Tuning my first antenna

So, I done gone and built my first antenna.

It's a simple directional antenna called a "yagi", designed to operate in the 2 meter amateur band (144-148MHz). The plans came from this QST magazine article. So far I'm out about $14 in parts and a couple hours' labor. (The plans are titled "7dB for seven bucks"; I chalk up the discrepancy to inflation and currency conversion).

The antenna works like this: there are three wire elements attached to a non conductive boom. The center element is the driven element; it is connected to the radio and is driven by it. The other two elements are called parasitic elements because they are not electrically connected to anything; they work by absorbing and reflecting the field produced by the driven element.

The longer element is called the reflector and the shorter one is the director. They work together to focus the field produced by the driven element, stretching it out along the axis of the boom and compressing it along the sides, in much the same way lenses and mirrors can be used to shape the light coming from a lamp.

But just like a musical instrument, building it is only half the job. Because an antenna is part of a resonant circuit, it must be tuned to match the radio to which it's attached. And the resonance changes with frequency, so the antenna must be tuned for the frequencies on which it will be used.

Now, I don't want to dive into a lot of electrical engineering here, partly because I don't completely understand it myself, so the short story is that a good measurement of how well your antenna is tuned is something called the standing wave ratio. Basically, if the antenna is out of tune, some of the power sent to it will be reflected back to the radio. The further out of tune, the more power is reflected. Too much reflected power will overheat and burn out the radio. The SWR is the measurement of how much power is reflected.

The resonant frequency is controlled by the length of the driven element. So tuning the antenna means changing the length of the driven element until the antenna resonates at the frequency you want. Since it's hard to make wires longer, I started with an antenna that was too long and trimmed the elements as necessary.

So how do you measure SWR? Why, with an SWR meter of course! Trouble is, I don't have one. However, a fellow operator was kind enough to lend me his antenna analyzer, which not only measures the SWR, but also the impedance, capacitance, inductance, and many other things. Since this is my first antenna, I'm not too worried about these things; I'm mainly concerned with building something that is an improvement over what I already have, without blowing up my radio, and an SWR below 2 will do that for me.

So I stood the antenna up on a step ladder in my back yard, hooked up the analyzer, and started tuning. Initially, the antenna resonated at about 130MHz, which is too low. I began trimming the driven element in 1/8" and 1/4" increments until I eventually got the antenna resonating at about 145MHz with an SWR of 1.6. This puts it nicely in the 2 meter amateur band.

How good is this? Well, before I packed everything away, I hooked the analyzer to the factory-built magnetic whip I have (A magnetic whip is an antenna which is magnetically stuck on the roof of your car). It resonates at 162MHz, has an SWR of 2.9 at 144MHz and an SWR of 2.4 at 148MHz. That doesn't sound very good, but if I were to give the manufacturer the benefit of the doubt, I would say that the poor readings are the result of the fact that a magnetic mounted antenna uses the surface to which it's stuck as a ground plane, which means it relies on a good electrical connection, something that is dependent on the type of paint used on the car (non-metallic paints will provide a poor electrical connection, which will hurt the antenna's efficiency.)

So while it's not the best antenna around, I'd say it's a good first try. Mainly I did it for the learning experience, and for that it was wildly successful, as it taught me an important lesson about antenna-feedline interaction:

The feedline gets in the way, electrically speaking. I originally had the coax feedline hanging down the mast in the center, which puts it inline and near to the driven element. On the advice of a fellow operator, I routed the feedline along the boom to the rear. This meant the coax was now running perpendicular to the antenna elements instead of parallel to them, which had a significant impact on resonance, impedance, and SWR. Future designs will take this into account from the beginning.

Portable shack: Powering up

I needed a power supply for my Yaesu mobile. Often times, such as when programming,  I find myself wanting to power up the radio in the house. However, since the radio is designed to run in a car, it wants 12 volts DC. 

To maintain flexibility, I've wired the Yaesu with an Anderson connector, and built a cable to plug into a car cigarette lighter socket. This allows me to drop it into a car easily. For use inside the house, I have a power supply from an electric cooler, which plugs into the wall and supplies 12 volts to just such a socket. 

However, this power supply isn't up to the task of supplying the current necessary to drive the radio at full power; I can turn the radio on and program it, but if I try to transmit at anything above minimum power, the little power supply shuts down. 

Now, lucky for me, a computer power supply can also supply 12 volts. And depending on the unit, it can supply quite a lot of power. But you can't simply plug a radio into a computer power supply.

For starters, the plugs don't match. The power supply has many special connectors for the computer motherboard, drives, etc., while the radio uses a different connector. (in my case, an Anderson SBS connector that I'd scrounged.)

Secondly, while this power supply is able to deliver 10 amps on the 12V rail, none of the components connected to it need that much current all by themselves. Thus, the power supply has several 12V wires going to different places. The radio, on the other hand, wants that 10A all to itself, which makes these little wires less than optimal, (and probably a fire hazard.)

And finally, there are a lot more wires than I need. The power supply can not only deliver 12V, but can also deliver 5V and 3.3V, for other uses inside a computer. The radio doesn't need any of these other voltages.

This is a power supply I rescued from a computer destined for the recyclers. The first thing I needed to do was remove the rat's nest of wires hanging out the back. To keep things clean, I wanted to desolder everything from the board and replace it all with a single Anderson SBS connector. Fortunately, the holes in the board are large enough to accept the 10 guage wire that's on my connectors; the challenge was finding a soldering gun large enough. The wires themselves will carry heat away from the joint, so you need a gun large enough to put heat into the joint faster than the wires can carry it away. A torch would do the trick, but it would also burn the board.

