The AM Forum

*** 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

Do you run anything but high power, Tom?

You could have found a nicer looking box.  ;D

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... ;D
And  I'll get the silver spray can out just for you.

T

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

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

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.

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

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... ;D

Tell Pascal I said hi.

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.

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

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

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

How about just using silicone HV wire for the link?

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.

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.

Hey Lichaloni!  

It took this to drag you out, huh?   ;D

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.

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,

*** 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

no paint?  ;D

just jokin hahha

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

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

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

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!   ;D

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.

Here's the final versions. I am now using an RG-213 coax stub and use the floating shield as the RF pick-up. The shield acts like a capacitor to sample some RF.

There is a single output version and a double output unit in the pics. The double uses TWO separate floating shields to feed the scope and mod monitor for total isolation.  I found the mod monitor loading the scope down so opted for two pick-offs on the same RG-213 stub.

Notice the longer stub lengths of the double unit. The two floating shields are about 4" long each and provide a very robust signal to the scope, about 25 volts p-p at 1KW. This reduces stray noise and fuzz if the signal is weak. The coax stub has the shield cut at the center and at the ends to make two floating pieces. Solder the scope and mod monitor leads anywhere that is convenient to each shield.

The mod monitor likes the bigger signal too. In addition, the scope and monitor now work down at the 10 watt  level quite well.

If less signal is needed, then simply make the coax shield shorter. Again, the shields float and do not touch ground - the only shield connection is to the scope hot lead (I use a X1 direct connection)  or mod monitor using a coax jumper for each.

There is little risk of sampler arcing now since the RG-213 is rated at a few RF KW. If it arcs, so would any other point on your feedline. Perfectly safe.


BTW, I checked the swr thru both units. The single unit is perfectly flat to 10M. The double unit, with the longer coax, is flat to 20M but I see a 1.2:1 swr at 28mhz. This is quite acceptable. The coax stub probably acts like a stripline with the floating coax against the chassis.

Thanks to Pete/CWA and Steve/HUZ for their coax suggestions as well as others in this thread who helped to evolve the design.

T

I've been using a similar (roughly same size enclosure) single output sampler with RG-213 for many years. I swept it years ago and, from memory, it was virtually flat from about 1.5 MHz to well over 200 MHz. I seem to remember a small bump somewhere around 95 MHz but it was fairly insignificant. 

One of the guys in my local radio club built a scope pickup by slipping a wire under the coax shield like shown in the old Monimatch articles in the ARRL handbook. They used a 20 gauge enameled wire under the braid and claimed it would handle a kilowatt.
I think I like your idea better. It is easier to build and the dual ouput is cool.

Bill

I just ran some careful measurements and found that the double sampler is putting out much more RF voltage than I thought. With a X1 probe I measure about 25 volts peak-to-peak RF at 1KW on 3.8 mhz. With my particular scope this is perfect, but other guys may want somewhat less RF voltage or use a X10 probe.

This extra voltage is great for driving the mod monitor when running low power as well as providing a clean, noise-free signal for the scope.

To decrease this voltage just make the floating pick-up shields shorter. The single sampler uses a 1" stub and supplies JUST enuff to drive the mod moditor at 300 watts. So, an RG-213 stub of around 2"-3" long might be a good compromise for all-around general ham use.

T

Or use a smaller coupling cap.

How about a load resistor at the input of the scope

Both of those would work.

Actually, I was going to add a pot in the box that went from the shield to ground. Maybe 5K would do it for a full range of attenuation.  But since I can use all of the available RF drive, I left it out.

Today I thought there was a solar flare. All the bands seemed way down. But when the locals started working Eu and Ja's on 75M and I couldn't hear the other side, I started looking. I had blown a safety 1/8A fuse that is in the splitter for the receivers.   The other night I was giving out such PW S-meter reports to the Euros that I was getting embarrased.   duuuhhhh.

T

Ah, Master Vu,
Please desclibe splitter.

Bill

Ho-K, Bill -

I use an FT-1000D transceiver and SP-600 simultaneously for receiving. The antenna switching is done internally between TX and RX inside the transceiver. During receive, I have a small solid state active splitter chip that splits the antenna into two paths - one for the FT-1000D and one for the SP-600.

During TX I have a relay that breaks the receive line to the splitter so that TX RF can pass around it without harming the splitter input. If something happens and TX RF gets to the spplitter, I have a 1/8A fuse in line. That has worked well for the times I've screwed up.

Works FB when I want to listen to hi-fi off the diode detector of the Sp-600 and still have the FT-1000D for QRM and AM DXing.
T

Thanks!

Bill