How to improve this RF sampler design

[varactor]

I wish to build a general purpose RF sampler I can place inline with the output of a HF radio (3-30 Hz, 50 watts max) to provide a signal that can be used to measure frequency. I'm a bit unsure of my design, and I would greatly appreciate any feedback you all have to offer! The sampled signal will be sent through a few feet of 50Ω coax and then terminated into a 50Ω load inside the frequency counter. The frequency counter will amplify the input and pass it into a series of flip-flops to divide it down to something a microcontroller can comfortably measure.

Rationale:

• Toroid: According to https://toroids.info/T37-6.php 10 turns at 30 MHz is about 50Ω, but 1 turn (resistance on the transmission passing through) is only about 0.5Ω
• Power: To evaluate power coming out of the toroid I hooked it up to a single 50Ω resistor and measured peak-to-peak voltage while transmitting 50W at 30 MHz and observed a sine wave with 7V peak to peak, which according to https://www.rfcafe.com/references/electrical/pwr2volts.htm means the toroid is delivering 0.5 watts.
• Resistor Network: To avoid running my 1/2 watt resistors so close to their spec, I put two in series locally (to ground) and two in series for sampling (one on each side of the sampling coax). When the RF sampler is disconnected toroid will experience a 100Ω load when the RF sampler is connected the toroid will experience a 50Ω load

Is this design reasonable, or are there some traps for young players™ that I should be aware of?

Thanks for your input!

[dietert1]

Did you check the frequency dependence? What you built is a 1:10 transformer and it transforms the voltage across the primary. As the primary is an inductor, it's voltage may be about 10x less at 3 MHz than at 30 MHz. With the sampling circuit as load the frequency dependence is probably less. Maybe one can run the ciruit with a near short on the secondary, like a 1 Ohm resistor.
How much signal do you need to monitor the transmitter?
Can't you just use a resistor? If you use a 5 KOhm resistor to feed the 50 Ohm sampling line it will also get 0.5 W at 50 W RF out. Sampling signal will be 0.5 Vrms, roughly frequency independent. You can split the resistor into two parts to reduce capacitive feed-through.

Regards, Dieter

[fourfathom]

That should work, but the sample output Z is closer to 150 Ohms than 50.

For a monitoring tap I usually just use a resistive divider.  A 5000 Ohm resistor connected to the transmission line connectors, and a 50 Ohm (or 47 or 51 Ohm) resistor to ground will give you about 46 dB of attenuation, or about 0.25VRMS with a 50 Ohm load at the sample point.  Or, 0.5VRMS unloaded.  That 5000 Ohm resistor will be dissipating about 1/2W so I would just put four 1.2K 1/4W resistors in series and call it good.

[varactor]

Thank you dietert1 and fourfathom for your input! I now realize I was drastically overcomplicating this task by creating a current sampler when really a voltage sampler would work just fine for a counter. As stated previously, directly sampling voltage does not suffer from the issue where current from the toroid would decrease as frequency decreases. I think I got on the wrong track by googling so many designs for SWR meters (where current directionality is critical). It seems "RF sampler" (what I was searching for) is a term often used for current sampling using an inductor, and an "RF power tap" is a term often used to describe dividing-down the voltage before measuring it.

I rewired my device as you described and added a series capacitor to ensure no DC made it out the sample port and it's working great!

I found these PDFs which describes how to build an RF power tap similar to what was described above:

• https://ae6pm.com/SCCARA-GRAM_Articles/A_40_dB_Power_Tap.pdf
• http://nebula.wsimg.com/6b769a49ac6df5b9377995f177c5a7e7?AccessKeyId=13A6205A294059260963&disposition=0&alloworigin=1

Thanks again!

[Hamelec]

the design of the last link cant work properly from 3-30MHZ, because its missing any frequency compensation and any matching to 50R.
frankly said: bulls..t for measuring purpose.

[langwadt]

the design of the last link cant work properly from 3-30MHZ, because its missing any frequency compensation and any matching to 50R.
frankly said: bulls..t for measuring purpose.

hm

[fourfathom]

the design of the last link cant work properly from 3-30MHZ, because its missing any frequency compensation and any matching to 50R.
frankly said: bulls..t for measuring purpose.

I can't open that link so I won't try to critique it, but freq compensation, or attenuation accuracy mat not be necessary if you just want a sample to drive a freq counter.

[Wallace Gasiewicz]

If you just want enough signal to power a freq counter, a few wraps of wire around the outside of the output coax at more than a few watts is usually quite adequate. It will also be quite enough to observe the signal on a scope or a spectrum analyzer. Just connect the wire to the BNC input of the counter with a small coax.
Sometimes just putting another connector, like a BNC on the back of the radio with a little antenna wire sticking into the chassis near the antenna connector is also quite enough.  This looks a lot "neater".
Both of these solutions do not interfere with your output signal paths much at all. You avoid interfering with the 50 ohm characteristic output of the transmitter and any possible SWR implications.
Some counter are sensitive enough to just be near the radio and use a small antenna on their BNC input.

[varactor]

If you just want enough signal to power a freq counter, a few wraps of wire around the outside of the output coax at more than a few watts is usually quite adequate.

Fantastic recommendation Wallace! I tested this just now and a few wraps of wire around the coax coming out of the radio produces over half a volt (PPV) into a 50 ohm load. I will definitely explore designs using this technique for my frequency counter applications -- thanks!

[Wallace Gasiewicz]

Remember that any freq count application requires a CW signal. or just an AM carrier and that the SSB signal will be changing since it is modulated. You can modulate the SSB signal with an audio generator at 1K or 1.5K audio signal and get the resulting plus or minus signal on the freq counter. It is more fun to use a spectrum analyzer for this though, if you have one.   
Also the signal from the wire wrap is not necessarily a 50 ohm input, but that should not matter.You can also use this to observe an AM signal "envelope" to see if you are modulating the signal correctly.