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 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.
