Evaluation of Homemade Hardline Coax
I have wondered how well home-made “hardline” coax could perform as an interconnect between PCB’s in a project enclosure. It seemed to me that for reasonably short lengths, you could make a coaxial interconnect with just a small copper tube, through which you insert ordinary hookup wire. At the ends, you solder the tubing and the wire to a PCB. Or to connect to the outside world you solder it to the backside of a bulkhead connector.
Having built the VectorAnalyzer60, I decided to use it to test the performance of such hardline coax made of #22 Teflon coated stranded hookup wire inserted through 3/32” copper tubing (hobby shop stock). The fit is reasonably snug, but the slipperiness of Teflon makes it easy to assemble.
I used 12” tubing, which is longer than I would normally use for an interconnect but should magnify any deficiencies. I started out with the tubing straight, and then wound it into a spiral around a 1 ¼” wooden dowel. I don’t anticipate actually using a spiral for an interconnect, but used the spiral to test the effects of bending.
Photo 1—Homemade hardline coax. Terminated
at small PCB on end. Attached to reflection port of
The wire inside the tubing prevented the usual kinking from bending, and made a smooth spiral. However, the winding undoubtedly causes some distortion to the shape of the tubing.
I attached a 49.9 ohm 0.1% resistor to the far end of the coax, between the center conductor and tubing, and attached the near end to the VectorAnalyzer60 for a reflection test. The results are shown in Figure 1.
Figure 1—Straight and Spiral Hardline Coax. The magnitude of the
return loss changed slightly when the coax was wound into a broad spiral.
The magnitude of the return loss of both shapes is respectable to
40 MHz. Phase above 40 MHz goes wacko on the spiral, probably
because of irregularities caused by bending.
Obviously, the coax did not have 50-ohm impedance. If it did, the return loss magnitude should be higher than 40 db. Nevertheless, it has respectable return loss up to 40 MHz when used with a 50-ohm termination, even in this 12” length. For shorter lengths, higher frequencies could be tolerated.
So what is its actual characteristic impedance? I used Ansoft Designer (free edition) to simulate terminated coax, and adjusted the coax impedance until I got a magnitude curve similar to Figure 1. The result was 40.5 ohms. So I terminated the coax in 40.7 ohms (the best I could do) and got Figure 2.
Figure 2—Homemade coax, now spiral, with 40.7 ohms termination.
The flatness of the magnitude indicates that 40.7 ohms is close to the
characteristic impedance of the coax, at least to 40 MHz.
If the coax had a characteristic impedance of 40.7 ohms, the coax and the termination together would appear to the VectorAnalyzer60 as a 40.7 ohm resistor, and would thus have a flat return loss of 19.8 db. Figure 2 indicates that the coax does indeed have a characteristic impedance near 40.7 ohms up to 40 MHz. Beyond that, things deteriorate. It is likely that the bending of the coax causes irregularities in the shape of the tubing, and areas where the wire insulation is compressed. That may cause the characteristic impedance to vary with frequency, and perhaps even to be not perfectly resistive.
So we have approximately 40 ohm coax hardline. But is it extremely lossy? Figure 3 shows the reflection coefficient of the 12” hardline coax terminated in a short. Ideally, the return loss should be 0 db. Any higher return loss represents signal loss in the round trip through the coax.
Figure 3—Return Loss of shorted coax. The phase starts near 180
degrees and declines, as it should. The magnitude is at worst 0.2 db,
representing 0.2 db loss in the round trip from input to termination
and back to input. That would be only 0.1 db per foot for a one-way trip.
The homemade hardline coax as described makes a fine interconnect in a 50-ohm system at up to 40 MHz in lengths up to 12”, even when significantly bent. For shorter lengths, the allowable frequency would increase proportionately. Where the input/output terminations can be made 40 ohms, the performance should increase substantially.
As a follow-up to the above experiments, I made another coax out of 12” of 1/16” brass tube, through which was inserted #30 silver-plated wire-wrap wire (probably Kynar insulation).
Figure 4—Return Loss of brass coax terminated in 49.9 ohms.
The return loss is much better, and based on simulation the characteristic
impedance of the coax was about 47 ohms. Bending the coax had some
effect, but this time there was no dramatic phase change at
Figure 4 shows that this coax has much better return loss, and has a characteristic impedance near 47 ohms. While bending the coax into a spiral had some effect there is no big change in phase. Most likely, the stronger, springier brass was less susceptible than copper to flattening during the bending process. Note that the phase changes smoothly, notwithstanding the apparent jump at the high end. Phase plots frequently give a false impression of dramatic shifts when the phase crosses the +/-180 degree boundary. If the final point were graphed as -180.5 instead of +179.5, the jump would disappear.
One might expect that this smaller, brass coax would have significant loss. The return loss of the shorted coax is shown in Figure 5.
Figure 5—Return loss of shorted brass coax.
The loss is much greater than with the larger copper coax.
Indeed, Figure 5 shows that the loss is significantly increased, having a round-trip loss of 0.8 db at 80 MHz, which is about a 10% power loss. Thus, while this smaller coax has a characteristic impedance quite close to 50 ohms, its usability is probably limited to shorter interconnects and lower frequencies.