[Elecraft] [K2] KPA100 - Antenna Help

[David Douglass]

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