rtl-sdr.com

Presumably that software will compare two audio signals from the two synchronised receivers?
Alan

The youtube shows a simple way of getting an equal distance. Measuring between antennas is not simple. I can not give you instructions but I think you will need two identical locked receivers and then compare the outputs. I do not know where to take the output to be able to preserve and observe or m...

Bad cable?
Bad joint on USB plug?
Alan

My first thought is that your TR7400 is faulty.
It would be the first time a RTL was better than a good proper radio.
Alan

It notches out constant tones.
Not good for data modes, it works too well.
Alan

What do you want to do?
Comparing signal levels to within about 1dB is easy.
Actual Field Strength will need a standard, few, if any, SDRs are calibrated.
Alan

What dongle? Does it have a bias tee? Is it switched on?
That would explain.
Alan

No easy answer.
The RTL is a poor receiver, is wideband, overloads so gain setting is crucial.
And may need filters to keep out of band signals away.
Spikes are common, often just spurii, not actually real signals.

See the shop, ebay, Google

Alan

It depends on what exactly a "sky-blue 820T2 & SDR module" is.
Is it advertised as doing direct sampling? Does it have an upconverter? Ordinary RTL modules go from 24MHz upwards.
Alan

This http://jcoppens.com/ant/qfh/img/coax/coaxconn1.jpeg seems to show it well.
Alan