RSP2 RF Protection Question

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W6JHB
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RSP2 RF Protection Question

Post by W6JHB » Fri Dec 22, 2017 6:47 pm

I'm awaiting delivery of my RSP2 - first time user here, although I have played around with an RTL-SDR dongle a little.

I have a ham radio transceiver (Elecraft K3) that I can operate/transmit CW, SSB, and FM on the 144 MHz band (2 meters). I want to use the RSP2 for (among other things) a receiver for the 440 MHz (70 cm) band. This would allow me to communicate through some of the ham satellites. To do so requires two antennas - one for 2 meters to transmit and one for 70 cm to receive. Since the satellites move, the antennas need to track them. Most folks use AZ-EL rotators of one sort or another to keep the antennas pointed at the satellite. This means that the two antennas would be only a meter or so apart, on the same non-conductive boom. It has been quite a few years since I've used satellite communications, but as I recall, one needs to be able to receive at the same time as one transmits (full duplex) so you can monitor your signal coming down from the satellite.

With the receive and transmit frequencies being so far apart, is there a chance of vaporizing the RSP2 when the ham rig transmits it's signal? I'll be running a max of about 75 watts of RF power output.
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Jim Bennett / W6JHB
Folsom, CA

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Re: RSP2 RF Protection Question

Post by Tech_Support » Fri Dec 22, 2017 9:45 pm

Hello Jim,
There is a strong chance that you could damage your RSP2. The separation in frequencies will not help you as it is RF power at the front end of your RSP2 that is the issue. Any more than +10 dBm (10mW) for short durations could end up damaging your device.
If you really need full duplex operation. you are likely to need a mast-head duplex filter to prevent RF energy from your 2m transmitter from damaging your RSP. This will need to take the form of a 2m notch filter or a 70cm band-pass filter. Whilst only a fraction of your transmit power will reach your receiver, I would recommend a filter that gives you around 40 dB of rejection at the 2m band.

If in doubt, measure the power present at your receive antenna using a power meter with no RSP connected.

Sincerely

SDRplay Tech_Support

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W6JHB
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Re: RSP2 RF Protection Question

Post by W6JHB » Sat Dec 23, 2017 8:52 pm

Hi - thanks for the reply and the info.

Not what I was hoping to hear, indeed. Placing a filter in the line would then preclude my being able to do the reverse - listening at all on 2 meters. Additionally, there are satellites with the uplink on 70 cm and downlink on 2 meters. I'll have to give some thought about how to resolve this one. While having spent close to $200 for this unit still gives me a great wide-coverage receiver, it seems like incorporating it into any sort of satellite communications setup is more problematic than I had anticipated. Sighhhh....... :(
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Jim Bennett / W6JHB
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vk3alb
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Re: RSP2 RF Protection Question

Post by vk3alb » Sun Dec 24, 2017 9:38 pm

Hi Jim,

I think you first need to assure yourself that your scenario won't blow out your RSP.

In this situation +10dBm is a HUGE signal and if S9 = -93dBm then according to support S9 + 103dB is the absolute maximum signal level you should ever measure at the input to the RSP.

If you don't have measuring gear you could get some sma or bnc style attenuators off ebay and stick them in line with the RSP starting with say 50dB (20 + 20 + 10) and do a couple of test transmissions. Try using say 1 W or 5W in the first instance until you get a feel for the levels.

This should at least tell you the scale of your problem and give you a starting point for designing a solution.

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Re: RSP2 RF Protection Question

Post by Tech_Support » Sun Dec 24, 2017 10:48 pm

That's an excellent suggestion Lou.

Sincerely

SDRplay Tech_Support

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vk3alb
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Re: RSP2 RF Protection Question

Post by vk3alb » Sun Dec 24, 2017 11:24 pm

Thanks Support,

It strikes me that 75W is quite a bit of power for satellite work but I don't know Jim's situation. I suggested low power TX because that will offer Jim nearly 19dB of extra protection during testing.

Assuming no losses in the system(s)

1W = 30dBm add 50dB attenuation = -20dBm = a safe testing regime to determine the coupling between the two systems.

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W6JHB
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Re: RSP2 RF Protection Question

Post by W6JHB » Mon Dec 25, 2017 11:34 pm

Hi Lou -

Well, yes, 75 watts is a lot if one is simply trying to work the LEO (Low Earth Orbit) birds - in many cases (from what I've read) 5 watts from an HT is sufficient if one is using even a hand-held directional antenna. However, as I recall (from many, many years ago) from working the satellites in highly elliptical orbits, 75 watts is on the cusp of not making contact. But at the moment I'm just trying to get the RSP2 installed, THEN see if I can copy any stations' downlink before I even attempt making a contact. Note that I said "trying" to get it installed. Can't get the device recognized by the OS. Fired a message to support but they are closed for the holiday. Going to post a question here on the forum in a few minutes.

73, Jim
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Jim Bennett / W6JHB
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Roger
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Re: RSP2 RF Protection Question

Post by Roger » Fri Dec 29, 2017 8:27 pm

Jim,

I can't imagine that two antennas that close will be able to operate in full duplex without a good RF filter on the receive side. Just too much RF coming into the front end of any receiver. With any broadband SDR, like the SDRPlay, you will still want some front end protection. If you want to operate half duplex you make the problem much simpler. You can use a RF relay switch like the ones made by Dow Key (about $30 to $100 on ebay) or you can buy a PTT activated switch from DX Engineering, Elad or MFJ. There are also designs on the Internet if you want to make your own.

For full or half duplex you might want to consider a Front End Protection box. Several companies make them but they only go up to 150 MHz. That might be because of insertion loss so you would have to contact the vendors. Maybe you will have to build your own. There are many designs on the Internet. Here are some links to the commercial vendors...

https://www.arraysolutions.com/as-rxfep
https://www.dxengineering.com/parts/dxe-rg-5000
http://www.crosscountrywireless.net/rec ... n_unit.htm
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OH2BUA
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Re: RSP2 RF Protection Question

Post by OH2BUA » Sun Dec 31, 2017 11:45 am

Dear Tech_Support,

RSP2 does have three antenna inputs, which is a really great thing.

Are they all equally vulnerable for high RF levels - or just the 'selected' one?

73, Jukka

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jon
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Re: RSP2 RF Protection Question

Post by jon » Fri Jan 05, 2018 2:47 pm

They are all equally vulnerable. it's important all antenna inputs are not exposed to high levels of RF at any frequency. We recommend 0dBM continuous or 10dBm for short periods maximum.

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