Pure Signal RF Sampler

To operate my station, an ANAN 100D and an SPE 1.3K Amplifier, with pure signal (Pre-Distortion), I need to sample the RF (RF-Sampler) from the amplifier output and feed a fraction of that signal back to the ANAN. Looking at the BiDirectional Coupler I made a while back (made a few) I decided to use the same principle. I'm not going to talk about the math involved, if you like to know how to, read
the article from David Knight, G3YNH, he explains the Mathematics of this project very nicely.

My Requirements are:
Input power handling:  > 60dBm (future proofing)
Sampled power: -50dB of through signal.
Frequency response: 1.8 - 50 MHz
Fig. 1


Pic. 1

If you look closely, you'll see a mistake I made ! Yep, the 10Ω R is positioned wrong! If you don't measure 50dB, you have R1 soldered incorrectly. I measured 46dB with R1 soldered incorrectly.


https://sites.google.com/site/vkonehw/home/homebrew/taps/rf-sampler/50dB%20Tap.jpg

Fig. 2

Well, 50dB it is. And nice and flat up to 70MHz. Plenty of room for the ANAN to add internal attenuation.

There is a small amount of asymmetry in the sampler due to the Faraday shield. This means that the input-output ports are not PERFECTLY interchangeable. Really not an issue at HF, so nothing to worry about. Also, a word about the Teflon tape, YOU DO NOT NEED IT ! I use it to make sure the the insulation of the wire does not scrape off on the edge of the toroide. So, it is an additional insulation for the protection of the wire. Wind the Teflon tape around the toroide and then add the 32 turns of the 30AWG wire. Checkout the BiDirectional Coupler page for more details on how o make the transformer.


Parts List:
1x short piece of coax (RG213, RG402, RG316 or similar) to make the link between the input and output.
Teflon plumbers tape (there is no need for this YMMV)
85cm of 30AWG enameled wire (depends on the toroide size)
R1 = 10Ω ½ W Metalfilm (at 1500W the dissipation is about 300mW, even at very high VSWR)
R2 = 39Ω ½ W Metalfilm


NOTE: It looks like the LO1230 is hard to get in EU, a close relative would be the FT50(A)-43. So if you have issues getting the LO, use the FT. Compare the AL value of the toroide from here LO AL Values with information from here Toroid Info


You can play with the following spreadsheet to get a view of internal attenuation values.

Sampler Calc



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