A four helix resonator band pass filter for the two meter band.
|This filter combination is suitable for use in the direct vicinity of a transmitting antenna from pagers, mobile communication or broadcast.
Pager signals and FM broadcast will be suppressed more than 65 dB !
Two units of 2-resonator-helix band pass filters in cascade (series), using a 1/4 wave interconnecting cable.
Remark: Using direct connection through male-male coupling, or through a 1/2 wave cable, the performance was less, resulting in higher insertion loss and less selectivity, especially below 142 MHz.
The 1/4 wave interlinking cable proved to be optimal.
Reflection damping (voltage):
Beware of cheap plugs and cable! On plugs check for loose center contacts, especially at T-connectors and 900 knee adapters.
Throw away if found.
Because the obtainable selectivity of this filter is so high, inferior quality of plugs and coax can be of negative influence on filter performance below 130 MHz and above 153 MHz !
Cheap (non PFTE-) plugs or cheap (single screened) coax can ruin the selectivity of this filter.
Never use PL259 etc. New good plugs cannot be cheap. Use only BNC- or N connectors with PFTE insulation (for instance Radial or Amphenol) and gold plated center connector.
When on the plug an UGxxxx number is visible, it is a sign of good quality. The tight connected N-connectors make even better screening contact than BNC. A must in critical applications.
Use double screened coax (Aircell7, RG214, Teflon=PFTE).
If the insulation of a connector or adapter melts when the center connector is heated with a
hot soldering iron?
Then the insulation was NO PFTE -> throw away.
NB: PFTE coax cable shows an higher insertion losses than non-PFTE cable.
So use PFTE cable ONLY when it must be used in a HOT environment. Not in low loss applications like here.
Experience showed me, that (Italian made) "RG213" antenna cable is HF-leaking. Do not use it for filters, or connections between filter and TRX.
Better use double screened RG214, Aircell7 or Aircom coax.