STUBS FILTERS

(1/1)

PY5EG:
WHAT IS THE BEST ARTICLE ABOUT AN PRACTICAL USE OF STUBS AS FILTERS FOR MULTI/ MULTI STATIONS

Steve Katz:
Can't recall last time I saw an article on this subject, but a coaxial line 1/4-wavelength long, open-ended, is a resonant stub trap; a line 1/2-wavelength long, short-circuited at its end, is also a resonant stub trap, and one that often is easier to control.

The Vf of the coaxial cable used must always be used in the length calculation, of course.  

The larger the diameter of the coaxial cable used, and the larger and lower resistance its conductors, the higher Q the trap can be.  As such, traps made of small-diameter cables are pretty useless; however, traps made of 7/8" hardline or Heliax work very well.

At the M/M stations I've operated, where I was involved in the filtering aspects, we've achieved at least 20 dB additional rejection by the use of 1/2-wave (end-shorted) stubs made of large-diameter coax like .875" hardline.

WB2WIK/6

LA1SJA:
Most texts deal with stubs for impedance matching rather than stub filtering.
One exeption is a book by the good old antenna guru Ronold W.P. King and his colleauges Mimno and Wing:
"Transmission Lines Antennas and Wave Guides"
McGraw Hill, N.Y. 1945.
They combine open and closed stubs to do both even and odd harmonics filtering.
It's out of print, so if you have an e-mail address to respond to I might send you a scan of a few essential pages.
Traditionally, stub filtering is done at the transmitter rather than in front of the receiver.
Stubs produce nice attenuation levels for the  harmonics with next to no effect on the fundamental frequency, but they tend to have a broad area with medium to little attenuation around the deep null at the specific frequency they are cut for, - even reaching into the adjacent amateur band, and thus they may be of limited use in front of a receiver, at least for neighbouring bands.
Have fun.
LA1SJA.

Russell Brown:
There's a wealth of info about stub filters at http://www.k1ttt.net/technote/techref.html#filters , and you may also want to look at the site http://www.arraysolutions.com/Products/wx0bbpf6.htm

Russ,W5RB

Dale Hunt:
Steve's response might be confusing:  it depends on
whether you want the stub to pass the transmit signal,
or to attenuate a specific other frequency.

A shorted quarter wave stub (or open half-wave stub)
will pass the design frequency and attenuate other
frequencies to some degree (except odd harmonics.)

An open quarter wave stub (or shorted half-wave stub)
will reject the design frequency (and odd harmonics)
but pass other frequencies with less effect.

So it makes a big difference what you are using for
the length of the stub - the desired frequency or the
interfering frequency.

The ideal situation is there the two frequencies have
a 2 : 1 relationship:  a shorted quarter wave stub for
40m will reject 20m, for example.  The same stub will
have little attenuation on 15m, though it probably will
help reject 80m signals.

Navigation

[0] Message Index