Hi Rick, I distinguish between the radiator and the feed method, and therefore I would not say "the compromises add up to make it work".
To me, moving a dipole's feed from center to the end (or to somewhere in between) doesn't change its radiating characteristic as a dipole. Whether the feed method is free of loss, or free of diverting power off of the dipole are complicated questions better answered by serious testing of an individual design than by simple rules of thumb. I haven't recently looked into the "box" of my Cushcraft vertical, but it has been reported that there is a separate impedance transformer and a separate common mode choke. A very low swr is achieved without a tuner, at full legal power (within bandwidth limitations; people have blown up toroids when going outside bandwidth on 40m on the R7). There is obviously not a high level of power lost in the "box", or it would not survive full legal power. Thus this is not like some antennas that sacrifice efficiency by swamping in order to achieve low swr. Therefore most of any observed loss of performance can be attributed to the radiator and its en vironment, in combination with some rf going on the outside of the feedline (which is not completely prevented by the one CMC in the box). Some end-fed antennas use a non-resonant radiatior. This invariably results in swr high enough to require a tuner. Since the R5/R7 has low swr and not very high losses in the "box", the radiator obviously is resonant on all bands, presenting a non-reactive impedance at the end. That impedance must be very high since it is at the end. The very high non-reactive impedance makes it comparable to the end impedance of a resonant halfwave wire dipole even if the Cushcraft radiator is in other respects different. That is my basis for assuming that the Cushcraft "box" can be compared to what you would put at the end of a wire dipole when moving the feed from its center to one end. An end-fed dipole can be resonant and present a very high but non-reactive impedance on multiple (higher) bands. This can allow a fixed, broad-band impedance transformer to provide low swr on multiple bands without a tuner. Yes, one has to be suspicious of possible losses in the transformer, and it takes a fight to stop rf from going onto the outside of the feedline. But since one avoids swr-caused losses in coax, and balun losses, and tuner losses with end-feed, I consider it an alternative worthy of serious consideration. If optimal protection against high voltages on the coax is a main goal, that just might tip the scale to make the end-fed dipole the winner. 73, Erik K7TV -----Original Message----- From: Rick WA6NHC [mailto:wa6...@gmail.com] Sent: Sunday, February 25, 2018 2:52 AM To: Erik Basilier <ebasil...@cox.net> Cc: elecraft@mailman.qth.net Subject: Re: [Elecraft] KPA500 faulting on high VSWR on power rise Hi Erik, I'll keep this short as the thread will likely be closed soon (or should). What I have up now is Field Day style, simple antennas (the inverted L for example, an 80M dipole, R7) shot into the trees so I could get on, pending the install of 'the real station', hi. The water pipe (300+' in two directions, deep enough to never freeze) is the L counterpoise, for now. It isn't ideal but it works and isn't meant for forever. I will be installing a lightning protection and counterpoise system with lots of copper, ground rods etc, which will come to box (for feed line, rotor control) at the tower end of the conduit to the shack (AND tie into the house grounding per code). The box will have hardline from the house, coax for the tower, matching network for the Inverted L and the surge and lightning devices on each feed (static or lightning stays OUTside). I'll also put an AC power outlet at the base, for occasional power tool use and a wifi web cam (may as well, I have to power the electric winch motor). I DX, I don't contest, so I don't need SO2R (other than the second rx for DX chasing on splits). Should that someday change, I'm blessed with the space for a tower farm or I'll put the EDZ up (kind of a favorite, I worked a LOT of DX on that dipole). With the KAT500 (or KPA1500), rapid QSY isn't an issue on the EDZ. No slam was inferred or taken on the Cushcraft; only that the compromises add up to make it work, but not as well as a tuned dipole. I'm not sure that an end fed wire antenna will work well on multiple bands without a fair amount of effort (remote tuner at least, absolutely a CMC to back that up). The voltages at radical SWR (non-resonant afterall) can be high. Isolating the radiation to the wire is a major issue, that antenna tends to want the feedline (coax commonly) as the counterpoise, bringing RF into the shack (and why I went with the inverted L which partly warms the worms). 73, Rick NHC ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
