much lower.


Flashback...

Okay, remember the scenario where I wondered about what length of doublet I could run and still have the same gain as an EFHW?  Where here it is, an 8 foot doublet (4 foot a side) with 825 ohm TL, 13.09 feet long up 11 feet off the ground for 20m. Hmm, think Buddipole. The TL is performing a matching function here or else the loss would be horrendous.






Wrap-Up

So let's put all of this into perspective shall we?

5 Watts delivered to a 0dBi gain antenna radiates oh, say what? 5 Watts? Yep.

On the other hand, let's put that same 5 Watts into the PPD (or any doublet up 1/2wave) with ~7.7dBi gain.

Using an antenna that provides 7.7dB additional gain, you get:

5 Watts = +37dBm.
37 + 7.7dB (on our 20m example) = +44.7dBm (29 Watts)

By just changing your antenna you've increased your signal strength as if you were running about 30 Watts on 20m at 5 Watts not to mention the increased receive capability.

The latest firmware mod from Elecraft for the KX3 provides for output power of 15 Watts on 80 through 20 meters.
15Watts = 41.8dBm.
41.8 + 7.7dB (on 20m) = +49.5dBm (88 Watts)
My Kenwood TS-2000 only puts out 90W.

When using a 5.8dBi gain antenna (like the PPD on 40m), we get:

5 Watts = +37dBm.
37 + 5.8dB (on 40m) = +42.8dBm (19 Watts)

15Watts = 41.8dBm
41.8 + 5.8dB (on 40m) = +47.6dBm (57.5 Watts)
There's your HardRock-50 amp.

Yeah okay, it's not QRP anymore but I didn't have to buy an amp if I wanted to go QRO.

More perspective.

I ran the numbers on my Novice 80m dipole I had up 35 feet fed with 100 feet of RG-8U and an MFJ-941D antenna tuner and 100 Watts from the shack. The mismatch loss of the coax was consuming most of my RF power on any band other than 80m, my radiated signal was in the milliWatt range! I am really QRO now. I ran a EZNEC and SimSmith model of what I had up and here is what was presented to my antenna back then.

Power Presented to my 80m Dipole
160m = 1 Watt
80m = 87 Watts
40m = 10 Watts
30m = 1 Watt
20m = 0.025 Watts
17m = 0.025 Watts
15m = 0.016 Watts
12m = 0.016 Watts
10m = 0.013 Watts

It can be seen that I was QRPp for most bands. And no, I don't have to give back my QSL cards that I got back then. What an idiotic statement.

Coming up:

The Park Portable Doublet System Revisited (Part 3)

"Yeah, But That Wide-Spaced Transmission Line Radiates."

72/73
WV0H Myron
Printed On Recycled Data

Saturday, January 16, 2016

The Park Portable Doublet System Revisited (Part 1)

2 Years On

Design notes of the MiniBalun Dual Ratio Balun



Back in 2013, I created a smaller version of Elecraft's BL2, I call the MiniBalun. I say smaller because it is physically smaller and can only handle 100 Watts as opposed to the Elecraft BL-2 which is designed for 250 Watts of power. My design goal was to allow for 100 Watt operation if the need ever arose. As I was prototyping binocular cores for the balun, I settled on BN-43-0202 (type 43) material as types 61 and 77 materials produced unacceptable loss of low-end frequency response (type 61) and excessive temperature rise (type 77). I also tried several different sizes but landed on the 0202 size, about 1/2" by 1/2" square and about 1/3" thick. You use two in the design so the dissipated heat gets spread out between two cores.

I measured the insertion loss using my Sark-110 antenna analyzer capturing separate open and short circuit csv files. In order to increase accuracy I performed an OSL calibration using homebrew calibration standards (and a commercial 50-ohm load) similar to the male BNC connector on the balun itself.


(I also performed reflective insertion loss measurements with my Rigexpert AA-600 but only wrote those down in my log book back then). The files were imported into AC6LA's Zplots as transmission line open/short files to compute the insertion loss over frequency. I used 1 foot and vf of 0.5 as placeholders in Zplots. (Although it may not matter what the actual values are as I'm only interested in loss and not time dependent functions such as phase, Chipman equation 7.29).

Below is a screenshot of the insertion loss results plotted in Zplots. The red line indicates loss in the 4:1 mode.


Below is a screenshot of the MiniBalun when in the 1:1 mode.

An insertion loss of 0.2 to 0.3 dB is the average loss on 20 meters and I consider this to be acceptable. I also confirmed the accuracy of this loss with back to back measurements of two identical MiniBaluns prototyped way back then as well. I set up my Kenwood TS-2000 at 100W with a Bird 43 wattmeter with a 100H slug, logged discrete data points in the HF band and found insertion loss results in the same neighborhood, around +/-0.1dB.

For a refresher on the addition of the coil to form a Balanced Line Tuner, please refer to my earlier post here.


Here is a greatly simplified schematic. Simplified but still contains the pertinent components. As with all good designs it has to start on a napkin, or maybe an iPad sketch book.



After thinking more about it I created a more formal schematic. By the way, do an internet search on S-match tuner, (or visit my blog post about it here http://wv0h.blogspot.com/2015/01/s-match-antenna-tuner-wvh-minibalun.html ) after rearranging the circuit components but keeping the circuit connections the same as shown in the logbook photo below, you will see that it is really the same circuit. There is added flexibility in having exposed input or output windings. Exposed windings allow the flexibility of connecting the capacitor to either the input for low impedance loads or the output side for high impedance loads.





The air variable ended up being a 110pF cap but it can really be anything greater than 100pF to allow lower band operation. If 20m is as low as you go, you can get by with a 35pF air variable cap.

Palstar BT-1500 makes the tuner (below) sporting the balun on the input side. It is a good idea because it keeps the balun matched to its design impedance, thus keeping the losses at bay. What differs in my design is the switchable 4:1/1:1 aspect of the input balun offering two ratios, increasing the possible tuning range of the BLT.




Coming up:

The Park Portable Doublet System Revisited (Part 2)

But Really? How Good Is It?

72/73
WV0H Myron
Printed on Recycled Data

Friday, December 18, 2015

2015 Peanut Power Sprint

Yahoo! Another 1st Place win for the Park Portable Doublet and band conditions were really poor as well. Good thing too as my limiting factor is speed, if the bands were better higher speed ops would have gotten a better score.




Myron WV0H
Printed on Recycled Data