This seems so in many cases but it is very doublet length dependent. I ran one case in TLW with 45' of 450 ohm line that came out as 13 ohms. With a 4:1 balun you'd probably be dropping this further. High current and most likely not a match.
The best suggestion is to model the antenna at height, get it's feedpoint impedances at bands of interest. Then go into a program like TLW with the 450 ohm line length and see what impedance results at the shack end. Adjusting 450 ohm length or doublet length may be required for an adequate compromise on the bands of interest. Without some analysis (or a proven design) it will be a crap shoot. 73 de Brian/K3KO On 11/15/2012 12:31, Jim Wiley wrote: > Ed - > > According to Elecraft, the KAT500 can match 10:1 SWR at 500 watts, on > any band 80 to 10 meters. It can also match 3:1 SWR at 1000 watts on > those same bands. The 160 meter and 6 meter capabilities are a bit > more limited, but it will still match a fairly wide range of loads. By > inference, 10:1 SWR at 500 watts between 3 and 30 MHz could result in > peak voltages in the range of 2250 volts and peak currents of 42 amperes > RF (one or the other, not simultaneously). This is pretty impressive > for a box that size. > > Using those voltages as limits, that indicates that the KAT500 could > handle power levels of about 250 watts at a 14:1 SWR. Any more could > result in damage. I would suggest replacing the 300-ohm line on the > antenna with 450-ohm ladder line, and inserting a 4:1 balun between the > antenna and the KAT500. I suspect that this will reduce the SWR > excursions, and may get you "inside the envelope" for what the KAT500 > can handle. > > A commonly used antenna is a doublet (dipole) fed with open wire, a 4:1 > balun, and a coupler. Most installations use a length of RG-8 coaxial > cable (typically 10 to 20 feet, but no longer than necessary) to "get > through the wall" - or from the coupler to the balun immediately outside > the building, then the open wire line connects to the balun at that > point and continues to the antenna. 450-ohm ladder line is usually a > better choice because it is designed for transmitting use, where 300-ohm > line is usually "TV lead-in" and not particularly suited for use at > higher power. Some transmitter rated 300 ohm line exists, and if you > have that type, it may work for you. As with all open-wire type lines, > keep the line at least 8" to 12" from metal objects if possible, more is > better, and avoid abrupt right angle turns. > > - Jim, KL7CC > > > > > > On 11/15/2012 2:41 AM, Goss, Ed wrote: >> Hi Jim, >> Thanks for the info. I am at work now and unable to post back to >> the group, but I appreciate the explanation. For the low bands, I use >> a 135 ft inv vee center fed with 300 ohm line, with a 1:1 balun near >> the shack. When my KAT500 starts to tune itself, I am seeing high >> SWRs, such as 14:1. After the auto tune cycle, it's 1:1. Perhaps I >> should be concerned that even with 100 watts, there would be >> conditions that overstress KAT500 internal parts with this type of >> antenna, but I want to be very careful about not damaging the tuner. >> I'm not quite sure how a user would know these limits, or if the >> KAT500 protects itself in any way. Perhaps the KAT500 is meant to work >> with antennas that present a somewhat better match to start with, like >> a typical 80 meter dipole which is cut for CW but the user wants to >> use all the way up to 4.0 MHz... >> --Ed--- >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2012.0.2221 / Virus Database: 2441/5394 - Release Date: 11/14/12 > > ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.2221 / Virus Database: 2441/5396 - Release Date: 11/15/12 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
