The old method of relying on word of mouth or simple flyers in the mail doesn't work very well these days. Turntables are a big thing now so consider trying to draw in that crowd to your radio meets by using some keywords. Same with amplifiers and the audio crowd.
Here are a few things that we do with the MSARC meets:
Start a mailing list. Have a sign up sheet at your swap meets and harvest email addresses.
Post your event to various online forums.
Advertise on Craigslist on several nearby cities
Post to various radio groups on Facebook.
If it does not already exist, consider creating a Facebook page for your club and posting your events on there.
If you are near a large city then they probably have a tourism site with a calendar of events. I took advantage of the Louisville events site to promote our next local swap meet:
https://www.gotolouisville.com/events-c ... swap-meet/Print up flyers and post them at various area antique shops and flea markets.
Consider starting your meets later in the day, say 9 or 10 AM. You are more likely to lure in the general public and collectors from father away. Unless you are retired or a dedicated enthusiast, most people don't want to wake up at 4 AM on a Saturday to attend a swap meet.