[TransAtlantic] K6EDX #4

[Bob Cooper]

K6EDX #4 Post

There seems to be some misconception of what a 'duct' is or does. Think of an almost lossless piece of large (HUGE! in fact) diameter coaxial cable, extending from your QTH to one hundreds, thousands of miles distant. Now give that 'cable' an elevation above sea/ground level (ASL/AGL), a physical vertical dimension (height) and a physical horizontal dimension (width). If your transmit/receive antenna is at the correct height (ASL/AGL) your transmit energy 'couples' into the duct and it transports your signal with very small incremental attenuation over whatever length as the duct exists to a receiving terminal (1) along the way, or, (2) at the terminus of the duct (end) - provided the station at the opposite end of the 'path' (imaginary cable) ALSO has their receive (and transmit) antenna at the height required (ASL/AGL) to ALSO be inside of the duct's vertical height parameters.

My apology for the basics here but based upon email responses, it is apparent not all, who should, understand the physical properties at play here. If you are (at either end of/OR along the duct) above OR below the duct's physical parameters (height or vertical thickness) no reasonable amount of energy on the transmit end will 'couple' (connect) you into this invisible-to-the-eye "waveguide."

Ducts tend to have a "resonant frequency range" and for us it is fortunate 100-200 MHz fits the "typical" duct resonance. I have experienced 50 MHz signals from Hawaii into California on ducting and 55.25 signals from Boston to the Turks and Caicos Islands (approaching 1,400 miles) but on average anything below the FM radio band in frequency (88 MHz not to be specific but as a class of signals) is non-resonant in a duct. So you can miss out on the high energy transfer efficiency of a duct by being (1) at the wrong height (too low or too high ASL/AGL), (2) the wrong north-south/east-west coordinate; simply beyond where the duct has formed), or, (3) on the wrong frequency. A percentage, fairly high in fact, of ducts 'resonate' well at say 400-900 MHz but not so good at 200(144) MHz. TV DXers can recount endless examples of this; signals at 600 miles on UHF, nothing unusual on VHF (including FM). The reverse also happens; FM (88-108 MHz or the weather radio channels in the 160-165 MHz band) are 'hot' and UHF is "dead." It pays to have a variety of receivers and the skills to monitor more than say only 144.300. 

All of this leads to my own attention to ducts from at least 1970 onward. In 1974-1979, it was my good fortune to be associated with CATA (the Community Antenna Television Association; and its monthly journal CATJ) within which several members including CATA head Kyle Moore owned their own two engine private plane. Kyle, and I, made dozens of short (under 1,000 mile) trips out of Wiley Post Field (Oklahoma City) and as he was a TV guy (operating cable TV systems) he understood when I climbed aboard with a battery or 12V operated color TV set and stuck a whip antenna to the Plexiglas window on his Bonanza. I can recount (will not) perhaps a dozen incidents when we lifted off the Oklahoma turf and he very slowly - much slower than he had been instructed by ground control - climbed through 400-600-1,000-2,000 feet to his assigned cruising altitude. The distance to, say, Lexington, Kentucky was just over 600 miles as we lifted aloft. "Stop - slow the climb down" I would instruct and for several tens of miles I could fine tune Lexington's channel 18 or 27 to allow him to fly and me to measure the duct's height. The answer was plus or minus 300 feet much of the time and with some skill he would drift up and down to allow me using his altimeter to define where the duct boundaries occurred (typically VERY sharply defined). Of course traveling to Ohio at say 800 feet elevation would have been a violation of (1) air traffic rules, and, (2) fuel consumption common sense so my 'measurements' seldom lasted for more then a few minutes. After settling at say 12,000 feet I'd urge him to drop 'back down' every few hundred miles and we'd run smack dab into the duct at the logical height virtually every time. "You are using up my fuel" was his usual chiding remark. Not to speak of why he was deviating from his 'assigned' altitude and needed to request permission to 'drop down to 800 feet' over say Joplin, Missouri!

Now move me to the 1980-1990 period. I now am VP5D and twice a month on average for ten years (200+ times) I flew in a Beechcraft D18 tail-dragger from Providenciales to Fort Lauderdale, and return; +/- 600 miles x 2. The pilot was the last of the surviving WW2 fighter pilot guys, 'Crazy Ed,' and he would do anything you asked as long as you gave him cash. Crazy Ed paid absolutely no attention to FAA/CAA rules, lied his way out of dozens (even hundreds) of official inspections with me standing beside him, and flying with Ed was akin to being a Japanese Kamikaze passenger. Once over the Bahamas his HUGELY overloaded D18 ("Weight and balance???" he would say to anyone stupid enough to ask; "If this sucker gets off the ground I pass!!!") with me in the co-pilot seat because the rear end was so badly overloaded with freight there were no seats left, we lost the entry door when the cargo bounced onto the loading door latch (only the cargo net strung in the doorway kept the freight on board with a gaping hole now where the door had been). Someplace down there in perhaps Eleuthera there is a D18 door buried in the coral. Or he would take paying passengers and stick them into the one seat crapper/loo (toilet). Once I tried to get in there through an aisleway jammed with freight and discovered a native Turks & Caicos lady with two kids crammed into the 'john.' You get the picture - Crazy Ed was totally off the wall. He 'loaded' each flight, especially from Fort Lauderdale Exec Airport down to Provo, from the front; "You are co-pilot" he would assign and then from front to back the boxes and Caterpillar D8 gears and foodstuffs and who knows what was jammed in until they reached the rear loading door. At that point a ground guy, after loading the 'john/loo' as well, would use all of his force to shut the door against the protruding boxes and freight. I cannot believe I did this - and still alive to relate the story - over 200 times!

The good times were when I had enough 'room' to operate my 12" GE battery operated TV set using a whip antenna 'Crazy Ed' had 'graciously' allowed me to install on his top fuselage - mostly because I was his best (and cash paying) client. I won't bore you with why he stopped in the Bahamas both to and from Provo but there were certain 'services' along the Bahamian chain which SOME passengers really wanted to use (read the book: "Television's Pirates" from Amazon.com). Getting to ducting; Ed was reasonably accommodating as to my '800 feet please' requests - subject ALWAYS to my promise to pay for the extra fuel these diversions would cost him. I can tell you that over a decade I witnessed dozens - tens and tens - of ducts especially in March-June, where we would lift off Exec Airport with me to tuned to a Fort Lauderdale or Miami UHF channel and 500+ miles later (or 300 miles later when we let down on a Bahamian island for 'service') as we descended down to Providenciales runway there it was - still there at about the same altitude but gone at sea level. These two generic examples sort of illustrate my point; either you are "in" the duct or you are "out" and if you are below - well - no reasonable amount of ERP (power) is going to get you in - or the other guy's signal out. Being inside the duct is paramount here - being below is to suffer a double whammy; first you are not in, and worse yet, your signal is being shielded from getting in by the moisture/temperature parameters of the bottom (or top if you are well elevated) of the duct. Think METAL waveguide here - those inside are home free; those "outside" (whether 'above' or 'below' ) might as well be in New Zealand; ducts, like metal waveguide, keep signals 'out' as well as 'in.' 

So this effort - to reach Europe on 144 MHz or above, if by the troposphere, is REALLY all about wave propagation. It is not about red blobs on Bill's maps or what happens between two distant points; it is about being conscious of what it takes to get 'inside the magic waveguide' (where power is not an ingredient; being in the right place at the right time is!)

Bob Cooper K6EDX/ZL4AAA