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I got an RF generator R&S HAMEG HM8135 that I use in a testsystem to generate FM and AM stimuli in order to check if FM antenna and AM antenna is correctly mounted on the testobject in a production line. Therefore I do transmitt with as low RF power as possible. Due to spacing and production considerations, I can only have one common antenna place quite close to the test object.

I do use a Schwartzbeck HFRA 5155 TX Loop antenna covering 100 kHz to 300 MHz, which will work for both FM and AM band range. HFRA 5155

However, if I look in the datasheet, I can see that the impedance around 100 MHz is about 50 ohm, but for 1MHz, it is only about 3 ohm. My test system works (I need so weak AM signal to detect if antenna is not solded correctly, but needs to run the RF generator at about 0 dBm), but my concern is that since I do have so low impedance at 1 MHz (AM test), there must be a lot of power reflecting back into the RF generator (R&S HAMEG HM8135).

Questions:

  1. What impact does the low impedance at 3 ohms for 1 MHz have on my RF generator? Can it be damaged?
  2. Do I need to add anything in order to protect the generator? I do not want to change the RF setup like using two antennas or switching. Just something simple.
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  • \$\begingroup\$ Look up the spec/datasheet of your generator, it will tell you the minimal impedance that may be connected. \$\endgroup\$ – PlasmaHH Feb 17 '16 at 20:18
  • \$\begingroup\$ At what frequencies are you going to be generating the test signals signals? Also looking at the data sheet this does not appear to be an meant for RF applications. \$\endgroup\$ – vini_i Feb 17 '16 at 21:19
  • \$\begingroup\$ Can't find any minimum impedance in the spec for the RF generator. \$\endgroup\$ – MattiasE Feb 17 '16 at 21:20
  • \$\begingroup\$ AM test frequency is 1 MHz (imp 3 ohm) with 0 dBm and FM test frequency is 100 MHz (50 ohm) with -30 dBm. Would it be better to put like 25-50 ohm resistor in series close to the antenna? I could perhaps accept more missmatch for the FM test in order to improve AM testing matching. \$\endgroup\$ – MattiasE Feb 17 '16 at 21:22
  • \$\begingroup\$ That degree of mismatch will change your signal level accuracy, which may impact the validity of your test. So you should consider matching issues for that. However, see my answer below, there is no signal generator safety issue from mismatch, by a long long way. \$\endgroup\$ – Neil_UK Feb 18 '16 at 9:18
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You need not be worried.

Commercial generators like that are designed to be run on the bench. That means it is assumed they will be left running, at max power output, with no output cable connected (open circuit reflects just as much power as short circuit!). Under those circumstances, 100% of the output power is reflected back into the generator. They just would not survive in a lab if they were less robust than that. Your antenna will not reflect more than 100% of the incident power.

That generator has a maximum output power of +13dBm. That almost certainly means that when programmed to 0dBm output level, it will switch an output attenuator in there, and be running its output amplifier at a higher power. The attenuator will absorb the bulk of the reflected power.

If you read the data sheet, it says it is protected against reverse power applied to the output of up to 1 watt, +30dBm. The reason for that is sometimes, idiots in the lab, when testing the receive side of a radio transceiver, would key up the transmitter. Ooops! Again, all lab generators have an element of protection like this so they last beyond the first week of use.

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