The Ultimate Homebrew Scope Sampler (and simple)

[K1JJ]

*** UPDATE Note:
After consideration from suggestions in this thread, the scope probe was modified using the inner conductor from RG-213 coax with the floating shield tapped off to the scope for better RF HV isolation.  The shield acts as an RF pick-up capacitor. The pictures below reflect the new design. There is a single scope version and a double output for both a scope and a mod monitor.  On the double unit, there are TWO separate floating shields that are separately tapped to provide isolation.  The double unit uses about 4" of coax per side giving a very robust signal to the scope.***

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For a while now I've wanted to improve on the on-air scope pattern I see on the scope. For scope sampling, I was using a capacitive divider - just a 1" piece of ungrounded wire running alongside the 50 ohm coaxial line. I've been noticing some minor scope picture fuzziness lately and wanted to improve on it.

I first tried a toroid that had the 50 ohm line thru its core with 5 turns wound on the core. The 5 turns connected to the scope. The results were marginal with some fuzziness, like harmonic lines on the scope. I tried a second core with different permeability with similar results.

Then I tried a different approach with air-core only. I ran a heavy #10,   2" long Teflon wire from SO-239 to SO-239 as the input and output for the 50 ohm coaxial line in the shack.  For a scope sample, I wound 6 turns of #24 Teflon around this 2" wire and grounded one end and connected the other end to the 3rd SO-239 for the output to the scope.

The results were marvelous!  I have ample output to drive the scope and the trace was sharp on all bands. I like the idea that the sample wire is grounded at the cold end. This keeps the scope clear of any anomalies.  The output level is close on all bands allowing little to no scope level adjustment needed as bands are switched.

Anyway here are a few pictures. You can see the "schematic" just by looking at the wiring - real simple and works FB at any power level. There are no resistors, caps or other stuff to burn up.

T

[KA8WTK]

Do you run anything but high power, Tom?

You could have found a nicer looking box. 

[K1JJ]

If you run a AM KW through it how do you keep from blowing up the scope
Have you tried it with high PWR?

No problem.

The number of turns determines the voltage output to the scope. The scope is seeing only a few volts at 1KW. I found that 6 turns made enuff voltage to set the scope's vertical input sensitivity at about 1 volt or so for 200 watts. (Using a direct X1 scope connection)    This way I can run QRP 5w or QRO 1500w and the scope will stay within its adjustable range.

The voltage on 50 ohms at 1KW is just a few hundred volts, so Teflon wire used for both wires is a safety factor of maybe X10.  No worries of arcing.


Bill, I still have that 2 watt series modulated AM rig that gets on from time to time, so cut me some slack...
And  I'll get the silver spray can out just for you.

T

[K1JJ]

Heck, if one is worried about the Teflon wire breaking down (I'm not) just add a .01 2KV disc cap in series with the scope probe lead inside the box. That will isolate it even further and still be perfectly RF transparent down to way below 160M.

T

[KA8WTK]

Bill, I still have that 2 watt series modulated AM rig that gets on from time to time, so cut me some slack... Grin

I remember that rig. It's amazing what 2 watts can do under the right conditions.

Tom, why the semi-circle in 50 ohm line? Just not enough room? I assume it could be a straight line like in the capacitive divider if there was more room.

BTW, I use your capacitive scope pick-up and it works great. Far superior to the HO-10s I was using. (Sold them all!)

Bill

[Steve - WB3HUZ]

But he's not coupling that much to the scope. Do the math. If he's getting 1 volt at 200 watts, he will only get a little over 3 volts at 2 kW. The scope is safe.

The voltage on 50 ohms at 1KW is just a few hundred volts, so Teflon wire used for both wires is a safety factor of maybe X10.

A few hundred volts is pretty hot   I assume that's with modulation applied and not just the carrier level.

Just don't go to 2KW

I agree though, K.I.S.S.

[Steve - WB3HUZ]

Probably not. It's a transformer with a single turn primary. Not a ton of coupling going on.

But he's not coupling that much to the scope. Do the math. If he's getting 1 volt at 200 watts, he will only get a little over 3 volts at 2 kW. The scope is safe.

Yeah, I misread his post, (few hundred volts at 50-ohms). There aint no way that wire to ground is 50-ohms.

[K1JJ]

I was assuming the worst-case scenario where the main 50 ohm Teflon covered wire arced over to the scope sampler.  But still, adding a .01/2KV isolation output cap is not a bad idea for those of us who like to call Pascal in Eu every night...

[Steve - WB3HUZ]

Tell Pascal I said hi.

[WA1GFZ]

The secondary is referenced to ground so a short will just kill the signal. Secondary reactance is nill so you can't generate much voltage. I would slide teflon tubing over the wire for more isolation though. Most Teflon large wire is only rated for 500 volts over mil range. Smaller sizes are 250 or 300 volts unless it is special order. I have flashed through insulation with RF.

[K1JJ]

Once you get your new toys finshed, I think you'll be quite able to do that yourself...

[Steve - WB3HUZ]

Let's hope. I'm sure Pascal has missed me. 

[K1JJ]

The secondary is referenced to ground so a short will just kill the signal. Secondary reactance is nill so you can't generate much voltage. I would slide teflon tubing over the wire for more isolation though. Most Teflon large wire is only rated for 500 volts over mil range. Smaller sizes are 250 or 300 volts unless it is special order. I have flashed through insulation with RF.

Good point about RF and Teflon. I've seen standard 40KV HV wire arc over from 4KV of 3.8 mhz RF. RF changes the game.

