Calculating delay spread from Spectrum analyzer channel frequency response function

Hello to all guys,
A bit of a help to my problem would be precious.

Description:
I use a pulse generator, a signal generator a mixer and an antenna to tranmit a pulse that is 400ns long,
at 900MHz.

I then use a spectrum analyzer to get the channel frequency response and the channel impulse response. Both are depicted below.
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The Resolution Bandwidth that I use is 5MHz since the pulse is 400ns.

The problem is that the delay spread is 75 ns and this nuumber is not sensible since the room is relatively empty and the measurements with the Network analyzer gave a delay spread of 7ns.


I can post my matlab code if u want to see it. it is the same that I used to process the data of the VNA and then the rms delay spread was correct.

Manos

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Also, guys what is the relationship between the frequency span and delay spread?
Does somebody know?