some laser cutting.....

some RC plane stuff, some other stuff.

Helical STL Scratchpad

A place to store/share construction data. Helical loaded magnetic loop, or “STL” (small transmitting loop) antenna designed by Rich K8NDS 

The blue (magenta) circle dimension is 39″ diameter. The circle is the mean center-line of the 2″ tubing around the octagon. The 40.44″ dimension is the PVC elbow centerline. to centerline distance.
Using PVC supplies from Lowes hardware store- the 45deg bend has 0.5″ insertion allowance on each end, and pipe end-to-end gap is about 2″ fully seated. That gives me a pipe length of 12.7″.  I’ll see how close that estimate is and report back.

.magant octogon1


Later that day…..

I made the frame, all glued up and dry. It came out within half and inch of the drawing, and plus/minus half inch symmetrical. I’ll make up the difference when wrapping the copper.



Tomorrow I’ll bring it inside and wrap the copper. I started gluing the PVC in the basement, but had to move to the freezing  garage as the solvent fumes were building up quick in the house.



Here’s some pics of the connector end of the gamma match. Brass tubing soldered to the N connector, heatshrink covered,  and clearance hole in the copper. Copper is .028″ thick.

loopfeed1 loopfeed2 loopfeed3



Soldering the connector base copper(.028″) to the copper wrap (.010″)  Not the prettiest, but it’s the contact area underneath that counts.


loopfeed 4




March 6 2015:

The vacuum variable cap arrived from Russia. Here’s the start of the cap mounting plate. I hijacked a kitchen cutting board and cut it to 16″ by 5.5″.  Lowes hardware supplied everything else.  Plastic electrical conduit clamps to mount the board on the pvc antenna, and steel 2″ pipe hangers to hold the cap. (part# 302045) I don’t know how they measure 2 inches, but it’s a near perfect fit on the 60mm vvc cap ends.  The copper foil ribbon will be folded and wrapped between the cap and clamp. No extra soldered connections.


vvc4 vvc5 vvc6




March 7 2015


The motor drive has reached a stall because I can’t find a suitable shaft coupler that adapts 12mm to .250″.

In the mean time I couldn’t wait to try this out. I started to manually tune the cap for highest signal level at 14.250 MHz, then I played with the gamma match for the lowest SWR, and back to cap adjust- – – -back and forth. At ground level I got 1.4:1 and heard stations a couple S units below my dipole.  Can’t wait to get this thing outside and motorized!!.

magloop2 magloop3



March 9, 2015


Still motor-less. I spent the first sunny above freezing afternoon trying to tune the Gamma Match feed. I tuned the cap to best swr at 14.2 mhz. Like others said, the match is different on the ground vs 6 feet high. Many times up and down on the tripod, tune and test…..  the best I could get is 2.3:1 at any 20 meter frequency. But the great news is it’s easily one S unit above my 20 meter wire dipole. Once I get things locked down below 2:1 I think it will just get better.


March 21

Finally locked down and soldered the Gamma match. I can get 1.5:1 or better SWR from 40 to 15 meters. I bought a RigExpert AA170 S11 analyzer and that really helped the process. Below are some screen captures from the Rig Expert AntScope app.

loopdata1 loopdata2 loopdata3 loopdata4



March 27: The Mini-6

I wanted to try a mini loop for portable and marine 6 meters.  Here are a couple of 1st version prototypes. I first made a solid core loop, just shooting in the dark for size, feed, and cap value. Ended up with a 10″ diameter of #6 wire, and a Johanson 22pF cap. It’s a low voltage cap for receive only, but I did get 2 watts into it to test SWR and pattern without it popping.

I then wound a helical loop on 3/8″ PEX tubing using copper tape of .500 by 1.5 mil dimension.  Still 10″ diameter, but now with 50 inches of copper tape (25 per side).

Both of these loops are tune from 40 to 90 MHz easily with the 22pf cap. The usable bandwidth of the helical wound loop is about 40% better being about 100KHz wide at under 2:1 swr !

Field testing in receive mode will be nex week out in the cornfields. I’ll compare it to a tripod mounted Buddypole dipole setup with the help of another portable operator.

