Co-ax Notch Filters

Gary, G0XAN, has built some coaxial notch filters for use at Special Event or contest stations where it's desirable to run two HF stations at the same time. The information originally came from an article by John Crowder G0GDU on the web site, although it doesn't seem to be available on the current site.

Coaxial notch filters are simply stubs on the transmission line between the trasceiver and the antenna. A quarter-wave stub will transform a short circuit at one end to a high impedance at the other end, and an open circuit at one end to a low impedance at the other. A half-wave stub will appear the same at both ends, i.e. an open-circuit stub will appear as a high impedance, a short-circuit stub will appear as a low impedance.

Since the main amateur HF bands are harmonically related, a single stub length can work as a quarter or half wave on several different bands. In this way, many pairs of HF bands can be covered by a small number of different stub lengths. A shorted SO-239 socket can also be used to convert an open-circuit stub to short-circuit, thus doubling its usefulness.

It turns out that a length of RG58 co-ax just under 7m long is a quarter wave at 40m, thanks to the velocity factor of the cable. It is also a half wave at 20m, 3/4 of a wavelength at 15m and a full wavelength at 10m. Adding an extra 7m to the stub makes it a quarter wave at 80m,or a half wave at 40m.

The notch filter kit consists of 4 lengths of RG58 co-ax:

  • A is shorted at one end with a PL-259 at the other.
  • B is open circuit at one end with a PL-359 at the other.
  • C has PL-259 connectors at each end (qty 2).

There are also two T-connectors to connect the filters into the transmission line, two in-line couplers to connect the sections of the stubs together, and a shorted SO-239 socket.

The following table shows which stub configuration to use when transmitting on each band, and which bands the stub will reject (so which band(s) you can set up another transmitter on).

Tx BandStubsFiltered Bands
80m C + A 40, 20, 15, 10m
40m C + shorted SO-239 20, 10m
40m C + B 80m
20m C 40, 15m
20m C + B 80m
15m C + shorted SO-239 20, 10m
15m C + B 80m
10m C 40, 15m
10m C + B 80m

It is very important when changing bands on the transmitter to ensure that the filters are also changed!

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