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 <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
