At 09:12 AM 12/10/2011 -0800, you wrote: >The down side of the 300 ohm twin lead folded dipole is that 300 ohm twin >lead usually is made from small wire and not mechanically very strong, so >it is not durable for fixed stations with varying weather conditions and >the wire is too small for anything but low power. With either the 300 ohm >twin lead or a folded dipole fabricated from #14 or heavier conductors a >4:1 balun is needed to tune well with a 50 ohm output or to transition to >coax. Then it is about as good as a well made dipole. > > >Willis 'Cookie' Cooke >K5EWJ & Trustee N5BPS, USS Cavalla, USS Stewart
Hi, No problem using good quality 300 TV twinlead with high power. Remember current is much reduced when feeding into a 300 ohm load vs a 75 ohm load. As I mentioned, I consistently used such an arrangement with a 4:1 balun feeding the folded dipole at 70 ft running with 300watts and fully plate modulated AM. In addition since the twinlead is operating with no standing waves there is no chance of magnified voltages and currents as a result of phase shifting. Never had a problem with RF power. Now to be fair TV twinlead was commonly available in at least 3 grades ... heavy, medium, or light. I am not sure what is available today in what is called TV twinlead. I understand it is possible to buy 300 ohm twinlead that is manufactured for power handling .... I don't know if anyone uses it for making folded dipoles (I suspect not). If one were to construct such a folded dipole for any HF band I can guarantee they will not have any power handling problems to today's legal power limit. I have also used TV twinlead to feed non-resonant doublets (100 inverted vee is a favourite) and used a 100watt AM transmitter (DX-100) with a balanced transmatch .... again, never had a power problem with the feed line. This arrangement is a bit tricky. Choosing a useable feed line length solves any power problems. However, you are correct about mechanical reliability. I did have to fix the antenna several times a year. At one time the cable manufacturers also made 75 ohm twinlead in both a lightweight and heavy duty version. The 75 ohm versions were more susceptible to current/heat damage due to the much greater current feeding an antenna with with a 75 ohm resonant impedance. The light weight version was only good for 25 to 50 watts but great for portable work as it was very light. It used to be the standard configuration for commercial portable radio service used by northern trappers es miners. Their radios only ran 3 to 5 watts. I used the heavy duty 75 ohm twinlead to feed a cubical quad for years ... again 100watts AM. Jim, VE3CI ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
