Tag: radio engineering

There’s a certain humbling quality to working at 472kHz. You pull up the math, realize a proper half-wave vertical would need to be over 300 meters tall, and then you have a moment. A long moment.

Then, you get creative.

Let’s dive into how we approached building a transmit antenna for the 630m band at our Kern County site. This post explains why we selected the inverted L configuration, how we leveraged the landscape to avoid a massive tower, what our simulations predict, and what we hope to learn once the antenna is operational.

472kHz is a pain in the neck but worth the trouble

At 472kHz, your antenna wants to be enormous. A half wavelength is around 315 meters — taller than the Eiffel Tower. So a “proper” full-size vertical is off the table for us mere mortals outside the broadcast engineering world.

But setting up the antenna isn’t just for the game. 

The 630m band is relatively uncrowded and, compared to HF bands, surprisingly understudied, partly due to antenna size limitations and partly due to site availability. 

Most existing research on LF and MF propagation comes from flat, low-elevation sites or coastal plains, where soil conductivity is high, well-characterized, and relatively uniform. Our site is none of those. At 3,500 feet above sea level in the Sierra foothills of Kern County, it sits in a natural RF-quiet bowl surrounded by mountains. It features rocky terrain, a known aquifer below, and almost no local interference. 

That combination of elevation, geology, and low-noise environment is rare. As a result, we generate propagation data that doesn’t exist anywhere else. But still, why go through the trouble?

At 630m, signal behavior is shaped simultaneously by soil conductivity, terrain geometry, and ionospheric D-layer conditions. We provide a clean baseline to untangle those contributions. Our unique site condition takes this antenna beyond a radio project to a data source for researchers who currently have to work around gaps in available datasets:

• Geologists and hydrologists can access empirical data on how 630m ground-wave signals interact with rocky, mountainous terrain and aquifer-influenced soil when modeling subsurface features. The data is thin in this discipline.
• Atmospheric and space-weather scientists studying D-layer behavior can gain long-term observations from RF-quiet sites. Most existing datasets come from coastal or urban-adjacent receivers with noisy baselines.
• Earthquake researchers investigating pre-seismic electromagnetic patterns can access data from a geologically active site. We’re in a seismically active country with interesting geology and a noise floor that lets subtle signals surface.

We’ll return to the impact on scientists and researchers shortly. First, let’s look at how the antenna itself can be built — affordably and without years of permitting hurdles.

Leveraging our site’s unique characteristics with an inverted L antenna

An inverted L antenna is exactly what’s on the tin: a wire that goes up vertically, then bends horizontally. The vertical section does most of the radiating work. The horizontal top section primarily loads the antenna electrically, helping it reach resonance at a physically manageable length.

For our target frequency, we aim for a quarter wavelength, about 150 meters of wire. But how do we gain meaningful vertical height without a self-supporting tower? Solving this challenge is where our canyon earns its keep.

We plan to run a support wire from rim to rim across the canyon, with poles on both sides. The antenna’s vertical section rises from the feedpoint at ground level up to that support wire, where it makes a 90-degree turn and runs horizontally to one side, while the other end provides mechanical tension. The setup gives us usable height, sourced from existing terrain, and nature does the heavy lifting.

Here’s the full geometry breakdown:

• North-south section: ~100m along the map, ~35m effective height
• East-west section: ~40m along the map, ~25m height
• Total wire length: ~150m (~quarter wavelength at 472kHz)

Another unique aspect of this build is that the antenna’s vertical section rises partly over solid terrain on the canyon rim, and partly suspended in open air over the canyon. While it’s possible to model an antenna over poor rocky terrain or in free space, current tools can’t cleanly model one that transitions between the two mid-wire. 

We’re in greenfield territory.

A conventional approach to achieving what we’ve outlined would involve a 30-meter self-supporting tower, a concrete foundation, permit applications, and a significant budget. Such hurdles are exactly why we created ClearSkyRF to help accelerate RF research and use amateur radio as a tool to generate scientific, geologic, and atmospheric insights.

This is also how we challenge the “this is how things are done” mentality. We’re not here to build the definitive, fully optimized, write-it-up-for-IEEE antenna farm. We create something good enough to generate real-world data, learn from it fast, and decide where to dig deeper. 

The unsung hero: a ground system that makes the difference

At low frequencies, a poor ground system will hurt your antenna efficiency more than almost any other design choice. The feedpoint of an inverted L antenna sits close to the ground, and its return currents flow through the ground. However, if that path is lossy, the power that should be radiating turns into heat instead.

Again, nature provides a solution: a clay soil layer with an aquifer underneath.

Our ground system consists of multiple grounding rods driven into the site’s clay soil section, combined with a radial wire network that spreads out from the feedpoint. We will build in the ability to connect or disconnect those radials independently, allowing us to run the antenna with and without them to directly measure the efficiency difference. Such a before-and-after comparison is more useful than any simulation, giving us real-world numbers for the actual soil condition.

For the baseline calculations, we use conservative soil parameters: a dielectric constant of around 13 and a conductivity of 0.0002 mS/m, typical for a rocky, mountainous terrain. In practice, we expect better numbers. The aquifer beneath our site will likely improve ground conductivity in ways that are difficult to predict precisely without direct measurement — understanding the difference is part of the experiment.

