I am not familiar with the protector you use, and how it is specified, but it seems to me that any such device intended for use on a transmission line must strike a fine balance between low breakdown voltage (for protecting your gear) and high breakdown voltage (to accommdate the power level in combination with swr). The higher the swr, the higher the max voltage, for a given power level. So, the manufacturer of the protective device should probably not just tell you a wattage rating, but also the acceptable swr level for that power. The higher swr level that is acceptable, the less protection is provided for your rig and tuner, etc. My guess would be that those protectors are meant for use at low swr. A multiband dipole fed with ladder line + balun + coax will have high swr on the coax at least on some bands, regardless of whether it is 1:1 or 4:1 or whatever. I would not expect the impedance to be close to 50 ohms on the coax. You might want to forget about the protection. Or, build a spark gap protector to be located on the ladder line. Or abandon the ladder+balun+coax approach and go to an end-fed dipole with transformer that can provide low impedance coax feed on all bands.
73, Erik K7TV -----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Rick WA6NHC Sent: Saturday, February 24, 2018 10:41 AM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] KPA500 faulting on high VSWR on power rise One exception is when the dipole is used on multiple bands, then a 4:1 may be required. That was the case for my 370' 80M EDZ (two opposing 5/8 wave, center fed). That is specifically a non-resonant antenna on any ham band but it performed best with the 4:1 instead of the 1:1. It was a compromise either way, this was the better choice since I needed that antenna to work on all bands. If it was only 80M, the 1:1 was the better choice. While I am not a fan of MFJ so consider it suspect in this case, I also believe that since the common mode choke (referred to as a balun in this thread) should GREATLY exceed the voltage expectations for simple overhead. The cost differences for higher ratings are not significant. In my stations, I use the 10KW rated devices from DX Engineering (KPA500 amp) and I've had no issues with them, even at extreme SWR mismatch (QRO on 160M into the above dipole, 20:1 unmatched with a Heathkit tuner). My new station will be entirely resonant, so a 1:1 CMC will suffice but I'll still use the high power devices. The bottom line here is that the antenna in this thread is not resonant (because of the feed choice, which is understandable for multiband use), the voltages presented will be extreme, component overkill (and a more qualified tuner) should be used. It's not a fault of the amp, but of the matching network. It is also KEY that the LEAST amount of coax is used (under 10' to the final) because the high SWR is present there as well, so the losses will be extreme and it could even exceed the voltage limits of the coax, depending on brand, age and type. Been there, done that (darned HOA limits). This also means that the coax should be tested, from time to time, until proven worthy. 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