My next goal is to get some 3/4″ plastic tube,and 5 mil copper to wind a new one, and search out and find either a 1KV standard variable or a small VVC and run it remotely. I only plan to run 25 watts when this is done.


Keep experimenting and 73


loop 6meter 1 loop 6meter 2 loop 6meter 3 loop 6meter 4

Jan 10 2015


BCP-C4F Connectors for sale

Canare BCP-C4F 75 Ohm BNC Connectors  Box of 86 connectors. $65 shipped CONUS


You can email me for payment info.  tom @ (this domain )

If you’re reading this- they’re still for sale.

canare1 canare2 canare3

Winter Project: Dynaflite Decathlon

My winter build is underway.  A few weeks in and I have it mostly framed up. 88 inch wingspan, 30cc motor and probably 13 to 15 pounds. Cutting and shaping all the wood is pure Zen relaxation.


front layout 1 firewall3

all bones 1 fuse bones 2 shavings1 shear web glue 1 tail group 1 tail group weight 2


Thanks for all the comments and questions. I’m happy some people are reading this!!.  You can email me  This site/blog is done in WordPress and hosted by Godaddy.

Antenna Projects

This is a new antenna I invented. At the lower frequencies it acts like a fat monopole, and at higher frequencies it’s a tapered slot antenna. it shows a very nice SWR below 2:1 from 150MHz to 10GHz. Of course I didn’t invent either type, but I’m  positive I’m the first to combine them into one antenna.

egg1 egg2 eggswr


Below are pics of a 12DBi helical. I made the helix support from laser cut lexan. The first 1/4 turn feed transition was the critical part in tuning.


100_1564 100_1563

Below are more variations of the tapered slot / fat mono.  The one in large blue foam is an extended low frequency model. With the larger ground plane and copper tape “top-hat” it’s now usable to 70MHz.

xlegg1 dualbandegg1 elipse with supports[1]


For my Ham Radio HF station I made my 4:1  balun for my Off-Center-Fed antenna. The core is a 140-77 ferrite, wound with teflon wire. Plots shown with 200 Ohms on output.  So far good for a couple hundred watts.

balun_open_sm balun_imp balun_swr mast2

Power supply puzzles for the Taranis

I was thinking about installing a 3S lipo in my Taranis.  The optimistic thought was besides longer run time, maybe there could possibly be a teeny tiny increase in output power resulting from a sloppy regulator that allowed more voltage to the RF board.
BAAAAAAA!!! Wrong answer Hans!
On the bench with a lab-grade 10A variable power supply showed different.   I ran this test 3 different times with different bandwidth settings, and different antenna orientations, and it’s repeatable and constant.  The RF circuit seems to be more efficient when the radio DC power is at a lower voltage.  Running at 12VDC shows almost 0.5 dB decrease in RF output power as opposed to 7.2VDC.   Maybe it means nothing, maybe it’s just my radio… what’s half a db between friends, I found later that at 12volts, there is about twice as much ripple and noise (.06v)  on the rf board voltage compared to 9volt input. One of the universal truths in radio design…Poor power supply=poor RF.



The next issue is notable.  You see 3 different color traces in the plot below. The Orange trace shows a baseline output plot from the Taranis with no audio playing.  The Yellow is a plot while audio wave files like – low rates, high rates,  gear up, ect were playing constantly at full volume.  The Aqua trace is the RF output while music is played at full volume. There seems to be about a 1.5dB decrease in power while loud audio is playing.    Again, I aint sayin’ nuthin…  Just thinking about power supply current shortcomings. I’m going to re-do this test after I record some constant-tone wave files.



Lastly below, for edumatainment purposes, and not just for X9D owners, I did a plot of antenna polarity coupling to show the  importance of never pointing your antennas at each other.  Think of the co-pol as both antennas pointing straight up, then laying ONE down 45 degrees sideways, then 90 degrees sideways, then finally pointing each antenna end-to-end.  You’re losing 13dB  by doing that.  That means your 100 mW signal is now 5 mW, and you STILL have to calculate the distance path loss to that faaarrr away model.  It’s brown short time.

taranis polarity


FrSky X6R innards

frsky x6r a frsky x6r b

I was working at the bench and heard a crunch under my bigass workboot. I accidently stepped on a receiver that must have fallen off the bench and cracked the cover off. Oh well good opportunity to check out the innards.  Looks like it survived just fine.

