Towertalk
[Top] [All Lists]

 ```There are two approaches.. If the tube you're sticking it into has an ID very close to that of the OD of the insulation of the inner coax, then you can probably use the databook values for the coax. If not, then you're looking at a bit more complex thing.. If you've got some magic way of holding the coax centered in the tube, then it can be considered as two coaxial capacitors in series, one with air dielectric, the other with whatever the coax used. If the coax isn't centered, it's a whole lot more complex. In any case, the formula for capacitance of coax is: C = 2 * Pi * Eo * Er * ln(router/rinner) * length Eo is the permittivity of free space 8.85 pF/m, Er is the relative dielectric constant: 1 for air, around 2-3 for most plastics router is the inside diameter of the outer conductor rinner is the OD of the inner conductor all dimensions in meters That should get you close... ----- Original Message ----- From: "Jim White, K4OJ" To: "Mirko Sibilja" Cc: "Gregg Seidl" ; ; "Floyd Sense" Sent: Saturday, June 21, 2003 2:00 PM Subject: Re: [TowerTalk] homemade beams > Do you know a location on the web where approximate lengths are > specified? I have used commercials antennas which this approach but > whenever it was time to design my own matching/gamma I took the easy > route and ordered a compete unit from Cushcraft and used theirs... > > I have not seen this as a sub routine on any of the antenna designing > software but mebbe I missed it... > > 73, > > Jim, K4OJ > > > > > Mirko Sibilja wrote: > > Another way to make the gamma match capacitor is to use piece of coax cable > > with braid removed, inserted into gamma tube. Calculate the neccessary > > lenght by cable's capacity/length unit. > > > > S57AD > > ----- Original Message ----- > > From: Floyd Sense > > To: Gregg Seidl ; > > Sent: Friday, June 20, 2003 7:15 PM > > Subject: Re: [TowerTalk] homemade beams > > > > > > > >>The gamma match cap is pretty easy. I've always made the cap from two > >>loosely telescoping pieces of tubing, covering the inside one with shrink > >>wrap tubing. That tube then serves as the gamma match tube and you adjust > >>the cap by moving the inner tube in or out. > >> > >>K8AC > >>----- Original Message ----- > >>From: "Gregg Seidl" > >>To: > >>Sent: Friday, June 20, 2003 10:14 AM > >>Subject: [TowerTalk] homemade beams > >> > >> > >> > >>>I would like to know if any one has built any of the beams in the 19th > >>>edtion Antenna book by the ARRL.I'm considering building either a 3 or 4 > >>>element 20 beam.I'm thinkimg about useing a hairpin match mostly because > >> > > I > > > >>>don't know how a gamma match works and where to find the cap that is > >>>needed.I know you need to insulate the driven elemnt from the boom on a > >>>hairpin but that I think could be done with some pvc or such,kinda like > >>>Mosley does.As a matter of fact I think I have an old Mosley center > >> > >>element > >> > >>>in the shed.Any help would be great.I know I could never come close to > >>>builing a JRC JST-245 but I think I could build a 4 element beam just as > >>>good as M2 or whatever and maybe save a few bucks to buy another > >> > >>JST-245,ok > >> > >>>maybe not!!! Gregg K9KL > >>> > >>> > >>>_______________________________________________ > >>> > >>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > >> > >>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with > > > > any > > > >>questions and ask for Sherman, W2FLA. > >> > >>>_______________________________________________ > >>>TowerTalk mailing list > >>>TowerTalk@contesting.com > >>>http://lists.contesting.com/mailman/listinfo/towertalk > >>> > >> > >> > >>_______________________________________________ > >> > >>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless > > > > Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any > > questions and ask for Sherman, W2FLA. > > > >>_______________________________________________ > >>TowerTalk mailing list > >>TowerTalk@contesting.com > >>http://lists.contesting.com/mailman/listinfo/towertalk > > > > > > _______________________________________________ > > > > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. > > > > _______________________________________________ > > TowerTalk mailing list > > TowerTalk@contesting.com > > http://lists.contesting.com/mailman/listinfo/towertalk > > > > > > _______________________________________________ > > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk ```