Ok I found a site for directions in making my own 1/4 wave 2m ground plane. It's roughly 19" tall, if I wanted say a 1/2wave would I just make it 38" tall, full wave 76" tall, etc???? I think there is more to it than that but I could be wrong. I think impedance would be a factor here. how bout ya antenna experts??
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homebrew antenna question
- Thread starter BIONIC_TEXAN88
- Start date
The 1/4 wave will be the easiest to make and match.A 1/2 wave will offer a very high impedance in the order of many hundreds if not thousands of ohms that must be transformed back to 50 ohms. The same thing applies for a full wave. Multi wave verticles are actually a series of 1/4 wave fed in phase and isolated from each other by design.You could try a 3/4 wave design as it will present something very close to 50 ohms at the feedpoint. As a matter of fact any odd multiple of 1/4 wave will but going really long brings other factors into play.try 19 or about 57 inches and see what happens.Anything else will require some sort of matching device that will probably introduce more losses that is gained by going bigger.
The quarter wave groundplane should work very well, with a minimum of trouble. It will have the same gain as a dipole, but should have a very consistent omni-directional pattern if mounted with nothing around it.
If you are in an area with repeaters within a few miles, even up to about 15 or 20 miles, a quarterwave groundplane will work just fine with an HT at around 5 watts. Farther if you have a higher power rig and good line of sight, not as far if you are surrounded by buildings.
Anytime you are homebrewing an antenna, the impedance is going to be an important consideration. Any change, even leaving your radials at a 90° angle rather than a 45° droop, can change the impedance and reduce efficiency.
Homebrewing VHF antennas is a great place to start--they are small, easy to handle, and usually quite cheap. The best way to start out is to use the plans and directions you can find on the internet. Start with simple antennas, and then you can move up to more complex types and experimenting.
You might also look at a simple J-pole design, like this one:
http://www.qsl.net/n6bg/thunt/jpole.html
They are basically a half-wave end-fed antenna, and are also simple to make. The performance of a basic dipole is about the same as a quarter-wave groundplane. There is a variation that adds a second half-wave element above the "J" that doubles the gain while keeping an omni-directional patter.
Also, remember that there is great value in getting an antenna up above the ground clutter. Holding a groundplane in your hand is much better than a rubber-ducky, but putting it a few feet above the highest point of your roof will help get the best performance from any V/UHF antenna.
Dave_W6DPS
If you are in an area with repeaters within a few miles, even up to about 15 or 20 miles, a quarterwave groundplane will work just fine with an HT at around 5 watts. Farther if you have a higher power rig and good line of sight, not as far if you are surrounded by buildings.
Anytime you are homebrewing an antenna, the impedance is going to be an important consideration. Any change, even leaving your radials at a 90° angle rather than a 45° droop, can change the impedance and reduce efficiency.
Homebrewing VHF antennas is a great place to start--they are small, easy to handle, and usually quite cheap. The best way to start out is to use the plans and directions you can find on the internet. Start with simple antennas, and then you can move up to more complex types and experimenting.
You might also look at a simple J-pole design, like this one:
http://www.qsl.net/n6bg/thunt/jpole.html
They are basically a half-wave end-fed antenna, and are also simple to make. The performance of a basic dipole is about the same as a quarter-wave groundplane. There is a variation that adds a second half-wave element above the "J" that doubles the gain while keeping an omni-directional patter.
Also, remember that there is great value in getting an antenna up above the ground clutter. Holding a groundplane in your hand is much better than a rubber-ducky, but putting it a few feet above the highest point of your roof will help get the best performance from any V/UHF antenna.
Dave_W6DPS
well, looks like I am gonna stick with the 1/4wave antenna. i used a 102" whip that I had already cut down to 20" for a scanner antenna. I am gonna cut it down to 19.2" (by calculation using 146mhz) and I have found some stainless tubing at work that I have cut (6) 20" ground radials and have them bent at 45 degree angles. My instructions said to use 4 ground radials but I had material leftover so I figure another 2 wont hurt.
Dave,
I'd agree with all you said except for the gain of a 1/4 wave antenna and a dipole, they are not the same at all. The dipole (a 1/2 wave) will have slightly more gain than a 1/4 wave.