I'll add some tubing and that .01 cap to be sure.  Don't tell Pascal I'm a HV pussy.

T

[K1ETP]

How about just using silicone HV wire for the link?

[WA1GFZ]

Don't waste your time with a cap, the secondary link is grounded
Many times you can clear up scope fuzz by loading the input with a resistor.

[Steve - WB3HUZ]

You can avoid all this by just removing a small section of the braid on the coax and wrapping the wire around the dielectric core. If the center never arc to the braid, then is won't arc to your coupling wire, no matter the insulation.

[K1JJ]

Hey Lichaloni!  

It took this to drag you out, huh?  

Yep, had I thought I needed more insulation safety, I wuda used a HV or coax inner from a RG-213.

I just finished adding in some more insulation and also a .004 uf at 2.5KV just for good measure. All bottoned up.  The scope picture looks outstanding now. No JSing anymore - just a clean picture.  Not sure why I put up with looking at the fuzz before.  Gawd that used to get me POed, esp when I switched bands.

Come to think of it, probably the problem was cuz I ran the sampler into a splitter - one went to the scope and the other to the mod monitor for audio monitoring. The monitor has a par res circuit that peaks the signal. That may have been the fuzz generator. But anyway, it's gone now. (listening for weak Yugo mobiles now, caw mawn)

T

BTW, Bill - you're right - the main 50 ohm wire should be straight thru.  I was playing with toroids before and needed more lead length. I shud have made it straight in the end.   Now I won't be able to sleep tonight worrying about it... sigh.

[WV Hoopie]

Mr. Vu et all,

Been using that exact RF pickup for over 30 years. Only needed one turn (plain solid #20 wire 600 volt insulation)  around the in & out section for a DX-100 and a Junkston "500". Voltage from the single turn feeds, via a switch, one of four toroids that are in parallel with a 100 puff air cap. One toroid is somewhere resonate near the middle of 80 meters, another for 40, 20, & 10. By adjusting the air cap, the vertical pattern on an old Eico 460 is adjusted. Output of this JS goes straight to the vertical plate of the CRT.  Plus there is a voltage divider circuit in the project box for the trapz pattern. Output of the divider circuit goes to the horizontal amp of the 460. Off position of the switch lets me view the received RF via the 50 ohm IF out of the R-390A's. That goes to the normal vertical amp input of the 460. Have to add this, my single turn is NOT in contact with the in&out hot RF wire! Just a loop about 1/4 in away!

Built another set-up using the same style pick-up for the mud fence desk kw. Little easier here, the Tectronix 2336YA can handle 100Mhz so the JS toroids, cap, etc., aren't needed. Output of the RF pick-up goes straight to Channel A. Project box with a voltage divider circuit designed to handle either hi or low power from the desk kw goes to Channel B for the trapz.

So, in most cases a single turn can get the job done with that style of pick-up. Add more turns if the scope wants more juice.

Craig,

[K1JJ]

*** UPDATE Note:
After consideration from suggestions in this thread, the scope probe was modified using the inner conductor from RG-213 coax for better RF HV isolation.  The pictures below reflect the new design and were also uploaded to update the the first post. ***

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Thanks for the suggestions guys!  The sampler looks better and is more bulletproof now using RG-213 inner conductor as the main thru-lead.

T

[K5IIA]

no paint? 

just jokin hahha

[Pete, WA2CWA]

Why not just leave the RG-213 braid in place and use that as the coupling element/wire 

[K1JJ]

After a lot more testing and trying many variations, I've found that letting the sampling wire cold end float through a resistor (as Brian suggested) gives an excellent way to increase or decrease the delivered scope voltage. I ended up with 500 ohms there to drive the mod monitor adequately. The scope used alone does not require as much voltage if it was the only device used and this resistor can be much lower. Adjust it to suit your own needs.

Either way, the scope picture is clean.

T

[K1JJ]

I've decided to give Pete/CWA's idea a try by using an RG-213 inner with the shield still on. The shield will act as a capacitive pickup.

In addition, I will build two units - one for the scope and one for the mod monitor.  I have been using a splitter which has been loading down the scope somewhat and causing fuzz. So separates shud solve the problem.  I like the idea of a capacitive divider and the shield which shud give plenty of pickup vs: a few wire turns.

Will post them later. Stay tuned.

T

[Todd, KA1KAQ]

You can avoid all this by just removing a small section of the braid on the coax and wrapping the wire around the dielectric core. If the center never arc to the braid, then is won't arc to your coupling wire, no matter the insulation.

What if you don't remove the braid? Will it still produce enough coupling for an accurate sample? Or will it give you a false sense of security by hiding the spikes through attenuation? Overload has worried me as well. Nobody wants to fry their monitor.

I remember the original version of this box Tom posted on here several years ago using two parallel conductors in a mini-box. Was planning to build one of those to go with the NEAR-Fest monitor scope before things blew up. Simple enough to build one box and put in inline with your antenna switching network to cover whatever transmitter is in use, too. Simple is good!

Yo 'roni!   

[Ed/KB1HYS]

I've used the capacitive pick-off (parallel verison) and a wrapped version for some time now.  Both have worked FB.  The wrapped version is installed in the home brew rig-antenna switch.  There are ports for 3 rigs, the antenna, the scope sample, and the reciever connection to the R-390. Also has the xmit-receive relay in there.  All inside a one-quart metal paint can... ain't pretty, but she works good.