K0UO's Rhombic Feed System
- skylarkcolo
- Mar 2, 2019
- 4 min read
Updated: Jan 19
Below photo is from the base of the feed point poles, which are about 25-30 meters tall. Note the open wire "Tapered Feed" or "exponential feed-line system" the wires coming down the poles provide a broadband match on HF, About 1 meter (39") wide at the top tapering down to less than 3" at the feed-point. Rhombics have a high input impedance of about 600-900 ohms, so it's not a trivial matter to achieve a match to 50 ohm coax while still retaining the very broadband nature inherent to the terminated Rhombic. This led K0UO to do some research on "tapered lines", the result of which is shown in the pictures above. After several prototypes, K0UO settled on the tapered 2-wire transmission line shown here, which is a modified exponential taper. The exponential feeder is used to match (not tune) the feeding either end of antenna using 600 ohm open two-wire line, from the control box to the 600-800 ohm Rhombic feed-point. It's basically the poor man's broad-banded transformer. As the figure implies, there is an exponential feed-line at the feed end and also at the termination end. The exponential feed-line allows the appropriate impedance across the entire useful frequency range of the Rhombic.
Looking up the pole from the ground: The exponential feed-line allows the appropriate impedance across the entire useful frequency range of the Rhombic.
The key concept with traveling-wave type antennas is that there are no standing waves, on the antenna itself, the current and voltage levels are the same everywhere along the antenna conductors, but you still have to match it to the fed-line. The Northern European on 40 meters is 5 to 6 wave-Lengths on long on each of the four sides, this may be the largest 40 meter Rhombic antenna in current ham use. The site also has two 1200' Vee-beams as part of the system. Up to 6 Beverage receive slow traveling-wave antennas are used in the winter months, each 1000 to 2000 feet.
The Rhombic antenna is a wide-band progressive traveling-wave (fast-wave) antenna, made of two acute-angle V-beams placed end-to-end and terminated in an open circuit or in a resistive load. Each side of the antenna is made of two legs of length "L" and as a whole the antenna has the shape of a rhombus, that is, the opposite angles are of the same value. The non-terminated Rhombic antenna is bi-directional, whereas the terminated Rhombic antenna is directional. The Rhombic antenna is useful over a wide frequency range. Although some changes in gain, directivity, and characteristic impedance do occur with a change in operating frequency, these changes are small enough to be neglected. So no waiting for a rotator to turn, the system has every direction, every band, every time.
Exponential feed-line or Taper Feed System
In essence, it's an elegant, physical solution to impedance matching that makes high-impedance antennas usable with standard equipment by creating a smooth, broadband transition without using a balun.
Key Components & Design
Broadband Linear Transformer: It acts as a continuous, wideband transformer that transitions impedance gradually from one value (e.g., 200 Ω near the ground/feed point) to another (e.g., 800 Ω at the antenna).
Construction: It typically consists of a tapered two-wire open transmission line where the spacing between the wires changes along the length of the line.
Open-Wire Line: Typically parallel wires separated by insulators (ladder line).
Taper: A logarithmic or linear change in wire spacing, often from a narrow spacing (e.g., 10mm) to a wider spacing (e.g., 300mm) at the feed point.
Length: Should be at least one wavelength
(λlambda 𝜆) at the lowest operating frequency for best results.
Chokes: A common-mode choke (or balun) is often added at the radio end of the coax to prevent feedline radiation. CMC
Key Specifications
Minimum Length: To be effective, the tapered section must be at least one wavelength long at the lowest operating frequency.
Impedance Range: Commonly used to match a 600–900 Ω antenna impedance down to a level that can be easily handled by a 4:1 balun (to 50 Ω coax) or a 600 Ω standard line.
Physical Example: A documented HF setup uses a 20-meter long tapered line, starting with 10 mm spacing and expanding to 300 mm at the antenna end.
Advantages over Standard Methods
Frequency Agnostic: Unlike quarter-wave transformers which are frequency-specific, an exponential line maintains its match across a very wide frequency range, preserving the broadband nature of the antenna.
Economical: Often referred to as a "K0UO's poor man's broadband transformer," it replaces expensive high-power wideband baluns with simple wire and spreaders.
Low Loss: By using open-wire construction, it minimizes the heat losses often found in magnetic-core transformers at high power levels.
The KØUO Rhombic Antenna Farm and Antenna Test Range: Home to the World's Largest amateur radio (ham), High Frequency (HF) Wire Arrays, miles of wire in the air and on the air daily.
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