Hi > On Aug 8, 2020, at 7:18 PM, Bill Byrom <t...@radio.sent.com> wrote: > > Yes, the weather can change the phase of the transmitted antenna signal > unless corrections are performed. From the WWVB (60 kHz VLF time/frequency > station at NIST in Fort Collins, CO) website at > https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwvb (see > second paragraph about automatic antenna tuning):
Yes … but …. There’s two antennas. They each have their own matching setup. Each are fed by this or that transmitter (= there are backups …). As long as everything is stable it all works fine. Wind blows and the antenna(s) move. Matching network ( one or the other ) responds. Since the other antenna is tightly coupled, the other matching network responds to the first one’s adjustments. They fiddle back and forth … While this is all going on, who knows what the phase is in this or that direction. Once it all balances out, you can do things to servo correct. Fun !!! Bob > >> Ideally, an efficient antenna system requires a radiating element that is at >> least one-quarter wavelength long. At 60 kHz, this becomes difficult. The >> wavelength is 5000 m, so a one-quarter wavelength antenna would be 1250 m >> tall, or about 10 times the height of the WWVB antenna towers. As a >> compromise, some of the missing length was added horizontally to the top >> hats of this vertical dipole, and the downlead of each antenna is terminated >> at its own helix house under the top hats. Each helix house contains a large >> inductor to cancel the capacitance of the short antenna and a variometer >> (variable inductor) to tune the antenna system. Energy is fed from the >> transmitters to the helix houses using underground cables housed in two >> concrete trenches. Each trench is about 435 m long. >> A computer is used to automatically tune the antennas during icy and/or >> windy conditions. This automatic tuning provides a dynamic match between the >> transmitter and the antenna system. The computer looks for a phase >> difference between voltage and current at the transmitter. If one is >> detected, an error signal is sent to a 3-phase motor in the helix house that >> rotates the rotor inside the variometer. This retunes the antenna and >> restores the match between the antenna and transmitter. > > > Changes to the antenna and tuning network affect the transmitted phase, and > of course phase changes over time affect the fractional frequency error. At > WWVB they control the actual transmitted phase so that it doesn't effect the > received frequency accuracy. >> The frequency uncertainty of the WWVB signal as transmitted is less than 1 >> part in 1012. If the path delay is removed, WWVB can provide UTC with an >> uncertainty of about 100 microseconds. > -- > Bill Byrom N5BB > > > > On Sat, Aug 8, 2020, at 3:15 PM, Hal Murray wrote: >> >> kb...@n1k.org said: >>> Same basic issue, lots of weird interactions and a need to keep the signal >>> very precise. Not as easy as it might seem. >> >> What does "precise" mean in that context? >> >> I'm not an antenna-nut. Can an antenna miss-match change anything other >> than >> the amplitude? >> >> How do you automatically tune something like that? The manual way would to >> twist the knob while watching a meter. If the meter goes down, you are >> going >> the wrong way. If it goes up, keep going until it starts going down, then >> back up to the peak you just passed. >> >> How do you even know that it needs tuning? Can you measure something >> accurately enough? If so, what? >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
