RG-8, RG-213, solid dielectric... 31 pF/ft.. If it's in a tube that just
snugly fits where the braid used to be (or, you left the braid on..) then
this number works.. if it's in a bigger tube, the capacitance is less.
----- Original Message -----
From: "Jim White, K4OJ" <k4oj@tampabay.rr.com>
To: "Jim Lux" <jimlux@earthlink.net>
Cc: <towertalk@contesting.com>
Sent: Saturday, June 21, 2003 3:11 PM
Subject: Re: [TowerTalk] homemade beams
> Now know why I wished there was graph/chart someplace with atypical
values!
>
> Kinda like the great chart K5KG does with lengths of dipole/loops/etc...
>
> An "easy reference" for gamma capacitors... I would use the aluminum
> tubing that was just big enough to accommodate dielectric...dunno what
> size that translates to.... and then.... what spacing for each HF band...
>
> Seems like someone could build some formulae into an excel spreadsheet
> for us QCAO types to find a reasonable guesstimate/starting point.
>
> Thanks,
>
> Jim, K4OJ
>
>
>
> Jim Lux wrote:
> > 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" <k4oj@tampabay.rr.com>
> > To: "Mirko Sibilja" <s57ad@amis.net>
> > Cc: "Gregg Seidl" <k9kl@gbonline.com>; <TowerTalk@contesting.com>;
"Floyd
> > Sense" <sentek@sprintmail.com>
> > 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 <sentek@sprintmail.com>
> >>>To: Gregg Seidl <k9kl@gbonline.com>; <TowerTalk@contesting.com>
> >>>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" <k9kl@gbonline.com>
> >>>>To: <TowerTalk@contesting.com>
> >>>>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
> >
> >
> > _______________________________________________
> >
> > 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
> >
>
>
>
|