Hi > On Aug 7, 2020, at 10:48 PM, Bill Byrom <[email protected]> wrote: > > Whit Griffith N5SU was the Chief Radio Scientist (or some similar title) for > Continental Electronics in the 1980's in Dallas. In around 1990 Whit gave a > slide show presentation to a Dallas Amateur Radio Club meeting at the > National Communications Museum, a project at the Dallas Communication Complex > in Irving started by Bill Bragg KA5PIP (SK) who became the voice of BIG TEX. > Whit showed amazing photos of the VLF transmitter and antenna they had > installed in Germany - I believe it was DHO38 at 23.4 kHz. The > helix/variometer room was huge, and the antenna used large cables strung > between tall towers as a capacitive top hat, and included an impressive > ground system. > > The radiation resistance of a VLF vertical antenna is very low. At 100 kHz > one wavelength is 3 km (9,843 feet). So if an antenna was 412 m tall (1,352 > feet), it would only be 0.137 wavelength long. The low radiation resistance > of such a short antenna requires a high antenna current. At high power levels > (1 MW or even more), the antenna current can be several hundreds of amps, and > this can produce very high voltages across reactive components of the antenna > system and matching network. > > The result is that high power VLF transmitting stations require a lot of > land, a very expensive and large antenna and matching system which retunes > itself as the weather changes the antenna impedance, expensive provisions for > lightning and high wind protection, an expensive transmitter, and a very high > cost for the utility power and maintenance. > -- > Bill Byrom N5BB > >
One of the WWVB transmitters *might* fit in a large garage. The antenna matching gear at the base of one of the antennas …. no way. It’s closer to the size of a small house. Of course as VLF stations go, WWVB is “low power”. Same basic issue, lots of weird interactions and a need to keep the signal very precise. Not as easy as it might seem. Bob > > On Fri, Aug 7, 2020, at 7:16 PM, Bob kb8tq wrote: >> Hi >> >> Back in the day, if you hung out for a while in the lobby at Continental >> Electronics, you would notice a >> model of an old style transmitter over by one wall. Go over and look a it >> for a a while and all the usual >> parts were there. Couple of big tubes, big matching coil insulators here and >> there. Eventually you would >> notice this tiny spec down by the bottom of the model … hmmm … wonder what >> that is? >> >> Eventually one might figure out that the tiny spec was a person. The model >> was of an Omega transmitter …. >> >> Bob >> >>> On Aug 7, 2020, at 7:33 PM, jimlux <[email protected]> wrote: >>> >>> On 8/7/20 4:13 PM, Bill Byrom wrote: >>>> See this 1961 IRE paper at the NIST website: >>>> https://tf.nist.gov/general/pdf/2303.pdf >>>> IRE merged with AIEE in 1963 to form IEEE. >>>> Figure 7 shows the calculated amplitude transfer of the ground wave signal >>>> vs frequency and distance. Note that for 100 kHz signals, the ground wave >>>> signal is reasonably strong at 2,000 miles but lousy at 5,000 miles. >>>> As this paper notes, the sky wave reflections are delayed, and this delay >>>> depends on the ionization state of the ionosphere along the propagation >>>> path. This delay is shown in figure 2. >>>> Figure 6 shows differences between daytime and nighttime propagation of >>>> pulsed signals. The received signal is a combination of the ground wave >>>> signal and one or more skywave signals (which are delayed with respect to >>>> the ground wave signal). >>>> -- >>>> Bill Byrom N5BB >>> >>> and such stuff is why Omega worked at VLF frequencies - none of that pesky >>> skywave - lambda=30km and you're ALWAYS below ionospheric cutoff. Alas, >>> they made some boneheaded mistakes like making one of the frequencies an >>> exact multiple of 60Hz. >>> >>> There is something positively Tesla-ian about Omega with high power low >>> frequency transmitters into physically enormous antennas - like the one >>> with the top hat across the fjord. None of this tiny L-band patch antenna >>> stuff inside a wristwatch. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
