Discone Antenna The following infos are related to a request by N7FWL. The discone antenna is not a good ham antenna, it is all right for commercial and military purposes. It is an omnidirectional antenna with wideband characteristics. It can operate over a 10 to 1 frequency range with, of course, some SWR variations over such a wide range. Since the antenna can operate roughly a 10:1 frequency range it will, more readily, radiate any harmonics present in the transmitter output. For transmitting use, and some receiving low signals, it is therefore important to use suitable filters for each frequency (band), to adequately attenuate the harmonics output or received high out of band signals. The radiation angle tends to rise after the first frequency octave, at about 5:1 in frequency the direction of radiation is above 45 degrees from the horizontal. I do not know the commercial models mentioned by Phil, N7FWL, neither I do have experiences on building such an antenna but I may suggest the following. The ideal construction, apart the full metal sheet, calls for 16 rod elements or a minimal of 8. The so wide band discone could be build with interleaved elements, 8 for the band of 40 to 400 MHz and 8 for 200MHz to 2GHz, two-in-one. Construction hints: A = Cone elements length equal to 1/4 wave at the lowest operating frequency ( 2,952/F(MHz) = length inches ). Cone angle = 60 degrees. f1 = 40 MHz and f2 = 200MHz B = The overall disc diameter equal 70% of 1/4 wave (lowest freq.) C = Top cone diameter, will be decided by the diameter of the coaxial cable utilized ( most purposes .5" ) D = Spang of the center of the top disc to the cone top, 20% of C, or .1" fo 50 ohm, use a suitable insulator which may be made of a potting resinr from ptfe or other stable low-loss material. \ B / _______\_/________ (40-400) D / - \ / ^ \ (200-2G) // \\ // \\ // \\ // \\ A // 60 \\ (200-2G) / \ / \ / \ / \ (40-400) I hope the drawing is understandable. Some mechanical details are on " VHF-UHF MANUAL " fourth ed., by G.R.Jessop,G6JP published by R.S.G.B. ( Radio Society of Great Britain ) Good work. Gian MODA - I7SWX This is a comment on Gian I7SWX's recent posting on discone antennas: About five years ago, when I moved to New Hampshire, I figured I needed some kind of broadband VHF antenna (50-450 MHz) as a space saver. I am not a mechanical person so I looked around for a cheap commercial version of the many discone designs I had seen in handbooks and articles. I found Hygain made one with three top elements and three radials (I forget the model # now). It was supposed to be for scanner use but a call to Hygain indicated that it could be used for low power transmitting as well. Anyway, I tried it out and it worked. Compared to a 1/2 wave or 5/8 wave groundplane, performance isn't great but it covers 144-148 MHz nicely without adjustments. It also is cheaper than some of the fancier versions now being marketed for hams. Bottom line: Come up with an application and then find an antenna that will satisfy the application. I knew the discone would not be a world-beater in terms of 2m DX so I wasn't disappointed when I couldn't work every repeater in New England with it. BTW, Gian's advice on harmonic and spurious radiation with regard to discones (or any broadband antenna) is correct. The discone will NOT discriminate between the real signal, the harmonic, or the spur. It will radiate all three very well, thank you. 73, MarStarin KB1KJ Is there such a thing as a HORIZONTALLY polarized discone antenna? I'd like to have transmit coverage of ham bands from 50-1300 Mhz in the horizontal plane (circular acceptable) with omnidirectionality. An overhead null is fine with me; a low angle of radiation is fine, too. A discone will work fine horizontally. In fact if you place two discones disc to disc and remove the disks you have a conical antenna which also has an extremely broad frequency response. The fan dipole used on HF is a "sparse" form of the biconical. You might try two biconicals mounted in the fashion of a turnstile antenna for omnidirectional horizontal polarization but you would run into problems with the half-wave phasing line between the two biconicals. Antennas like the turnstile, Alford array, and big wheel can approximate omnidirectional horizontally polarized coverage but only over a limited frequency range. 73 de WA4VZQ Barry 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 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 in 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 discones 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 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 dre 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_, which 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!). The discones as sold by Icom and others for use mainly with wideband VHF/UHF receivers like the Icom R-7000 are not just simple discones. They would indeed cut off below 100 Mhz or so, except that a whip and loading coil has been added to extend coverage to lower frequencies. How *well* this works is, of course, a matter of debate. Phil I may suggest to mount a discone horizontally, for the omni directionality I do not know, maybe two discones could be mounted as a dipole, in this way you will have a very broad...
wrobert2