I took the homebrew QRP 1 m diameter small transmitting loop out to a local summit for some fun and was surprised with some fantastic propagation, on 40 meter nonetheless. This was one of my first DX contacts on the band, and with a small compromise antenna and QRP power, nonetheless.
Small transmitting loop antennas are relatively simple and fun to build. Here’s a little helpful info to get you on the right path to building.
https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx
http://webclass.org/k5ijb/antennas/Small-magnetic-loop-K5IJB.htm
Also check out @Amateur Radio VK3YE’s videos here on YouTube – he has done lots of work with small transmitting loops.
Here’s the hardware I use to make the larger part of the loop. Pretty simple bar-stock aluminum folded out straight and then bent into a circle. Works great!
1/16" x 1/2" Flat Aluminum Bar: https://amzn.to/3trTdAX
Mini project box: https://amzn.to/3h8FeNP
6-32 x 1/2" Stainless Screws: https://amzn.to/3tmqI7G
6-32 Nylock Nuts: https://amzn.to/33lLnyj
#6 Flat Washer: https://amzn.to/3h9YJWy
Feeding and matching a loop takes a bit of trial and error, and some specific electrical components – primarily variable and potentially also switched capacitors that can handle very high voltage and currents. Study up before you build to optimize your design.
If there’s interest, I can create some videos that go more into my strategy for building and matching these types of antennas. It’s a fun build!