Interesting, I didn't know Tek had discontinued their VNA. It's (still) a rather crowded market. From a message I posted a few months ago to an email list where the same question came up:
(snip)There's a massive selection of VNA options in the $10K-and-under price class. A few years ago, tempted by Copper Mountain's evident success, I built a 6 GHz 4-receiver USB VNA with the intent of jumping into the commercial market. By the time I finished the prototype hardware, the bang:buck ratio was no longer competitive with some of the other offerings, so I ultimately killed the project. In the course of making that decision, I did an informal market survey, which now includes more than a dozen competitive manufacturers:
- Arinst
- Deepace
- Anritsu's Shockline and SpectrumMaster series
- Keysight P937xA Streamline series
- Obviously the Keysight FieldFox qualifies
- Megiq
- Even MiniCircuits now offers a 6 GHz USB VNA (eVNA-63+)
- Copper Mountain
- Pico Technologies, as discussed here
-
Tektronix TTR506A- LA Techniques
- Transcom / Setup Electronica T6
- Siglent (not handheld, but much lighter and cheaper than some of the older competitors)
And that doesn't even count the low-end offerings in the NanoVNA, LibreVNA, PocketVNA and similar families, or Henrik Forsten's nicely-executed homebrew job at
https://hforsten.com/improved-homemade-vna.html. Ultimately, none of them including my own was a suitable replacement for my old 8753C/85047A outfit. Most were either too expensive, lacked important features, specifications, and/or quality, or had some combination of the above drawbacks.
If I were looking for a portable unit in the 4K range, I'd probably look for a used FieldFox. But if you don't actually need any of the FieldFox's specs or unique features, it would be crazy not to start by spending a few hundred bucks on one of the newer Asian offerings and see if you outgrow it. Failing that, the Megiq is probably a decent choice, going by Shahriar's review on his Signal Path channel, and the new ones from Tek, Keysight, and Mini-Circuits look intriguing as well. None of them will be as cost-effective as a used HP, though, and certainly not as cost-effective as the newest generation of Nano-class models. Not even close.
(end snip)So, since you're in the position of having outgrown the low-cost Asian imports, my vote would be for an 8753C/D/ES or used FieldFox. You can probably buy two or three of those for what the Mini-Circuits model costs.