A few comments. 180 degree radiators are almost always avoided as at high power the base can literally be explosive. Theoretically 225 degrees would yield the most radiation towards the horizon but presents two problems. The first (for class A stations) is the secondary lobe that causes self interference skywave. The second issue is self loading where the radiator is longer electrically than it is physically. In such instance a 225 degree radiator is actually electrically longer causing the main lobe to lift off the ground and break into additional sidelobes. For that reason, class A stations opted for a compromise height of about 195 to 200 degrees. This lowers the base impedance over 180 degrees while limiting secondary skywave lobes to an acceptable level and staying in a safe area as far as self loading is concerned. Some class C (former class IV) stations use 225 degree radiators, though.
Bandwidth is partially a function of the size of the tower face and also the design of the matching network. The problem becomes challenging when radiators get down around 60 degrees. There are two directional class A stations I can think of - WBZ Boston and WWL New Orleans. Both are very much alive and well and still quite successful. Sent from my iPad > On Dec 18, 2018, at 2:43 PM, Fred Jensen <k6...@foothill.net> wrote: > > My mini-dissertation strictly applies to Class A [so-called "Clear Channel"] > stations only, who almost universally use single, omni-directional verticals > since they are not required to protect any other stations at night. There > were a few Class A's on the coasts that did use directional phased arrays > since there weren't many/any listeners out at sea but I don't know if any of > them are left. Non-Class A's generally employ 2 or sometimes 3 phased > verticals to achieve directional patterns at night to protect co-channel > stations, putting null(s) in the direction of their service areas. > > Class A's, such as KFI, seek to maximize the coverage in their service area > [i.e. maximize the area covered with an adequate field strength]. Neither a > 90-degree vertical ... current max at the bottom ... nor a 180-degree ... > current max half way up the tower ... will do that. At 640 KHz, a half-wave > is ~730 ft. > > The obsession with VSWR in ham radio is a bit surprising. I can't really > remember when the term became common, or when I first saw a VSWR bridge, but > I do know that I never saw one in SE Asia in the mid-60's. These days, high > VSWR seems to equate to "antenna that doesn't work." > > 73, > Fred ["Skip"] K6DGW > Sparks NV DM09dn > Washoe County > >> On 12/18/2018 12:38 PM, hawley, charles j jr wrote: >> Interesting, I had an analog computer course exercise at the U of I Urbana >> when we had two verticals separated and phased to define the coverage >> pattern. I don’t remember the length of them. It’d be interesting to recall >> what it was. >> >> Chuck >> KE9UW >> >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to w...@w2xj.net ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
