[Antennas] Vertical beams

[cboone at earthlink.net]

For HF, vertical arrays can be made with normal verticals and properly fed in/out of phase for highly directive patterns (AM Broadcast stations do the same thing with their towers but their systems are much more complicated than what most amateurs need)...IF you can put up 1/4w on the band in question, that makes it easy....but shorter verticals (loaded, etc) can be used IF each vertical is tuned to the same freq for a 50ohm match...in AM BC use, short towers may or may NOT be matched to 50ohms depending on the system (I Had one AM with towers 66degrees tall; 90deg is 1/4wave)...needless to say, the base Z was BELOW 36-50 ohms...and they were originally fed with open wire line...later we rebuilt the array with Heliax and had to redo all the tuning networks at the towers AND in the phasor cabinet that made the array work as designed (main lobe in the proper direction, NULLS in their proper place to protect cochannel stations)..In an active array, both elements are driven...so you dont have a reflector or director..but a non-active array (with a parasitic reflector or director) is easier to construct.

Easiest active HF "Yagi" is two quarter wavelength verticals (or similar) spaced 1/4wave or 1/2wavelength apart and fed with 90 or 180 degree phasing in the coax system...Look in any AARL Hnadbook and you will see the simple but effective patterns you can get..an ACTIVE array is more effective than a driven element and passive reflector, etc...better F/B ratio and more gain in the main lobe...
(in AM station, the lobes are NOT always inline with the towers...the famous 12tower array east of Dallas (the former 1190 KLIF, now KFXR) has 12 tower5s...two rows of 6 each...the pattern is BROADSIDE to the towers with the main lobe at around 270deg true..and VERY narrow (about 20 deg wide; the joke was the signal went down Commerce St in downtown Dallas but didnt touch the curbs!)...5KW into the array gets you 75KW Effective Radiated power (ERP)...go 5 miles off the back side of the array, you can see the towers but CANNOT hear the station! THATS a null!

Chris
WB5ITT
(Former Chief Egr of 1190 Dallas :)


-----Original Message-----
>From: r miles <greenacres113 at charter.net>
>Sent: Feb 22, 2010 11:40 AM
>To: antennas <antennas at mailman.qth.net>, towertalk <towertalk at contesting.com>
>Subject: [Antennas] Vertical beams
>
>I want to web search them. Anyone know the exact name they go by? The ones with a split driven element & a reflector or  director some of the DXpeditions are using.
>K9IL
>