Saturday, January 24, 2015

I'll Take a BLT in a Box, To Go, Please.

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.





Now I'm ready for a rainy day.
72,
Myron
WVØH


Wednesday, January 21, 2015

S Match Antenna Tuner

I had another project on my list and was wanting to build the S Match Antenna tuner. PAØFRI highlights this design on his picturesque website, http://pa0fri.home.xs4all.nl/ATU/Smatch/smatcheng.htm


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.  

1-22-2015 Update...
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.

73,
WVØH
Myron 

Saturday, January 10, 2015

W3NQN 15/20/40m Filters

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. 

More to follow.


73,
Myron 
WVØH

Wednesday, January 7, 2015

Loss in a Deployed End Fed Wire and 9:1 Unun

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)
7.1               62-j418
10.1             166+j214
14.1             3249-j89
18.1             148-j274
21.1             172+j339
24.9             644+j1304
28.1             4118+j921
50.1             227+j237

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)
7.1              5.9
10.1            1.5
14.1            3.7
18.1            2.1
21.1            2.1
24.9            4.0
28.1            4.7

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.

Myron WVØH


Friday, January 2, 2015

W3NQN Filters

Ever since I saw that QST article featuring W3NQN, Ed and his band pass filter design, I've always chickend out because of the agreement I made that it was too hard to build.

Since winding some trifilar and quadrifilar ununs I have noticed that the W3NQN band pass filters were utilizing similar architecture at the in and outputs. So why not take a crack at it?

From: http://www.kitsandparts.com/W3NQN_May_June_1998_QST.pdf


I started with a 20 meter filter as this to me would be the most useful out at Field Day. I didn't have the T130-17 cores as published so I decided to investigate the contents of my junk box and see what I could substitute. 

Using the free "mini Ring core" calculator v1.2 from DL5SWB, it was found that I could use a T68-7 (μ = 9) to achieve the same inductance, roughly 1.3μH for the shunt reactors and the same -7 cores for the series reactors. Still using five trifilar turns but with 19 AWG copper enameled wire instead.

I didn't have two 90pF capacitors for the shunt caps but did have four 180pF 500V dipped silver micas. I didn't have a 36pF capacitor for the series element so I gambled on using a 39pF 500V (which actually measured 38pF) instead, and it was fine.

The T68 size cores are nice as they fit in the aluminum boxes I had as well. The Amidon catalog shows that a T68 sized core can handle 88 Watts. Perfect, as that is all my rig can put out on a sunny day! I have no idea what test condition that assumes, so I'll experiment.

After about 4 hours at the bench, voilà! A filter. I'll have to find a two-port VNA to see what the stop band looks like but so far no complaints on the VSWR. 


I did in the meantime, set up my Bird wattmeter and dummy load to see what insertion loss I have under power. Maybe do some temperature rise tests as well.


I ran my TS-2000 into my Bird 4311 and a 100W DB Products dummy load to calibrate the through path. I installed the filter before and after the wattmeter to get the insertion loss measurements.


We have a low loss filter. The power input was 50W and 48W came out the other end for an insertion loss of 0.177dB.

The warmest temperature recorded was around L1/C2 at about 50C after a couple of minutes keydown at 50W, I ran the power up to 88W for a couple of seconds and there was no arcing or smoke!

After the testing I put it in line with the KX3 for an on air power check and it seems to be pretty transparent at that power level.

Lessons Learned: The actual values of capacitance and inductance are not that critical. The reason why Ed split up the center series inductor is so that there are no series resonant concerns with one larger core and single winding. There is greater adjustment range to tweak the tuning around. The "humps" in the pass band SWR response can be shifted around by playing with the windings on the series cores. It makes for a finer adjustment. 

The aluminum box was manufactured by Hammond and it's rough dimensions are 3-1/2" long x 1-1/2" wide x 1-1/4" tall. And the end from a bamboo tooth pick from a hamburger joint was used to isolate the series inductors from the housing about 0.125 inches. 

The lid doesn't affect the response as I tried to maintain as much separation as possible.


Now on to another band. What should it be? 15m? 40m? Hmmm, decisions decisions.

Monday, December 29, 2014

Ununs, Ununs, and more Ununs!

Paging through the Sevick books and having some extra time on my hands, I decided to create some Ununs that Jerry Sevick, W2FMI (SK) made. 

Focusing on the 9:1 and 16:1 Ruthroff designs, I experimented with Ferrite and Iron Powder toroid cores of various sizes and permeabilities. It took about 8 tries to learn the sensitivity of the number of windings vesus the size and permeability to get the frequency response I wanted.


What I learned was for best mid band performance, don't go crazy winding as many turns as you can cram on the core, 4 turns is best for the quadrifilar 16:1 unun. Shown here in the lower left (labeled "best"). It was wound on an unidentified core but the AL value comes in at 100nH/1000t and places it near the 61 mix category. Back to back power loss measurements at 100W prove that the best insertion loss at the lowest VSWR point (20m) is at about 0.25dB per two ununs. The 2:1 VSWR bandwidth is 6.3 to 36 MHz for a single unun, slightly less when placed back to back.

My power test told me that they only get slightly warm (110F) when applying 20m, 100W, for a couple of minutes. Perfect for portable ops with 5-15 Watts.

The FT-114-67 and 43 ununs on the right were too narrow and I have come to conclude that they contain too many windings. 

The iron powder cores yielded virtually the same results and since they are in shorter supply than my ferrite selection, I abandoned production.
 Now, the next thing, figuring out how to field test them. 

Spring! Hurry up.

WVØH
Myron

Friday, December 26, 2014

Back to Back 1:64 (Really 1:32) Transformer Power Test

I read a blog post the other day showing a neat little transformer that can provide a 1:64 transformation from 3.5-30 MHz. Hmm, sounds good so why not build two and measure the insertion loss? Okay, I'm game.



So here they are, two FT-114-43 toroid cores wrapped like the photo depicted. I couldn't find any 100pF caps so I used 120pF ones instead.
So next I calibrated the through path on my TS-2000 set at 5 Watts and terminated into my Elecraft DL-1 dummy load and measured with my voltmeter.


The through path is obscured by the impedance mismatch caused by the transformer so I needed to subtract the loss of the TS-2000 internal antenna tuner. For these calculations it was about 0.6 dB of loss at 14.100MHz.

The back to back transformer measured loss was at about 2.0 dB, about 1 dB per transformer at 20 meters.

I went down to 40 meters and repeated the test, about 3 dB loss (1.5dB per) and thought maybe the capacitors may be causing some grief. So I ran the test without them.


It seemed to help but only slightly. The back to back loss at 7.1 MHz was 2.8 dB (1.4dB per) and on 20m, 2.3 dB (1.15 dB per) loss on 40m.

************ UPDATE 12-27-2014 **************

I discovered I wound an extra turn on the input making it 3 turns on the primary instead of the design shown as 2 turns. It made it worse! The loss increased substantially and actually got it to self-resonate on 20m good VSWR with no load! 


And for grins I put power through it.


Almost 4-dB on 80 meters, 1.5 to 2.5 dB loss on 40 and 20!

Bottom line: Don't waste your time with this one.

Myron
WVØH