Article 7292 (236 more) in USENET>rec.ham-radio: From: parise%uit.span@VLSI.JPL.NASA.GOV (Ron Parise) Subject: Discone Antennas Message-ID: <881024062708.874@VLSI.JPL.NASA.GOV> Date: 24 Oct 88 13:27:08 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Discone antennas are indeed very impressive for just about any frequency range. I do not believe that one can be build which will cover 25-1300Mhz as RS and others claim. A 10 to 1 bandwidth ratio is all that can be expected from the design. The low end cutoff frequency is determined by the length of the cone (slant length). The cone is 1/4 wavelength at the lowest operating frequency. The response of the structure degrades very rapidly below that freq. The length of the elements on this recent flood of discones on the market is such that it's low end cutoff should be about 100MHz. You might also notice that they specify that it can be used for transmitting on 144,220, and 440Mhz, even though they claim it is good down to 25. They do not claim it can transmit on 50Mhz, which supports my claim that it just won't work below 100. I think they just claim the wide bandwidth to make it appeal to the scanner crowd. I started playing with dicones about 10 years ago and I currently have on that covers 48-480MHz, just right for 50, 144, 220, 440, and everything in between with < 2:1 SWR. I would like to build an HF discone for 7-30MHz at some point. They give excellent low radiation angle response over the whole HF spectrum and are really not that big (relatively speaking). Building discones is quite easy as they are not particularly critical in their construction requirements. You can find a set of simple cookbook type formulae in the radio amateurs handbook for designing your own. Happy disconing! Ron From: Pete_Simpson@MERCURY.CEO.DG.COM Subject: Discone antennas (and Ethernet coax) Message-ID: <58.016695@adam.DG.COM> Date: 25 Oct 88 12:24:16 GMT Sender: daemon@ucbvax.BERKELEY.EDU Reply-To: Organization: The Internet Lines: 58 There was an article in 73 Magazine a year or so ago (I'll see if I can find the exact issue) which described how to build a discone for 30 - 1200 Mhz. Since the discone only has a working frequency range of about a decade, the author designed it to cover 120 - 1200 Mhz and added a cut-down helical CB whip to the top of the antenna. This whip, together with the mounting pole, formed a coaxial dipole for 30 - 50 Mhz. I built it. It's made of 1/8" brass welding rod (about $6 worth), has 8 radials (in both disk and "skirt") and the hardest part of building it was getting the place where the radials connect machined. It would appear that it's not possible to build a discone without at least a good drill press & maybe a lathe. Anyway, I got a friend who works in the machine shop at MIT to do it for me. The article had all the details & he just whipped up the pieces for me out of scraps around the shop. It works fine. I have a VHF - UHF TV mast mounted preamp at the antenna (covers from channel 2 to old channel 83, about 56 - 900 Mhz or so) to fight feedline loss. I have a friend who used one of these on his Grove Scanner Beam. They have a lousy noise figure (about 6 db is typical) but plenty of gain so you can use cheap coax. The RSGB VHF - UHF handbook has drawings for another version of the VHF - UHF discone (I like their junction hardware a little better but it's tougher to machine). I've looked at commercial discones (I spotted one on a building at Spaceport USA when I was in Florida this spring and counted radials as the tour bus drove by - I believe there were 8 but I've seen them with 16). It was made of welded aluminum, painted white. About 1/2" - 3/4" dia. radials. Also in Florida, just outside Ft. Lauderdale, there's a commercial HF radio facility in a field next to the highway (talk about antenna farms!). I made my wife take a detour and did some quick photography. Lots of rhombics but the most impressive thing I saw was an HF discone. Picture a 60 ft dia ring of 60 ft tall phone poles with a wire connecting their tops in a circle. From this circular wire, about every foot or two along its circumference, wires run down to a plate about 6" off the ground in the exact center of the circle of poles. (there's your cone...I assume the disk was buried under the soil). There's a horrendously expensive book ($80), put out by Artech House (Norwood, MA) called _Shipboard Antennas_, whic is basically a catalog of Navy antennas with all their specs. It's chock full of discones (I looked at it at a recent trade show & would have bought it if it wasn't so overpriced as to be silly!). Anyway, good luck; they're not that hard to build & they do seem to work pretty well. No way is it worth the $70 that Heath and Radio Shack are charging & it is possible to build it yourself. Besides, you get to have people stopping by your house and asking why you put your Christmas tree on the roof... BTW, Ethernet coax: The teflon-coated kind seems to be pretty resistant to weather. PL-259 connectors fit perfectly & the loss figures are similar to RG8 (1.0 dB/100 ft at 50 Mhz, compared to 1.2 for RG8). The latest Belden Master Catalog (885) has the loss characteristics on page 186 (up to 50 Mhz.) 73 de KA1AXY Pete Simpson simpson_p@mercury.ceo.dg.com