This paper presents the design and implementation of a novel SDR based massive MIMO testbed with up to 70 nodes built at Tennessee Technological University. The deployment can reach a $30 \times 30$ antenna MIMO scheme. With this testbed, we are able to measure the channel matrix and compute the achievable rate of the massive MIMO system using experimental data. The measured channel capacity is linearly increasing with the number of antennas of the base station. We also demonstrate the channel reciprocity including the circuits impact from the transmitter and receiver. We show that the Vandermonde channel model is more realistic to describe the massive MIMO architecture than the widely used Gaussian channel model, in terms of capacity. By adjusting the range for angle of arrival $α$ and the base station antenna distance $d$ during the simulation, we find out the Vandermonde model agrees with our measured capacity at a certain $α$ for each selected $d$ and the $α$ is very close to that of the experiment deployment. It is the first time that the feasibility of Vandermonde channel model is demonstrated by the experiment for massive MIMO.