Wednesday, April 19, 2017


The last few Field Days, the South Plainfield Amateur Radio Club has been plagued by that perennial old bug-a-boo ........ stations interfering with each other.  Even QRP transmitters at close proximity can wreak havoc on each other! This year, I intend to do something about it. Since we typically run only two HF transmitters, I was thinking of building a set of these K4VX filters.

I've spoken with my good friend Bob W3BBO about this in the past, and he put the bug in my ear about using coaxial stub filters, instead. He has some experience using them and he told me they worked well for him.

I did some Googling and searching of the ARRL Web site last night and found this: as well as this YouTube video:

As I'm going to be replacing the coax out to the Butternut as soon as the weather cooperates, I'll have a useful purpose for the old coax, instead of just tossing it.  It would end up being a lot cheaper than purchasing toroids and capacitors and enclosures and antenna sockets needed for the K4VX filters. All I need for the stub filters would be some PL259s (which I have) and a few T-connectors which I can get at a local hamfest.

If they don't work as well as I'm hoping, then maybe a combination of the K4VX filters along with the stubs can be used next year.  But it's already mid-April and time is growing short. Field Day will be here before you know it.  And even just the stub filters will be better than the nothing we've been using.

72 de Larry W2LJ
QRP - When you care to send the very least!


  1. I'm all for saving money, but

    (1) Well-designed filters with inductors and capacitors can do a much better job than open or shorted coax stubs, because of losses in the stubs. Now if you use heliax it might be a different story.

    (2) Your ancient coax is presumably being retired because it has become more lossy (water?) than it once was. If so, it will make worse stubs than new coax.

    TANFL. But sometimes you pay for lunch and it's not as good as it might be.

    David, VE7EZM and AF7BZ

  2. The coax is being changed because of age and because I knicked (but did not penetrate through) the jacket in one area, close to the antenna.