Build a 23cm Band Dish Feed Antenna (1296 MHz)
A surplus satellite dish or offset-fed parabolic reflector converted to 23 cm amateur radio operation is one of the highest-gain antennas a home builder can realistically construct. A 90 cm dish at 1296 MHz produces approximately 28 dBi of gain — equivalent to a 16-element Yagi stack — from an antenna that costs nothing if a surplus dish is available and under £20 to feed with a home-built dipole-splash-plate or helix feed. This guide covers the theory, feed design, and installation for the two most practical 23 cm dish feed types.
A parabolic dish antenna focuses incoming electromagnetic waves to a single point — the focal point — regardless of where on the dish surface they arrive. The antenna gain depends primarily on the dish diameter (in wavelengths) and the aperture efficiency, which is determined by how well the feed illuminates the dish. A perfect dish with perfect illumination would achieve 100% aperture efficiency; in practice 50–65% efficiency is typical for home-built systems, giving real-world gain 2–3 dB below the theoretical maximum.
where η = aperture efficiency (0.55–0.65 typical), D = dish diameter (m), λ = wavelength (m)
| Dish diameter | Theoretical gain (100%) | Practical gain (60% eff.) | 3dB beamwidth | Source |
|---|---|---|---|---|
| 60 cm | 32.3 dBi | 25.1 dBi | 2.8° | Small Sky dish |
| 80 cm | 34.8 dBi | 27.6 dBi | 2.1° | Standard Sky dish |
| 90 cm | 35.9 dBi | 28.7 dBi | 1.9° | Large Sky dish |
| 120 cm | 38.3 dBi | 31.1 dBi | 1.4° | Motorised dish |
| 150 cm | 40.1 dBi | 32.9 dBi | 1.1° | Large offset dish |
| 180 cm | 41.8 dBi | 34.6 dBi | 0.9° | 2m prime focus dish |
Before designing or mounting the feed, you must locate the dish's focal point precisely — the feed element must be positioned at this point to maximise gain. An incorrectly positioned feed (defocused dish) loses gain at a rate of roughly 0.5 dB per mm of offset from the focal point at 1296 MHz — positioning accuracy of ±5 mm is the practical minimum target.
Calculating focal length for a prime focus dish
For a 90 cm prime focus dish with a depth of 18 cm: F = 0.9² / (16 × 0.18) = 0.81 / 2.88 = 0.281 m = 281 mm from the vertex (deepest point) of the dish. This is where the feed element should be positioned along the dish axis.
Finding the focal point experimentally
A practical method for any dish: hold a microwave source (a 1296 MHz signal generator, a PLO, or even a strong satellite signal) at the expected focal distance along the dish axis and scan the feed position while measuring output power with a spectrum analyser or receiver. The position of maximum received power is the focal point. On a clear day, a hand-held thermal detector can find the focal point of a dish by focusing sunlight — the hot spot is the focal point.
Offset dish focal point
For an offset dish, the focal point is not at the geometric centre of the physical reflector surface — it is offset from the physical dish axis. The feed arm of the original LNB on a satellite dish points toward the focal point. Measure the feed arm length from the centre of the dish surface to the LNB input point — this distance (typically 250–350 mm for a 60–90 cm Sky dish) is your focal length. The feed must be mounted at this same position, angled to point back toward the dish centre.
Interactive Calculator: Dish Gain & Feed Positioning23cm Dish Gain & Feed Design Calculator
The dipole and splash plate (also called a dipole-reflector or scalar feed) is the simplest practical feed for a 23 cm dish. It consists of a half-wave dipole at 1296 MHz with a circular reflector plate (the "splash plate") behind it. The reflector plate focuses the dipole's radiation pattern into a forward hemisphere, matching the illumination angle required to optimally fill the dish surface without excessive spillover past the dish rim.
Dipole dimensions for 1296 MHz
Splash plate dimensions
- Diameter: 0.75λ = 174 mm is the typical optimum — large enough to direct most radiation forward, small enough to avoid reducing the illumination angle excessively
- Dipole-to-plate spacing: 0.25λ = 57.9 mm (quarter-wave behind the dipole)
- Material: 2–3 mm aluminium sheet, cut as a circle — copper sheet or FR4 PCB with full copper pour also works
Cut two 55 mm lengths of 3 mm brass or copper rod. Drill two holes through a small brass or aluminium block (10×10×15 mm) at 3 mm diameter, 20 mm apart. Insert the dipole rods into the two holes, leaving exactly 55 mm protruding from each side of the block. Solder each rod firmly in its hole. Thread a length of RG-316 semi-rigid coax through the centre of the block — the inner conductor connects to one rod, the outer braid to the other. A female SMA connector mounts on the block body for the coax input.
