In amateur radio, antennas often become a compromise between available space and desired performance. The RA9SUS simple loop antenna is a refreshing exception. Designed for operation from 14 to 30 MHz, this compact HF loop antenna delivers surprisingly reliable results while occupying very little physical space. For operators restricted to balconies, terraces, or indoor installations, this design opens the door to serious HF operation without large wire runs or ground systems.
The attached diagram clearly shows the elegance of the concept. A closed rectangular loop with a total perimeter of about 4.7 meters, carefully tuned and symmetrically fed, forms the entire radiating structure. Nothing excessive, nothing ornamental—just solid RF engineering.
Working principle – RA9SUS simple loop antenna
At first glance, the antenna appears uncomplicated. However, its effectiveness comes from careful electrical design rather than physical size. The loop has a total perimeter of approximately 4.7 meters, which makes it electrically small across the intended HF range.
Instead of relying on resonance alone, the antenna uses a variable capacitor (12–495 pF)to achieve tuning. This allows the same loop to be adjusted across multiple bands, providing flexibility without mechanical changes.
The antenna uses two-wire ladder line configured in a rectangular loop. Think of ladder line as the backbone of this whole setup. If you’ve never worked with ladder line before, it’s that parallel wire transmission line you might have seen on older TV antennas. It has much lower loss than coax at HF frequencies, which is one reason this design works so well.
To transition from the balanced loop to a standard 50-ohm coaxial feed, a symmetrical matching unit is used. This step is essential because it prevents common-mode currents on the coax and keeps noise pickup under control.
Frequency Coverage and Operating Characteristics
The RA9SUS simple loop antenna comfortably covers 20 meters through 10 meters using a single antenna structure. That alone makes it attractive for operators with limited space. However, like most small loops, it requires retuning when changing frequency.
The bandwidth at any given tuning point is relatively narrow. Rather than being a disadvantage, this characteristic improves selectivity and helps reject strong off-frequency signals. Consequently, operation often feels cleaner, especially in crowded HF bands.
Tuning the antenna becomes part of the operating process. First, the capacitor is adjusted for resonance, and only then is the transceiver fine-tuned. Many experienced operators find this approach both intuitive and satisfying.
Receive Performance
One of the most noticeable benefits of the RA9SUS simple loop antenna is its low noise reception. Small loop antennas primarily respond to the magnetic component of the RF field, which makes them less sensitive to common electrical noise sources.
Because of this, signals that are buried in noise on long wire or end-fed antennas often become easier to copy. In addition, the loop exhibits mild directional characteristics, allowing some degree of noise reduction by repositioning or rotating the antenna.
This behavior makes the antenna particularly well suited for urban and indoor environments, where noise is often the limiting factor rather than signal strength.
On transmit, expectations should be realistic. The RA9SUS loop will not outperform a full-size dipole installed at height. However, within its intended role, performance is consistently respectable.
With moderate power levels, reliable contacts can be made across the covered bands. Many operators report stable SWR, predictable tuning behavior, and effective radiation for both local and medium-distance QSOs. Therefore, for balcony and attic installations, the antenna delivers exactly what it promises.
RA9SUS simple loop antenna – Construction Considerations
Construction of the RA9SUS simple loop antenna is straightforward, but precision matters. Mechanical symmetry should be preserved throughout the loop, especially around the tuning capacitor and ladder line connections. Even small imbalances can reduce efficiency and increase noise pickup.
The air-spaced variable capacitor is strongly recommended, particularly for power levels above QRP. Solid electrical and mechanical connections are equally important, as losses quickly add up in compact antennas.
Since the design does not require a ground or radials, installation options remain flexible. The loop can be mounted vertically or horizontally and can be used indoors or outdoors with equal success.
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