Band Pass Filters
 
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Band Pass Filters

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tomkhaz
(@tomkhaz)
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Joined: 4 years ago
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I want to set a second HF antenna at my house, but my testing tells me I'm going to have interference, even running a rig QRP. For instance, one radio running JS8Call 40m QRP, and another running 20m SSB.

I believe band pass filters could be the answer, so I'm looking for advice and perhaps a source of dependable and simple BPFs.

I see fancy, and expensive BPFs on places like DXEngineering. I also find many Chinese versions on EBay. I'd appreciate any advice or experience you can share.

Thx.

Tom - KF0AER


   
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(@wr0ot)
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I have on a few instances setup two stations at my house for my Wife, Son, and myself to operate simultaneously for contests.  I can say in my experience Band Pass filters rated to handle your full transmit power are the main key in running multiple stations in close proximity.  Another thing that can help some is using cross polarized antenna's, this can get you some more isolation between antennas.  Use a vertically polarized antenna on one station and horizontal polarization on the other.  This will not help nearly as much as the band pass filters, but it will help some.  In my use case at the house, we have a FlexRadio 6400 at the house hooked up to a few different attic dipole antennas.  For contests, we setup an ICOM 7300 on the kitchen table, and run a coax out the back door to a temporary ground mounted full 1/4 wave vertical antenna with a bunch of ground radials.  For this setup the spacing between the Vertical antenna and the Dipole antenna is at most 30-40 feet.  I will insert the band pass filters between the radio and the antenna for which ever band each station will operate.  I use a short 2-4 foot coax jumper between the radio and the band pass filter to keep the filter near the radio.  If either station wants to change bands, it means swaping band pass filters for the correct band.  We never run both stations on the same band.  The band pass filter passes the whole band so they will do nothing to help when using the same band on each station, and you would just have the same issues you are having now.

The radios you are using can make a difference also in how much cross station interference you will experience.  In my setup no matter which combination of bands we use between the two stations, I have never witnessed any noticeable band interference with the FlexRadio being induced from the IC-7300 transmissions.  How ever on the IC-7300 we on occasion can see (on the spectrum scope) or sometimes hear a small amount of interference from the FlexRadio transmissions.  This comes from the difference in the quality of the band selection filters built into the radios we use.  The more expensive radios usually come with better filtering built in than the cheaper ones.  This is one of a handful reasons the big contest stations us the really expensive HF radios.

If you are going to be transmitting at power levels near the ratting of the BPF you will want to be aware of your SWR ratio for your antenna.  The higher the mismatch between the radio and the antenna will cause higher voltages across the filter capacitors and inductors.  if the voltages across the components in the BPF go to high you can damage the filter.  Either make sure you build matched antennas, use and external antenna tuner, or buy larger over ratted filters to handle the miss match.  If you are going to use an external tuner for antenna matching then insert the bandpass filter between the radio and the external tuner.  The tuner will match the load of the antenna, and the BPF will see a good 50 Ohm load between the tuner and the radio and everything will be happy.

Good Idea:

Radio > BPF > Ext Tuner > Antenna

Bad Idea:

Radio > Ext Tuner > BPF > Antenna

I have a couple 100 watt rated Filters from Low Band Systems that I been running full 75 Watt FT8 transmissions through to an antenna with a 2:1 or better SWR match and have had no problems.  When they were available they have some 300 watt filters that only cost a little bit more that I picked up just to play things safe.

Right now DX Engineering does not have much in the way of band pass filters in stock.  Not sure if we will see Low Band Systems back in the market any time soon.  I think a good choice right now would be to look at Array Solutions at https://www.arraysolutions.com/filters   There are other options out there just do some internet searching.

I know the feeling about spending another 300-400 dollars for a pair of filters that only cover one band each, just to run a second station at a time seems like a hard cost to swallow.  Having done it though, it works great, and was well worth the money. having a set of filters for each of the ham bands adds up fast.

