@rogersstuart
I was prompted by oz2cpu's sneaking suspicion that the main PCB may not have been an exact replica of his original 4 layer board and the great importance he places on it meeting this requirement, to verify whether it was a 4 layer board or not. You answered the question honestly and I thank for clarifying that point.
I sought further advice from oz2cpu before making any decision to cancel the order since your stated eBay policy is "no returns", denying me the opportunity to 'buy and try' as you've just suggested. Cancelling the purchase whilst this was still possible seemed the best option all round and I thank you for deferring to my cancellation request and the speedy refund.
Since the Belden ribbon cable undoubtedly accounts for most of the high levels of crosstalk shown by the graph you took the trouble to publish, it's impossible to say how much of a problem the lack of the inner groundplane shielding layers would be in this case.
Since oz2cpu had stressed the importance of a 4 layer PCB, I deferred to his judgement over your claim that this wasn't really that important after all. You may be right but there is no way to prove this, short of actually testing using twisted pair or coaxial cabling between the main PCB and the pods.
One way around this issue with your version of the main PCB, would be to wire up a pod using twisted pairs or coaxial cables and generate a new crosstalk graph for comparison against the Belden cable you're supplying as a courtesy to those with a more modest initial performance requirement.
Provided you've made it clear up front that the main PCB is only a two layer version of the original design and spell out your reasons for this cost cutting choice, backed up by the new graphical evidence obtained using twisted pair or coaxial cables, I should think you wouldn't have any further problems in this regard.
Supplying only a single 'starter pack' of test clip leads isn't such a bad idea since the quality and price of these test clips can vary considerably and it's best to economise here and allow the end user to make their own choice in the price/quality equation.
No returns is just a psychological thing and part of doing business on eBay. I do not want to pay for shipping to Europe. There is no such thing as no returns on eBay once someone opens a dispute. I would prefer to leave people feeling satisfied even if that means a loss here or there.
I feel like I should not have responded because the thread has been derailed now. I saw mention of opening a dispute and you sounded upset, so I said something. I was worried that someone might act maliciously assuming that I'm a bad actor out to scam people.
I agree with you about the high level of crosstalk. I'm not sure what it should be. At best it is -15 dBm below 100 MHz, at worst it is -8-7 dBm. Though, I have to admit that I'm not sure if I measured it correctly. I soldered female DuPont connector pins to SMA connectors and measured S21 using my 75 OHM VNA that was calibrated with 50 OHM Rosenberger "standards" in a 50 OHM system impedance to the end of the cable. I had to bend the pins a little and jam them in there. It was pretty sketchy. I have been told by the designer of the 8753E that it is acceptable to mix impedances like this. Recently I have found that adding pads can clean up a trace and improve accuracy. I would measure it that way now and might see a slight improvement.
I also normalized the data from 50 ohms to 120 ohms based on the 120 ohm cabling that is specified. I'm not sure if it was correct to do so.
As someone had mentioned, the clips that I include are the cheap kind. You know, I wish I could provide good ones but I couldn't find a supplier for them at a low enough price. I say that you get 8 but you would actually receive 10 in case 2 are bad. I count out 10 for each bag and inspect them all. It's really just tacked on so someone can immediately test their build. It's just as cheap for you to buy more on Aliexpress as it is for me to. I don't have a specific markup for this product. All I can say is that I'm selling it for at least half of my calculated price so I'm trying not to add extra things or make it more complicated.