Cut a 174 mm diameter circle from 2 mm aluminium sheet using a jigsaw or angle grinder with a cutting disc. Drill a 10 mm hole at the exact centre for the coax/support tube to pass through. Deburr all edges. Lightly sand the surface to remove oxidation. The splash plate does not need to be perfectly flat — minor distortion of 5–10 mm does not significantly affect performance at this scale.
The dipole should be positioned 57.9 mm (λ/4 at 1296 MHz) in front of the splash plate — this is the spacing that maximises the forward gain of the dipole-reflector combination. Use a 10 mm aluminium tube as the support and spacing structure: the tube passes through the hole in the splash plate and is fixed with a nut on the rear face. The dipole block attaches to the front end of the tube. Verify the 57.9 mm spacing with a calliper before final assembly.
Attach the completed dipole-splash-plate assembly to the dish's feed arm at the focal point position. For an offset dish, the feed arm must be angled so the dipole points back toward the dish centre — the original LNB mounting bracket provides the correct angle. Remove the LNB and mount the new feed assembly in its place using the same hardware. Verify the dipole is at the focal distance from the dish surface vertex. Run the SMA coax or a short RG-316 pigtail from the feed to an N-type or SMA connector at the bottom of the feed arm.
A short helical antenna is an alternative feed that provides circular polarisation — important for EME (Earth-Moon-Earth) operation where the polarisation rotation through the ionosphere and the Moon reflection makes linear polarisation impractical. A 3-turn helix at 1296 MHz provides RHCP (or LHCP depending on winding sense) with approximately 10–11 dBi gain and an illumination pattern well-suited to dishes with f/D of 0.35–0.50.
Helix feed dimensions for 1296 MHz
The helix is wound from 4–5 mm OD copper wire or tubing over a 73 mm diameter fibreglass former. Right-hand winding (clockwise when viewed from the front, i.e. from the direction of radiation) produces RHCP. The ground plane is a circular aluminium plate of 174–232 mm diameter with the helix feed point at the centre.
Parts List — Dipole-Splash Plate FeedMaterials for 23cm dipole and splash plate feed
With 28+ dBi of gain and a 1.9° 3 dB beamwidth (for a 90 cm dish at 1296 MHz), the dish must be pointed very accurately to make any contact. A 1° pointing error at 28 dBi costs approximately 1.5 dB — noticeable. A 2° error costs over 6 dB. For terrestrial contacts, careful manual alignment to a map bearing is usually sufficient — radio towers and hilltop beacons are large targets that tolerate ±5° of pointing error before significant loss occurs. For EME, accurate celestial tracking is essential.
Azimuth and elevation readout
At minimum, install a compass rose and an inclinometer (protractor with a plumb line) on the dish mount to allow repeatable pointing. Commercial TV dish mounts provide azimuth and elevation adjustment screws — these are adequate for terrestrial work. For EME, a motor-driven alt-azimuth or equatorial mount with PC-based tracking software (WSJT-X includes a Moon tracking function) is the standard approach.
Finding the focal point by optimising received signal
A 1296 MHz beacon (listed at beacons.amateur.ch), a local repeater, or a signal generator held by an assistant at a known distance provides the reference. Connect the feed to a wideband receiver or spectrum analyser capable of 1296 MHz.
Slide the feed assembly forward and backward along the feed arm while observing the received signal level. Maximum signal indicates the feed is at the focal point. Note this position and fix permanently. The optimum position should be within 5 mm of the calculated focal distance.
With the feed at the optimal position, slowly rotate the dish in azimuth while recording received signal level vs. bearing angle. The 3 dB beamwidth (points where signal drops 3 dB from maximum) should match the calculated value from the gain calculator. A measured beamwidth significantly wider than calculated indicates feed spillover or a mispositioned feed — both symptoms of being off the focal point.