 

John - WR0OT

 


   
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tomkhaz
(@tomkhaz)
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Joined: 4 years ago
Posts: 22
Topic starter  

@wr0ot Thanks John. I have been studying up and I appreciate the advice and experience. The big BPFs at DXE and other places are overkill for me. I'll not run more than 100W so small filters should be good for my use.

For example, I'm thinking of a 14Mhz band pass on my FTdx-3000 to work 20m. That should make my QDX happy on 40m running a digital mode.

I also read about the best order for the devices and it seems to be the consensus. Good to hear your experience with SWR and all that.

Of course, there are many identical Chinese single band BPFs on Ebay for a reasonable price. I may give one of those a try to dip my toe in the water. Have you heard anything about those?

I have no room to physically separate the wires in any good way. I know that's asking for more problems. I'll see if it's all worth messing with!

Tom - KF0AER


   
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(@wr0ot)
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The circuits that make up a band pass filter are rather simple and don't have much complexity.  The equations are very well known for calculating the exact values needed for the inductors and capacitors used in the circuits.  The big problems comes in the fact that there needs to be a fair amount of precision in matching the actual components in the filter to the calculated values.  As the values of your components in the circuit begin to deviate from the correct calculated values the filters ability to reject out of band signals quickly diminishes.  You also start to get all sorts of phase issues and other problems creep in quickly.

The biggest difficulty in making a band pass filter is going to be obtaining or building capacitors and inductors that are actually the values you calculated.  In the past when I have purchased cheap capacitors in large lots I have seen so call 5% tolerance capacitors be as much as 50% off from their printed specification.  Usually in these cases all the parts I ordered of the same value from the same batch were all with in 1-2% of each other, but they were all 50% of from the value listed on the part.  With out a good meter to measure actual capacitance and inductance for your parts, you don't actually really know what you are getting.  Some times the parts you order are spot on and some times they are not.

The good quality bandpass filters are usually have there inductors hand tuned, again so that the inductors actual inductance matches what the circuit was calculated to use.  Once everything is in place and tweaked, some sort of glue is usually spread across the components to keep them from moving and shifting.  Once everything is assembled the circuit would need to be verified with a Spectrum Analyze, or a Vector Network Analyszer.  Both of witch are not cheap pieces of test equipment.

The big problem with any of the cheap Chinese electronics online is that very little effort is put into design and assembly.  Most stuff is really the result of some random person copying a circuit design from any place they can find one, ordering some cheap PCB to be made, stuffing it with parts and soldering it.  Maybe some basic test if at all, and then slapped online for sale.  We all know the name of the game here is profit, not precision and quality.  Like I said the circuit for a BPF is simple and well know.  This lends it's self well to the ebay market, however precision is a different story.

I know I have seen lots of really cheap BPF circuits out there for receive only applications.  Be aware that the voltage levels across the filter at even just 1 watt of transmit output power are substantially higher than receive voltage levels.  5-10 watts of power handling is a lot easier to meet, than 100 watts, and maybe watt you are looking at can handle QRP levels.  I have not looked at what is really needed for QRP filtering.

I could easily imagine some of these cheap filters really only actually giving you maybe 10-20 db of out of band rejection where a good filter is going to be more like 40-60 db or more.  Maybe that is enough to resolve your issues.  If they are that cheap maybe give it a try.  Start with low powers first.  Would hate to see a blown capacitor or shorted inductor cause any radio damage.  Keep a close eye on the SWR at the radio to be aware of any mismatches quickly.

It is obviously a lot more time and effort to build your self, but there is a lot of information online on building your own PBF's.  Like I said nothing has changed in this area for many decades.  If your ARRL membership is active check the QST archives even back into the 80's.  There have been lots of articles in QST over the years on making BPF's capably of handling transmit level power.  But for me this is what just drove me to just spend the 150 bucks per filter, call it a day, and just get on the air.

Good luck in coming up with a solution.  Please let us know how the cheap filters work out for you if you go that route.

 

John - WR0OT

 

 

@tomkhaz


   
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