SO2R and multi-op contesters know that BPF (band pass filters) at each operating position are mandatory. On receive the BPF reduces interference from other transmitters and on transmit it reduces out of band noise and harmonics.
Although it is possible to forgo BPF (and I have!) the benefits far outweigh the expense. With low power the interference is so objectionable that many band combinations are unworkable, especially in small stations where the antennas are close together. High power exacerbates the problem and presents severe risk of receiver damage.
The latter occurred a few times in my own experience with high power multi-op contesting decades ago when BPF usage was uncommon. The lamp fuse in the pre-amps of some rigs of that era saved many contest efforts (and bank accounts). We always kept one or two spare transceivers on hand for emergencies. Technically adept team members replaced lamp fuses during their off time.
Today there are BPF with excellent performance:
- High return loss (low attenuation and SWR) across the pass band
- High out of band rejection (low return loss), and notch filters for adjacent bands, including the all important second harmonic
- High efficiency (low power dissipation within the pass band)
- Automatic band switching by transceiver band data, CAT or software control
- Bypass when not needed and for non-contest bands (leave them inline between contests)
- Power rating available from 100 watts to legal limit (ICAS or continuous duty)
A full set of BPF can be expensive. Strangely enough the big gun multi-multi stations have an easier time
of it because typically there is a dedicated operating position per band
which greatly reduces the switching requirements and thus can lower the
cost compared to smaller stations while maximizing performance.
Because they are a specialty product commercial choices are limited and prices can be quite high. Used products sell quickly because they are in demand by the budget conscious. For low cost there are kits or just PCBs with instructions for building your own. You trade your time for money saved. Tuning these high performance devices requires a VNA and knowing how to use it.
Until now I have operated SO2R at my own station without BPF. This limits me to low power or QRP, and even then there are frequency and antenna combinations that must be avoided. Since my objective is to do better with SO2R and to host multi-op contest operations with legal limit power the time has come to add BPF to my station.
For my station, antenna system and contest objectives I can trim the tree of possibilities to find the optimum solution. Before I come to that it will help to discuss the several alternatives. Each has its pros and cons and implementation requirements. I've condensed the alternatives to just 3 and there are variations within each.
There is more than one way to implement BPF in a station. Perhaps the most flexible option (1) is a switchable 6-band BPF -- the HF contest bands are 160, 80, 40, 20, 15 and 10 meters -- plugged into the transceiver. The BPF need only handle the transmitter power for low power or QRP contest entries, or the even lower drive power for a high power amplifier. Oddly enough you can get by with BPF rated for 100 watts if you are high power, yet you need the 200 watt rating for low power.
The transceiver or other control system selects the appropriate BPF for the band, or bypass the BPF for non-contest operation and for non-contest bands. Manual switching is not recommended since you're sure to forget and that can damage the BPF if the transmitter does not fold back power fast enough due to the high SWR. If you hear silence from the headphones check which BPF is selected! Even so it is a reasonable way to start and automation can be added later.
BPF can be placed between the amplifier and the antenna switch (2). This is a more expensive solution since the BPF must dissipate perhaps 10x to 20x the heat compared to low power and the design to achieve high performance is more challenging.
A benefit is that harmonics and other out-of-band noise generated by the amplifier -- all amplifiers do this so transmitter quality alone is insufficient -- are removed by the high power BPF, something that is not possible for low power BPF. Many contest stations with low power BPF compensate with switchable coax stubs after the amplifier to notch the second harmonic. Although the stubs are inexpensive it takes time to design and build them, and to place them correctly.
The high power BPF switching system must handle full legal power. It most always must be home brew since high power commercial filters are sold individually. Power lost in the BPF cannot be recovered; in (1) you can increase amplifier drive power to compensate.
Many high power BPF require cooling fans and that increases shack noise unless they are located remotely. Automation to select the correct filter is more critical than with low power. If the filters are outdoors power must be provided for the fans. In fact, all BPF can be remote if you have confidence in your automated switching system.
Switching and harmonic stubs can be avoided entirely by placing the filters after the antenna switch (3). This can be complicated in stations where one or more of the antennas are multi-band. A common example is tri-band yagis. However when all your antennas are mono-band this is a good choice. It can be economical since only one high power filter per band is needed since, other than in rare cases, no two operating positions are on the same band at the same time. That is, you need only one set of BPF.
