double dipole, good or bad idea?

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KK4ELO

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this is just a brainfart,but here goes.

question 1) i thought of MAYBE trying two dipoles for 2m. one oriented n-s, one e-w, for the same radio, using both at the same time, and finding some way to match them up for the radio.

or,

question 2) would it be possible to direct four beams in four different directions, using one radio? (four directions is not needed, maybe only two. but just a thought.)

is this possible, or am i thinking of burning up a radio, vs. getting an omni?
 
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n5ims

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1.) Much of 2 meter work uses vertically polarized antennas so a single dipole oriented vertically will give you better results than your E-W & N-S dual-dipole combo would and there would be no need to fuss with any matching network to get them both in proper phase.

2.) You might be able to get a dual or quad beam antenna working, but getting them all in phase might be problematic. If you didn't get the phase just exactly right, you might get worse results than with a much lower gain single antenna.

To help explain, some Cable-TV companies used to need an antenna system that not only have good gain in one direction, but eliminate same channel signals from the opposite direction (one example I know of needed to pick up their primary station on channels 3 and 12 that was 70 miles to the west while blocking stations on channels 3 and 12 that were 90 miles to the east). What they ended up with was stacked high gain antennas pointing west with a lower gain single antenna pointing east that was 180 degrees out of phase with the stacked set. This provided good gain to the west (using the stacked set) and canceled the station to the east (using the out-of-phase antenna pointing east). They did have seperate antennas for the channel 3 station and channel 12 station. There was no ghosting on the primary stations to the west, but when they signed off we had a great signal on the station to the east until they signed off about an hour later. Please note that these were custom designed commercial antennas with a custom designed phasing harness, not some off-the-shelf models hooked up using some random hunks of feed line.

I do know of a repeater system that had an omni-direction antenna to handle their local coverage and a beam to enhance coverage over a nearby large lake (mainly the bridge crossing the lake, but coverage was pretty broad over most of the lake area). What they did was feed the two antennas to seperate receivers and use a standard voting receiver system to select which one fed the repeater (the transmitter power was sufficient to cover the entire area well). While this probably isn't what you have in mind, it might be one option if you really need that kind of coverage.
 

LtDoc

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Either will work, and have been around for a while. Try googling a 'turnstile' antenna. See if that's your 'double-dipole' idea. Connecting two or more directional antennas is certainly possible. The 'trick' with that is the phasing lines, how they are connected. They would certainly give more gain than just a common omnidirectional antenna. But, so would a vertical array type omnidirectional antenna. The common 'base' omnidirectional antennas are all vertical arrays I think, usually fiberglass enclosed. The more elements in those arrays the more gain (and also the taller the thing is). Or, another option might be something like the old CB 'SuperScanner' type antenna but for 2 meters. It would amount to a switchable set of two element beams, which can also be omnidirectional depending on how you want to arrange the switching.
Lots of possibilities.
- 'Doc
 

AA1LL

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Like LtDoc says, a turnstile antenna will get you more omnidirectional coverage, in azimuth. It is horizontally polarized at the horizon (elevation=0) which is non responsive to the more commonly used vertical polarization. To make a turnstile, you need to feed the two crossed dipoles with a quadrature 3dB hybrid. At higher elevation angles (>30 deg) the result is circular polarization and this type antenna is used for satellite communications so the spacecraft's antenna orientation can change as it flies by without affecting reception.

With different phase shifts other than 90 degrees between the array of two crossed dipoles, various patterns result, but none with a very useful shape.
 

prcguy

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A basic turnstile with one element fed 180 deg out using a 1/2 wavelength line (in 75ohm coax for matching) will give you a horizontal polarized omni. Feeding one element with a 1/4 or 3/4 wavelength line will create circular polarization without needing a 3dB hybrid.

The basic turnstile that's horizontal omni pol will also be less gain than a single dipole broadside because half the power is sent to one dipole that may not be facing you.
prcguy


Like LtDoc says, a turnstile antenna will get you more omnidirectional coverage, in azimuth. It is horizontally polarized at the horizon (elevation=0) which is non responsive to the more commonly used vertical polarization. To make a turnstile, you need to feed the two crossed dipoles with a quadrature 3dB hybrid. At higher elevation angles (>30 deg) the result is circular polarization and this type antenna is used for satellite communications so the spacecraft's antenna orientation can change as it flies by without affecting reception.

With different phase shifts other than 90 degrees between the array of two crossed dipoles, various patterns result, but none with a very useful shape.
 

zz0468

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What's not really addressed completely here is the question of polarization. If you're wanting to work FM repeaters, vertical polarization is used. If you want to try SSB and CW on the lower end of the band, that's all done with horizontal polarization.

That alone should determine which type of antenna to use.
 

prcguy

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Absolutely on target. My comments were aimed at not needing a 3dB hybrid to make a Turnstile.
prcguy


What's not really addressed completely here is the question of polarization. If you're wanting to work FM repeaters, vertical polarization is used. If you want to try SSB and CW on the lower end of the band, that's all done with horizontal polarization.

That alone should determine which type of antenna to use.
 

KK4ELO

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alot of good replies. thanks for your help. i thought of trying this on 2m to see how it works, and if it pans out, i was gonna try it with murs. my reasoning behind it is that i only need two directions mainly. by the way, this will be simplex, no repeater working.
 

zz0468

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alot of good replies. thanks for your help. i thought of trying this on 2m to see how it works, and if it pans out, i was gonna try it with murs. my reasoning behind it is that i only need two directions mainly. by the way, this will be simplex, no repeater working.
Ok, so it's simplex. Is it to be FM or SSB? FM is predominantly done using vertical polarization. SSB is predominantly done using horizontal polarization. This is an important detail when choosing what antenna to put up.
 

LtDoc

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You have all kinds of options depending on exactly what you want/need. They run from simple to very complex. Best advice I can think of is to find all the information you can about antennas, the various 'styles' etc, and then decide which will suit your particular purpose the best. But then, that's what you're doing asking questions here, right? :)
- 'Doc
 
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