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"Desmoface" ... anyone think a verticle dipole would work. ..a vertical dipole for 10... Steve kb8viv How about a J-Pole? ..one using a mast with the 1/4 wave parallel section added 1/2 wave below the top. Real easy. No radials. John Smith - KD5YI As others have already noted, vertical dipoles do work whether center-fed or end-fed (a.k.a. J-pole) without radials. As already noted, hang it as high as you can manage - just the normal advice for any antenna except N.V.I.S. If you want to pursue the J-pole route, go to CQ magazine's web site http://www.cq-amateur-radio.com/ and download George VE3ERP Murphy's freeware HAMCALC ham radio software package. (lower left corner) (Download it just for general principles.) Program about-23 of 250+ programs is Murph's version of my version of Gary N3GO O'Neill's J-pole program from his Communication Quarterly article on end-fed half-wave antennas. Go to Steve AA5TB Yate's website http://www.qsl.net/aa5tb/ under "Antennas" for more info and a link to Gary's article on how J-poles and Zepps really work versus the popular view. The program gives starting lengths. For 28.5 MHz: Using 300 ohm twinlead: halfwave radiator wire = 196.7 inches series matching 300 ohm section = 78.6 inches to the 50 ohm tap point shorted shunt stub 300 ohm section = 6.6 inches Total length is about 23.5 feet if you hang it all up vertically. The not-quite-quarter-wave matching section can be horizontal with the halfwave radiator vertical if you need/want to run it that way. Gary's article, the program or a Smith chart show why the commonly used 450 ohm ladder line is not the as good a choice for matching as 300 ohm. Have fun. Measure and cut metal and plastic, hang it up, apply and withdraw RF energy. Don't just do computer calculations (as I do most of the time (sigh)). Cheers, 73, Ron McConnell N 40º 46' 57.9" W 74º 41' 21.9" FN20ps77GU46 [FN20ps77GV75] http://home.earthlink.net/~rcmcc |
"Desmoface" wrote in message ... How about a J-Pole? Low angle radiation. The bottom end can be connected to earth ground. In fact, I constructed one using a mast with the 1/4 wave parallel section added 1/2 wave below the top. Real easy. No radials. thats also an option..i wonder if'n I'll need radials with a vertical dipole?? Steve kb8viv As others have replied, no you don't. I once made a 1/4 wave vertical and was listening to a local conversation on my IC-2AT when there occured a lightning stroke from a thunderstorm a few miles away. Although the mast and radials were grounded, the conversation disappeared for about 5 seconds and then faded back in. Electrical charge on the ungrounded vertical element. But I have never had this problem with a J-Pole since all parts of it can be at earth ground. I've also used folded vertical monopoles with the same results. In my old age I have probably become paranoid about ungrounded driven antenna elements. I know that some (maybe all) commercial antennas are designed with networks or other devices which does the grounding. If I were to build a simple ground plane today, I would make it a bit shorter than resonant and add a shunt inductor across the feed point. That way there can be no static accumulation on the element. By the way, we sometimes get sand/dust storms here. I have seen and heard the repeated discharges from the center pin to the shell of a PL-259 on a RG-8 line coming down from the roof during a sand storm. Anyway, this is not to convince anyone that a J-Pole is a cure-all or even the best choice. I just wanted to tell about what drives my choices and tell about a couple of my experiences. Anything you choose will be just fine, as long as you are aware of its properties. Have fun. John - KD5YI |
KB8IV wrote:
"I`m wanting a good 10 meter antenna for local stuff and was thinking about a conventional vertical...then I thought maybe I could do without the radial system---." Good thinking. I worked for an oil and gas company which was assigned a 33 MHz frequency shared with a few others in the same industry but available to us almost at any location within the U.S.A. We made good use of the frequency for land mobile operations with dozens of fixed stations and hundreds of mobile units. It worked very well for the radio "line-of-sight. Usual FM base station power was 500 watts. The mobiles were 50 watt units. Higher base station power is justified by noiser receiving conditions in a mobile. In our international operations we had many HF installations. Some were AM and some were SSB. I made a trip to Bolivia to relocate a HF station from a site which was being seized by the government to accomodate homeless rural people who were descending upon La Paz to seek their fortunes. Relocation of the station was easier said than done but was ultimately successful. We had offices in La Paz and in Cochabamba and we were drilling in the Chaco Jungle. While in Bolivia, I got a request from Argentina. They wanted an aircraft beacon installed on Tierra del Fuego, local land mobiles that worked, and radio between Tierra del Fuego and Buenos Aires. Strikes frequently interrupted normal communications. The Peronistas wanted Juan Peron back and the strikes were part of their agitation. So, I went to Argentina. In Argentina, we had RCA and RCA Radiomarine SSB equipment. Our aircraft were equipped with Collins equipment which was dependale and our pilots knew how to use it. The RCA SSB was a problem. The automobile units had plastic coil forms which melted in normal service. This was far removed from the equator. It was near the antarctic and damn cold. Operations were a problem. Base stations were used to communicate between land bases, vehicles, boats, and tankers taking on oil we had found and were producing. Multiple crystals meant operators often couldn`t communicate because they were often switched to the wrong channel. They also were mystified by knobs identified as "speech clarifiers". Fortunately we had some 5-watt Motorola Handitalkies on our stateside 33 MHz FM frequency, on loan to our Argentine operation. After I installed the low-frequency aircraft homing beacon, the next problem was getting reliable communications between our main bases in Rio Grande and San Sebastian, almost 40 miles apart. This is a land where hurricane force winds blow nearly every day. The wind is so prevalent it is relied upon for aircraft