A little deeper
No filter is perfect. There is attenuation in the pass band and on transmit that means heat. Filter components must withstand the RF currents and voltages and safely dissipate the heat. At 1 kilowatt a pass band attenuation of -0.3 db equates to 70 watts of heat. That's a lot to dissipate from within a small metal box!
Performance worsens with frequency since the capacitance across the relay is constant but the reactance is frequency dependent. I referred to this problem in my Beverage switch by recommending against the simple reed relay setup for other than low frequency use.
Since antenna switch isolation is not ideal it is helpful that the BPF be between it and the receiver. Out-of-band RF from the other transmitter that leaks through can then be removed by the BPF. However, this does not work for harmonics since, for example, the second harmonic of a 40 meter transmitter is not attenuated by a 20 meter BPF.
That 20 meter harmonic must be removed by a filter on the 40 meter signal path after the amplifier (or transmitter if there is no amplifier). The filter must be a high pass BPF, (2) or (3), or (1) with a supplemental coaxial stub notch filter on the amplifier output. Of course this is not mandatory since you may be able to live with a stronger harmonic on 20 meters by avoiding frequencies close to it. The avoidance area is wider with SSB (and RTTY to a lesser extent) since the second harmonic is 6 kHz wide.
This should also be an incentive to clean up your transmitter since SSB splatter and CW key clicks will wreak havoc on other bands, annoying you and your multi-op team members in addition to everyone else on the band. It is poetic justice should you suffer from your own poor signal.
I'll stop here; I said a "little deeper" not a lot deeper. Filters are not my expertise so I will leave further technical detail alone. Diving into the weeds of Cauer and Chebyshev filters and multi-filar coil windings is for others to deal with. Luckily the ham community includes people with the knowledge to design high performance BPF and we can benefit from their work, only needing to pay attention to the metrics and correct application in our stations.
No product recommendation
I deliberately avoided recommending specific products. I don't have sufficient experience with the many available alternatives. To ease your product search here are links to a few BPF commercial products. There are others, including kits. These are not recommendations and the order is random:
There do exist technical and comparative reviews of some commercial products. All that I've seen are of uncertain quality or are good but not up to date and therefore obsolete, so I won't link to any. It is better to consider products used by successful contesters and not the opinions of a friend or the ham you happen to share a coffee with. Talk to them.
Many contesters build their own BPF from kits and the performance depends on how well they did it, including proper use of a VNA to
optimize them. As noted earlier, antenna switches can compromise
performance so don't be seduced by high out-of-band rejection specs that may not be achievable in your station configuration. You must carefully design your station to realize the full benefits of a high performance BPF.
I do recommend BPF with notch filters for adjacent bands. The ultimate rejection of a BPF is not enough to avoid noticable interference on portions of adjacent bands. They cost more but this is not the place to choose economy over performance. Your contest scores will reflect your choice, good or bad.
I
researched options and spoke to a few hams I respect and I decided to build the BPF from kits. The precision tuning
should prove interesting. If I have to buy a VNA I will do so. The NanoVNA is inexpensive and widely available from many manufacturers. Several friends offered to lend me theirs.
This choice (1) will
give me the flexibility to handle multi-band antennas, reduce interference due to switch cross-talk and
alleviate concerns with high power filters, which would have to be outdoors with the antenna switch. Time is marching onward and I need to be ready for the coming contest season.
My choice
Although
an indoor task I want to work on my BPF solution this summer and
early fall rather than leave it for the winter. With
tower and antenna work progressing well I believe I can free up enough
time to get it done. Well, that's my objective but my track record of keeping to schedule is not good. This is a hobby and I do what I can.
My original plan was to
buy commercial products rather than build my own from bare PCBs that are
available. Some are very good, but they are not inexpensive. Although I
built this into my budget I can find uses for the money saved by going with kits. I do not have a VNA and I my
skill at tuning these filters is untested. The filters are mechanically and
electrically straight-forward but precision is paramount to achieving
ultimate rejection and low pass band attenuation.
I now have a kit in hand for a single band low
power BPF. If the build is successful I will continue along my chosen path and build the other 11 kits (!) and the switches
for selecting among the 6 BPF in each unit. I'll say more about the kit after it's built and tested. Some readers may recognize it from the picture.
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