There is a great set of apps that illustrate the Smith Chart on the web: http://www.fourier-series.com/rf-concepts/smithchart.html This illustrates the use of a stub: http://www.fourier-series.com/rf-concepts/flash_programs/SmithChart3/smithchart3.html of course knowing where to put a stub and how to use the charts is the trick to success cheers, W6 MTF
--- In Repeater-Builder@yahoogroups.com, allan crites <wa9...@...> wrote: > > Dave, > I thought I'd do a bit of investigation using the rough VSWR numbers you > provided earlier, and here is what I calculated with my trusty Smith Chart > and calculator: > 1) the length of the o.c.coax (I used RG-214/U in my calculations) you used > on your 1st try to reduce the VSWR to 1.5 was about 3.7". > 2) the VSWR being 2.0 relates to a Z=51+j35 ohms > 3) the impedance matching line needed to be added to the existing coax is > 23.6" of > RG-214/U. > 4) the o.c. stub needed to be added at the Tee adapter on the xmtr end of > the additional line is 5.18" of RG-214/U > 5) if one wanted to use a s.c. stub then the impedance matching line needed > to be added to the existing coax is 13.325" of RG-214/U > 6) and the s.c. stub length needed is 8.14" of RG-214/U at the Tee adapter > All impedance matching was done here with only VSWR measurements which can be > obtained using a Bird WM and an arbitrary length of o.c.coax (o.c. coax is > preferred because one can easily shorten it by cutting with a side cutters or > I use a hand held hedge clipper) and the aid of a Smith Chart, a compass for > drawing VSWR circles, and a 4 function calculator. Are you listening Norm? > AC >   >  > > --- On Sun, 5/9/10, WA3GIN <wa3...@...> wrote: > > > From: WA3GIN <wa3...@...> > Subject: Re: [Repeater-Builder] Commercial VHF antenna on 2m > To: Repeater-Builder@yahoogroups.com > Date: Sunday, May 9, 2010, 11:20 PM > > >  > > > >  > Thanks, >  > I'll get back to you but it won't be immediately. The site is secured and > access is very limited. >  > Dave > > ----- Original Message ----- > From: allan crites > To: Repeater-Builder@ yahoogroups. com > Sent: Monday, May 10, 2010 12:17 AM > Subject: Re: [Repeater-Builder] Commercial VHF antenna on 2m > >  > > Dave, maybe I can help you with your impedance matching if you could provide > me with some information. I need to know: > 1) the freq of operation > 2) the VSWR at the xmtr end of the coax line feeding the antenna > 3) the VSWR of the line when you added the Tee adapter and open circuit coax > stub > 4) the type of coax used for the stub (50 ohms, solid dielectric, foam, > polyethylene or teflon, the impedance, and the length in inches. > When I get this I will put it into my Smith Chart program and see what I can > find. > AC > > --- On Sun, 5/9/10, WA3GIN <wa3...@comcast. net> wrote: > > > From: WA3GIN <wa3...@comcast. net> > > Subject: Re: [Repeater-Builder] Commercial VHF antenna on 2m > > To: Repeater-Builder@ yahoogroups. com > > Date: Sunday, May 9, 2010, 10:09 PM > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >  > > > > > > > > > > > > > > > > > > > >  > > > > > > DB224... no access to the > > antenna. We got to > > live with it as it is ... just trying to make the > > transmitter happy till a time > > comes when we can either tweak the antenna or replace > > it. > >  > > Thanks , > > dave > >  > > > > ----- Original Message > > ----- > > From: > > allan crites > > > > To: Repeater-Builder@ > > yahoogroups. com > > > > Sent: Sunday, May > > 09, 2010 10:13 PM > > Subject: Re: > > [Repeater-Builder] > > Commercial VHF antenna on 2m > > > >  > > > > > > > > > > > > > > Dave, > > Is this a 154 MHz colinear antenna you are > > wanting to use on 146 > > MHz or one like a DB 224 with 4 exposed > > dipoles? > > If the 4 exposed dipole type, just what makes > > you suspect that > > there is "a lot of loss" when an antenna > > made for 154 MHz is used > > at 146 MHz and how do you expect to tune the > > dipoles without > > compensating for the harness impedance matching > > sections mismatch > > also. > > Allan Crites wa9zzu > > > > --- On Sat, 5/8/10, WA3GIN > > <wa3...@comcast. net> wrote: > > > > > > From: > > WA3GIN <wa3...@comcast. net> > > Subject: [Repeater-Builder] > > Commercial VHF antenna on 2m > > To: > > Repeater-Builder@ yahoogroups. com > > Date: Saturday, May 8, > > 2010, 11:13 PM > > > > > >  > > > > > > Hi > > folks, > >  > > Several weeks > > ago I posed the question of > > using a Commercial VHF antenna that was resonant > > on 154Mhz on > > 146.745Mhz. We tried it today. The SWR was > > a bit over 2:1 on the > > repeater freq. We installed a T connector after > > the cans and used an > > open stub to try to match the line...got it down > > to 1.5:1, wouldn't go > > any lower. > >  > > We think the > > height of the antenna makes > > up for what we suspect is a lot of loss in the > > antenna. The previous > > location of the repeater antenna was 100ft ASL > > and this location is > > 525ft ASL. Maybe one day we'll get a chance > > to retune the four dipole > > antenna. > >  > > Thanks to all > > that provided ideas for > > this project. > >  > > 73, > > dave > > wa3gin > >  > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
