Enough protection with antenna switch?

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Radioham
Posts: 3
Joined: Wed Jan 31, 2018 11:01 am

Enough protection with antenna switch?

Postby Radioham » Wed Jan 31, 2018 1:41 pm

I have my RSP connected to the same antenna as my transceiver using a Daiwa manual antenna switch. The isolation value of this switch is 60 dB, according to the specs. When I use my transmitter (max. 100 W.) I see an alarming strong signal on SDR Uno: S9+50 dB. I did not expect such a strong signal because the input of the RSP is shorted by the switch. However, it seems that the specs of the switch are correct:
100 W= 50dBm; 50 dBm-60dBm= -10 dBm = S9+50 dB.
In reaction to an earlier -similar- post it was stated by tech support that an antenna switch would offer enough protection since the SDRplay would be isolated.
Still I wonder if that's the case with this particular switch, knowing that 0 dBm is the recommended max (= S9+73 dB).
Last edited by Radioham on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Tech_Support
Posts: 364
Joined: Mon Jun 01, 2015 7:00 pm

Re: Enough protection with antenna switch?

Postby Tech_Support » Wed Jan 31, 2018 2:31 pm

Hello Radioham,
-10 dBm will not present any long-term risk to your RSP. 0 dBm is also a safe level.

Sincerely

SDRplay Tech_Support

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vk3alb
Posts: 28
Joined: Sat Jul 01, 2017 11:43 pm

Re: Enough protection with antenna switch?

Postby vk3alb » Wed Jan 31, 2018 7:27 pm

I think it's important that every user take heed of what radioham has found. Doing a measurement like this put him in control of protecting his own gear and gives him some understanding of his own systems performance.

One thing he could do is add a second changeover relay in the RX leg that switches the RSP to a small 50 ohm load or indeed even to ground. Care must be take if this is done with the bias tee enabled. This would add the isolation of the second relay to the 60dB offered by the Daiwa switch.

Another way might be to install a step attenuator in line with the RX leg and switch in some extra attenuation.

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Lou
VK3ALB

Roger
Posts: 151
Joined: Fri Dec 29, 2017 7:59 pm

Re: Enough protection with antenna switch?

Postby Roger » Wed Jan 31, 2018 10:42 pm

Radioham wrote:I have my RSP connected to the same antenna as my transceiver using a Daiwa manual antenna switch. The isolation value of this switch is 60 dB, according to the specs. When I use my transmitter (max. 100 W.) I see an alarming strong signal on SDR Uno: S9+50 dB. I did not expect such a strong signal because the input of the RSP is shorted by the switch. However, it seems that the specs of the switch are correct:
100 W= 50dBm; 50 dBm-60dBm= -10 dBm = S9+50 dB.
In reaction to an earlier -similar- post it was stated by tech support that an antenna switch would offer enough protection since the SDRplay would be isolated.
Still I wonder if that's the case with this particular switch, knowing that 0 dBm is the recommended max (= S9+73 dB).


I also own a Daiwa manual 2-way switch which I use to switch from the RSP to an Icom transceiver. It has about 70 dB of isolation

100W = 50 dBm; 50 dBm-70 dB = -20 dBm; S9 on most receivers is -73 dBm so -20 dBm is equal to S9 + 53 dB

I am going to build a relay switch box which is activated by the PTT of my Icom. It will use two relays; one to switch the antenna between the Icom and the RSP and another to short the RX input to the RSP during transmit. A protection circuit will also be used to protect the RSP from strong signals from other transceivers in the area. I will need to wire it up with coax or a stripline in order to maintain the 50 ohm impedance. I plan on using a manual toggle switch, instead of an RF splitter, to select the desired radio for receive. Most of the time it will be set to the RSP but when I have a QSO I will flip it to receive on the Icom because of the processing delay you get with an SDR radio.

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Radioham
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Joined: Wed Jan 31, 2018 11:01 am

Re: Enough protection with antenna switch?

Postby Radioham » Thu Feb 01, 2018 3:34 pm

Thanks for the replies to my post. Thanks Roger for correcting the S9-equivalent in dBm.
According to Tech Support I don't have to worry at this signal level but it seems I am not the only one who does (despite this reassurance).

