Analyzing an Absurd Attenuator

I made a last minute decision to enter the CW NAQP (North American QSO Party) this past weekend. My contesting ambitions are muted during the warm summer months and my station is a shambles, as those of you following the blog will know. The weather on the weekend was so hot and humid that staying indoors with air conditioning was too tempting.

With several antennas not available, a competitive effort was off the table. I am also several months out of SO2R practice. On a whim I decided to enter in the QRP class. By thus lowering my expectations I could relax and enjoy 10 hours of summer contesting.

Choosing QRP caused one small problem. The primary radio, a FTdx5000, can only be lowered to 10 watts. I ran into this once before and I built a 3 db attenuator capable of dissipating 5 watts of CW or SSB; it wasn't robust enough for key down, and that isn't necessary. Not expecting to do QRP again with that rig I repurposed the aluminum enclosure for another project. 

I did keep the core of the attenuator -- a π resistor network -- storing it in a small plastic bag. I should get rid of lots of junk I've accumulated over the years, but the attenuator hardly takes up any room. The greater danger was misplacing or losing it because it's so small. But there it was in a drawer of the operating desk and I pondered what to do. 

I was in no mood to spend the time punching holes in another enclosure for a one-time use. So I improvised. With a couple of SO239 jacks, wire and a few minutes solder, I produced the monstrosity below.

No, you should never do this! RF circuits need be wrapped in a conductive enclosure to keep RF in and to keep RF out. But it worked after a fashion. With a calibrated load on one end the SWR gradually rose with frequency, reaching about 1.15 at 30 MHz. That's pretty good, and better than I had any right to expect. The power meter read about 5 watts, so the primary objective was met.

The 3 db of attenuation on receive was no inconvenience: any signal so weak that 3 db would make it inaudible is someone I'd never be able to work with QRP anyway. On bands below 10 meters even that is irrelevant since atmospheric noise dominates the SNR.

Knowledgable readers will see that there is are potentially serious drawbacks of the open air design, related to those already mentioned. Any nearby noise source would leak into the receive path, and on transmit the leakage could affect the receiver of the second radio (for SO2R).

Tuning across the 10 meter band where the problem should be most severe, I heard a number of electronic noises that are likely coming from computers in the shack and other electronics in the house. The spurious signals were strong enough that I decided it would be worthwhile to deal with it. After about 30 seconds of not-so-serious thought I invented version 2. It took another 5 minutes of my time.

This proves that monstrosities can be made into truly absurd monstrosities. A strip of aluminum foil stolen from the kitchen was spirally wrapped around the attenuator and firmly taped to the SO239 jacks for a conductive seal. The exposed leads and resistors connecting the centre pins were taped to avoid shorts when the foil was wrapped.

Yes, this is a monstrosity but it is a better performing monstrosity. The spurious signals dropped at least 20 db. The SWR also improved, though only slightly. The contest start was rapidly approaching so I declared the project a success and spent several minutes readying the rest of the station. The attenuator worked flawlessly during the contest.

A couple of days later I contemplated what to do with it. I can spare the connectors so it could go back into storage. I didn't expect the foil to survive storage so I prepared to remove it. I stopped and wondered how it truly performs. I pulled out my VNA and put it to the test before ripping off the foil.

I measured S11 (SWR) and S21 (insertion loss). I did the S11 measurements with a calibrated load. The result changed a small amount when I added a coax jumper to facilitate the S21 measurements. The test setup is shown above. I did the measurements with the foil covering first to best reflect its performance during the contest, but I'll show the VNA plots in the order I built the attenuator. The foil is too fragile to survive a second wrapping.

The coax jumper is not the best, which explains the higher SWR than what I measured when it was first built. The calibrated coax tails from the VNA are too stiff and short to connect to both ends of the attenuator without using the jumper. An SWR of 1.2 on 10 meters is pretty good. No tuner is necessary to prevent the transmitter from folding back power, and in any case the antennas are responsible for most of the higher SWR where it does occur.

Insertion loss is slightly less than the required -3 db to drop 10 watts down to 5 watts. The power meter read very close to 5 watts on both 20 and 10 meters, so either the meter is misreading or the transmitter is not quite accurate on the power setting. Setting and measuring RF power is something of a black art and accuracy requires careful test methodology and calibrated instruments.

My measurement of the RF power is good enough for amateur work, and for a one-off application at that. In the worst case the BPF add an insertion loss of from -0.2 to -0.5 db, depending on band, so I was easily at or below 5 watts even if I started with 5.5 watts.

This plot is with the aluminum foil wrapping. The foil has a negligible effect on the insertion loss, which is no surprise. The improvement of the port impedance is more significant. The reduction of common mode and radiation into and out of the attenuator are likely responsible. The measurement data support what was already evident from the sharp reduction of spurious signal reception when the foil was added.

Many readers will have no doubt thought up ways to improve the attenuator. Well, so have I, but that was not the point of the exercise. I wanted it to be quick and functional for a single operating event. For example, with 15 minutes of work I could have run wires between all 4 corners of the jacks to improve it mechanically and electrically. I deliberately avoided doing that.

Finding the optimum balance between excess effort and poor performance is never easy. Others in the same predicament might lean either toward higher quality or shoddier construction. I simply find it interesting how much can be accomplished with a minimum of effort when you understand the underlying physics and mechanics of what you're doing. 

The less you understand the more you tend to overbuild or underbuild. The same is true of almost everything in our stations: antennas, towers, small accessories and more. A little knowledge goes a long way. Amateur radio offers an ideal playground to learn, and to practice what you learn. 

I hope you enjoyed this (slightly) silly midsummer distraction.