Antenna readings are all over the place  - Page 1
Forum section: electronics / rf-microwave

author: kellogs
author_rank: Regular Contributor
timestamp: May 22, 2025, 08:12:52 am
content: Hello,Inside the car, I have a PCB bowtie balanced antenna and trying to get some impedance readings. It is an electrically short antenna for 433 MHz. The readings vary as much as +/- 15% as I move only the cable with the antenna taped down, or when I move the antenna around the cabin, or whatever else I try.There is a 1:1 balun used rated to 500 MHz for which I have measured a phase difference of 170 - 178 degrees for the 403 - 453 MHz range. Also, I have tested the same balun with a few resistors and capacitors (non radio grade) and have observedstable readings.From what I have gathered both the balun and the nanovna + calibration are good. Is it normal to see this behaviour ? If it is, then how on Earth can I match the impedance for such antenna ? My receiver is so deaf it only catches 1 in 100 preambles from no more than half a meter away...Thank you

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 23, 2025, 01:08:15 am
content: You have too many obstacles in near field. Your car is made of metal, which detunes your antenna.Any metal object within a few feet will detune your antenna. You can use a calculator online to figure out where the near field region of your antenna is.

author: radiolistener
author_rank: Super Contributor
timestamp: May 23, 2025, 03:03:32 pm
content: yes, antenna very sensitive to any conductor, magnetic material or even dielectric within near field zone which is usually wavelength or smaller, depending on antenna size, but not less than wavelength / pi. For 433 MHz it will be 0.7 meters. For proper antenna behavior you need to remove any things within that radius from antenna.Also note that the car cabin effectively acts as a reflector of electromagnetic waves. Therefore, placing an antenna inside it leads to effects similar to positioning a light bulb in a mirrored room, predicting the signal propagation under such conditions becomes highly challenging.

author: A.Z.
author_rank: Super Contributor
timestamp: May 23, 2025, 04:30:49 pm
content: Quote from: EggertEnjoyer123 on May 23, 2025, 01:08:15 amwould you be so kind to post some photos of the antenna (and balun) to allow better understanding how it's made and how it's installed ?Also, if possible, connect the antenna to a VNA and check its impedance (r+x) not all bowtie antennas have a 50 Ohm Z0

author: coppercone2
author_rank: Super Contributor
timestamp: May 26, 2025, 02:49:44 am
content: antenna reading all over the place. First thing I thought is that its working right if its doing thatIf it was not all over the place I would think something is shorted

author: antenna
author_rank: Frequent Contributor
timestamp: May 26, 2025, 03:15:07 am
content: The only way moving/touching the coax cable affects SWR is if the coax shield is part of the antenna (common-mode current).The bandwidth of a bowtie is not something you need at 433MHz, so why that antenna?. I would be tempted to make a slot antenna on the windshield using foil tape.Slots are kinda neat. If you get the slot length close, you set the feed point impedance by the feed distance from the end and then adjust the frequency by extending the other end of the slot. It is surprising how well the match is maintained moving just the far end. I made a foil tape slot antenna on a notebook with 1/4" copper tape and it was easy to tune and also seemed to care very little about me touching the coax or having things near it.

author: A.Z.
author_rank: Super Contributor
timestamp: May 26, 2025, 08:52:57 am
content: Quote from: coppercone2 on May 26, 2025, 02:49:44 amok, I give up.