Short and sweet. Operated for 2 hours. Worked 14 stations. Some MN QSO Party on 40. Used the MFJ-2289 Big Ear antenna up 13 feet at apex, KX3 running 5 Watts and temp was 58 degrees F. So much for freezing. In fact last nights low was 57°.
Here is the 14 QSOs I logged.
Score = 14 x 6 x 2 x 4 x 2 = 1344 points
So pretty fun except it wasn't very cold, which was a good thing because I didn't feel too well coming off a cold.
A post on the Internet said that AC6LA's Zplots can automatically generate vf, |Zo|, insertion loss for you!
So the thing I was missing from my 26 AWG transmission line graph was the velocity factor and |Zo|. Well, since I calibrated to 725 Ω with a conventional transformer that resulting plot of a 50 Ω graph was pretty boring so I just plotted the loss and velocity factor on one graph.
As you can see the velocity factor is pretty constant over the HF band and the loss was as I reported before, maybe a smidge lower because my line length was actually 101 feet long and Zplots can normalize to 100 feet automatically.
Ever since posting loss curves about using my #26 (19/38) stranded wire I use for my Park Portable Doublet, I've wondered about selecting another verification method to cross check my work. My friend John, KN5L has been evaluating some transmission lines, investigating loss, and coming up with loss verification methods that are useful for this purpose. While his investigation has been with balanced 300-Ω window line Universal Radio sells, mine is over 800-Ω.
Higher impedance line will exhibit lower loss using smaller conductors than equivalent conductors at lower impedances because of the I^2*R property in Ohms law and I use that property to maintain portable status for this gram weenie. I compiled the data into a chart that looks like this from a previous post. Plotted the data a did a curve fit. It is supposed to be a straight line on log/log paper but there may be some errors in my measurement system that may account for this non-linear response but the general principle is there.
So I've always wondered about how I could use a second opinion to verify what I measured is close enough. Well, today John writes that I can get an approximation using RF resistance numbers from a website compiled by VE3EFC, located here.
In his tables, the RF loss can be made into a ratio of characteristic impedance to RF loss resistance. The characteristic impedance can be found by, (#26 wire).
Zo = 276 x log(2S/d), where S (S = 8.25, d = 0.016) is the spacing and d is the diameter of the wire,
Zo = 276 x log(16.5/0.016) =832Ω
Since the total resistance of the wire is comprised of DC and AC (RF) resistance we will add them together.
DC resistance = 34.43 ohm per 1000 feet for #26 (19/38), so for 200 feet we get 6.9 Ω.
Now, the 15 MHz RF resistance for #26 stranded is similar to one gauge size smaller because of the stranding and surface irregularities so #28 at 15 MHz yields R = 34.83 Ω per 100', or 69.66 Ω per 200', the up and back distance. Plugging all of that into a loss ratio and "dB-ing it", we get,
Loss at 15 MHz = 10 log((832 + (7+70) / 832) = 0.38 dB
The chart shows just over 0.4dB loss at 15 MHz, and that is close enough for me.
Since I use only a third of that line in my deployment of the Park Portable Doublet, the loss is 0.128dB. Couple that to a MiniBalun/BLT with less than 0.3 dB loss at 15-MHz and we have acceptance in the last dB club, finally weighing in at 0.43dB loss of the tuner and TL up to the antenna feedpoint. With an antenna that has around 6-dB broadside gain on 20m and with 5 Watts (+37dBm) applied to this antenna system, the receiving station would say that I am running around (36.9897+6-0.43=42.56dBm) 18 Watts.
I was wondering about when I wanted to operate my Park Portable Doublet when it rains. My MiniBalun BLT isn't exactly waterproof let alone weather proof. Field Day certainly can be rainy, even here in Colorado.
I decided that a waterproof enclosure would be just the ticket. So here it is, one clear plastic box with a nice seal to keep the rain out.
So I redrew the circuit. (Pardon the hand drawn schematics).
The MiniBalun BLT provides common mode choking to help reduce common mode currents, the S Match design may not.
Mine is a bit more compact. Oh yeah, you can make one out of an Elecraft BL2 and some coilstock.
As I look over the initial schematic diagram the original circuit reveals itself to be balanced L match in its topology. Not that it's a bad thing it's just a different way to skin the same cat, impedance matching. Ultimately the need to cancel the loads reactance and equal the leftover resistances, a conjugate match.
