Low power longwave transmitter (DCF77) - Page 1
Forum section: electronics / rf-microwave

author: Zbig
author_rank: Frequent Contributor
timestamp: March 02, 2020, 06:13:56 pm
content: I have several radio-controlled (DCF77) clocks around the house but where I live, the signal is no longer usable - probably due to increasingly crowded RF spectrum and interference from all the SMPSs everywhere, etc. so the clocks rarely - if ever - are able to sync anymore. My idea was to build a small AM transmitter with just enough power to cover my apartment but being an RF noob I am, I think I've underestimated the challenges of transmitting on long wave. Basically, I don't have much clue how to build an antenna and of what type to get a shot at broadcasting DCF77 apartment-wide and whether that's possible at all. I've did some research but failed to find any meaningful information on transmitting on longwave with an antenna smaller than half the city (and maybe that's the clue right there...). Is that perhaps possible with a "mag-loop" antenna? Anything other? Or should I just call it quits? I know I am able to generate proper DCF77 frames as I've built a working proof of concept that is able to fool the clock into adjusting to the arbitrary time of my choosing. The problem is, I use a few turns of random wire as my "antenna" and it has to basically touch the back of the clock's case to work so I guess it just works thanks to capacitive coupling more than anything else.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2020, 07:01:10 pm
content: It should be illegal to transmit at that frequency with enough power to effect more than the very close vicinity.Usually the antennas are magnetic antennas. As one is in the near field, it makes a difference wether one generates a magnetic or electric field. So if at all generate a magnetic field to couple to the antennas in the near field.This may be acceptable for short time use to set the clocks once a week or so.

author: SeanB
author_rank: Super Contributor
timestamp: March 02, 2020, 08:08:04 pm
content: How about an external magnetic loop tuned to DCF77, feeding via a simple low gain buffer stage into the house, with another smaller untuned loop near the clocks, to give them a stronger signal over the interference.

author: johnkenyon
author_rank: Regular Contributor
timestamp: March 10, 2020, 03:42:50 pm
content: Avoid the issue (where possible) by disconnecting the RF receiver and injecting a base band signal.I have a similar issue with large WWVB (read US spec) capable clock, which is only within range of the UK MSF signal - Same frequency different encoding.The 60kHz receiver PCB is separate from the main control board, and has power/shutdown/and data connections.When I get around to it, my intention is to disconnect the 60kHz receiver board, feed the 60kHz MSF signal into an Arduino, decode it, apply a 5hr fudge (the clock expects UTC time data, and subtracts 5/6/7/8hrs to display local time), then tap out the data in WWVB format directly into the receiver.I might not even bother with the MSF if I can feed the Arduino with NTP over a wifi connection (or use an ESP module to do the whole job)John

author: Lord of nothing
author_rank: Super Contributor
timestamp: April 01, 2020, 01:53:01 pm
content: Someone told me the "normal" Clock just use the Signal one a Day or so to Calibrate them self.So you might need to broadcast 24/7 to get the Signal right.Another problem is you should have an accurate Time Signal. Maybe you will use a GNS Receiver for.

author: johnkenyon
author_rank: Regular Contributor
timestamp: April 02, 2020, 02:48:44 pm
content: Quote from: Lord of nothing on April 01, 2020, 01:53:01 pmOn my "large WWVB (read US spec) capable clock", the receiver is on a separate board, and from vague memory only gets turned on when the clock wants to synchronise.(I can't remember whether this is done by actually turning the board on, or if there is a separate "enable" pin.The clock itself waits a while for a valid signal, so a WWVB emulator which can start up from dead quickly would work (this would preclude "direct" GPS, but querying a NTP over wifi or other wireless media would work)