Once I had removed all the wires, I soldered the Anderson connector onto the board. There's just enough wire to carry the connector outside the case, where I could bolt it to the chassis (as a strain relief.)

The next task was to provide a way to turn the power supply on. This is an ATX power supply, which means it's normally controlled by the computer. The computer is able to turn itself on and off, but it also means that there isn't a switch on the power supply; there's no way to turn it on unless it's connected to a computer.

Well, a little research on the internet showed me that if you tie one of the wires to ground, the power supply will come on, and breaking that connection will turn the power supply off. So I drilled a hole in the back of the case, installed a toggle switch, and connected it between this lead and ground.

This power supply is able to drive my Yaesu at full power (75 watts) without any problems. It supplies just a shade under 12 volts, which is less than the car, but the radio doesn't seem to mind. 

Portable Shack: The Next Generation

My first design for a portable radio shack has died on the drawing board.

After a bit of thinking, I decided that a single box to contain all my radio gear would simply be too big. Starting with a 30kg battery and adding a few radios would have resulted in a box that was just too much to haul around in a practical manner.

Instead, inspired by the design of the case for the Spilsbury SBX-11 that was given to me by a friend, (similar to the one pictured on this page,) I decided to build a case for each radio, and one more for the battery. In addition to making it easier to pack everything, since it's easier to move several small boxes than one huge one, it also provides more flexibility. For example, if I'm going on a short road trip, I can take the 2m mobile and plug it into the car's power outlet instead of lugging around a bunch of extra stuff (battery, HF radio, antennas, etc,) that I won't need.

Well, I just finished the first of these cases. It's designed to hold my Yaesu FT-2900R 2m mobile radio. The box is constructed of 3/8 plywood, rabbetted and joined with glue at the seams. Inside the case are several ripped-down pieces of lumber which act as spacers, so the radio is held snugly in the case while allowing room for air circulation. There's a space behind the radio so the antenna cable has room to turn around and exit via the front opening without getting damaged.

The microphone hanger is attached to the lid of the case, so that the mic can be stowed inside the lid when not in use. The case is waterproofed, so it can be left out in the rain without risking damage to the radio. (The only thing left to accomplish this goal is to add a gasket to the lid seam.)

I've made the wiring connections as modular as the case design. The power cable is short, fused, and terminated with an Anderson SBS connector. It will be connected to a longer cable which will supply power from a battery, or a car's power outlet; using the Anderson connectors means I can build different cables for each power supply application, and interchange them between my radios. I'm also going to get a small extension for the antenna line so that I can connect an external antenna from the front, instead of having to remove the radio from the box each time I set it up and take it down.

With this setup, I can set up a station nearly anywhere, in only a couple minutes. It's even suitable for dropping into a vehicle to provide mobile service, as long as I don't have a front passenger. And while it's not as portable as my handheld, it also has fifteen times the output power, so I can see it being taken to a lot more places now that it has a nice-looking overcoat.

Chasing RFI: The Workaround

The only thing better than seeing a project through to its conclusion is not having to do the project at all.

I had been reading this document, which does a very good job explaining RFI, while I tried to find some time to hunt down the source of the interference in my house. While I was doing that, the good folks who brought us CHIRP made some improvements to the software that allow me to expand the squelch range on my Baofeng GT3, which meant I could cut out the bursts of static while still hearing actual signals.

"But hold on a sec," you say, "what exactly is this squelch thing you're talking about?" Squelch is a circuit in a radio receiver that turns off ("squelches") the audio when a signal is not being received. When a strong enough signal is detected, the "squelch is opened" - the audio circuit is turned on so you can hear the received signal.

Not all radios have a squelch circuit, but the ones that do (all that I've seen at least,) have an adjustment. This allows you to set the strength of signal that is required before the squelch opens. Ideally, you want to adjust the squelch to be low, so that you don't miss a transmission. However, there are times when you need to turn it up, such as when you're in an area with a lot of RF noise.

The problem with the UV5R series of radios (of which my GT3 is a member), is that the squelch is notoriously useless. Bursts of static will trick the radio into thinking that there's a signal present when there isn't one, and the squelch adjustment is very narrow (there is no noticeable difference between level 1 and level 9).

So, back to CHIRP. The 10 levels of squelch available in the GT3's menu (0 thru 9) are mapped to values inside the radio's programming. In fact, the radio can set the squelch anywhere between 0 and 127. At the factory, the 10 levels in the menu are mapped to these internal values.

Once I installed the latest daily build of CHIRP and looked at the squelch mappings on my radio, I discovered that levels 1 thru 9 (0 is mapped to 0, which disables the squelch circuit altogether) were mapped to values between 17 and 33, in increments of 2. On the advice of this page on the Miklor website, I changed the values to range between 24 and 64, in steps of 5.

After dumping the modified image to my radio, I moved to a part of the house where I previously had static. I turned the squelch up one step at a time until the static stopped. While I was doing this, a couple other folks were having a conversation on the local repeater that I was monitoring. I was able to confirm that my new squelch setting got rid of the static while still letting the signal from the repeater through.

So my time-consuming hunt for RFI sources has been called off. And a good thing too, since I have a bunch of other projects on the go, and I don't really have the time to go chasing RF gremlins.