At Long Last, The CUB!!! (And how to make a dipole for it!)

 At long last, I have you now, Cub! After 2 or 3 weeks of waiting for it, I'm hoping it will be worth the wait.

In the meantime, I looked over the PDF instructions to see what I was working with. I will need some alignment tools, and some might just be rudimentary. I did manage to find my NanoVNA in a drawer on my workbench, so this week has been good... up until I began running a low grade fever and felt like I was going to pass out at any given moment.

Not sure what happened, but you can all breathe - it was NOT Covid. My guess is that I got overheated at band practice or something else. I'm not quite sure what happened with me, but I'm feeling just a bit better.

I am so excited to put this thing together and possibly document it for the YouTube channel! But what should I do for it in the meantime???

Well, one thing you can do is build a dipole antenna, the most simple and quite effective antenna for any radio, especially QRP rigs. Here's the best thing - you don't need to graduate with antenna theory to know how to build one. It's just two elements branching off of a balun, ran to the radio with coax. You might already have the materials in your house already! As a tip, speaker wire works best for QRP antennas. It's what I use with my EARCHI antenna.

Okay then...let's get started.

1. Let's do some math!

Now it's time for everybody's favorite subject: math! The formula for a dipole, in feet, is as follows:

468/f = L

Whereas f is your frequency and L is the overall length. To find the length of both legs, divide L by 2. For me, I need a 20m antenna. It's best to make your antenna for the center frequency of the band or the radio. The MFJ 20m Cub has a good range of 60KHz, and I'm going to shift the lowest frequency up to about 14.030. The top will be around 14.090, so that makes the middle frequency roughly 14.060 - right around the QRP calling frequency! Let's calculate that:

468/14.060 ≈ 33.3ft

Divided by two, each leg will be:

234/14.060 ≈ 16.6ft (or 33.3/2!)

Realistically, these lengths are not true to a ruler, so we'll run it a bit longer. I'll put my antenna at about 33.5ft. During the tuning process, you can cut the length so it will be resonant.

2. Cut wire and materials

For the antenna elements, find some speaker wire. I use insulated, solid core wire for my end-fed EARCHI. I've heard others use stranded wire, but I'll leave that up to you. We know that we'll need 33.5ft for the overall length, but you'll only need to cut 2 pieces of wire measured at 16.75ft for each element. 

Some other things you'll need:

• SO-239 chassis mount
• RG-8X or RG-58 coax
• Balun - you can make one yourself or buy them premade. I'll talk more about this later
• Mounting stuff and common hardware
• Common sense - like don't EVER place an antenna near powerlines
• Solder + soldering iron

3. Build It!

There's really no need to explain this step, but I'll do it anyways. Below is what your final product should look like:

First, build your antenna body. You can use PVC pipes or anything else to create the body. Start by drilling a hole in the bottom for your SO-239 connector. Next, drill holes on the sides near the top for your element binding connections. On top, I recommend you drill a hole for an eyelet bolt for hanging. Inside, solder one wire to the center of the coax connector and solder another wire to the outside/ground of the coax connector. Take these wires and connect them to the element binding connectors. Attach your elements, hoist up the body, and you have a dipole now! Again, if you use a balun, make sure it matches the antenna appropriately. A 1:1 should work fine for any QRP applications.

If you don't have a balun, you can make one with the coax! In fact, the MFJ Cub manual mentions using coax as a balun. This information is pulled directly from it:

Coax Balun:

15m: 6', 8 turns
17m: 7', 8 turns
20m: 8', 8 turns
30m: 10', 7 turns
40m: 15', 6 turns
80m: 20', 8 turns

This means that for 20m, I'll need 8ft of coax to make a balun with 8 turns in a loop, kind of like winding up an extension cord. 

Now, operating the Cub is a straight-forward process, much like the Pixie - just plug it in and play. You'll need to add a BNC jack or use an RCA to UHF adapter if you don't use the RCA jack (which I wouldn't recommend using). This rig does not have a built in keyer, so you'll need to use an external circuit. One that I'd recommend which has a side tone buzzer with it can be found here (courtesy of Ernest, PA3HCM). This uses any Arduino board that you have, and as a recommendation I'd use a Nano.

Connect the Cub to a 13.8V power supply. I'd use a low-amperage one, like 500mA or even 1A. You won't need 15A to run this device. If you get a wall wart, make sure it's a transforming power supply and not switching. I've had horrible RFI with switching on my higher powered radios. On a QRP rig, it would render the radio useless with RFI and switching noise.

Remember, you only have a limited amount of tuning range (about 60KHz), but it beats the Pixie's fixed tuning any day. If you want to shift your frequency window, you'll need to readjust the circuit to fit your desired operation window. For me, 14.030 works well since I'm a General. The top is at 14.090, but in between it all lies the common Feld Hell Club frequencies, which I'm bound determined to use! When I'm not operating Feld Hell, I can also operate CW here. It fits my needs perfectly, but you may have different needs. Perhaps you're an Extra who has full reign of the band - you'll probably want yours at 14MHz even (I'd recommend setting it at 14.005MHz so you don't set it to 13.9999. The law is pretty strict!). Maybe there's some contest frequencies you love to use. 

A lot of you are probably wondering why you'd ever want to operate QRP. For me, it gives me the opportunity to operate anywhere. The MFJ Cub can fit snugly in my backpack - so can a 20m dipole with some coax and a mini key. If so desired, I can take a 12V wall wart as a power supply.

You can also 3D print your own mini keys or buy surplus ones from the days of the Cold War. One really interesting one is a style of Soviet key. I'm sure this style was made from the 1940s or 50s up to the late 1970s. They can be found for reasonable prices on eBay. They look a bit like this:

I really wish someone still produced this style of key - it's compact and lightweight. Of course, one could make a 3D printed design of this.

For now, I wait to build the Cub as I need to order more tools like insulated screwdrivers, a power supply, and maybe a shortwave receiver. I've got to get back into SWL - my current shortwave receiver is a real piece that doesn't function well at all. Then again, it was another skimp purchase.