4
$\begingroup$

I have a Daiwa CS-201A two position switch. The single side is connected to by quarter wave vertical (40m). The A and B sides are connected to an IC-7300 and TS-830S.

The switch is claimed to have 60dB of isolation. It also shorts the unused port to ground.

If I transmit 100W (50dBm) from the TS-830S to the antenna, am I correct in that about -10dBm will leak to the other side into the IC-7300?

I tried it and saw about +40 over S9 on the 7300. The service manual for the 7300 says never give it a signal over 20dBm (0.1W).

Am I safe doing this? How can I tell if it’s too much?

UPDATE: I did some testing with an Elecraft XG3 signal source. It produces signals at -107dBm (1uV), -73dBm (S9), -33dBm (S9+40), and 0dBm.

Following the instructions using the 1uV signal and a DMM on the speaker output, the MDS is around -135dBm and -142dBm with the preamp off and set to 1, respectively.

Also, the -73dBm signal is showing just shy of S9 on the S meter. That worried me a little, but I saw in the August 2016 QST magazine article that the ARRL test showed an S9 with a 70uV signal. So there's an error of about 3dB (which is also the S unit increment on this rig's meter).

So I think my receiver is still okay.

I also tested the isolation of the switch by hooking the 7300 to port A, the signal generator to port B, and a dummy load to the common port. With switch in the B position (allowing a direct path from the signal generator to the dummy load), I set the signal output to 0dBm and noted the S meter deflection on the 7300. It was around S5.

I don't know exactly what signal level that is, but it's less that what it read when I fed -73dBm directly into it. So even if we overestimate the received signal at -73dBm, that's still at least 73dB of isolation- if I'm thinking this through correctly. This was done on at a frequency of 7.020MHz. I think the switch claims an isolation of 60dB at 600MHz. It appears the isolation improves as you go down in frequency.

Added a bounty for feedback on the tests and the data. Is it really possible for the switch to have that good of isolation? Is there anything else I should look into? Do the tests seem valid? Am I in the clear running two rigs on this switch?

$\endgroup$
  • $\begingroup$ Manual says no. What makes you think it's wrong? $\endgroup$ – Zeiss Ikon Mar 26 at 11:49
  • 1
    $\begingroup$ Respectfully, manual doesn't say that. $\endgroup$ – Aleksander Alekseev - R2AUK Mar 26 at 12:33
  • $\begingroup$ @AleksanderAlekseev-R2AUK "The service manual for the 7300 says never give it a signal over 20dBm (0.1W)." Seems to me +40 over S9 exceeds the manual limitation as quoted by the questioner. $\endgroup$ – Zeiss Ikon Mar 26 at 13:20
  • 5
    $\begingroup$ S9 is -73dBm. +40 over S9 is therefore -73 + 40 = -33dBm. The manual says nothing over 20dBm. That's like +93 over S9. Can you imagine if a receiver couldn't handle anything over S9+20? All it would take is someone in the next zip code over to turn on their amplifier. They could remotely let the smoke out. $\endgroup$ – Paul Mar 26 at 14:09
  • 1
    $\begingroup$ For the bounty, could you perhaps say a little more about what you're looking for, especially as there is currently an accepted answer? $\endgroup$ – Kevin Reid AG6YO Apr 16 at 20:04
4
$\begingroup$

S9 corresponds to 50 microvolts or -73 dBm. Thus S9+40 is -33 dBm. In theory, since it's much less then 20dBm your 7300 should be fine. HOWEVER I believe that +40 is just a maximum level your transceiver can show. Given that the attenuation of your switch is 60 dB and you are using 100W (50 dBm), -10 dBm will leak. It's much more than -33 dBm but you are still in the safe range.

From personal experience I used a DIY switch with measured 58.7 dB attenuation in the same manner with a 100W transceiver and an RTL-SDR, which maximum input is 10 dBm. RTL-SDR showed a large signal level as well, but worked just fine.

This being said I wouldn't recommend to trust the manufacturers claim regarding the attenuation level of the switch. It's always a good idea to measure it using a low-power signal generator (if you don't have one you can solder a simple Clapp oscillator) and an oscilloscope (or your transceiver).

$\endgroup$
  • $\begingroup$ Yeah -33dBm suggests even better isolation. I doubt that. Maybe the S meter isn't all that accurate (are they ever?). Also the max scale on this radio's S meter is +60. But maybe it's software capped to 40? Is there any way to tell if there's damage to the front end? $\endgroup$ – Paul Mar 26 at 13:04
  • 1
    $\begingroup$ It is not unusual to have a poorly calibrated / designed S meter. For instance, it may show an accurate S9 value but everything above and below is not very accurate. You can easily check it by generating X dBm value (generated / measured using some device you trust) and comparing it with S meter reading. If you radio receives signals and sounds OK I wouldn't worry that it actually suffered the damage. Just in case you can examine the look of the components to make sure nothing is damaged. $\endgroup$ – Aleksander Alekseev - R2AUK Mar 27 at 7:03
  • $\begingroup$ I updated my question with some details on tests I performed with the rig. $\endgroup$ – Paul Apr 15 at 15:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.