[TowerTalk] New MF Broadcasting Array

[Gene Smar]

Frank et al:

     The feedpoint arrangement reminds me of my crossed-dipole turnstile array I used for a bit during the 80's to listen to  OSCARs on 2M.  The two dipoles were fed from a common coax and connected to same via a 1/2 WL coax cable connected as a 4:1 balun.  The dipoles were then mounted over a metallic screen 1/2 WL below it.  The purpose was to generate a very high TOA for the antenna, similar to one of the antenna model patterns shown.  The net result was a reduction in polarization loss on the downlink as the satellites' signal rotated between space on the ground.

     I don't quite understand why the MF broadcaster would want to generate CP MF signals; the ionosphere would surely ignore any polarization (RH or LH) and put its own spin on it, so to speak.

     The design more closely looks like a NVIS (Near-Vertical Incidence Skywave) design.  I've been messing with one of these designs for the past few years, mainly for RACES and MARS HF circuits.  For those who aren't familiar with NVIS antennas, they launch MF and HF signals, up to 10-12 MHz or so depending on MUF, etc., into the ionosphere at very high TOAs, typically >70 degrees.  The refracted MF/HF signal then returns to earth in a cricle out to several hundred miles from the transmitter with very little attenuation.  This provides 1. higher signal strength at the receiver and 2.  coverage into the <skip zone>, that is, the area around the transmitter beginning where the ground wave signal from a vertical antenna peters out and ending where the first F-hop signal from a high dipole would normally touch ground (maybe 700-1200 miles away.)  

     I erected a pair of crossed inverted Vs last year for the Scouts' JOTA exercise near here.  The dipole wires were cut for 40M and 20M, and the center was at 19 feet above ground.  With 80W I was able to work into the Left Coast and Europe on 20M and 17M (not really NVIS coverage) and throughout the US midwest on 40M from Olney, MD, about 20 miles north of WDC.

      FYI:  http://www.w0ipl.net/ECom/NVIS/nvis.htm .


73 de
Gene Smar  AD3F


From: donovanf at starpower.net
Date: 2007/07/04 Wed AM 08:30:38 CDT
To: TOWERTALK at CONTESTING.COM
Subject: [TowerTalk] New MF Broadcasting Array

Lets hear some comments about this new MF broadcasting array:

http://www.broadcast-transradio.com/Horizontal_Task_for_TRANSRADIO.pdf

73!
Frank
W3LPL
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