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I’m not sure what your project is but you may want to look at a variable RF single samplers like a Bird 4275. There are also less expensive units that have sliding plungers (eBay). Here is a video showing the use of a Signal Hound SA44B with RF sampler (video is about setting modulation level in a packet TNC). Not sure if this video applies to what you want to do but it may give some ideas.
Hi Hank, what a coincidence. I actually watched exactly that video a day or 2 back. It really helped clarify a few things for me.
See my reply to Justin above. Does that pretty much agree with what you’re suggesting? Although I’d want a Directional Coupler, not a Single Sampler like the Bird 4275, because I want to be able to measure reflected power (i.e. SWR). Also, the Bird is good from 20mHz to 1GHz, whereas I want something more in the range of DC (or 1mHz at most) upto about 500mHz.
So if I’ve understood all this correctly, I should start hunting for a Dir Coupler which accepts ~10W in, has -30dBm coupling loss and is good across 0-500mHz. Know of such a thing?
Best, Dave W7BOT
Dave,
A few things:
1) The attenuator we sell is only rated for 2 watts. You would need a 10-20 watt attenuator.
2) Typically a directional coupler is used to monitor transmit power. This way, you only lose a small amount of power and you can monitor it while in use. Depending on the “coupling loss (typically 20 dB) you may want additional attenuation. I would recommend aiming for between -10 dBm and 0 dBm to make decent measurementsHi Justin, thanks for the heads-up about the 2W limit for the 20dBm attenuator I bought with my SA44B. That little detail slipped my attention. Now looks like that was money wasted.
Yes, my learning over the past few days has also led me to the Directional Coupler as the way to go to monitor a QRP rig while it TXs.
Now here’s the rub. You guys advertise/sell a Mini-Circuits ZFDC-10-5 which I was considering. Looking at the data sheet, I find Max RF Power Input = 0.5W and Coupling Loss = ~10dB.
Could you check my logic to see if I understand this correctly? If I want to use that unit, I would first have to attenuate my 10W QRP rig output down to 0.5W or less? i.e 10W = 40dBm –> 0.5W = 27dBm, so I need a 13dBm attenuator (let’s round off and say 15dBm)?? Now I’d have ~0.3W (25dBm) at the input of the directional coupler, which is in-spec for the unit. Finally, since coupling loss is 10dBm, the output at the sampling port that goes to the SA44B would be around 15dBm, which is in-spec for the SA44B. But you guys mentioned it starts compressing above 10dBm, so I need an additional -5dBm attenuator in the sampling line going to the SA44B to bring signal down to the ideal 10dBm?
Is all that correct? The problem with such a setup is that the PEP to the antenna on the output of the coupler is then only around 0.5W whereas the PEP out of the QRP rig is 10W.
It seems to me I need to find a directional coupler that can ACCEPT the full 10W PEP (40dBm) from the QRP TX (without any attenuation needed on the input). Hopefully I can find a coupler with a coupling loss of -30dBm. Am I right in understanding the 10dBm signal that remains at the sampling output is OK to feed straight into the SA44B without the need for any further attenuation? And I still get a full 10W PEP going thru the coupler to the antenna?
Thanks in advance for your help.
Best, Dave W7BOT
Hi Andrew, thanks for your quick reply.
I’m not opposed to buying more RF limiters if needed. But you lost me when you wrote
If you knock the input down to +10 and then add this +12dBm limiter to the front end
How do I do that? I have ~8W (~40dB) at the TX output, then -20dB with the Accessory Kit – USB-SA44-ABA -20dB pad I already own. This brings me to +20dB. How do I go the extra distance to +10 even before I “… add this +12dBm limiter to the front end …”
