EMRFD Message Archive 13592
Message Date From Subject 13592 2017-02-16 04:25:35 Baris Resistive bridge BarisRegards,Hello,I'm trying to build the resistive bridge in Fig 7.43 on page 7.24. I've seen couple of references to this circuit on the web (like http://www.vk2zay.net/article/97) but the value for R1 and R2 confused me, are they really 62 ohms? Anyone used this bridge up in the 144 MHz region? 13593 2017-02-16 07:36:39 John Marshall Re: Resistive bridge Boris,
The bridge works by comparing the impedance you are measuring to a "Known Termination". Ideally, R1 and R2 should be the same as the Known Termination, but 62 ohms allows good performance with either 50 or 75 ohm Knowns. Rather than a specific value, it's much more important for these two resistors to be matched as closely as possible and for the Known and Unknown arms of the bridge to be symmetrical. I built one years ago using 1/4 watt resistors that works at 432 MHz.
13594 2017-02-16 12:19:58 E-P Mänd VS: [emrfd] Resistive bridge
Hello Baris: It is a bridge, resistor ratios are important, not values. So = R1 / Known = must be the same as R2 / Unknown. R1, R2 = 68 ohms is good for either 50 ohms system or 75 ohms system, don’t worry. I’ve constructed similar bridges, goes up to 1 GHz with short connections and proper lines (CoAx, CoPlanar or stripline) to connectors (BNC or SMA), but use SMD resistor and capasitors only. Even better, if you are interested in only 50 ohms system, use R1 = R2 = 50 ohms and add third 50 ohms resistor to make a three-port divider to keep all impedances same. – Kindly, EePee = OH2NFI, member of ARRL, too.
13595 2017-02-16 12:54:11 Tayloe, Dan (Noki... Re: Resistive bridge
In the bridge, there are three resistors in 3 out of the four legs, with the 4th leg being the antenna under test.
If the three resistors are 50 ohms and the antenna is 50 ohms, then one side of the “H” bridge looks like 100 ohms (50 + 50) and the other side looks like 100 ohms (also 50+50). Two 100 ohm resistors in parallel is 50 ohms.
Thus, the transmitter sees 50 ohms if the antenna is 50 ohms. If the antenna is very high impedance (i.e., “open), then the transmitter just sees (50 + 50) in parallel with (50 + very large) or just 100 ohms.
If the antenna is shorted, the transmitter sees (50+50) in one leg and (50+0) in the other. The net impedance is 33.3 ohm.
If you are measuring 50 ohm antennas, use all three resistors as 50 Ohm, and that will serve to protect the transmitter by limiting the load variation seen (33.3 to 100 ohm worst case).
From a measurement point of view, all you need is to have the two resistors in the one leg be the same, and the resistor in the second leg be the same as your target antenna impedance (i.e., the forth “resistor” in the bridge).
- Dan, N7VE