FrSky Receiver size, and X9D Pot repair.

A comparison of assorted receiver sizes. I thought the Hitec was small, but damn….rcvr compare angle rcvr compare top1 x6r coin


A combination of bad soldering joints, and my opening the case too much and disturbing wires resulted in failure of the aileron  pot wires.  The pics show overwicking of solder on the red wire (indicated by the kink), which made it very inflexible. Upon opening  the dust cover, I also found the brown wire detached.  Last pic shows the repair.

broken pot wire 1 broken pot wire 3 broken pot wire 4

Taranis speaker hum… one of many solutions.

Just to set this straight…  This is the greatest radio since Marconi asked “Can you hear me now?” Since the beautiful Hedy Lamarr said “Jam THIS!”  This is an RC transmitter that has every switch and function user assignable, almost unlimited mixes, and plays music while some sexy fembot announces receiver signal strength, lipo voltage,  and GPS coordinates  !!! The audio hum was hardly there. I had to hold the radio within 12″ to hear it. It didn’t even bug me, but everyone was talking about it.  It’s like you live somewhere for years and one day your wife points out the stain on the ceiling…  Jeeez thanks!  Well I had the tools and figured what the hell.  I have the “B” model, and it already had the “Scott’s Special Speaker Mod”

I didn’t fall off the turnip truck yesterday…..  That happened weeks ago, and as a paid RF noise chaser I had some clues. The hum changed on how you held the radio. It also changed with antenna position.  When I opened the radio, the hum was VERY variable according to which wires and ribbons were moved. That led me to believe there is/was a lot of RFI noise inductive coupled into the whole wire harness.  Usually I find that the noise is from DC/DC switching converters.  Next in line is the processor clock. That’s what I found on the Taranis.  The inter-board ribbons were pretty hot with EMI from 160MHz to 600MHz. The CPU clock frequency RFI has sub-harmonics like a big ugly comb generator response. These harmonics get into the DC bias lines, and pollute all the op-amps and audio amp chips.  Even though they’re in the micro volt level, they get amplified up with the audio signals you want.

Since I don’t have a schematic,  I didn’t want to do a lot of probing and testing to find where to install filter caps and inline induction. That would cost millions of dollars and government forms would have to be filled out.  A simple, quick way of reducing the transfer of RFI between system boards seemed like an easy try.  Noise is born on the processor mainboard, and I want to strangle it on it’s way to other boards.

For some fun I display a sample from the “big box o’ ferrites”  EMI arsenal. Rings, clamp-ons, beads,  plates, tape. Actually the plates are an idea I had in mind to epoxy on the bottom of the boards, and/or to fit and position between the boards when the case is closed. The smallest beads could work, but I’d have to disassemble the ribbon connectors to get them on. I decided on the ferrite tape.  I fount the hottest ribbons and gave them a few wraps of ferrite.  The probe is so sensitive that I could isolate the two noisiest wires from the ribbon and just wrap those. The ferrite tape seemed to absorb a good amount of RFI, and the noise reduction from one end to the other was about 15dB  The audio hum was so reduced that I had to hold the radio at my ear to even acknowledge it.

Please click on the pics for the full view. I show the EMI response before and after, some pics of how I wrapped the ferrite tape around the ribbon cable, and a pic of my DIY near-field EMI probe. The description and diagram of the probe and be found here.




After a couple days of test, I finalized and improved . The tighter the ferrite tape is against the wires- the better. I re-wrapped them and tied it up with waxed lacing string.

Even though the tape mfg claims surface resistivity in the MegOhm range I also put a square of foam over the board so it wouldn’t rub against it when the case is closed.

I found out this also reduced the headphone hum.


 Here’s the ferrite tape fitting. The upper ribbon cable gets two pieces of 2″ long tape wrapped around all wires.  The lower ribbon in the pic-  separate the last two wires from the ribbon, and wrap them with a piece of tape lengthwise. 

Here are some links to ferrite tape manufacturers / distributors.


This EMI / noise mitigation was fueled by beer and rushed to press. Next time I do a mod I’ll have the video camera and whiteboard set up :P :)

IF you’re not a Taranis owner, check it out here-

Take Care,