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.

Chasing RFI: Introduction

If you've been following along, you'll remember one of the reasons I'm building a portable RF shack is that my house is full of RF interference which is causing me problems. I'm going to try and track down the source of the interference, eliminate it if I can, and hopefully learn a bunch of stuff along the way.

Right now I'm in the very early stages of troubleshooting. Shortly after I got my handheld (a Baofeng GT3 Mk2) I noticed that it would break squelch in certain areas of my house while tuned to the local 2m repeater. When the squelch opens, all I hear is static.

My first thought was that some piece of computer gear was causing the interference, so one morning while everyone else was asleep, I tried to do some rudimentary investigation. I placed my radio in a location where it was picking up the interference, then I shut down my server, and cut power to my network stack. The noise went away. A little further experimenting pointed me to the ethernet switch.

However, changing the switch to a different model resulted in the same interference. Also, it seems that the interference is only present if there's an ethernet device connected to the switch.

A little later on, a friend of mine loaned me his SDR (software defined radio). I plugged it into my computer, installed HDSDR, and took a look at the waterfall display. I could easily see signals present all through the 2m amateur band, spaced at something like 30-50Khz intervals. Reading on the internet doesn't point me to any definite sources.

At this point I'm devising some more rigorous scientific testing so that I can conclusively narrow down the source of the interference. I'll post the results of my experiments here, so stay tuned.

Why a portable shack?

While I would love to have a fixed station in my house, I'm coming up with more and more reasons to forsake the basement radio room for a portable radio shack.

That's not to say that I won't use my radios at home, but I think I'm going to be doing more with the radios while out of the house than in it, and it's easier to accomodate the portable shack at home than it would be to take a permanent station on the road with me.

For starters, I don't have the space in the house to dedicate to a bunch of radio gear. While there are many places where I could set up a radio rig, they all get used for something else at some point. This means having a shack I can set up and take down easily is important, and then I'm halfway to a portable unit already.

Secondly, I don't have the space outside the house for a long antenna. We live on a city lot with neighbours on all sides. Our back yard is completely open; any mast would stick out like a sore thumb.

Thirdly, my house is full of RF trash. I'm going to do some work to narrow down the source and try to filter it out, (stay tuned for more on that,) but the simplest answer is just to get out of the house.

Of course, there are downsides to going portable. I'm going to be limited in power since I'm designing everything to run off a 12V battery. Since I'm not lugging a generator around, I'll be limited in the time I can spend on the air. I'm also going to be limited to portable antennas.

But then again, those limitations present some interesting engineering challenges.

The Router, Revisited

One of my readers asked for more pics of my router table hack, so here they are.

Here's an overall photo of the complete setup. Note the ad-hoc debris collection system. (the router cuts off bits of wood of various sizes, ranging from dust to large chunks.)

Here's what it looks like from below. The table surface is doubled up to bring it to the approximate height of the table saw table, and then a couple rails are added so the router table drops in between the rails of the saw table.

Here's a close-up of the table surface. I used a spade drill bit to provide a place for the bolt heads to drop in so they don't sit proud of the table surface. The hole for the router bit was cut on the drill press with a hole saw. It looks small, but there's no point in making it any larger as the router body stops me from using any larger bits anyway. The grooves in the table surface were accidental; I placed the router on the table surface to change the bit, and because I didn't use hardwood plywood for the table, the force required to break the chuck loose pressed the router body into the table.

A view from the back.

A different kind of routing table

My day job is in IT, so the routers I normally deal with don't usually produce sawdust.

However, I'm currently working on finishing the baseboards in my kitchen so I can move on to building my portable radio shack, and that meant it was time to dig out my old router and find a way to make it work for me.

I bought the thing many years ago to trim counter top laminate, used it for one job, and then put it away. It's such a huge bulky thing that I've never bothered to dig it out. I even have a router table, which I've never used. (My router doesn't fit, and I've never gotten around to modifying the table to suit. I also don't have space in my garage for another table.)

However, the router is the perfect tool for cutting the baseboards to fit around the stairs, specifically the little relief cuts so the trim will fit around the metal bullnose on the front of each step. And so began the project to find a way to make the router useful. And by useful I mean it can be set up in a minute, stashed out of the way quickly when not in use, and doesn't take up a lot of space when stored.

The result is what you see below. The router table is composed of a sheet of 3/4" plywood, that has a couple braces on the bottom so that it can sit on the rails that extend from the side of the table saw table. The table saw fences can be used in some situations, and I can clamp the vacuum near the bit to collect the sawdust.

The portable router table. It's being used, even while jammed in a corner.

Using this rig, with the flush trim bit, I can easily whip off a dozen identical pieces of trim for wrapping the stairs. When I'm done, the router simply lifts out of the saw table, and I can throw it on a shelf.

A Portable Ham Shack for DXing on 2m

The title of this post sucks, but it's what I've got for now.

Ham radio is a multi-faceted hobby. There are just so many things you can do with a radio set and some math.

One of the things ham operators like to do is try to make contact with other stations that are far away, something known as "DXing" (in radio jargon, "DX" means "distant station"). The farther away a contact is, the better.

The weapon of choice for hunting distant contacts is usually an HF radio. This is because the HF bands (between 3 and 30MHz) will reach around the world thanks to the fact that the radio waves will skip off the ionosphere. This allows an operator to effectively bend his or her signal around the curve of the Earth.

My problem is that I don't have an HF set, and there's currently no room in my budget to buy one. So while my wife is pleased that she won't have to put up with a radio shack in the basement and the backyard suddenly sprouting a forest of antennas, I'm not going to be DXing over HF anytime soon.

However, there is another possibility. Many operators are talking to distant stations on VHF radios, in the 2m band. While the range is not as great (VHF signals punch right through the ionosphere, so they won't reach the other side of the planet,) operators are reporting contacts as far away as 1000km thanks to other atmospheric effects like tropospheric ducting.

I already have a pretty good VHF set, which I was thinking of installing in my RV trailer. However, that's going to limit me in terms of when and where I can use it (for example, I won't be using it in the winter and if we're camping, I can't get out of everyone's way to go play with my radio.) Ideally, I'd like something a little more portable so I can lug it up to a nearby mountaintop when I'm out camping. And since I'm not using HF, I don't need a huge antenna, so a portable radio shack is not out of the question.

Most portable VHF radios find themselves mounted in a vehicle. For various reasons, that won't work for me, so I'm thinking of building a portable radio shack. Basically, a box that can be lifted into or out of the back of a pickup truck by one or two people. It would house a radio (or eventually two), battery, and have space to store a portable antenna. A place to put a 20W solar panel would also come in handy and would make the rig ideal for emergency situations.

As for the antenna, my early research is pointing me in the direction of a Yagi-Uda. Something that I can easily disassemble to store inside the box. Same for the mast; I want some height, but it also needs to be portable (ideally, stowed in the box beside the antenna.) This means flimsy; hopefully I can come up with something that can be attached to the side of a vehicle for stability.