>Your 10m of coax with a velocity factor of about 0.66 might be an electrical > half wave to this system. Oooops! That would be more like a full wave and less likely the problem. I would still recomend adding a little coax to see the difference. BTW, 1.4 : 1 is a pretty good match. I seldom worry with anything better than 2:1. John Ferrell 6241 Phillippi Rd Julian NC 27283 Phone: (336)685-9606 Dixie Competition Products NSRCA 479 AMA 4190 W8CCW "My Competition is Not My Enemy" ----- Original Message ----- From: "John Ferrell" To: Sent: Sunday, December 29, 2002 3:19 PM Subject: Re: [OT]: (Stupid) Antenna question.. > Here is a couple more thoughts: > Reduce power for testing to about 5 to 10 watts. This is a safety measure in > case you are dumping power into something in the structure. House wiring, > plumbing, downspouts, etc, can easily be accidently resonant on 15M. > > Your 10m of coax with a velocity factor of about 0.66 might be an electrical > half wave to this system. That would show your transmitter an infinite > impedance. If it is not too difficult, ADD about 5 meters of coax at the > trasmitter end to assure that it is not performing as a half wave line. Be > certain to not coil it so it becomes a choke. > > I assume the antenna feed point is difficult to get to. If it is not, take > your dummy load to the antenna end of the coax and check the SWR. If that is > good consider a matching stub across the antenna terminals. It is messy to > set up because it takes a helper. I usually use twin lead tv antenna wire > for matching and an open line is easier than having to short the stub again > everytime you cut it. Test at low power for safety! > > > Dipoles are theoretically 70 ohms > Transmitters are usually 50 ohms > Most coax in amateur work is 50 ohms > > AND all of these numbers only work when they are matched! > > > John Ferrell > 6241 Phillippi Rd > Julian NC 27283 > Phone: (336)685-9606 > Dixie Competition Products > NSRCA 479 AMA 4190 W8CCW > "My Competition is Not My Enemy" > > > > ----- Original Message ----- > From: "Milan Pavlica YU7XW" > To: > Sent: Sunday, December 29, 2002 1:30 PM > Subject: Re: [OT]: (Stupid) Antenna question.. > > > > > > John Ferrell wrote: > > > > > The math is all theoretical, a great place to start. Antennas, > especially > > > transmitting antennas can be humbling. > > > > Really? I didnt see that ;-) > > > > > My thoughts: > > > Is the antenna location to close to the ground? > > > > Nope. This is the story. > > It was 1/2wave 3.5MHz diple which resonated very well. > > Now i wanted to build 21MHz dipole, cuted the wire - and it's doesnt work. > > Antenna is located under roof of my house, and coaxial cable length is > about > > 10m - exactly above my room where radio is located is feed of antenna. > > > > > Is the coax shorted or open? > > > > open > > > > > Could you be coupling to something in the near field of the antenna that > > > affects its resonance? > > > > i dont think so. > > I just checked radio on dummy load and radio is OK > > > > > Sometimes drooping the ends of a dipole will help bring the match closer > to > > > unity. > > > Baluns are more popular than effective. My Cushcraft A3S Multiband beam > is > > > fed directly but they specify that you use six turns of the coax on an 8 > > > inch diameter as an rf choke at the antenna end. This discourages rf on > the > > > outside of the coax. > > > > > > How long is the coax run? It can make a difference. > > > > -- > > http://www.piclist.com hint: The list server can filter out subtopics > > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > > > > > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics