Do external signals affect the S11/SWR measurement of an antenna?
For example, if I try measuring the S11/SWR of an HF antenna with a SWR meter or a Vector Network Analyzer in the middle of a field day with many transmitters near the antenna, or try measuring a 2.4 GHz dipole at a lab with many wireless access points, is it possible that the instrument sees a higher reflected power, thus giving me invalid S11/SWR results?
If it's a possibility, what can I do to reduce the measurement error? Shielding is often impractical for a large HF antenna. Is a SWR meter less vulnerable to this problem than a VNA due to its higher output power?