50 Ohm Resistor Networks for Dummies

It's called a dummy load. No seriously, it is.

What's it for? Well, it gives the radio a place to send its power without radiating RF.

Why? It provides a nice consistent load for tuning the final amplifier stage on my radio before I transmit. It can also be used for testing the radio after a repair. Also, some radios transmit while receiving a program download from a computer (though they shouldn't.) This gives you a way to prevent such transmissions from interfering with others.

Why not just disconnect the antenna? Well, the radio forms one half of a resonant circuit. The antenna forms the other half. The most efficient circuit is one where both halves have the same impedance. For amateur radios, that ideal impedance is fifty ohms.

If you have an impedance mismatch, not all of the energy makes it from the transmitter to the antenna. The law of conservation of energy says that the energy that doesn't make it to the antenna has to go somewhere, and in this case, it gets reflected back to the radio. When it gets back to the radio, it gets turned into the heat in the final amplifier stage. Enough heat, and the radio dies.

As far as impedance mismatches go, a disconnected antenna is about as bad as you can get. That's why you should never run a radio without an antenna attached. When you fire up the transmitter, all the energy goes to the antenna connection, and finding nothing there, is reflected right back to the final amplifier. Since all the output from the radio gets instantly turned into heat, failure happens pretty quickly. The 50 ohm dummy load gives the power a place to go. It still gets turned into heat, but the heat is in the dummy load instead of the radio. And the dummy load is built to shed heat.

My dummy load is a network of 1k ohm resistors in parallel. The net resistance of this network is 50 ohms. The resistors are each able to dissipate 3 watts of power, so together the network can handle 60W.

However, my VHF radio is capable of putting out 75W, and my HF radio can pump out 100W. So, the resistor network sits in a 1L paint can filled with mineral oil. This will allow the resistors to take more heat before failing.

This build is based on a design by K4EAA. He also sells the pack of resistors needed to build it. (Not just any resistors will work, since some are built of a coil of wire. Such wire-wound resistors are OK for DC, but as soon as you pass RF through them, they act like inductors instead of resistors, and their impedance changes. These are metal-film resistors which have the same impedance at any frequency.)

Now, if only I could remember to switch back to the antenna after tuning the finals...

Hot Tubes in the Shack

These days, there's a ridiculous number of directions one can take in the hobby of amateur radio. But in the beginning, "amateur radio" meant someone banging out Morse Code on medium wave or shortwave.

Shortwave (HF) is the bread and butter of amateur radio. Before there were satellites, before transoceanic cables, people were using HF radios to communicate across the world. And they still do, since HF signals can get from their source to their destination without any equipment in between.

The main drawback is that the propagation of HF radio waves around the world is heavily dependent on atmospheric conditions. And this is where the challenge lies.

Since shortly after I got my license, I've been wanting to play with HF. I scored well enough that my license permits transmission on all the amateur bands. However, an HF rig isn't cheap. I'd need at least a few hundred dollars for a basic used rig, and with everything else that's going on right now, that just isn't in the cards. 

Enter the community. My situation came up in passing during a discussion over our weekly lunch meeting, and a fellow operator was more than happy to loan me one of his old rigs that had been languishing in his basement for some years. The following week, I helped him lug a Kenwood TS-530S and an AT-230 antenna tuner from his pickup to mine.

The tuner is especially important to my situation, and I'll get to it in a moment, but first I want to yak about the radio itself. Kenwood built these radios throughout the 1980s (I can't find a date stamp on my particular unit so I don't know when exactly it was manufactured.) They're what's called a hybrid rig, meaning that most of the electronics are solid state, but it uses vacuum tubes rather than transistors for the final RF amplifier (the part that puts the power out to the antenna).

Kenwood hybrid final compartment. Photo courtesy of k4eaa.com

Vacuum tubes have been largely replaced by transistors, even in ham radio, because they don't consume as much power, are smaller, and last longer (tubes have a lifespan, much like light bulbs, and radios using tubes are designed so that the tubes can be easily replaced.) However, tubes still find their uses in large RF power amplifiers, microwave ovens, audiophile amplifiers, etc.

The Kenwood hybrids are sought after among the ham community. (As an example, I recently found an eBay listing for a TS-530S selling for $300CAD - and it's broken.) While they're a simple rig, lacking many of the features in modern digital radios, they make up for it by being easy to use, robust, and easy to repair. The radio's much larger and heavier than a modern radio of equivalent functionality, but large discrete components are easier to remove and replace if something goes wrong.

It's actually a perfect starter rig for a ham just getting into HF.

For starters, it doesn't feel like a toy. I had an opportunity to use an Elecraft KX3 recently, and it wasn't much bigger than my handheld. The Kenwood lets you know that it means business. It's twenty kilos of radio sitting on my desk.

