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Hi all,
Pretty new at this, so here goes a dumb question.
SA44B has a warning on front panel that states max signal must not exceed +20dBm. If I understand correctly, that means I can connect a QRP ham band TX which outputs no more than 1/10 W of RF power (0.1W = +20dBm, right???).
I have a QRP rig that outs 10W max. So, I’m planning to use the Accessory Kit – USB-SA44-ABA – which includes a ’20dBm pad’. I’m guessing the 20dBm pad is an attenuator.
Am I right in assuming the 20dBm pad drops the 10W (+40dBm) QRP signal to +20dBm (0.1W)?? Therefore it’s safe to connect 10W QRP rig thru the 20dBm pad to the SA44B … AND not worry about frying the front-end?
I understand tolerances are too close for comfort in my equation. Good news is that QRP rig ACTUALLY outputs less than 10W (maybe 8w) so as long as my assumption above is correct about the 20dBm pad, the 8W out of the QRP rig WILL NOT fry the SA44B front-end?
Best, Dave W7BOT
Hi Dave,
Your conversions look good.
That said, I would not operate your 10W transmitter with the 20dB pad into the SA44B. +20dBm is a hard limit. I would add another 10-20dB of attenuation as well. I would try to stay below +10 dBm, the SA44B goes into 1dB compression at about 16dBm for some frequencies.
We also sell an RF limiter you might be interested in, the VLM-63-2W-S+. If you knock the input down to +10 and then add this +12dBm limiter to the front end, I think you would be safe from costly front end RF damage.
Regards,
AndrewHi 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 …”
Right now you have 20dB of attenuation. I recommend 30dB of attenuation and an RF limiter.
The RF limiter will protect the front end of the SA, the 30dB of attenuation is necessary to bring your 40dBm signal down to 10dBm where the SA is no longer in compression and can accurately measure the input. Our RF performance specifications only hold to +10.
Regards,
Yes, I recommend the two things.
The limiter does not come with an additional attenuator, that would be separate.
I didn’t think to ask, what frequency are you operating at? Our limiter only goes down to 30MHz, so if you are lower frequency than that and you wanted a limiter, you would need to find a different part.
Hi Dave,
Maybe other forums members will be able to help, I’m not familiar with parts outside what we provide. You also might have more luck on a dedicated radio forum.
Regards,
AndrewDave,
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 measurements.Hi Dave,
I’m not sure what your project is but you may want to look at a variable RF signal 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.
https://www.youtube.com/watch?v=xIqGWz89mWk
Hank
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
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
You can check Mini Circuits for an acceptable coupler. You might have to get 2 or more to cover your frequency range. Look for connectorized models.
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