Construction of the
multiband Fuchs antenna

multiband Fuchs circuit These multiband antenna was designed by Frank Lamprecht, DL7AQT, and first published in the QRP-Report [1]. The Fuchs Antenna got its name from the Austrian radio amateur named Fuchs, who first decribed it in 1928. It was a monoband endfeed half wave dipole. The length of the antenna should be ½ λ or a multiple of it. For 3.5 MHz you need about 41 m. But you can match these wire lenght with Frank's development also on all others bands between 10 and 80 m!
 
The following both tables shows the length you need for a given number of halfwaves (½ λ) per band. The length of about 41 m is a good compromise. With the Fuchs circuit it is possible to use these 41 m long wire on all bands between 10 and 80 m.
 
  3,550 MHz ½ λ = 42,25 m ≈ 1 · ½ λ at 41 m
  7,025 MHz ½ λ = 21,35 m ≈ 2 · ½ λ at 41 m
10,125 MHz ½ λ = 14,81 m ≈ 3 · ½ λ at 41 m
14,050 MHz ½ λ = 10,68 m ≈ 4 · ½ λ at 41 m
18,080 MHz ½ λ =   8,30 m ≈ 5 · ½ λ at 41 m
21,050 MHz ½ λ =   7,13 m ≈ 6 · ½ λ at 41 m
24,900 MHz ½ λ =   6,02 m ≈ 7 · ½ λ at 41 m
28,050 MHz ½ λ =   5,35 m ≈ 8 · ½ λ at 41 m
 
With the Fuchs circuit it is also possible to use an 21 m long wire on bands between 10 and 40 m (without WARC bands).
 
  7,025 MHz ½ λ = 21,35 m ≈ 1 · ½ λ at 21 m
14,050 MHz ½ λ = 10,68 m ≈ 2 · ½ λ at 21 m
21,050 MHz ½ λ =   7,13 m ≈ 3 · ½ λ at 21 m
28,050 MHz ½ λ =   5,35 m ≈ 4 · ½ λ at 21 m
 
Tuning is very easy. The first step is, to find with the circuit the point of maximum noise (loudest signal) in your receiver. Use the taps with the maximal L/C ratio (more L = higher Q). Now with a small transmitter signal, the coil link is switched to lowest SWR. Switch the coupling section of the Fuchs circuit to get the brightest signal at the LED RF detector (or lowest SWR if your transmitter has a built in SWR Meter). Ready! The Fuchs circuit is equipped with an output indicator. The LED in the indicator glows only at the point of resonance.
 
Is there a kit for this antenna? Sure! The QRPproject sells a kit named "Fuchs 80-10" with all parts and an english manual. The circuit fits into a 7 × 5 × 2.5 cm plastic enclosure. A BNC plug plus a BNC to BNC connector gives the flexibility to use it with any rig which has a BNC antenna jack.
 
[1] Frank Lamprecht, DL7AQT: Fuchs-Antennen für den Multibandbetrieb. QRP-Report 1/2002, S. 13-14