The other thing that makes it perfect for a starting ham is that it's all manual. The most modern thing about this rig is the digital frequency readout. There's no automatic antenna tuner, and you have to tune the final amplifier stage whenever you switch frequency bands. Having to learn about all this stuff teaches you things about how radios are built, and how signals propagate through your antenna and through the air. Things that you won't learn on a modern rig like an Elecraft K3. Don't get me wrong; I'd love to have a K3 in my shack, but it's like giving a calculator to a fourth-grader before teaching them long division. There's always the risk that they'll take the easy way out, and miss an important learning opportunity.

And learning is the reason that I got into ham radio in the first place.

Of course, this radio isn't without its drawbacks. For me, the biggest one is the lack of portability. Between being physically large and heavy, and being power-hungry, this rig isn't going camping with me. My goal of contacting hams on the other side of the world while sitting next to a campfire by a lake in the bush will have to wait until I can afford a smaller rig like the Yaesu 817D or an Elecraft KX3. However, for now, that's more than made up for by the fact that I'm on the air, with nothing more than a piece of clothes line strung between my house and my shed.

And that's where the tuner comes in. A radio and its antenna each form two halves of a resonant circuit, and to maximize power transfer from one half to the other, each half must have the same impedance. Since the impedance of an antenna is a function of the length of the antenna and the frequency of the signal, (this is an oversimplification, but it works for my purposes,) antennas have to be constructed with the proper length and spacing in order to work. Any random length of wire usually won't do, because it will be too long or too short, and this will cause RF energy to be reflected back to the radio. This reflected RF energy gets turned into heat in the radio's final stage, which eventually burns it out.

The tuner makes up for a badly tuned antenna by adding additional resistance or reactance to the antenna side of the resonant circuit so that it appears to be perfectly matched to the radio. This allows the radio to push maximum power to the antenna without risking damage. The drawback is that, since the antenna isn't truly resonant, some amount of transmitted power is soaked up by the tuner. The further out of tune the antenna is, the more power is wasted.

The silver lining of this cloud is that a tuner allows an operator to use any random length of wire as an antenna. It's not as efficient as a tuned antenna, but it gets my signal out into the air without risking letting the magic smoke out of my borrowed tubes. Twenty bucks worth of clothes line and a couple screw-in anchors has me talking to folks a few hundred kilometers away.

Cold Shower, anyone?

It all started on Easter weekend.

Friday morning, we got the trailer ready to go. I had waited until the last minute to fill the water system, because it was still freezing at night and I didn't want to risk any burst pipes. When the time came to fill the water heater, I opened the valves, only instead of being greeted by a healthy rushing of water, I heard only a hiss and trickle. Water issued from the hot water taps in a slow drizzle, rather than the rush I was expecting.

I'd run into this problem last year, and I thought I'd fixed it. Something was wrong with the bypass valves. Since I wouldn't need the valves until this fall, I figured I'd just take them out and deal with the problem when we got home. So I removed the valves and tossed them into the garage.

That evening, we're all set up at my sister-in-law's, and the kids are getting ready for bed. I turn on the pump so they can use the toilet. The pump starts running. And doesn't stop. My wife calls from outside, "there's water running out the side of the trailer!"

Crap. I quickly kill the pump and start investigating. The water heater has given me trouble in the past; judging by the soft floor around the heater I'm not the first victim, either. Unfortunately, since I'd removed the bypass valves, I had no way of shutting off the water supply to the tank. So, we couldn't use the water, and I had a wet floor. Again.

The next morning, after everyone got motivated, I took a closer look. Thankfully, my dad was more than willing to pop out and give me a hand. He brought with him an air compressor, and after blowing out the water lines, we started looking for the source of the leak. After ruling out the pipes, we pulled the tank itself. Hooking up the air compressor to the water tank made the pinholes in the bottom stand out really well. A quick trip to a local RV shop, and I had the fittings I needed to close the water system, without the tank. Now, we had cold and cold running water.

After returning home, I started exploring my options. While my wife would be perfectly happy losing the hot water tank and gaining some more storage, I liked the idea of having hot running water for things like washing dishes or brushing my teeth. So I looked into the cost of repairing or replacing the tank.

At this point I had no idea if the tank was repairable. Pinholes usually indicate corrosion; the material around the pinhole might be so thin that attempting to weld the hole shut may very well make the hole bigger. A skilled welder might be able to give me decent odds on the success of a repair, but before I had a chance to talk to one, I looked into the possibility of replacing the tank altogether.

Surprisingly, the design of RV water heaters hasn't changed much in 30 years. While features have been added (like automatic ignition and gas-electric units,) the dimensions have remained the same, and I was in fact able to find a brand-new unit that looked and operated exactly like my old one. However, it cost several hundred dollars, which was way out of my price range. A new heater was worth about half of what I'd paid for the trailer in the first place, which meant that if my current heater couldn't be fixed, I'd be going without hot running water.

