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...@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, 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 > > > > > > > > > > > > > > > > > > > > > > > >
