The Radio Amateurs' Emergency Network

Effective 2m/70cm in-line filters

Many RAYNET groups are increasingly suffering interference from nearby PMR and pager sites, as well as other RF sources. A recent review of talkthrough reports confirmed that this problem was not getting any better but the solution to the problem is, of course, effective filtering of the receive signal.

Groups around the country have had considerable success in the past with band-pass filters for 2m and 70cm from one particular manufacturer, DCI. Originally available in single band versions, these filters are also available as dual band units increasing their usefulness in cross-band talkthrough situations. They are highly effective filters but somewhat costly with some members adapting simple bandpass networks from former PMR gear as a cheaper alternative. 

Recently though some alternative suppliers of filters have come into being, usually manufacturing filters from discrete inductors and capacitors rather than the helical resonator type made by DCI. This greatly reduces the price of the filters albeit at the cost of reduced filtering performance as can be seen in the graph below though the size of the filters is significantly reduced as well as the risk of mechanical damage in transit.

Comparison of filters

Practical Use

Both kinds of filters have been used in environments where stations and/or talkthroughs have been in very close proximity to hilltop PMR sites, and in temporary control rooms with multiple PMR systems being used in the same room and on the same mast as the RAYNET equipment. 

The DCI filters are more likely to filter 100% of out of band interference, even when using amateur equipment with "barn door" receivers. The relative cost of these units though can be a deterrent to some groups purchasing them, especially if multiple Local Authority sites are likely to need some filtering. Alternative units have less effective filtering but dependent upon the source and level of interference, some protection is also gained from the frequency split between our operations and the interfering signal and the net effect of suppressing the interference remains the same.

Filter Loan Scheme

To help Groups choose the most appropriate filter for their needs, the Network has purchased a 2m and a dual band 2m/70cm filter from Cross Country Wireless for loan to groups to enable them to test the level of filtering required. This represents a cost effective compromise since we are reasonably confident that the filters will survive multiple passes through the postal system and help groups identify both the type and magnitude of the interference they need to suppress. If the loan filters reduce but not totally suppress interference then a DCI is probably a better choice, if they kill the interference altogether then the more expensive option is not required. What we can't cure though is the increasing amount of in-band QRM from computer systems which neither filter will be effective against.

The filters are available for loan from the Technical Team on the following terms;
  • Loans will only be made to member groups of the Radio Amateurs Emergency Network.
  • There will be time limits on loans depending upon demand.
  • The Network will pay the postage and packing to get the filters to the Group
  • The group pays the postage and packing for the return of the filters either to the Technical Team or the next nominated recipient of the filters.
Loans will be made on a first come first served basis and requests should be made to The Technical Team .

Contacts

DCI's agent in the United Kingdom is Waters & Stanton plc. Readers may also be interested in more technical background on the filters from DCI's own Web page, which includes Network Analyser plots for all their filters.

Cross Country Wireless offer a number of filters for sale through their website www.crosscountryradio.co.uk where more technical information is available. Mentioning that you are a RAYNET member prior to purchasing may also get you a pleasant surprise since the proprietor is also a member !

For further information on these filters, please post an e-mail to The Technical Team

The different filter types shown with a AA battery for size comparison

Image of DCI Cavity Filters and Cross Country Wireless lumped circuit ones.