Switching between 2 RF couplers, anybody?

JJ4SDR
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Switching between 2 RF couplers, anybody?

Postby JJ4SDR » Sat Apr 01, 2023 1:53 am

Re-did the math that Scott had provided, to determine good RF attenuation levels.

I do have an Xtronic coupler sampling at -44dB (+an adjustable sampler combo) that I can use with all of my amps. I just like the idea of having to manually switch among antennas, depending on the band. One of the amps (RF2K-S Solid State) has an internal coupler sampling at -55dB (this works with all 4 ANT OUTS) and there is no need for any manual ANT switching.

Now, looking at the math, at 1,500W out, I need to have an RF sample attenuated by 47.76dB and, at 1,000W out, the sample needs to be 46dB attenuated.

Regardless of amp (assuming that the SWR is 1:1), wouldn't an RF sample at -55dB be too high and PS might have difficulty correcting? Granted, I haven't too much trouble. However, on some bands I can't put out more than max 1,000W out which suggests that the RF coupler output is 9dB too high. Consequently, I am seeing red and yellow flashing fairly frequently before it turns green when I am putting out max 1,000W. I am hitting 0 or even 3 at times with the meter in the "ALC Group" position.

Perhaps, I have the Xtronic coupler/adjustable combo and the internal to the RF2K-S amp on an SMA switch, so that depending on which band I am on, I switch between the 2 RF couplers for "optimal" correcting. Or, am I overthinking this again :roll: ?

Juha
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w-u-2-o
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Re: Switching between 2 RF couplers, anybody?

Postby w-u-2-o » Sat Apr 01, 2023 1:58 pm

Your math is off by 1dB ;) Example: 60dBm (1KW) - 13dBm max. safe signal level = 47dB minimum attenuation.

But no big deal, what's 1 dB among friends? :D

More to the point, you are overthinking it again. But it's good to be thinking about these things! So many people don't :?

Quoting from my old post on PureSignal levels:

2. On older radios, step attenuator levels of below 7dB or so generally result in poor performance because internal crosstalk dominates the external feedback signal. The newer the radio, the less of an issue this is. However, better to have it (a higher feedback signal level) and not need it then need it and not have it.

You don't have an older radio (like a 100D or a 200D) so you've got even more margin.

Yeah, it sucks that certain amp makers decided to use a -55dB coupling factor even though "everyone" told them to use -50dB. At least Elecraft got it right! Nevertheless, the situation is not that dire. If you are driving to 60dBm (1KW) peak then you'll see +5 dBm peak at the output of the coupler and that will result in a step attenuator setting of 5 + 14 (preamp gain) - 3 (target ADC input level) = 16dB, which is plenty fine (give or take a couple of dB, the "book" values for everything are obviously subject to some unspecified tolerances, e.g. the RF2K coupler may be -55±2dB depending on freq., but we don't know exactly).

With that 16dB step attenuator value in hand, now we can see there is plenty of headroom to handle the extra 1.8dB going to 1500W, and we can also see that dropping the power from 1000W to 100W, a 10dB change, will still put the step attenuator at 6dB. So you are OK. If you go barefoot just switch to the internal coupler inside the 7000 instead.

To summarize, you are fine, and any slight, very slight, degradation in linearization is, IMHO, worth the convenience factor of using the amp's built-in antenna switch. Nobody but you will know that your IMD is degraded a dB or two.

I'm curious: why on some bands can't you achieve more than 1000W?
JJ4SDR
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Re: Switching between 2 RF couplers, anybody?

Postby JJ4SDR » Sat Apr 01, 2023 2:54 pm

Scott,

Glad you like the "extra dB" :lol:

Summa summarum; I am OK with using minimum attenuation of 47dB whether I put out 1kW or 1.5kW (in fact, I often stick with 1.4kW), correct?

How do I "switch to the internal coupler" to use PS? Does the PDF document on PS detail this and checking/unchecking of boxes on forms (if needed)? I use PS when running amps, but sure don't mind putting PS to use while bare-foot either.

On 2 bands, I use an End-Fed-Half-Wave (EFHW) antenna that works great for me. It has a 49:1 UNUN at the feed-point that is rated for 1.6kW PEP (I am not really wanting to test that rating though). That is the only reason for not wanting to run more power. I do operate CW as well and have not observed rising of the SWR which would indicate saturation in the ferrite material in the UNUN.

Juha
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Re: Switching between 2 RF couplers, anybody?

Postby w-u-2-o » Sat Apr 01, 2023 3:19 pm

JJ4SDR wrote:Summa summarum; I am OK with using minimum attenuation of 47dB whether I put out 1kW or 1.5kW (in fact, I often stick with 1.4kW), correct?

As long as you stay below +13dBm on any input to the ANAN you are OK. Personally, I'd recommend a few dB safety margin. Hence the general recommendation for a total of 50dB loss in the coupled path at 1500W. Don't forget most amp's are capable of generating more than 1500W peaks and that pretty much all the power meters people have and use are average reading power meters so you are not exactly sure where the peaks are at.

On any Orion MKII or later hardware, if your S-ATT on TX is getting above 24dB you know you are in the danger zone.

How do I "switch to the internal coupler" to use PS? Does the PDF document on PS detail this and checking/unchecking of boxes on forms (if needed)? I use PS when running amps, but sure don't mind putting PS to use while bare-foot either.

Heck, I can no longer keep track of what documentation people are using and what it says. There's a lot of stuff out there various folks have written, some good, some bad, some right, some wrong, and I don't know what you or anyone is reading unless you include a link to it.

The answer is: on 7000-based hardware (including Andromeda) the routing for PS feedback is controlled by Settings > General > Ant/Filters > Antenna "Rx Bypass on TX". If this is checked then during TX the feedback is expected on the Bypass input on the back of the unit. If this is unchecked then the feedback is sourced from the internal coupler inside the ANAN.
JJ4SDR
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Re: Switching between 2 RF couplers, anybody?

Postby JJ4SDR » Sat Apr 01, 2023 6:21 pm

Thanks Scott and, in particular, I appreciate the reminder about the 24dB S-ATT limit. I was hitting 28dB and I got it now set so that it hits 23dB. This is interesting in that - this is obviously a theoretical number - the total attenuation from the sampler is now 55dB (at approximately 1kW PEP out). I will make sure that I will increase that when using the internal coupler in the RF2K-S amp, when putting out 1.4kW, just to be extra careful.

I referenced the PDF document titled "PureSignal 2015‐05‐12" that I downloaded from this sub-forum.

Juha
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