Seeking Your Interest In RoadTesting Ethernet or SMA Eval Kits

While I'm interested, I don't have the skill set to do what I'm thinking - so I picked "not interested".  HOWEVER, for someone that does have the skill... 

My scenario is the USB control connection between a HF radio and laptop computer.  The setup uses a SDRUno and the laptop runs SDRPlay software.  The radio is a Kenwood TS-590.  The laptop screen is projected onto a larger screen so more people (ideally visitors) can see what is going on.  If we use a cheap USB cable to connect the radio to the laptop, the software crashes when we key the microphone.  The USB cable that works is higher quality and has ferrite chokes on each end.  With optical isolation, would the quality of the USB cable matter?
Just thinking out loud. 

Hi Mike,

Isolation would definitely help. I'm wondering, if the root cause could be absence of a balun on the antenna end of the SDRuno. It could be worth upgrading the balun (or string a load of ferrites on that antenna cable). Otherwise, ferrites on the USB cable at a minimum would be needed (might be worth adding a couple more, just to be on the safe side).

Hi Mike,

Isolation would definitely help. I'm wondering, if the root cause could be absence of a balun on the antenna end of the SDRuno. It could be worth upgrading the balun (or string a load of ferrites on that antenna cable). Otherwise, ferrites on the USB cable at a minimum would be needed (might be worth adding a couple more, just to be on the safe side).

Hi shabaz .  I agree that my problem has to be something like common mode current.  I tried using it today with the same setup that worked back in January and it's not working.

The SDRUno actually gets its input from an output on the Kenwood.  I've got a 6" RCA to SMA cable from the radio to the SDRUno.  Maybe I need a longer jumper to move the whole system away from the radios.  There IS a chance that the last time it worked, I did not have the VHF radio turned on.  That could be my nemesis today.  Our State is having their QSO Party today so I just let the two guys that wanted to use the station go crazy.

I'll take a crack at troubleshooting it a bit later in the contest.  I do have an RCA to something homemade cable that I can ferrite the dickens out of and adapt to SMA.  My only 1:1 balun is SO-239 to wire.

I was thinking about it a bit more over dinner, and came to a similar thought! that maybe for demo purposes you're directly connecting, perhaps with an attenuator in-line? Sometimes the cables used can leak a bit, so it could be worth trying a different cable in case there's an issue there (e.g. double-shielded, or semi-rigid could be options), and passed through some ferrite cores. This one would be very good for HF: 431167281. (it has 10mm hole size, so the coax could be looped through it a couple of time; would be even better to use at least two of them, so 4 loops).

     7mm, but I've got a few.  Great fit for RG8X.  Unfortunately, this is RG59/U - but it will have to do for now.

I'll know in a couple of hours.

The problem wound up being in the USB providing rig control.  This cable already has a large ferrite with the USB looping through it 5 times.  Additional ferrites did not solve the problem BUT moving the USB from the 3.0 plug on the radio side to the USB 3.0 plug on the opposite side allowed it to work.  Better shielding on that side?  I still have some RF shielding I bought a while back.  Maybe I'll give that a try..

Hi Mike, 

Interesting! Glad you're finding what works. Also, just curious, is the box an attenuator?

Depends how it's built of course, if it's super-high attenuation in there (e.g. 30dB or more) the RF might be coupling unexpectedly despite the attenuation.

I found that out when trying to attenuate by 100dB or something (I wanted to do a similar thing, feed a transmitter into a radio receiver), and ended up using separate 20dB and 10dB attenuators (each with their individual metal cylinders) all chained up, and it was totally impractical admittedly! Partitioning inside the enclosure (e.g. walls of copper-clad PCB) could be another idea.

Recently I found this nice thing on eBay (but it's not high power, so would still need a higher-power attenuator to front it):

I've not tried it out yet! Hoping it works. 

EDIT: This would be another way to do it; using a coupler to get less of the signal (and pass the rest into a large dummy load), but this one only offers 15dB reduction, and only supports 4W. Maybe there are higher-power DIY versions suitable for HF use, but I've never checked to see.

Hi shabaz .  The box is just a lazy man's way of terminating a SO-239 connector.  The bolt on the side is because I gave up trying to land the shield on the aluminum.  It's just a ring ternimal, bolt and nut.  I bought a number of the bodies thinking to build attenuators but they turned out rather poor.  Right now, I only use attenuators for radio direction finding - so I purchased a few and made one out of strip board that attenuates 46dB.  The calculator says 116dB but the coupling says... "I don't think so." 

I've been experimenting on and off with trying to attenuate 500mW to 750mW signals.  I have intentionally bought and made inefficient antennas.  You would be surprised how effective a 3-stack of 30dB attenuators works as an antenna.  Wouldn't a coupling loop near the dummy load act like a potential transformer?  It would be hard to get accurate level readings without plotting a bunch of known values.  I may have to try that out.

I've had that same experience.. trying a high-value attenuation and getting nowhere near it! 

I was wondering how to seal each stage, in the easiest-to-assemble way I could think of. I sketched the following, it still needs work, and might not be the best way.

If (say) it was a 4-stage (where each stage shouldn't be more than about 25 dB) then that's 100dB of attenuation if desired.

The idea being that each section could be boxed with a screening can. The resistors (or at least some of them) would be large (e.g. 1210 sized) SMD, so that they could dissipate some power.

Once the cans are soldered (with a high-power iron), the adjacent cans joining each stage would need to be bridged with solder too, to prevent RF finding a way to the outside world between the stages.

The cans are cheap in quantities of 10. Slightly taller versions would be better, but this is all I could find and might be OK:

The SMA connectors could be soldered on the other side, and then solder used to fillet all the way around the edges of each connector. Also every via hole could be soldered over if required!

Don't know if it's worth building though, because it would be hard work to solder the cans without a high-power iron. Also it would only work up to a few hundred MHz.

Regarding a coupling loop near an attenuator (if it isn't fully shielded), or close to coax braid, it would work, but would be hard to know what the attenuation would be (and it might vary over frequency). But might be good enough!