U/Ls are "grass roots" and do not require radios/transponders. They are limited to what class of airspace you can enter, but I think I read they recently got approved for another class of airspace but I'm too out of the loop to remember it all.
The grass field I learned to fly out of had no tower or permanent radio, so people just used visuals for organizing who was landing or taking off, that's the purpose of the circuit.
I don't take claim to the data or descriptions that I included below, but I geocoded a map of the airports/strips in Ontario by their status and you can see it here:
http://batchgeo.com/map/5885c0f2cf38d852bdf84fc737c26342
AERODROME STATUS
Certified (Cert)
An aerodrome for which an airport certificate is issued, requiring the operator to maintain and operate the site in accordance with applicable Transport Canada standards. Regular inspections are conducted by Transport Canada to confirm compliance.
Registered (Reg)
An aerodrome listed in the Supplement which is not certified as an airport. Registered aerodromes are not subject to an ongoing inspection program. Pilots intending to use these aerodromes should obtain current information from the owner/operator.
Military (Mil)
An aerodrome that is owned and operated by DND and is not certified or inspected by Transport Canada. All military aerodromes require prior permission (PPR) for civilian aircraft. The utilization of any DND aerodrome/heliport, including those listed as abandoned, as well as, DND facilities for the purpose of storing petroleum products (POL), is strictly prohibited without written approval of DND.
NOTES:
Prior Permission Required (PPR)
Where the acronym "PPR" is shown, the aerodrome owner's or operator's permission is required prior to use, except in cases of emergency.
Prior Notice Required (PN)
Where the acronym "PN" is shown, the aerodrome owner or operator is to be notified prior to use in order that current information on the aerodrome may be provided.
Landing Fees
Where "Ldg fees" is listed, the aerodrome operator charges a fee to all users for using the aerodrome. The exact fee can be established by contacting the operator.
**CUSTOMS (AS MOST CUSTOMS ARE NOT 24 HOURS AND HOURS OF CUSTOMS AVAILABILITY CHANGE FREQUENTLY, PILOTS ARE ADVISED TO PHONE WELL IN ADVANCE TO CONFIRM HOURS)
AOE refers to Airports of Entry, and designates all aerdromes where customs services are available. Aerodromes with capacity limitations are indicated by a number preceded by a "/", e.g., AOE/44.