The Ultimate Homebrew Scope Sampler (and simple)

[K1JJ]

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

[Pete, WA2CWA]

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. 

[KA8WTK]

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

[K1JJ]

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

[Steve - WB3HUZ]

Or use a smaller coupling cap.

[WA1GFZ]

How about a load resistor at the input of the scope

[K1JJ]

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

[KA8WTK]

Ah, Master Vu,
Please desclibe splitter.

Bill

[K1JJ]

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

[KA8WTK]

Thanks!

Bill