In addition to antennas and amplification, it is almost always important to band-limit an OCAR. This is necessary both to stay within one's license restrictions as well as to avoid wasting output amplifier capacity on unwanted signals. For the 900 MHz OCAR, we are now using a two-section filter K6PZB constructed from standard water pipe. These help to keep pagers, cellular telephones and other out-of-band signals from negatively impacting operation.

Single Section Copper Pipe Coaxial Filters

• 900 MHz
• Measured response of a 900 MHz filter
• construction details
• 1200 MHz
• 1200 MHz filters (wide) 2 GHz sweep, 1 dB insertion loss
• 1200 MHz filters (narrow) 100 MHz sweep, 4 dB insertion loss

The coupling loops can be rotated to provided the desired response.

This kind of filter is very easy and inexpensive to fabricate while offering quite good performance for OCAR use. Coupling loop size can be increased to provide even wider bandwidths. Filters of this type also have application in weak signal (EME for example) operation where sensitive, low noise input preamplifiers need to be protected from large out-of-band signals which might cause spurious signals to raise the noise floor.

While these single section filters work well, we have designed and built a two section similar design in the same piece of copper pipe. This produces a flat-top and wider filter along with steep skirts.

Two Section Copper Pipe Coaxial Filters

900 MHz , made from two of the single section filters and end coupled.

Photo of 2-section filter

• 900 MHz 2-section filter (2 GHz sweep)
• 900 MHz 2-section filter(100 MHz sweep) adjusted for slightly narrower passband

Filtering at an Intermediate Frequency

In cases where an OCAR is desired to operate over only a narrow bandwidth or when extremely good shape factors are desired, it is possible to downconvert the OCAR band to an intermediate frequency where the approriate filter is easier to fabricate. After passing the signal through a filter it can then be upconverted to the original frequency using an isolated output of the same local oscillator used to downconvert.

For example, if it is desirable to have an OCAR at 145 MHz with only a 100 KHz bandwidth, an inexpensive handheld radio might be used. The 10.7 MHz crystal filter could be replaced with a standard FM broadcast filter and the filtered output converted back up to 145 MHz with an isolated output of the same LO. Futher amplification would then be applied as with on-channel filtering.