I prefer quarter wave antennas myself. Less noticeable too. Neil - WA6KLA
vmckever wrote: > > Hummmm, Now I know why I use a 3db gain mobile antenna. > > Vincent N6OA/2 > ----- Original Message ----- > From: "Mike Perryman K5JMP" <[EMAIL PROTECTED]> > To: "[EMAIL PROTECTED] com" <[email protected]> > Sent: Saturday, April 30, 2005 10:17 AM > Subject: [Repeater-Builder] antenna's > > > First thing... this ain't the place for this type of antenna question... > > there are yahoo groups specifically for this type of discussion. Begging > > the listowner's pardon... I will address the question at hand... > > > > I guess the first to ask is what exactly are you trying to accomplish? > > You > > haven't provided enough information to even begin formulating an accurate > > answer to the question. Taken literally at face value... yes, you can > > build a splitting/phasing harness quite easily. > > > > For those in the know... Please pardon the enormous bandwidth... ;-) > > Just delete this message now and move on.... > > > > I first considered responding that you should get an 11m radio to go along > > with the 11m idea. But that is rude, and un-called for... so I have just > > sat idly by to see how far this would go. It looks as though it isn't > > going > > to die a quiet graceful death, so here goes... > > > > Phased Directional Antennas 101 > > > > Your question... > > > >> Someone has told me their is a co-phasing harness > >> available for two meters has anyone heard of this. to > >> unite two antennas. In the same way truckers do on > >> CB's > >> Thanks > >> > > > > > > You can create a 50ohm "match to split" by using RG-59 is 75ohm (and is > > lossy as hell at 2m, but for the sake of argument please bear with me.) > > cable cut to an odd multiple of a 1/4 wave length combined with a > > T-connector. > > > > Feed the T with standard 50ohm line. Assuming a purely resistive load, > > the > > two lines in paralell will present a resistance of approx 37.5 ohms. This > > is close enough. And I am NOT going to get into complex impedances here. > > > > The first thing you need is manufacturers data for velocity of propagation > > for the line you intend to use... an electrical wavelength is not the > > same > > as a physical wavelength.. It will be physically shorter than just > > calculating a wavelength mathematically. Reason being that no cable is > > 100% > > effecient. This is where the velocity factor comes in. A better quality > > RG-59 will have a velocity factor of something near 82% (0.82). To get to > > where you need to be.. divide 300 by the desired frequency in mHz.. The > > result is one physical wavelength. (please pardon the round-off error) > > > > 300/freq in mHz= 1 wavelength physical length in meters - ie; 300 / > > 146.52 = 2.047m > > > > multiply this number by 0.25 for the 1/4 wave length we spoke of > > earlier... > > 2.047 * 0.25 = 0.512m > > > > This is where velocity of propagation comes in to the equation... you now > > multiply the 1/4 wave physical length by be velocity factor of the > > specific > > cable you plan to use... we spec'd RG-59... so the velocity factor is > > approximately 82% > > > > 0.512m * .82 = 0.42m - this is the electrical 1/4 wavelength @ 146.52 mHz > > using RG-59. > > > > Using odd multiples of this number will keep you on a current node, but > > only > > at 146.52 mHz... if you change freq by very much, the whole thing falls > > apart. Staying on the current node is important... you will just have to > > trust me here. As I am not going to delve into the reason's why. > > > > Figure out how much distance you need to cover between the 2 antennas... > > how far from the split to the connector at the bottom of the antenna. > > For > > this discussion, we will use 5 meters (physical) for each half of the > > phasing harness.... to get to the antenna from the T. We will call this > > the > > "cable run" distance. > > > > Next divide the cable run by the 1/4 wave electrical length. > > > > 5 / 0.42=11.9 quarter waves.... we need to stay on the current node by > > arriving at an odd multiple, so we must add another 1/4 wave electrical... > > > > So each cable run will be 13 electrical quarter waves.... (odd number to > > stay on the current node) > > > > 13 * 0.42 = 5.46m or 17.91 feet. > > > > Cut your 2 cables to exactly this length, connector tip to connector tip. > > Attach one end to your antenna, and the other to the T connector, then use > > 50 ohm line from your radio to the T connector. > > > > This arrangement (method) is only good for the specified frequency used in > > your calculations. If you change frequency very far.. the match will not > > be good, and that will "piss your radio off" ... so to speak. Bad match > > = > > power "fold-back" or even worse smoked finals. > > > > Now you have a phasing harness with 0� phase shift that will present a > > workable load at 146.52 mHz.... Placement of the antennas presents a > > whole > > new world of problems. > > > > Assuming you place them exactly 1 wavelength apart (rough guess for > > mounting > > on each mirror of a vehicle), fed in phase & unity current, the resulting > > pattern will resemble a squashed four-leaf clover, with most of the signal > > being radiated to the sides.... > > > > (I tried at first to send this with a EZ-NEC plot of the pattern, but > > yahoo > > threw-up. If it is desired, contact me directly.. And I can supply bothe > > the file and the plot) > > > > So I guess my question now is WHY? Is this a "coolness" thing? > > You will most likely acheive around 2.7 to maybe 2.8 dB of gain.. and a > > horrible pattern... > > Why not be cool by being "smart" and buy a higher gain omni antenna? You > > will be more effecient, avoid all the headaches, and the pattern and match > > won't go bonkers when you change frequency.... > > > > Enough said.... > > Sorry for the rant guys... but it was a good exercise in practical > > application of theory. > > > > Mike > > > > > > ----- Original Message ----- > > From: "Warren Beaul�" <[EMAIL PROTECTED]> > > To: <[email protected]> > > Sent: Thursday, April 28, 2005 2:10 PM > > Subject: [Repeater-Builder] antenna's > > > > > >> Someone has told me their is a co-phasing harness > >> available for two meters has anyone heard of this. to > >> unite two antennas. In the same way truckers do on > >> CB's > >> Thanks > >> > > > > > > 73 > > Mike > > K5JMP > > www.k5jmp.us > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > Yahoo! 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