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Circuit and Partlist RF-Bridge


Jim, N2ADR programmed the outstanding feature to use the HiQSDR as VNA (Vector Network Analyzer).
With this feature you can measure in thru-mode filters, amplifiers a.s.o and in reflective mode antennas, impedances and impedance matchings and much more.

RF-Bridge complete


To use the thru-mode, you have to use just a attenuator in the range of 6..25dB not to overload the HiQSDR RX input, the device under test (DUT) located between TX-output and RX-input.
To use the reflection mode, you must additionally use a directional device like a directional coupler or (better) a resistive RF bridge.
The RF bridge is nothing new - the principle is well known for many years and features wide bandwidth, very good directional properties (>42dB from 1MHz to 1GHz in this case) and simple construction. BUT: it is not usable with power.

To measure impedances, VSWR, antenna matching or behaviour of antenna properties or other devices, power is not needed, so the first choice is a precise resistive bridge.
The operation frequency ranges from approx. 1MHz flat and wide up to GHz range with a directional factor of >40dB.

Only the coupling to the measuring RX (speak the HiQSDR RX) is a little bit a challenge. The diagonal signal of the RF bridge is symmetric and this symmetry must under no circumstances de disturbed to be precise.
On the other hand, the RX input of the HiQSDR is a asymmetric 50Ohm coaxial input.
To convert from symmetric to asymmetric, a balun can be used.
To guarantee a flat frequency response over a wide range, special care must be taken.
The used balun is a ferrite loaded coaxial balun, well known for years too and often used as mantle current blocking device in antenna systems.

To ease the building of the RF Bridge, i (DB1CC) made a small PCB wich is divided in two parts: the bridge part wich holds the bridge resistors and three SMA-connectors for reference Port (50Ohm Load) the RF input and the connection to the device under test.
The second part holds a connector to HiQSDR RX and an (optinal) pi-type attenuator of 6dB to protect the RX input.
Both PCB parts must be soldered into a shielding case, the ferrite loaded coaxial balun and its compensation line (a seconds coaxial cable wich has only the shield used) are soldered between these two pcb parts.
Over the two coaxial cables, 22 multi-aperture (ribbed) cores are placed.

RF-Bridge PCB

Complete Circuit overview

Complete Circuit of HiQSDR RF-Bridge from Helmut DB1CC HiQ RF Bridge Circuit

RF-Bridge circuit


Count Manufactor Manufactor Part No. Case Distributor Part No. Distributor
4 Diverse SMA, LP1.57mm LP 1.57mm 526-5785 RS-Components
4 Diverse 100E 1% 805 Diverse
2 Diverse 75E 1% 805 Diverse
2 Diverse 150E 1% 805 Diverse
22 Würth Ribbed Ferrite Core 7427051 Würth
2 Diverse sleeveless (tinned) coax RG178 Diverse
1 Technologie 2000 HiQ-RF-Board Technologie 2000


  • Datasheet Ribbed Ferrite Core
Ribbed Ferrit Core