Antenna Tuning with VNA: how much additional VNA port protection needed? - Page 1
Forum section: electronics / rf-microwave

author: indeterminatus
author_rank: Contributor
timestamp: June 04, 2021, 03:28:18 am
content: Hi,note that I am a beginner with respect to electronics, and RF specifically. I would like to tune/characterize/inspect an antenna; for now, let's assume the antenna is supposed to work at around 1 GHz, but I'm interested in a general solution for all kinds of antenna sizes (well OK, let's stick to the ones that I would realistically get my hands on).I would like to connect the antenna to a VNA (S11 measurements).What I'm lacking is a good idea (ideally, calculation) how much (additional) input protection between the VNA and the antenna is needed.What I know so far/have gathered:I have the specs of the VNA ready, so I know my "target"EM waves with frequencies "outside" the visible in the VNA hit the VNA's frontend nonetheless, so a surge at 800 MHz might fry my frontend even though I'd be looking at a very narrow band around 1 GHzstatic build-up in the antenna (several kV) and feed line capacitance (in the order of nF) can be problematicroughly calculating the power emitted when transmitting (I know enough that nothing is simple with electromagnetics)What I am missing is an idea how much "external" power such antenna can pick up (that was not fed into the antenna by the VNA in the first place). I live in an urban environment, I do not know of any strong transmitters nearby, I do not have a microwave oven, and the (both physically and electrically small) antenna would be connected via the shortest path possible to the VNA in the inside (think unshielded apartment, no fancy lab).I do not have a field-strength meter, and I'd rather not rely on one alone -- after all, a sudden surge that might be picked up by the field-strength meter would be picked up by the antenna as well, and fed into the VNA (if connected) before I'd be able to react.My question: How could I determine the theoretical maximum an antenna can pick up, so that I can put appropriate attenuation between the antenna and the VNA?

author: Bud
author_rank: Super Contributor
timestamp: June 04, 2021, 04:22:32 am
content: You should not use an attenuator for S11 measurements, as you will be measuring a poorly loaded attenuator and not the antenna.Static is a real threat and can kill devices front end, but commercial VNAs have robust protection.VNA input is 50 Ohm which is fairly low impedance so you'd need to induce substantial RF current on it to develop any sizeable voltage that could damage the VNA, like being very close to another high power transmitting antenna. I would not worry about that sitting in a city apartment.

author: TheSteve
author_rank: Unknown
timestamp: June 04, 2021, 04:34:11 am
content: The only thing I would add to what Bud said is to ensure there is no static buildup on the antenna before connecting it to the VNA. Short the antenna connector briefly before connecting it. This is mainly an issue with larger outdoor antenna's. And ideally it's best to have antenna's that appear as a short at DC.

author: hendorog
author_rank: Super Contributor
timestamp: June 04, 2021, 05:36:19 am
content: You can get good return loss measurements with a low value attenuator on the port as long as you calibrate at the end of it.The downside is that you lose twice the value of the attenuator in dynamic range. For antenna measurements that could be ok with a lowish value attenuator (below about 6 dB) and it might give you peace of mind.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 04, 2021, 11:17:37 am
content: Quote from: indeterminatus on June 04, 2021, 03:28:18 amI have no idea about the theoretical maximum but imagine a direct hit with lightning would be up there. Maybe something in the following video would help you.