Hi, Im 300ohm who designed the NAROD (the VHF-HI/UHF) version of the Gray-Hoverman antenna. (I also have a fractal uhf version posted in the forum too, because a lot seem to be fractal obsessed, heh) I found this thread amusing, so I have to jump in, heh.
I can see on the DHC forum who 300ohm, you refered to, is? , lol lol lol]
I only did that because I knew I'd someday confuse hd fan, heh.
The reason is actually that most TV antennas use a folded half wave dipole instead of a half wave dipole
The reason Ive read that they used 300ohm impedance is that back in the old days, 300 ohm twin-lead was the size they could comfortably and cheaply manufacture. (there seemed to have been a big clamor for the use of 600 ohm line back then) Consequently all TV antennas were designed to use 300 ohm twin-lead for a good impedance match. Even now, all the commonly available cable and baluns are set for that standard. If designing for 75 ohms, a 1 : 1 balun would be needed anyway, saving nothing. With the NEC2 computer modeling engine I use, you input the desired impedance rather than relying on the intrinsic impedance of the antenna itself. The mismatch shows up as VSWR, which you then tweak the design to have the lowest possible figure. (The NET gain figures take that into account. If NET gain is not posted, assume RAW gain.)
Could you give us more details in regards of the performance of this antenna in the High section of the VHF Band (Band III, Ch 7 to 13).
All that info and more can be found at [Link: digitalhome.ca]
The NEC files are also posted, so that any antenna modeler can look at them, copy and paste it into a NEC modeling program and improve upon it if he so chooses. The excellent and totally free 4nec2 program can be downloaded from Arie Voors website at [Link: home.ict.nl]
The NEC2 engine that it uses was developed by Lawrence Livermore Labs for use by the U.S. Navy on a Cray 1 supercomputer. The program was extensively used and tested by them for about 2 decades before being released to the general public domain in the 1990's. With PC's being so many times more powerful now, the program really comes to life.
have created an item that is inherently elitist, isn't it?
Heck no. The postings and plans and parts lists are in plain, simple, down to earth language that anyone should be able to understand. I would argue that anyone who isnt willing to get their hands dirty by building their own antenna is an elitist snob, heh.
Since corporations will generally sell their grandmothers if that will make them money, why aren't they reintroducing at least the earlier design?
Well, why are a lot of the "new and improved" antenna models coming out, worse than the ones they will replace and cost more ? Go figure. The "new" Coke maybe ?
I built one of these too using a 2x4, 12 gauge copper romex, and 6 wood screws and washers.
Wood is not a good thing to lay antenna elements on. Plastic is much better and even with plastic, you want to have minimum contact with the driven elements. And avoid having the washers intrude into the feedgap space, that in effect narrows the feedgap spacing.
Last edited by 300ohm on April 9, 2009 00:28.