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I'm planning to connect an SDR in 'parallel' to a modem used to transmit and receive a radio signal on 2.4/5.8 GHz.

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The SDR that I want to use is the HackRF one, which can receive up to -5 dBm in input (or +10 dBm if an amplifier is disabled). The modem I'm using is set to output 30 dBm (1 W) while transmitting, so I can't directly connect the SDR, the antenna and the modem on the same feed. Is a circulator sufficient to solve my problem?

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Would it be best to use a RF limiter?

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Another problem I will have is that slitting the signal from the antenna means losing half of the power so I will have to use an active amplifier to compensate, are amplifiers that amplify only in one direction even a thing?

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  • \$\begingroup\$ From the perspective of the antenna, both the SDR and the modem represent 50 Ohm loads, do they not? \$\endgroup\$
    – user57037
    Commented Jan 3, 2023 at 17:34
  • \$\begingroup\$ Describe the use of modem and SDR, more Infos needed \$\endgroup\$
    – Antonio51
    Commented Jan 3, 2023 at 20:08
  • \$\begingroup\$ @mkeith yes that's the idea \$\endgroup\$
    – TheRealBen
    Commented Jan 4, 2023 at 8:47
  • \$\begingroup\$ @Antonio51 the SDR will have to measure the RSSI of the incoming signal and possibly also log it. The modem has to receive the signal for most of the time and once every second it has to transmit a small amount of information. \$\endgroup\$
    – TheRealBen
    Commented Jan 4, 2023 at 8:51

2 Answers 2

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A circulator is not suitable because it will only couple received RF from the antenna to the SDR -- not to the modem, which has to both receive and transmit.

If you're not trying to use the SDR anywhere near the two modem frequency bands, a frequency-selective diplexer would work, but I don't think that's a readily available component and would have to be custom designed.

If you have a logic signal that corresponds to when the modem transmits, and don't mind having the SDR blanked during transmit intervals, you could use a simple RF switch.

It may be simpler to abandon the single antenna / single feedline configuration in favor of separate antennas, especially if you want to use the SDR at frequencies away from the 2.4 / 5 GHz bands of the modem.

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  • \$\begingroup\$ I need the modem and the SDR on the same frequency, this will be an antenna pointing system (using step tracking). I need the SDR to precisely read the signal power and possibly to log the signal. I don't like the double antenna solution because I'm worried about how precisely they would need to be aligned. The antenna that I will be using is going to have a 25dBi+ gain so a very narrow beam. \$\endgroup\$
    – TheRealBen
    Commented Jan 4, 2023 at 8:42
  • \$\begingroup\$ @TheRealBen That extra info helps. Yes a single antenna is best. Can you get an RSSI value from the modem firmware? If not, then an RF switch is your best bet. \$\endgroup\$
    – Mark Leavitt
    Commented Jan 4, 2023 at 17:15
  • \$\begingroup\$ I can get the RSSI from the modem but I'm worried about the precision and the refresh rate of it, in the datasheet is not specified anywhere. I was looking into terminated vs non-terminated switches, as far as I understand, terminated switches absorb as much power as the antenna, thus the antenna is receiving half of the power that the modem transmits, instead non-terminated switches reflect the power back into the modem, with the risk of damaging it, am I correct? \$\endgroup\$
    – TheRealBen
    Commented Jan 5, 2023 at 16:56
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    \$\begingroup\$ Use a nonterminated switch to interrupt the feedline to the SDR, as close to the feed junction as possible. The switch should open only during transmit intervals of the modem, and there will be a good 50 ohm match for the xmtr. During receive, you're going to end up with a 3 dB loss due to splitting of the signal and impedance mismatch. The only way to avoid this is to forget the SDR and use the modem's RSSI, which I'd suggest trying first. \$\endgroup\$
    – Mark Leavitt
    Commented Jan 5, 2023 at 17:31
  • \$\begingroup\$ Seconding this, your modem's RSSI is probably calculated on some "quality of reception", something more relevant to operation than actual receive power. You could tell your power meter (you're not making much use of the SDR as SDR, so I'm going to call it a power meter, not a radio) that it has fantastic reception just by having an interfering device nearby, doing heavy crosstalk. But that wouldn't increase receive signal quality for the modem. \$\endgroup\$
    – Marcus Müller
    Commented Jan 24, 2023 at 17:40
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If the modem works at only one frequency,
a transmission line lambda/4 and 2 diodes would be "sufficient".

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