The hardest part of using any antenna, no matter what it's length, is impedance matching. Some are easier to match than others, that's why you see so many of those (1/4 wave groundplane, for instance, a dipole for another). Others are not quite as easy to make at home cuz the matching 'device' isn't as easy to figure/make.
- 'Doc
I'd agree with all you said except for the gain of a 1/4 wave antenna and a dipole, they are not the same at all. The dipole (a 1/2 wave) will have slightly more gain than a 1/4 wave.
The hardest part of using any antenna, no matter what it's length, is impedance matching. Some are easier to match than others, that's why you see so many of those (1/4 wave groundplane, for instance, a dipole for another). Others are not quite as easy to make at home cuz the matching 'device' isn't as easy to figure/make.
- 'Doc
If a 1/4 wave has 50ohm with the radials down at 45 degrees if you lengthen both the radiator and radials to 5/8 wave will the impedance change too if so sounds like an antenna analyzer is the way to go.
try scaling down the Avanti Sigma4 into a 2 meter version. it's already been done successfully at least once that i know of.
W5LZ said:
Isn't a 1/4 wave ground plane the same as a 1/2 wave dipole just with the ground element at an angle for impedance matching? If a dipole is two 1/4 wave elements center fed with coax running off at 90 degrees and a ground plane is two 1/4 wave elements center fed with coax running parallel with the lower elements at an angle aren't they the same in radiating mass making db gain the same?
the quarter wave antenna or quarter wave monopole is a whip antenna that behaves as a dipole antenna. it is fed in the lower end, which is near a conductive surface which works as a reflector. the current in the reflected image has the same direction and phase that the current has in the real antenna. the set quarter-wave plus image forms a half-wave dipole that radiates only in the upper half of space. far from the antenna and near the ground, electromagnetic fields and radiation patterns are the same as for a half-wave dipole. the gain is the same as that for a half-wave dipole.
No, a 1/4 wave antenna isn't the same as a 1/2 wave dipole, either in input impedance, radiation pattern, or gain. Best way of showing anyone that is by using an antenna modeling program to see the differences. Most of those programs will break things down into the various things most people are interested in, input impedance, gain if any, and patterns. Or, find some of the 'old' radiation graphs in all the radio books. The total lengths of a 1/4 wave groundplane and a 1/2 wave dipole may be the same, but so are some Fords and Cadilacs. Certainly doesn't mean they perform the same.
- 'Doc
- 'Doc
i figure i can convert my m400 over for use as a 6 mtr. vert. as i don't have one.in any event it will give me something to do since another storm is inbound. 1/4 waves work suprisingly well if in the clear.
Sorry, a 1/4 wave antenna is still not the same as a 1/2 wave antenna, in performance, shape of the radiation pattern, or gain. They are simular in the same way that all antennas are simular. The same way that a vehicle with four tires, a motor and seats are simular. There's still a hell-of-a difference between a Ford and Cadilac. Same for the 1/4 wave and 1/2 wave antennas.
Don't take my word for it, ask Cebic... (rec.radio.antennas). See what he says. Or try his modeling program.
- 'Doc
Don't take my word for it, ask Cebic... (rec.radio.antennas). See what he says. Or try his modeling program.
- 'Doc
Any and all pattern plots I have seen show the 1/4 wave to have slightly LOWER gain than a 1/2 wave at the horizon.At a higher takeoff angle of around 40 degrees or more however it may show slightly higher gain due to the higher angle of radiation.
we're not talking about a 1/4 wave antenna and a 1/2 wave antenna, we're talking about a 1/4 wave (monopole, grounded marconi, etc) antenna and a half wave DIPOLE, both vertically polarized.
the series input impedance of the two vary by less than a factor of 2:1, they're both center fed, the voltage and current amplitude distribution in both the 1/2 wave dipole and the 1/4 wave monopole, grounded 1/4 wave marconi, etc. are identical and on terra firma there is no appreciable difference in gain.
the series input impedance of the two vary by less than a factor of 2:1, they're both center fed, the voltage and current amplitude distribution in both the 1/2 wave dipole and the 1/4 wave monopole, grounded 1/4 wave marconi, etc. are identical and on terra firma there is no appreciable difference in gain.