Building a meandered inverted-F antenna for SW reception - Page 1
Forum section: electronics / rf-microwave

author: prabhatkarpe4
author_rank: Contributor
timestamp: October 24, 2024, 11:20:06 am
content: I came to know that a meandered inverted-F antenna (MIFA) can be used to reduce the size of a long antenna and it's often used in mobile phones to make the antenna smaller.So, can MIFA be used to get SW reception for a radio receiver?What is the method and formula to build the antenna?

author: A.Z.
author_rank: Super Contributor
timestamp: October 24, 2024, 05:30:59 pm
content: try asking yourself a question; given that such antenna has been around for quite some time, why nobody used it for SW/MW ?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 25, 2024, 12:59:39 am
content: What is the wavelength at WiFi frequencies?What is the wavelength at shortwave?

author: rf-fil
author_rank: Regular Contributor
timestamp: October 25, 2024, 05:03:31 am
content: A MIFA structure would work OK, if its size is proportional to the frequency. For HF, it would probably be at least a meter across, maybe a few meters. Now.. electrically short antennas like that are sometimes used on HF, when there is no room for anything better. The reason why a MIFA / PIFA and similar structures are not normally used on HF is that you don't really need them on HF. A simple short stick with a loading coil at the base, or other similar matching circuits are OK. A SW broadcast receiver is another example - they just use a short whip, and a high impedance receiver pre-amp to get it to work - no need for complex antennas.With 1GHz and above, matching circuits are harder to design, so you use the MIFA / PIFA / etc to electrically lengthen the antenna, to make the matching circuits easier.Ultimately, these small antennas all have fairly low efficiency, so people use electrically larger antennas wherever possible.Also, for HF reception, a magnetic loop antenna is also small, but has some advantages: they don't pick up local RF interference as much, are somewhat directional and can be narrow band. A loop antenna is a much better choice if you don't have room.As for formulas - there aren't any. People use EM software ($$$) to design these. But also... you can just build one, and use a VNA to measure its impedance and then design a matching circuit for it.

author: fourfathom
author_rank: Super Contributor
timestamp: October 25, 2024, 06:54:04 am
content: Reception and transmission are quite different when discussing antennas. Yes, they have reciprocal behavior on TX and RX: gain, directivity, etc. But on TX we are trying for maximum power coupling and this usually means we want a tuned antenna. But on RX we may not care about optimum power transfer. Instead, we want the receiver sensitivity to be somewhat better than the background noise, and it doesn't take a well-matched antenna to do that. "Official" reference antennas used for atmospheric noise floor measurements are often only two meters in length, omnidirectional and vertically oriented, connected to a very high impedance preamplifier. An antenna like this can be used to measure the "rural quiet location" atmospheric background noise from 1 to 30 MHz -- you gain nothing from more sensitivity than that.Of course a directional antenna can have big advantages in reducing noise and interference from some directions, and help with dynamic range problems and intermod products. And polarization can make a huge difference. Also FWIW, most people do not have a quiet rural location, but instead have a huge amount of EMI and locally-generated noise. And the common-mode noise pickup on the shield of the coax cable interconnect can often be a major problem, not cured by just a few ferrite cores. It can get pretty complicated.