Some more measurements. When I completely disconnect my RSP 1A from the antenna switch, it still gives a S9+28 dB (!) reading on my SDR Uno.
It seems to be related to the proximity of the antenna (about 3 meters). From my 20 dipole (about 20 meters from the house) I still get a S9+8 dB signal into the RSP. So just disconnecting is not enough to protect the RSP completely. These measurements were made with a 10 W carrier.

Of course when you unscrew the RSP from the switch it is no longer connected to ground. Adding a ground connection attenuates the signal to S6 for the 20 m dipole, so 26 dBm. Surprisingly the effect was not so large for the nearby antenna (about 15 dBm).
Still wondering how the RSP could pick up so much without an antenna, I decided to unscrew the small SMA/coax adapter on the RSP. This made a 10 dB difference to the incoming signal. So this adapter also works as an small antenna.

So there is more to protecting the RSP than just disconnecting it from an antenna. It picks up quite some signals, even in its RF-shielded box.
But at the measured levels with my set-up I understand I don't need to worry. I will try not to.

Greetings.
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EA1HET
Posts: 1
Joined: Mon Jan 08, 2018 3:44 am

Re: Enough protection with antenna switch?

Postby EA1HET » Tue Feb 06, 2018 6:32 pm

Hi there,

I got in touch with EA4TX, that is a professional electronics engineer, to discuss this fact. He has a past experience doing something similar for contest stations. Basically, what he has dedigned it's a front end limiter with protections in terms of coax line (passive relays). He's going to publish the final work on his website soon. I know from him that electronics project was sent to production line already, so, we can have another solution to protect our SDRs soon

Hope this helps!

Best regards.
Last edited by EA1HET on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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KD0RC
Posts: 2
Joined: Tue Sep 18, 2018 8:09 pm

Re: Enough protection with antenna switch?

Postby KD0RC » Fri Nov 02, 2018 5:25 am

I just built the Nu6X TR switch from the Aug QST. It limits the RF voltage to 1.8 Vpp to the RSP1, regardless of the input voltage (65 Vpp at 10W from my K2). This, of course, lights up the ADC Overload indicator. I assume that this level of RF is completely safe for the RSP1A. Am I correct in this assumption? What RF Voltage would be the max for safety?
Thanks,
Len
KD0RC
Last edited by KD0RC on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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glovisol
Posts: 172
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Enough protection with antenna switch?

Postby glovisol » Fri Nov 02, 2018 8:34 am

Hi Len,

The RSP play specification for RF input is:

0 dBm - 1 mW - permanent maximum.
+10 dBm - 10 mW - short periods.

0 dBm correspond to an RMS voltage of 0.223 V and to a peak-to-peak voltage of 0.632 V.
+10 dBm correspond to an RMS voltage of 0.707 V and to a peak-to-peak voltage of 2V.

Therefore the 1.8 Vpp you are applying to the receiver is too much for safety and you should consider additional RF attenuation when transmitting.

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Radioham
Posts: 3
Joined: Wed Jan 31, 2018 11:01 am

Re: Enough protection with antenna switch?

Postby Radioham » Fri Nov 02, 2018 12:22 pm

Just a short follow-up. I have been using the set-up described in my first post (with the manual switch) now for 9 months without any problems. So indeed there is enough protection with that antenna switch.

73.
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KD0RC
Posts: 2
Joined: Tue Sep 18, 2018 8:09 pm

Re: Enough protection with antenna switch?

Postby KD0RC » Sun Nov 04, 2018 11:35 pm

OK, I figured out what is going on... I built this TR switch because the author of the article specifically called out the RSP1. I read on the QST In Depth page that he designed it for approximately +5dBm output - DOH!!! He mentioned that a 0dBm unit could be built using Schottky diodes, but the ones he had in mind are only available in surface mount so he went with 1N4148s... I ordered the Schottkys and will just tag short leads to them so they fit the board. In the meantime, I lowered the gain of the RF amp in the TR switch to give 1.2 Vpp out and added a 6dB attenuator between the switch and the RSP1A so that I should be around .6 Vpp (just under 0 dBm) into the RSP. I will report back when I get the parts in and tested.

Thanks for your quick and precise response to my question, I really appreciate it!

73,
Len
KD0RC
Last edited by KD0RC on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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