Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Two Meter Longwire Antenna Question
I would like to make/experiment with a longwire antenna
specifically for the two meter fm band. Is there an optimum length to try & still keep an omnidirectional pattern? Also the optimum amount of radials and will i need a matching unit? |
#2
|
|||
|
|||
Two Meter Longwire Antenna Question
On Wed, 03 Sep 2008 03:06:01 GMT, "Spin"
wrote: I would like to make/experiment with a longwire antenna specifically for the two meter fm band. Is there an optimum length to try & still keep an omnidirectional pattern? Also the optimum amount of radials and will i need a matching unit? The nice things about VHF and UHF wire antennas is that they can be easily built and easily modeled with one of the numerous NEC2 programs available. The short wavelength means that you can generally ignore ground effects. However, you do have to consider the mounting structure (i.e. tower) in your calculations. I have no way to answer your questions as you didn't specify what you're trying to accomplish or what you have to work with. Optimum for what? VSWR, gain, radiation angle, pattern, what? Not enough info. As penitence for this capital crime, you are sentenced to download and install EZNEC, 4NEC2, or other NEC2 modeling program: http://www.eznec.com (Free demo is good for simple antennas) http://home.ict.nl/~arivoors/ (lots of good examples included) http://www.si-list.net/swindex.html (old list but still useful) Plug your proposed antenna into these programs, and see what it will do in the way of pattern, VSWR, radiation efficiency, bandwidth, etc. Incidentally, the 3D antenna patterns are cool. Yes, you will have to learn a bit about antennas, learn how to run the program, and learn a few new profane incantations to get it working. However, I assure you that it's worth the effort. Once you get something reasonable on the computah, then built it and do some measuring. -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com # http://www.LearnByDestroying.com AE6KS |
#3
|
|||
|
|||
Two Meter Longwire Antenna Question
"Spin" wrote in news:tanvk.364$1a2.205@trnddc04:
I would like to make/experiment with a longwire antenna specifically for the two meter fm band. Is there an optimum length to try & still keep an omnidirectional pattern? Also the optimum amount of radials and will i need a matching unit? I am guessing that you want vertical polarisation, and omni directional pattern in the H plane (azimuth). As an end fed unloaded vertical wire over perfect ground is lengthened than about 60% of a wavelength, the low angle major lobe splits and significant power is radiated in higher angle lobes. You probably don't want that. So, in the sense that a longwire is an endfire antenna, it is not suited to your need for a vertically polarised omni antenna for FM operations. A 5/8 wave ground plane isn't a bad antenna, and about as long as is useful in a straight radiator. It is an example of a good antenna that is not resonant, debunking the myth that resonant antennas always work best. It does not have a 50 ohm feedpoint, and is reactive, so some impedance matching is needed for 50 ohm line, but it is not difficult. For higher gain with omni pattern, you should consider a colinear array. They are very popular, but mostly of commercial manufacture. Owen |
#4
|
|||
|
|||
Two Meter Longwire Antenna Question
"Owen Duffy" wrote in message ... "Spin" wrote in news:tanvk.364$1a2.205@trnddc04: I would like to make/experiment with a longwire antenna specifically for the two meter fm band. Is there an optimum length to try & still keep an omnidirectional pattern? Also the optimum amount of radials and will i need a matching unit? I am guessing that you want vertical polarisation, and omni directional pattern in the H plane (azimuth). As an end fed unloaded vertical wire over perfect ground is lengthened than about 60% of a wavelength, the low angle major lobe splits and significant power is radiated in higher angle lobes. You probably don't want that. So, in the sense that a longwire is an endfire antenna, it is not suited to your need for a vertically polarised omni antenna for FM operations. A 5/8 wave ground plane isn't a bad antenna, and about as long as is useful in a straight radiator. It is an example of a good antenna that is not resonant, debunking the myth that resonant antennas always work best. It does not have a 50 ohm feedpoint, and is reactive, so some impedance matching is needed for 50 ohm line, but it is not difficult. For higher gain with omni pattern, you should consider a colinear array. They are very popular, but mostly of commercial manufacture. Owen If you increase a single monoples length beyond 1/4 wave, the takeoff angle of radiation compresses closer to the horizon for a gain improvement. Of course now it isn't resonant but so what, all you have to do is finish with a loading coil to make it look like a 3/4 w antenna and it has a resonant low impedance again. If you increase the length beyond 5/8w though, high angle lobes begin to appear and the low angle lobes become less intense. Another way is to stack elements and feed them in phase so that their patterns reinforce. There is a point of diminishing returns though due to losses in the matching and feed systems. Commercial base antennas up to about 10db gain are commonly available. A favorite UHF mobile antenna has 2- 5/8 sections stacked one over the other. Because the 2m and 440 FM sections of the Ham bands are very close to harmonically related, this antenna can be constructed to double as a 2m antenna just short of 5/8 for that band too. I bought a commercial #9007 KD4BUM vertical for $20 that is 5/8 over 1/4 wave collinear at about 9' long. Works great as the mount is interchangeable with HF hamsticks and CB whips and has quite a bit of gain. It is nothing but a fiberglass pole with #16 or 18 wire that goes up 1/4w then windings for a phase reversal + the matching section for the next 5/8 section to look like 3/4w. Very simple in construction for such excellent performance and still short enough for mobile use. |
#5
|
|||
|
|||
Two Meter Longwire Antenna Question
On Tue, 02 Sep 2008 20:38:12 -0700, Jeff Liebermann
wrote: As penitence for this capital crime, you are sentenced to download and install EZNEC, 4NEC2, or other NEC2 modeling program: http://www.eznec.com (Free demo is good for simple antennas) http://home.ict.nl/~arivoors/ (lots of good examples included) http://www.si-list.net/swindex.html (old list but still useful) One more free antenna modeling program. MMANA-GAL: http://mmhamsoft.amateur-radio.ca/mmana/ I just blundered across this one. Plenty of examples to plagerize. Optional NEC2 interface using engine from 4NEC2. Looks nice so far. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Question is 'it' a Longwire {Random Wire} Antenna -or- Inverted "L" Antenna ? | Shortwave | |||
Longwire question | Shortwave | |||
longwire attachment question | Antenna | |||
Longwire question | Shortwave | |||
Question, Longwire | Shortwave |