Re: Protecting (tr)uSDX against high SWR
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I also have a Q-Match manual tuner that has a tune mode with a resistive bridge that keeps the SWR below 2:1 under all load conditions. I agree, this type of tuner will protect these simple FET QRP finals.
Dave W6VYC
On Tuesday, June 25, 2024 at 06:15:17 PM PDT, HOWARD POMERANTZ <howpom@...> wrote:
I have a ZM-2 that I built. If anyone wants to borrow it to try it out with your (tr)uSDX or other let me know.
The best way to protect? (tr)uSDX against high SWR is to use a proper antenna tuner like ZM-2 ATU that will protect the finals during the antenna tuning.<image.png>
73 Ned AC6YY
Connie,
I agree completely. What is the effect on the spectrum, particularly on SSB, which may easily be distorted due to the diode. I think that the failure mode is due to the high voltage as the 3 FETs can dissipate the couple of watts of reflected power.
It could also be neat fix if the diode is more or less transparent under normal operating conditions. It might be interesting to run an experiment with the diode biased with the peak operating voltage on the FETs using a VNA to see what its RF characteristics are in the HF bands. I have never seen an RF amp. that uses this kind of protection.
Dave W6VYC
On Friday, June 14, 2024 at 12:39:57 PM PDT, Connie W6EFI via <stillinger= [email protected]> wrote:
Is there a side-by-side test documented anywhere???? Ie, showing that the radio is destroyed without the zener diode under the same conditions otherwise??? I couldn't find one.?? They're cheap -- you'd think someone would love to blow one up for the views.
I've done a little searching and all I can find as the origin for this recommendation is just one guy with a self-assembled kit who blew his radio up in an unknown way with unknown power even though he had a tuner inline and was presumably tuned up, and he doesn't know what the root cause was, and then said he'd install the zener just in case.
As for WB2CBA (Barb), he just shows that his radio still works.? He didn't do a side-by-side test.??? And it's not a TruSDX.?? Maybe he could have painted it blue or stuck a bandaid to the back of the radio instead and shown that the radio still worked after a high-swr test.?? Look!?? Painting it blue works!
DL2MAN recommends against adding the zener.?? (His recommendation is boring, just use low power for tuning.? Personally I tune by ear before any kind of tx'ing.?? Works great if you have the ear and the tuner.) ?? Also I had a friend (sadly SK now) who gave his TruSDX a hard workout for a good while and told me that it was pretty robust against mismatch.
I don't think "voltage reflected back into the radio" is exactly the problem with mismatched impedances anyway so maybe the zener works, but maybe not for its breakdown threshold; maybe it just changes the tuning.?? Or maybe it works as well as a bandaid.??? Gotta show me some measurements or a side-by-side test before I'm convinced.??
For all of these reasons, and the fact that it's a cheap radio, and any mod activity introduces unknown risk (which is why regression testing exists), I think I'll save myself the trouble for now. ? I am still open to to the idea and interested to see if any more evidence comes in, in which case it's easy enough to add the zener.???? Or if I blow up this radio in which case you can give me a good Nelson "haha!"?? (Simpsons reference)
73,
Connie? W6EFI
Connie Stillinger
650-380-2018
On Friday, June 14th, 2024 at 10:37 AM, Doug Hendricks < ki6ds1@...> wrote:
Thank you Hiroki for your validation.? ?I heartedly encourage everyone who has usdx radios,? including the one from Ed Fong to do this simple mod.
-- Connie KN2EFI
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David W Crocker