One of the most important items in a Contest Station is the filtering solution. Due to the proximity of the antennas, bandpass filters should be highly considered – first due to high QRM between the stations and second to protect the radios front end.
There are a lot of options available in the market. They are basicaly divided in two categories: The High Power BPF Filters and Low Power BPF Filters.
The High Power BPF Filters, such as well known Ranko´s BPF (4O3A) were designed to be used just after the linear amplifier. In this configuration, all the high power signal passes through the filter. This is desirable to better attenuate the harmonics amplified by the linear amplifier.
Low Power BPF Filters, are placed between the rig and amplifier or the rig and antenna switch. They are cheaper than the High Power ones.
The bandpass filters are “monoband” devices i.e. they can be used only on the actual radio´s selected band. That means, if the radio is on 20 meters band, the BPF placed after the radio should be a 14mhz BPF. Although this is simple to understand, many people get confused on how to use them. Just imagine them as a “part of the tuned radio”, if the radio is tuned to 28mhz, a 28mhz BPF filter will be used.
That being said, we can look forward on some brands and analyze some important factors while selecting a filtering solution. They main factors are: Insertion Loss and Attenuation Curves.
The Insertion Loss is a number that reflects the loss (in dB´s) due to the device inserted in the transmission line. Good BPF´s should not have a Insertion Loss bigger than 1dB (better 0.5 or below that).
The Attenuation or Response curves is a curve which represents the BPF´s attenuation (in dB´s) in response to the frequency variation.
In all cases, these characteristics may vary a lot mainly due to the filter design used. There are a great number of different filter designs: Butterworth Filters, Chebyshev Filters, Bessel Filters, Eliptic Filters etc. Each design vary its characteristics: bandwitdh, insertion loss, response curve, riple etc. More about filter design and topologies can be found on this link.
For the Amateur Radio enthusiasts, there are a good number of brands that manufacture these filters. For Low Power, they are: Dunestar, OM-Power, Hamation, Low Band Systems and VA6AM. For High Power, they are: 4O3A, VA6AM and Low Band Systems.
Since our station profile is set to be a Low Power Contest Station, than set our focus on the Low Power filters available in market. Some factors were quite importants on choosing the filters. Besides the technical factors some other factors as financial, transportation, acquisition logistics got to be important too.
During our research we compared the attenuation curves between the different brand LP filters. You can check it ou by bands here: 10m – 15m – 20m – 40m – 80m – 160m. All the numbers were taken from the manufacturer´s web site. You can download all the table data in Excel format here.
In terms of insertion loss, the Dunestar BPF600 claims about -0.5db average insertion loss across the BPF band. The Hamation filters did not provide I.L. numbers on their website. Here also the LBS filters seemed the better I.L. between them with best than -0.5db in all bands. As can be seen the Low Band Systems BPFs are the LP filters with greater adjacent band attenuation. The QST Maganize published a review on LBS filters in March – 2017. Click here to read it.
Besides the great numbers of the LBS BPFs, some other factors were important on the decision to purschase these filters. On July 10th we ordered a 6 pack monoband filters by LBS. Andrei, RA6LBS, was very kind and proposed to delivery the pack himself during the WRTC event in Germany. The brazilian WRTC competitior, Alex PY2SEX is going to pick it up with Andrei and ship them to Brazil – hopefully before CQWWs contests.