David, Is this a new KPA100 from Elecraft, or did you purchase the kit from a previous owner. What I am getting at is -- do you have the latest T/R switch components installed in this KPA100? Blue toroid cores at RFC1 and L16. If the toroid core at L16 is red, you have the older T/R switch design, and it is susceptible to a parasitic oscillation above the 40 meter band. The KPA100UPKIT is the cure should you have the lower level KPA100.
73, Don W3FPR David Douglass wrote: > Hi All, > > A bit of a strange one here.. > > Short story, I have just built and installed a KPA100 internal amp for my K2. > All is going well with the set-up and I have been using fine with a multiband > vertical I have at home. > > Problem is I put up an end-fed 1/2 wave vertical to use in a local contest > this weekend and I am seeing a strange phenomenon!! Below is a series of > observed events. > > 1. Erected antenna, checked SWR with analyzer (SWR 1.2:1) > 2. Connected to K2/100 via about 10m of RG-213 and checked SWR using 10w = > SWR still 1.2:1, so far so good. > 3. Turned up power (50w), checked SWR, getting Hi Refl reading on K2, and > external meter, blown fuse in K2/100 lead. > 4. Replaced fuse, check again at 10w OK, turn power up to 11w and engage > KPA100, get Hi Refl.. > 5. Check K2/100 using dummy load and my other multiband vertical, all OK at > 10w and 100w. > 6. Use K2 at 10w to drive my external amp, to 400w to check the 1/2wave > antenna and all OK, SWR 1.2:1 > 7. With the KPA100 engaged the power seems to way overshoot on the K2, hence > the blown fuse. However, this doesn't happen into a dummy load or other > antenna. > > Looks like my K2 with KPA100 doesn't like the 1/2 wave end-fed antenna at > all, but for the life of me not sure why. > > I first though I must have burnt out the 1/2wave matching circuit somehow on > the antenna, but obviously this is OK as it will handle 400w. The end fed 40m > halfwave is home-made, and is basically a airwound coil with a couple of > doorknob capacitors in series to match the high impedance at the end of the > 1/2w wire to 50 ohms. In short it is a beefed up Par-End Fedz antenna. > Speaking of which, I have a couple Par-End Fedz antennas, which I plan to > check to see if I get the same problems. > > Any suggestions would be appreciated. > > David, VK2NU > > ______________________________________________________________ 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
