My First Antenna Project

I have a big attic. When we bought this house, and realized just *how much* attic space there was, I knew I would find something creative to do with the space. This was before I got into ham radio, but the giant attic (in spite of the rain gutters messing with my antenna patterns) has definitely been the most convenient place to explore antenna ideas. Currently my attic is home to a 40 meter full wave loop that resonates at 10, 20, and 40, but more about that later.

Getting Started: A NanoVNA, wire, duct tape, and a dream

My first purchase, even before the IC-7300, was a cheap vector network analyzer (VNA), the NanoVNA. As an RF engineer, the VNA is the key to virtually every adjustment and tuning. The NanoVNA is affordable, and seems to work well (although mine is starting to act up on me…) and has already been invaluable in my antenna work so far.

My first antenna was simply a ~20 meter full wave loop of 16 AWG insulated copper wire. I had plenty of space in my attic. I used a 1 1/4″ type 61 toroid I bought from Amazon for a few bucks, and wound 5 turns for the 20 meter loop, and 5 turns for the RF coax cable. With my nanoVNA, I could see my 19 meter guess was too short by about 10% (the VSWR “dip” was 10% lower in frequency than I wanted), and it was closer to 75 ohms. This impedance was low for a full wave loop. It wasn’t really behaving like a full wave loop in free space, but it wasn’t in free space, it was in an attic with nearby gutters and wiring. I added 2 meters of cable, changed to 4 turns on the antenna side of the balun, and I was on the air! With this antenna, I made SSB contacts all over the country, even into Hawaii!

A few things I found along the way that helped:

1. For a transformer, a good rule of thumb is about 200 ohms of inductance at the lowest frequency (or 4x the impedance), if practical. 4-5 turns seemed about right. The data sheet on the toroid you select should specify inductance at a specified number of turns.
2. Nearby wires and gutters will detune your antenna. That’s the price you pay for building in the attic, or even close to the house. For me, at least, the convenience factor wins. Retune and move on!

Expanding it to 40 meters

Twenty meters is a great place to start, I think. It’s a busy band, so something’s almost always happening. But I was curious what else might be out there, so I expanded my attic full wave loop to 40 meters, filling the perimeter of the entire half of the attic and even requiring some meandering. It was inductive through the entire 40 meter band (I suspect a little mag loop action from all of the nearby parasitics in the attic), but I was able to add a capacitor (built out of 4 inch x 8 mil mat tape and 1 inch copper tape) and match it at 40 meters while resonating at 10m and 20m well enough for my antenna tuner to be happy.

I discovered just how awful my noise floor was at 20 meters compared to 10 and 40, which will lead into my second antenna…

Conclusion

If you’ve never built an antenna, it’s not that hard! Get yourself a cheap VNA, read an article or two, and try something! Your radio might even have a built-in VNA function. Your VNA will tell you not only how well you did, but how to fix it from there! Sweep the VNA from about half to about twice the expected frequency. Find the lowest VSWR point, and WRITE DOWN the frequency and impedance reading. Change element lengths, rewind your balun, try stuff and see what happens! The wire is cheap but the knowledge and experience gained is invaluable.