K1TTT Technical Reference

One important thing to remember with either the k2tr style stub filters or other bandpass filters is that they only stop the harmonics generated by the radio and/or amp.  If something outside the shack is rectifying the stub won't help. I had that here for a while when i had a bad connection on the 160m inverted v, had big harmonic on 80m that nothing would filter out.  One way to try to figure out if that is the problem is to tune to the harmonic on the rx (carefully with attenuator if it is really bad) and start turning the power down.  If it gets to a point and suddenly dissapears you probably have something rectifying outside the shack, or possibly inside, I recently had 2 mfj swr meters go bad and arc internally for some reason... what a noise that made! at least until the smoke escaped.  Check coax connectors also.  If outside, the problem is often wx related, being better when things are wet and worse when dry.


Generic formula for lengths:


> > I've been compiling some data to build the stubs here and was

> > wondering, what is the formula for determining the stub length.

> > Is it just cut and try or what.

> > I tried 234/F and for 40 meters came up with 21.75 feet.  Most of

> > the posts say 23 feet open or shorted work best.  Where did the

> > 23 foot figure come from.  I have a bunch of RG-6 CTV coax here

> > that I'd like to use, but it has foam instead of plastic

> > dielectric.  How to figure the length of stubs using the foam

> > instead of plastic?  Thanks

> > Tom W7WHY


> quarter wave in coax (ft) = (246/F) x (velocity factor)


> where F is freq in MHz


> velocity factor:

> RG 213/8/58 = .66

> RG 6 = .75

> RG 8X, foam RG 8 = .78

> 9913 = .89


> Some of the foam dialectrics may vary from nominal VF.


> I think the best way to put a quarter wave stub exactly on freq is to

> temporarily connect a 1 inch piece of wire from the shell to the center

> pin of the stub's PL-259. Arrange this wire into an "L" shape, making

> a little loop with the center pin and shell of the pl-259. Lightly couple

> a dip meter into this loop. This works on an open stub, which is handy,

> because you'll be cutting little pieces off to bring it up to frequency.


> Dave Hachadorian, K6LL

> Yuma, AZ

> K6LL@juno.com