tirsdag den 27. august 2019

Lower SWR on Helix13 from Wimo

The QO-100 satellite uplink is in the 13-cm band. I'm using a 21-turns helix antenna (Helix13 from Wimo) for uplink antenna to the NB transponder. This RHCP helix antenna works fine except for bad SWR.

Fig. 1. Helix13 antenna from Wimo (picture from 2009).

Fig. 2. Remove this alu-tube.

First I removed the alu-tube from the antenne. This tube is intended for impedance matching. It is not needed any more, since I'll use a different matching method.

An N-connector is fitted where the alu-tube was before. You must drill holes in the helix reflector for the flange of the N-connector.

Fig. 3. Dimensions of the impedance matching plate.

I made an impedance matching plate for the antenna. The plate is cut from thin copper sheet. The length is 1/4 turn and it is mounted above the helix reflector (see figure 6). This matching plate converts the helix impedance to 50 ohms.
Fig. 4. Purpose of the holes.

The impedance matching plate extends the helix wire. The plate is soldered to the N-connector's center pin (green hole). The red hole is used for fastening the alu wire. You can use a short bolt for this. I soldered the helix wire to the matching plate using a short brass tube. Before soldering, the brass tube was pressed with a pipe wrench to make it fit the helix alu-wire.

Fig. 5. The new N connector.

Fig. 6. The impedance matching plate.

Fig. 7. Click picture to enlarge.  

Notes to fig. 7: Distance A is 3 mm above the reflector. Distance B is 7 mm above the reflector.

Fig. 8. Measuring SWR.

The graph in fig. 8 begins at 2.0 GHz and ends at 2.7 GHz. The marker (yellow triangle) is at 2.4 GHz.

The SWR at 2.4 GHz was measured to 1.08 which is an excellent value. All SWR values between 2.0 GHz and 2.55 GHz were below 1.2.

My antenna analyzer is a hand-held instrument N2201. It covers 137.5 MHz to 2.7 GHz. The N2201 has a smaller brother: N1201. The difference is that N2201 is equipped with an additional power meter. The less expensive N1201 would be sufficient for this SWR adjustment.

Vy 73 from OZ1BXM Lars
Webpage: oz1bxm.dk

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