SO2R Antenna Band Filtering

Continuing our discussion about SO2R technical discussion brings a focus to the concerns of cross band interference between your two radios.

Having one transmitter spewing energy at full power while trying to listen on the other radio’s receiver is daunting. Unless you have an Elecraft K3, your radio’s ability to perform adequate band pass filtering is probably not sufficient for the 2nd and 3rd harmonics from the transmitter resulting in severe de-sense of the receiver signal chain.

Orienting the antennas to minimize their interaction will help. If you are lucky you may well achieve good isolation between at least some of the cross band combinations.

To help radios avoid this interaction, a few manufacturers have products which provide excellent band pass filtering.

Enter the tunable (or selectable) band pass filter.

I found the following manufacturers with band pass offerings:

There may be others so look around. Note some of the above manufacturers also have single band band-pass filters available. These might be appropriate inline near mono-band antennas. However, it is likely you are using one antenna for a few bands if you are just getting started in SO2R.

If we modify our station system diagram (shown in the previous post) by adding the multi-band band pass filters we get…

A simple SO2R setup including two band-pass filters
A simple SO2R setup including two band pass filters

I personally own an ICE 419B. I like it fine. It offers front panel controls as well as remote operation via signal inputs.

In the “Basic SO2R” you are unlikely to have automatic band switching capability (we’ll cover that in a future post). So having front panel controls makes sense. Not all models have front panel controls so you need to be sure you understand what you are purchasing if you decide to drop the coin on BPFs.

Contest Bands Only
There’s one thing you need to understand very clearly about these Amateur Radio Band-Pass Filters; They are for contest bands only. They only allow 160, 80, 40, 20, 15 and 10 meters through. WARC bands won’t work.

The ICE 419B has a “no filtering” mode. I don’t think this means the filters are removed. I am pretty sure it just allows all filters channels through in parallel. This suggests only the contest bands get through this device either one at a time or all at once.

The point is this. If you design these filters into your shack, you may well have to do something extra to operate in the WARC bands. Contest stations and regular stations have competing requirements. Keep that in mind.

Mixed Blessing
Adding filters to your ham shack is a great way to ensure your two SO2R radios have as much isolation as possible. As you continue to evolve your shack you will most certainly want to automate these filters so they track the selected band on the radio. Until that happens understand you will now have one additional thing to twiddle when you change bands.

Is filtering really need with modern rigs?
There are reports the Elecraft K3 provides sufficient filtering to prevent interference from the second harmonic of a lower band. True? I have no idea. If I had a K3 I would still have filtering until I know for certain I won’t damage the front end with the energy.

The market has provided band-pass options for contest stations for quite some time. The models appear to be mature and ready for your application.

Next we will examine how to take advantage of more than two antennas.

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