And then I had a chance to talk to a welder at a local machine shop. Our conversation gave me confidence that a fix might be possible after all, so I left the heater with him for a couple of days. I got it back with a large patch welded over the entire bottom of the tank, which allayed any fears of corrosion-related failures in the future.

While the tank was out getting repaired, I examined the water damage. It was extensive, but I managed to repair it well enough. That's covered in another entry.

The tank had originally been wrapped in fiberglass wool insulation with a cardboard outer coating. Needless to say, this didn't survive the leak; the wool held the water like a sponge and the cardboard disintegrated. I'd pulled off the soaking, rotting insulation before repairing the tank, and before I installed it, I wrapped it in the foil-coated bubble wrap you can buy in a hardware store. Two layers made the tank too large to fit into the hole, but one layer doesn't seem to be adequate; the tank feels pretty warm to the touch, which means that a lot of heat is being lost through the tank walls. However, now that the tank is installed, I can reinstall the second layer and hold some more heat in the tank (instead of letting it out into the trailer, which is not the most desirable thing in the middle of the summer.)

I got the tank reinstalled, and then figured out what was wrong with the bypass valves. The handle of one of the valves was not properly lined up, which meant the valve never opened or closed properly. I realigned the handle, and the valves work properly.

So now I've got hot and cold running water again.

Baby Journey Part 2: Routine?

It's now 23 weeks.

A couple weeks ago we had an ultrasound in our local hospital, live-streamed to the medical team at BC Womens' Hospital in Vancouver. The ascites is unchanged. It's both good and bad news. It's not going away, but it's not getting any worse, either. I took that as good news; I need all of it I can get right now.

We just finished a set of appointments in Vancouver. However, this time we had a little bit of time to prepare for the trip; enough lead time that my wife was able to get in touch with a social worker in Vancouver who arranged for us to stay at the Ronald McDonald House on campus. Not only that, we qualified for financial support from the Variety Club. It honestly never occurred to me that this kind of help was available; I figured I'd have to make do on my own, and I was struggling to figure out how I was going to make ends meet while spending all this money to drive to and stay in Vancouver. I can't thank these guys enough for the help they've provided me. I've got enough to worry about without trying to figure out how I'm supposed to pay for all of it.

The first appointment was another two-hour ultrasound, scheduled for first thing Monday morning. The hospital bent their usual schedule a little for us so that we could reduce the number of days we spent out of town, and I could reduce the amount of work I would miss. The ultrasound was difficult, but it came from a completely unexpected source.

Puffy capacitors.

The baby was more or less fine; his condition is unchanged from the last scan two weeks prior. Our ultrasound tech was great. However, luck of the draw gave us the oldest ultrasound machine they had, and it had decided that it would slow down and freeze after ten or fifteen minutes of operation. That means our scans were regularly interrupted by a fatal error, and a five-minute reboot of the machine. It's about ten or twelve years old, which puts it in the time frame to be a victim of the capacitor plague. Not that I can say for certain, since I didn't open the machine up to take a look, but the manufacture time and the symptoms come together in a nice burst of serendipity.

After our ultrasound, we met with our social worker, who made sure we had a place to stay for our next appointment in four weeks' time. I gotta hand it to her. For us, the actual hospital appointment is but a small part of the entire experience; since we're out of town, there's so much more to worry about. Things like where we're going to sleep. She takes care of those things for us so that we don't have to worry about them, and instead we could worry about other things, like what we were going to eat at the cafeteria.

I find it amusing that the cafeteria has all sorts of healthy choices for beverages, but they carry Diet Coke instead of regular Coke.

Anyway, so with lunch out of the way, we met with a neonatologist. He presented us with the latest theory, that our baby has leaky lymph vessels. Normally, the lymph vessels collect lymph and return it to the circulatory system, but as it was explained to us, the lymph vessels in our baby are not properly formed, allowing lymph to leak out and collect in the abdominal cavity. Since the lymphatic system operates at a low pressure, the buildup eventually stabilizes when the pressure reaches equilibrium, which is why the baby's belly swelled up to a point and then stopped.

This condition is not uncommon, and normally corrects itself within weeks or months after birth. However, the fluid buildup must be drained shortly before birth, so that the baby is able to properly take his first breath. This is normally done with a needle through mom's abdomen, and since this procedure can't be done during active labour (because the abdominal wall is contracting and this would disrupt any procedure involving a needle through the abdominal wall,) we're looking at a caesarian section shortly before our due date. After birth, we're told the baby will have a drain installed, and then spend some time in the NICU while the lymph vessels close up. At that point, the drain is removed and we can go home.

Prior to that, though, the doctors are going to start doing nonstress tests to keep an eye on the baby and determine if any early intervention is needed. The first of these tests will happen on our next trip to Vancouver in early June; any earlier would be pointless as an intervention would carry little chance of a positive outcome anyway.

We returned home with a generally good feeling, and our next step all planned out. And since my next trip is nearly upon me, I should close this entry so that I can start the next one.