Antenna measurement using Leader LDM-815 Dipmeter
Connect the RF bridge to the RF-OUT connector on the dipmeter. Adjust the Dipmeter to the expected measurement frequency and set the needle somewhere in the middle. Then, adjust the 'Sensitivity' control on the RF Bridge to have Full Scale deflection on the meter.
Now connect the antenna to the Zx port on the RF Bridge. Connect the antenna withouth the use of (long) transmission lines. Adjust the dipmeter frequence to where the needle on the RF bridge has the lowest value.. This is the frequency where the antenna impedance most closely resembles 50 Ohm. Now replace the antenne by a 75 Ohm terminator and adjust the meter on the RF Bidge to indicate an SWR of 1:1.5. Connect te antenna to the Zx port again and read the SWR. For the J-pole Antenna from the graph below, optimum SWR 1:1.3 occurs at 144,2MHz.
Indicated SWR with open Zx port and with J-Pole antenna connected
Coax cable in between the SWR meter and the antenna can influence the obtained SWR value. Use a cable that is much shorter than a quarter wavelength at the measurement frequency. For RG-58 (v=0,66) and the Leader LDM-815 (f^ = 250MHz) it is always safe to use a piece of 15cm coax.
Antenna measurement with the homebrew RF impedance bridge.
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Disclaimer I will not be responsible for damage to equipment, your ego, personal injury or worse that may result from the use of this material and material found on any links on my pages. You are responsible to make sure that your use any of my designs is legal in your country. |
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Copyright: Erwin Gijzen |
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Version: Oct-2007 |
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