Simulation result with a pleasant surprise

Before stringing a single wire, we modeled the antenna to check resonance, impedance, and radiation pattern. The numbers came back encouraging. At 472kHz, the model shows:

• Impedance: R ≈ 6.8Ω, jX ≈ 3.5Ω — low resistance, as expected for a short antenna, which means a matching network will be needed between the feedpoint and standard 50Ω coax
• SWR curve: Clean resonance across the 630m band
• Gain: ~1.1 dBi — spectacular for this class of antenna

The radiation pattern also demonstrates a notable null of around 6dB of attenuation to the west. Follow that bearing, and you end up pointed at the antenna farm on Mount Wilson, north of Los Angeles. The dense cluster of high-power broadcast transmitters would otherwise contribute significant interference to any receiver in Southern California.

The null isn’t pure luck. The orientation and directionality of the horizontal top section determine how the pattern tilts and which direction the null goes. Run it north-to-south, and the null points one way. Flip it south-to-north, and the pattern flips with it. 

We chose our orientation deliberately to put that null on Mount Wilson. In effect, we have some directional control over a 472kHz transmit antenna. As far as we know, no one else on the planet operates an electrically steerable antenna on the 630m band.

What this antenna will tell us

Once in the air, this antenna will be a transmit reference for experiments that are difficult to run anywhere else. These include:

Propagation characterization

By transmitting WSPR beacons on the 630m band and tracking their reception, we can build a propagation map based on our site’s characteristics, including elevation, terrain shielding, and soil, and compare the results against model predictions.

Ground-wave vs. sky-wave separation

At 472kHz, both propagation modes are active, and they behave differently over distance and time of day. Our low noise floor and known antenna geometry will allow us to distinguish them more cleanly than at a noisier site.

Soil and subsurface sensing

Ground-wave signal strength over distance is a proxy for soil conductivity. The gap between the assumed soil parameters and our measured antenna efficiency provides insights into what’s happening in the ground beneath us, including potential aquifer influence. The data will show us both ends of that equation, while the radial experiment will further sharpen the picture.

Atmospheric and space weather correlation

The 630m band is sensitive to D-layer ionospheric conditions, which respond to solar activity, geomagnetic storms, and some terrestrial weather patterns. Long-term continuous monitoring from a quiet, elevated site generates a unique baseline that makes anomalies visible and attributable.

Seismic monitoring potential

Pre-seismic EM anomalies in the LF and MF bands have been reported in the literature for decades, but the evidence remains contested — partly because clean, continuous datasets from geologically active, RF-quiet locations are rare. We’re in a position to contribute to that record in a way most sites can’t.

None of these is guaranteed. Some may turn out to be dead ends. That’s the point — the minimum viable experiment exists to explore possibilities without betting an entire facility on one hypothesis. And the inverted L antenna doesn’t have to do this alone.

Building a multi-antenna experiment

The inverted L is a transmit antenna. But what makes it genuinely useful as a research tool is how we pair it with other antennas. 

We’re designing a regimen that alternates transmission and reception across other antenna systems on our site. The combination opens up comparisons that a single antenna can’t provide on its own.

On the receive side, we have two complementary tools. The Giga Loop is a full-wave 630m loop that will eventually encircle over 15 acres of the property. It picks up signals from all directions with a large effective aperture. Its size makes it unusually sensitive at these frequencies, and running it alongside the inverted L antenna provides a direct comparison between a compact transmit antenna and a large receive loop on the same signals, at the same site, at the same time.

We also have the receive-only, directional Beverage antennas aiming at nine regions across the globe — domestic targets split across the West Coast, Midwest, and East Coast, plus international bearings toward Europe, Asia, and beyond. Beverages are low to the ground and highly directional, making them poor transmitters but excellent weak-signal receivers. They enable us to pull a signal from a specific direction while rejecting noise from others.

With these antennas working together, we can run multiple experiments with different perspectives simultaneously. For example, we may transmit on the inverted L antenna and receive on the Giga Loop and Beverages. We may also switch orientations, connect or disconnect ground radials, and log the variations.

Questions and answers will emerge: Does the signal received on a Beverage aimed northeast match what the propagation model predicts for that bearing? Does the Giga Loop see the same fading events as a directional Beverage, or different ones? When an anomaly shows up (e.g., atmospheric, seismic, or otherwise), does it appear across all three antenna systems, or only some?

No single antenna answers those questions, but our combination can.

But talk is cheap…

The theory is sound, and the simulation looks promising. The physical build is the next step: getting the support wire across the canyon, rigging the poles on both sides, and laying out the radial ground system. After that comes the matching network, as the ~7Ω feedpoint impedance needs a transformer to interface with standard 50Ω coax and the transmitter.

The initial tests will be WSPR beacons at low power. We’ll watch for spots, compare against the modeled radiation pattern, and measure efficiency with and without the ground radials connected. Then, we’ll publish what we find — the good, the surprising, and the awkward.

We’re not a university lab with a five-year funding cycle and a crane on call. The goal isn’t the perfect antenna. We will build the first good-enough antenna that starts generating data no one else is collecting. We provide the testing ground to help ambitious projects get off the ground without waiting years.

Stay tuned to see what we find out.