Being the founding member of "The Last dB Club", I wish to make it clear that the charter of this unique club is to remove all inefficiencies down to the last dB, having said that anytime you pass RF through a medium you incur loss. A conventional transformer can have loss or it can have low loss, it depends on how it was designed. An autotransformer tends to be less lossy if properly wound so given the choice, I prefer to use an autotransformer for impedance matching circuits, if I do not need DC isolation from "goes into" to "comes outta". That said, the Last dB Club cannot endorse this design.
I started this endeavor with the intent of just seeing if I could build a scaled down version of W3NQN's famed filters. I started with the 20 meter filter and scaled the size of the toroid cores down from the T130 size to the T68 size. In doing so, the inductance values had to be recalculated to obtain the same response. I got lucky on the 20m filter because everything dialed right in. Not so much on the 15 and 40m filters.
Think outside the box.
My lessons learned include prototyping the components outside the hobby box. You will find 2 variable capacitors with 3/4" clip leads to allow tuning of the shunt caps (C1,C3) very helpful. I used two ARCO 427 capacitors. The series capacitor isn't as critical and can be left alone.
Once you have the ability to adjust the shunt caps, all in the world is fine. These two adjustments are kind of like adjusting an antenna tuner, rocking back and forth, tweeting and peaking. Once dialed in, unclip them and measure with a capacitance meter, then insert that value of capacitance. Reinstall the whole works in the box and attach to the SWR meter and do final tweaks on the center two inductors to dial it in.
I spent about 3 hours coming to a tuning solution for the 15 and 40m filters.
I haven't measured the insertion loss yet but the way the SWR reads, it should be pretty good.
Ever since playing with the ubiquitous 9:1 unun there came a point in time when I wanted to find out how lossy this antenna system really is when deployed with a seemingly random wire length. There are many lengths of wires used with a 9:1 unun but most fall in between 30 to 40 feet. One website wants you to add some coax on the unun without any chokes to create a counterpoise. So I chose to model a 35 foot piece of wire and a 15 foot piece of #12 to simulate the shield of a piece of coax. Since many people use a telescoping 33 foot pole, I modeled this wire suspended from the tip top sloping down to about 15 feet away from the base to the unun and an additional 15 feet of coax to the rig, all in line.
Here are the antenna impedances seen by the 9:1 unun assuming an average ground and Copper loss from 40-6m.
Freq (MHz) Zin(Ω)
Transmission Line Transformers are well understood devices having been characterized extensively by Dr. Jerry Sevick (W2FMI, SK) and his publications indicate a need for a load to be mostly resistive and closely matched to the characteristic impedance of the unun, in this case 50:450 Ω. If you deviate from the load impedance of 450+j0 Ω you will suffer mismatch loss. A loss that will dissipate power in the unun core and windings.
Well how much loss is there?
Well, to answer that let's identify potential loss mechanisms of the antenna system. First, the mismatch loss of the unun itself including the mismatch loss of the attached coax. So to calculate the MML of the unun first we enlist the help of an online calculator to reveal the mismatch loss.
Plugging in the Zin values of the 20m EZNEC result we obtain a MML of 3.7dB.
3.7dB? Yes, over half the power will be lost in the unun alone! So if you started with 5W (36.99dBm), you are now down to (36.99-3.694)dBm = 2.1W applied to the antenna.
Okay, well 2.1 Watts will get out, right? Sure it will but you forgot to add the mismatch loss of the coax.
So if we have a 9:1 unun attached to a 3249-j89 Ω load, what is the impedance seen on the input side of the unun? So, I measured it. I found a resistor that closely approximated the load impedance and read the unun input Z directly on my AA-600. It was 109-j139 Ω.
Plugging in that value into our handy dandy chemandy calculator we get, oh my, another 3dB loss.
So 36.99-3.694-3.109 = 30.187dBm. That's 1-Watt folks. 1 Watt applied to our antenna wire, 4 Watts burned up in coax and unun.
I ran through the same calculations to see what the MML of just the unun was and here it is:
Freq(MHz) MML of unun(dB)
I could perform the additional loss calculation of the coax but why? I'm going to stop here.
So where does that leave us and how could we improve the antenna system? Well, there is very little you can do to eliminate the loss given the antenna configuration here.
Since 35 feet of antenna length represents a near halfwave length on 20m, you may be able to devise an impedance matching device to match to the high input impedance such as an end fed half wave tuner on 20m. Then have different wire lengths to get to that "magical" halfwave length wire on each band.