There are several calculators on line, that might help a person decide on cablr types to use. <http://www.ocarc.ca/coax.htm> which shows a numer of cable types to select from, and you input the length, frequency, and wattage, plus the SWR/reflected power ratio. Even at 450 MHz, an SWR ratio of 1.5:1 shows low loss in fact. The type of cable can make a tremendous difference. Tuners/matchboxes do not normally make the antenna radiate any more power, but make the output of the radio happier seeing a better matched load
I people saying that a dipole is 50 ohms at the center feed, but I had been taught/learned that it wos closer to 75 ohm (more like about 72) at the resonant frequency. A folded dipole tend to be, if my memory is corect, on the order of 300, give or take. used to be able to easily fiond these facts in the Radio Amateurs handbook, but they don't have as much information in the newer versions. Someone mentioned 30 ohm coaxial cable, where do you find this type? I don't recall ever seeing any info to speak of, unless you can dive deep into older catalogs like Belden had. back to the baic dipole, using 50ohm cable on a 72 ohm dipole will still be under 1.5:1 at resonant frequency, so loss is _not_ enough to worry about. Worry more about using bad cable, poor connectors, especially when not installed properly, and mounting antenna too close to metal or other items that can affect it. When making power readings, these should be done first into a good dummy load. I have one that will handkle up to 150 watts with a huge heat sink, and shows, at lower frequencies, less that 1.1:1 SWR, and even low at 450 MHz. Use short jumper to the meter. Then add duplexer, and take second reading. This shows the duplexer loss, which can be figured in Bd loss In my case, about 100 watts from the PA, to about 70 from the duplexer cans, is about the rated 1.5Db loss for the model duplexer. Then subtract figured loss from the cable, and add the gain factor for the antenna. This latter is where it gets confusing. many manufacturers, especially for ham bands, rate in DBi, and not DBd, which can be incorrect. As an example, I have a Diamond X510NJ rated at 11.7 DB gain. this is DBi, and I found, somewhere, information showing about 9 or 9.5 DBd gain for the antenna on the 440 band. I wonder what the actual basis is for the Db, maybe a 1/4 wave mobile whip? It can be misleading when you don't know the basis for the gain factor. Now, to the folded dipole. Does it in fact have gain over the straight dipole? if so, what is the gain factor in reality? What is the gain factor for 2 phased together, and so on? YMMV Wayne WA2YNE On Wed, 04 Jun 2008 19:36:13 -0500, Laryn Lohman <[EMAIL PROTECTED]> wrote: > --- In [email protected], Wayne <[EMAIL PROTECTED]> wrote: > >> Note that forward power can sometimes read higher than true if > there is >> more reflected power. > > > Very true. Power coming back down the line is RE-reflected in the > transmitter and adds to the forward power, producing an *awesome* > forward power reading. > > >> And to answer something that one person came up with a while back, >> decreasing the reflected power does not add it to the true radiated > power. > > > ...Another way of saying that reflected power does NOT subtract > directly from the power that is radiated by the antenna. > > > > -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/ ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Repeater-Builder/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> 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/
