Idea: Optimizing simple Yagi antennas? - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: December 06, 2024, 04:41:21 pm
content: Has anyone studied what happens to the radiated power if you just add reflector or director elements to an unmatched dipole. Gain goes up but antenna impedance goes down. Do you always need impedance matching to get more power to a receiver. Is there already info about optimum design.A job for a VNA, I think.

author: A.Z.
author_rank: Super Contributor
timestamp: December 06, 2024, 10:58:39 pm
content: http://on5au.be/content/a10/ant36.html

author: LM21
author_rank: Regular Contributor
timestamp: February 12, 2025, 04:10:19 pm
content: I made a simple test.I used a 440MHz dipole at VNA output end and some dipole few meters away as a receiver. NanoVna shows that dipole impedance rises from 60 ohms to 70 ohms when I brought wire behind the dipole. It was slightly longer than the 440MHz dipole. The path loss was always very close to 35dB. I thought that extra elements lower the antenna impedance and rise the gain.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: February 13, 2025, 04:48:33 pm
content: Quote from: A.Z. on December 06, 2024, 10:58:39 pmLink doesn't work.

author: CaptDon
author_rank: Super Contributor
timestamp: February 13, 2025, 11:29:15 pm
content: I feel your measurements are still being done in the 'near field' environment. You probably need a distance of 50 feet or more. Also the apparent gain is bought by directivity. You are bringing the available power from a broad radiation pattern in to a more concentrated energy beam. Your 35 dB path loss may change a small amount, but the power in the side lobes will change both shape and field intensity to a much larger degree.

author: A.Z.
author_rank: Super Contributor
timestamp: February 14, 2025, 09:08:45 pm
content: Quote from: ahbushnell on February 13, 2025, 04:48:33 pmtry herehttps://web.archive.org/web/20230219030200/http://on5au.be/content/a10/ant36.html

author: LM21
author_rank: Regular Contributor
timestamp: February 15, 2025, 09:04:22 am
content: Quote from: CaptDon on February 13, 2025, 11:29:15 pmThe distance was several wavelengths, and a formula from net told that receiving end was in far field. But something is clearly wrong.The formula "multiply the diameter of the antenna squared by 2, then divide by the wavelength of the radio wave." 2(0.3*0.3)/0.7 gives 0.3m.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 15, 2025, 10:54:04 am
content: Here in the UK, radio amateurs are required to comply with EMF regulations to limit exposure to members of the public to RF fields. As an experiment, I calculated the field strength at ground level from my 2m 6 element Yagi, then went across the road with a dipole and hand held spectrum analyser to measure the level. Perhaps not surprisingly, the measured level when converted to Volts/metre field strength matched with the calculated level at that distance. At the time I had the loan of field strength meter and while the readings were nothing like as stable as those from a horizontal dipole and SA, it did match my calculations too.You can do a lot with antenna modelling and the results really do match actual measurements. Have a play withhttps://eznec.com/SJ

author: A.Z.
author_rank: Super Contributor
timestamp: February 15, 2025, 12:33:15 pm
content: at 2m on the 2m band ?consider the fresnel area

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 15, 2025, 12:34:46 pm
content: The transmit antenna was 11 metres above ground, the receiving antenna was at head height, both horizontally polarised.SJ

author: A.Z.
author_rank: Super Contributor
timestamp: February 15, 2025, 02:11:53 pm
content: Quote from: Solder_Junkie on February 15, 2025, 12:34:46 pmrepeatconsider the fresnelhttps://en.m.wikipedia.org/wiki/Fresnel_zone