Most surplus satellite dishes are in good physical condition — the reflector surface is an aluminium pressing with a galvanised or painted steel frame. Before converting to 23 cm use, assess the dish surface for dents, deformation, and surface oxidation. Small dents (under 5 mm depth) in a 90 cm dish cause negligible gain loss at 1296 MHz — the wavelength is large relative to the dent size. Large deformations or visible distortion of the parabolic curve will reduce gain and should be straightened if possible.
Remove the original LNB and LNB bracket arm. On most Sky dishes, the arm is bolted through the dish edge — remove the bolts and save the hardware for reuse. Clean the dish surface with a mild detergent wash. Check all frame bolts for tightness — loose frame joints allow the dish to flex under wind, shifting the feed position and causing pointing and gain variations.
The 1240–1300 MHz band offers a range of operating modes. The primary weak-signal segment around 1296.0–1296.2 MHz uses SSB and CW for terrestrial tropo contacts. Digital modes (FT8, JT65C) have become increasingly popular for marginal-path contacts. EME is active on 1296 MHz with a dedicated segment around 1296.000–1296.025 MHz where JT65C, Q65, and CW are used.
| Segment (MHz) | Mode | Activity |
|---|---|---|
| 1240–1243 | ATV | Amateur television — wideband analogue/digital video |
| 1296.000–1296.025 | EME | Earth-Moon-Earth weak signal — CW, JT65C, Q65 |
| 1296.100 | SSB | International calling frequency — tropo DX |
| 1296.200–1296.500 | SSB/CW | Terrestrial DX activity |
| 1296.500 | FM | FM calling (UK/Europe) |
| 1296.900–1296.975 | Digital | FT8, FT4, WSPR weak-signal digital |
Can any satellite dish be used for 23cm amateur radio?
Most surplus satellite dishes work well at 1296 MHz provided the reflector surface is in good condition. The key requirement is that mesh apertures are smaller than ~23 mm. Standard Sky/Freesat 60–90 cm dishes are ideal — they are high quality, widely available as scrap, and use the offset geometry that is actually better than prime focus for avoiding feed blockage. Larger dishes from motorised systems also work but need more substantial mounts.
How do I find the focal point of an offset dish?
Measure the distance from the dish surface centre to the point where the original LNB input flange was located — this is the focal length. Alternatively, use the formula F = D²/(16d) for a prime focus approximation, or find the focal point experimentally by scanning a known signal source while moving the feed and noting the maximum signal position. For most Sky dishes, the focal length is approximately 280–320 mm.
What power can I run into the dish feed?
The dipole-splash-plate feed handles any power your transmitter can produce — the limiting factor is the SMA connector (rated 0.5 W continuous at 3 GHz for standard types; 50 W at 1296 MHz for quality PTFE-insulated types). Use N-type connectors if running more than 10 W at 1296 MHz. The helix feed can handle similar power levels. The dish itself has no power limit — it is a passive reflector.
What feedline should I use from the dish to the shack?
Use LMR-400 or better for any run over 5 metres. At 1296 MHz, 10 m of LMR-400 loses approximately 2.5 dB — significant but acceptable. RG-213 is not suitable at 1296 MHz (loss ~8 dB per 10 m). Many serious 23 cm operators mount a low-noise preamplifier (LNA) at the dish feed point and use a longer run of LMR-400 or hardline from the LNA to the shack, as the LNA overcomes the feedline loss and dramatically improves receive sensitivity.
How accurately must the dish be pointed for EME?
For EME on 23 cm, the Moon must be within the dish's 3 dB beamwidth — approximately ±1° for a 90 cm dish. The Moon's angular diameter is 0.5° and it moves at about 0.5° per minute, so pointing must be updated every 30–60 seconds for a fixed dish. Most EME operators use motorised mounts with computer tracking software (WSJT-X, MoonTracker, or SatPC32) that automatically track the Moon's position based on the operator's GPS coordinates and time.
What is the difference between a dipole-splash plate and a helix feed?
A dipole-splash plate provides linear polarisation — good for terrestrial SSB/CW contacts where the distant station uses the same polarisation, and simpler to build. A helix feed provides circular polarisation — essential for EME where Faraday rotation randomises the signal polarisation, and useful for satellite work. For a general-purpose terrestrial 23 cm dish station, the dipole-splash plate is the simpler choice. For an EME station, a 3-turn helix feed is strongly recommended.