I prefer to look at SWR as the ratio of the magnitude of the load impedance to the magnitude of the characteristic impedance of the transmission line, or the reciprocal of this ratio, whichever gives a number of 1 or greater.. This corresponds to the radius of a circle on a Smith chart. An ideal 1/4 wave vertical over a ground plane has an impedance of something like 32+j0 giving an SWR of 1.56. Although transmission line losses increase somewhat with SWR, i don't believe this is significant. More significant, in my view, is the mistuning of the output network of the transmitter by its being loaded with a load impedance that is different from that for which it was designed. The load impedance seen by the transmitter is the antenna driving point impedance "rotated" around the Smith chart by the transmission line length (with the radius dropping somewhat due to line losses). If we have a 50 ohm line and a 50 ohm load, we "rotate" around a circle of radius zero as we travel from the antenna (the load) back to the generator (the transmitter). Thus, the transmitter still sees 50 ohms. If instead we terminate the line in our 32+j0 load, we can go 1/4 wavelength down the line and the impedance gets "inverted". Instead of being 50/1.56, it's 50*1.56 or 78 ohms (note that we still get the same SWR). This impedance then loads the output network. It is transformed by the output network to an input impedance that loads the output device. Any load impedance other than that for which the network was optimized will result in a nonoptimum load on the device (typically giving less output power and possibly adversely affecting efficiency, possibly increasing dissipation to where the device can be damaged). Besides antenna driving point impedance, there is the radiation pattern to consider. A vertical antenna over a ground plane will have an omnidirectional horizontal pattern, but will have a vertical radiation pattern that depends upon the height of the radiator. Each increment of the radiator can be thought of as another very short antenna with a current flowing through it. The phase and magnitude of that current varies depending upon where the point is, and the length of the radiator. The resulting fields are added up to get the overall vertical pattern. There's a fair amount of data on vertical radiators (admittedly the data is at about 1 MHz) in the NAB Engineering Handbook... (While you're there... see my chapter on transmitter control systems :) Harold On Thu, 26 Mar 1998 13:26:31 -0000 Eric H writes: >Just add my 2 cents in here. I used to be an Amateur Radio operator, >so the >problem is SWR. This is the forward voltage vs. the reverse voltage >down >the antenna. SWR is a function of the wavelength (frequency) of the >signal, >and the length of the antenna. An SWR of 1:1 means that you have no >signal >returning to the transmitter. A 3:1 SWR is very bad, meaning you are >getting more power returning to the transmitter than is going out. >You can >purchase a SWR meter at HAM stores. It may be a good idea to get the >maximum signal out the antenna. > >Eric Hvinden- AME IMAGING >-----Original Message----- >From: Justin Crooks >To: PICLIST@MITVMA.MIT.EDU >Date: Thursday, March 26, 1998 4:19 PM >Subject: Re: Programmer sold, RF freq's > > >>Yeah, I experienced the same problemwith 418 MHz Abacoms. Which >antenna >>are you using? They >>must be EXACTLY the right length for maximum range. (PIC driven, of >course, >>for picky PICLIST members). The whip antena gave us the best >results. If >>I remember right, 16.5 cm for 418MHz, 15.5 cm for 433MHz (1/4 wave, >you can >>do the math). Use a thick, straight piece of wire or something. We >even >>went so far as to use a digital caliper when cutting the antenna >(with good >>results. Of course, we were probably overly anal). Good luck!!! >> >>---------- >>> From: John P. Leonard >>> To: PICLIST@MITVMA.MIT.EDU >>> Subject: Programmer sold, RF freq's >>> _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866] Date: Thursday, March 26, 1998 7:55 AM >>> >>> Hey, I sold the programmer; thanks for the numerous offers. >>> "Is he toilet-trained, union..." sounds like the frequent comments >about >>> drummers made on the bass players list group "The Bottom Line" but >hey, >>> not by me :) >>> >>> Please allow a brief [OT] excursion RE: 418MHz vs. 433MHz: >>> >>> I'm playing with the Lynx Tech. 418MHz eval. kit and a 433MHz pair >from >>> Abacom Tech. and have had serious difficulty achieving anywhere >near the >>> ranges claimed by either. The Lynx pair will barely reach 30-40 >ft. out- >>> doors in direct line-of-sight. (Spec. sheet says "...a highly >reliable >>RF >>> link capable of transferring serial data at distances in excess of >300 >>ft. >>> is formed.") I'm awaiting a return call from Lynx's tech. support; >I >>MUST >>> be doing SOMETHING wrong. >>> >>> If Bob Blick or anyone else would care to continue this discussion >via >>> private E-mail I am interested. (I know if it ain't PIC-PIC-PIC >it'll >>> piss somebody off; and don't want to see -SNIP-BITCH-SNIP- about it >for >>> the next week.) >>> >>> Yeh, I'm having a REALLY bad day, >>> John >> > _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]