operation. Commercial air service to Rio Grande was cancelled when the wind was calm as the runways were too short for take-offs and landings in the calm. Commercial flights used DC-6`s (Aerolineas) and C-46`s (Austral). Our company flew a DC-3 back and forth between Buenos Aires and Tierra del Fuego. We had Beechcrafts on the Island. For line-of-sight we needed elevation for 33 MHz antennas. We had welders, line pipe, steel handbooks and I had a slide rule. So, I went to work and produced guyed towers at both ends of the path. They were a little over 100 feet and I put vertical homemade resonant vertical centerfed dipoles for 33 MHz at their tops. These were connected by RG-8 with the 5-watt radios in the offices. Voila! We were in business with full quieting. No bandswitching. No clarifiers. No melted coils in the Motorolas. We had a second office in Rio Grande but it didn`t need to talk to San Sebastian. So I made a short self-supporting line pipe tower for that office. It was about 30 feet. It too had a vertical dipole and a 5-watt Motorola Handitalkie. When I made a call on that radio, an oil field supply company in Midland Texas who shared the frequency came back to me! We`re talking real DX and it was solid. As has been said, that`s the way it is when the band is open. Let me correct a statement in this thread that the loweer dipole element is a radial. It is not. It is an axial and it radiates. Radials are supposed to be balanced so that they do not radiate. It is true that radials provide a 2nd antenna connection as does the 2nd dipole element. That is where the similarity ends. For the radio connection with Buenos Aires, the public correspondence station, Radio Pacheco, seemed to work during telephone strikes. So we went to work. It operated on certain HF channels. Al Hopson, our chief pilot in Argentina was also a ham and he had had recognized a Hallicrafters HT-20 that Glen McCarthy had left behind in the Chaco Jungle of Bolivia when we bought his consession. Al put the HT-20 away for safe keeping and asked me if I wanted it. He fetched it for me. I went to Buenos Aires and visited Radio Boliche, "Barato y chi chi". While there I bought a 2nd-hand Eddystone receiver that worked good and was cheap (barato y chi chi just as their slogan said). Back on the island again, it was out with the slide rule and my precious copy of Ed Laport`s "Radio Antenna Engineering". When we gave Buenos Aires a call with that HT-20 connected to that rhombic, we really rattled their cans. The were very slow to believe that we were so far away. We ordered the FM stuff they needed to replace all the SSB crap, and left them delighted with the improvements already made. Yes. Centerfed vertical dipoles work fine without radials. They have nulls at their tip ends which reduce mutual impedance with stuff in those directions. Centerfed antennas are mostly independent of the earth at close range when low-angle radiation is considered. Best regards, Richard Harrison, KB5WZI |
Yes. Centerfed vertical dipoles work fine without radials. They have
nulls at their tip ends which reduce mutual impedance with stuff in those directions. WOW!! That sounds like the makings of an Indiana Jones story line hehe..Thanks for the entertaining and informative post..73's de kb8viv.. steve |
I wrote:
"Centerfed antennas are mostly independent of earth at close range when low-angle radiation is considered." Sorry I left out the word "vertical". Certainly a low horizontal dipole is not independent of earth, but is closely coupled to the earth. Best regards, Richard Harrison, KB5WZI |
the vertical dipole and radials
Just picture a 1/2 dipole horizontally -- then rotate it 90 degrees (or stand it on end) -- same antenna -- different polarization. So this picture should clarify why "No Radials". The lower end is the other half of the dipole - it radiates in conjunction with the upper half. Others have advised you about the radiation from "Radials" -- 73 From The Wilderness Keyboard |
But, don't get it up to high. EZNEC shows that when the bottom end is a half
wave length above ground, the TOA goes up over 40 with not much improvement in gain. The vertical half wave dipole seems to be at it's best a quarter wave above ground (meaning the bottom end is only 8 feet up on 10). So, the top insulator should be up about 24-25 feet. When installed at this height, the TOA is below 20 and the gain is almost the same as the higher version, roughly 1.4dbi. The antenna does not require radials. "Keyboard In The Wilderness" wrote in message news:PCzLb.55730$m83.35174@fed1read01... the vertical dipole and radials Just picture a 1/2 dipole horizontally -- then rotate it 90 degrees (or stand it on end) -- same antenna -- different polarization. So this picture should clarify why "No Radials". The lower end is the other half of the dipole - it radiates in conjunction with the upper half. Others have advised you about the radiation from "Radials" -- 73 From The Wilderness Keyboard |
Jim,
I don't know where you get this from. I just ran EZNEC on a 10 m vertical dipole with the following results: A) Bottom at 8 feet, gain at 5 degrees is -2.44 dbi. B) Bottom at 42 feet, gain at 5 degrees is +2.75 dbi. If you want to work locals, the greater height will also help to get clear local obstructions, like small hills Tam/WB2TT "Jim Leder" wrote in message ... But, don't get it up to high. EZNEC shows that when the bottom end is a half wave length above ground, the TOA goes up over 40 with not much improvement in gain. The vertical half wave dipole seems to be at it's best a quarter wave above ground (meaning the bottom end is only 8 feet up on 10). So, the top insulator should be up about 24-25 feet. When installed at this height, the TOA is below 20 and the gain is almost the same as the higher version, roughly 1.4dbi. The antenna does not require radials. "Keyboard In The Wilderness" wrote in message news:PCzLb.55730$m83.35174@fed1read01... the vertical dipole and radials Just picture a 1/2 dipole horizontally -- then rotate it 90 degrees (or stand it on end) -- same antenna -- different polarization. So this picture should clarify why "No Radials". The lower end is the other half of the dipole - it radiates in conjunction with the upper half. Others have advised you about the radiation from "Radials" -- 73 From The Wilderness Keyboard |
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