Bandpass Filters On Hand

Introduction

There are a number of bandpass filters bought from eBay plus a few from Minicircuits, and one special order, on hand at HawkRAO.

The bandpass characteristics of these are given below for reference. Having plots of the filters assists in identifying those filters which may be useful for radio astronomy. In that service they may be suitable for reducing noise/RFI at the 'sky frequency' or further down the RF chain at an intermediate frequency (IF). No attempt is made to rigidly classify which purpose each filter will be used for - but some possibilities are mentioned.

For excitation a broadband noise source is used and the bandpass output displayed on a spectrum analyser. The broadband noise source used has a higher output level than the one used for the Sawbird LNA bandpass plots. This is necessary to provide sufficient output level for the spectrum analyser in the absence of an LNA.

HI Line Filters

There are a couple of different filters on hand which should be useful for HI line observations (1420 MHz).

Custom 1420 MHz

This filter was obtained from Marcus Leech (CCERA : https://www.ccera.ca/) some years ago.

It shows a 3 dB bandwidth of about 30 MHz and is nicely cased in copper tubing with endcaps. This was used during past HI line observations here at HawkRAO.

NOTE: this filter could also be used for filtering the down-converted (IF) frequency (1428 MHz) of a Ku band LNBF with a 10.75 GHz LO observing the 12.178 GHz Methanol Maser line. The reduction from the nominal 1 GHz bandwidth of the LNBF IF signal to about 30 MHz bandwidth before being recorded by an SDR should be beneficial.

Minicircuits VBFZ-1400-S+

This shows a much wider 3 dB bandwidth (~150 MHz) than the previous filter and so although bought for use in HI line observations it is probably more suitable for wideband pulsar observations.

eBay H-Line Filter - 1420 MHz

This shows a much, much wider 3 dB bandwidth (~600 MHz) than the previous filters.

Note that the frequency span has been increased from 250 MHz for the previous two HI line filters, to 1000 MHz - 4 times wider.

Although also bought for use in HI line observations, it is probably more suitable for general rejection of the mobile phone frequencies below 900 MHz.

eBay 902.50 MHz

This filter was bought (along with similar 915 MHz units) for a project to observe pulsars at a higher frequency than the current frequency of 436 MHz.

That project never eventuated.

eBay 1.85 GHz

Why this filter was bought is unknown - or at least forgotten. At least it shows a pretty flat passband and nice 'square' shape to the passband. The 3 dB bandwidth is about 60 MHz.

eBay 1.54 GHz

Once again - why this filter was bought is also unknown or forgotten. At least it also shows a pretty flat passband and nice 'square' shape to the passband. The 3 dB bandwidth is about 40 MHz.

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