Bandpass Filters

Discussion of amateur radiosport activities.

Bandpass Filters

Postby Jamie_WB4YDL » Wed Jul 20, 2011 4:13 pm

One of the biggest things going in contesting right now, is the addition of a second radio in what is known as SO2R operation. This abbreviation stands for Single Operator, Two Radio. The basic idea is this - there are times during a contest, no matter what mode of operation, that there are empty moments where, for example, you are calling CQ, that you could use to advantage by listening for multipliers or others calling CQ on a different band. In fact, you could make a contact between calls of CQ on this second radio. This technique involves listening to two radios simultaneously - usually Radio 1 in the left ear and Radio 2 in the right ear. This contest operating technique has incredible potential to increase your scores dramatically. You don't have to spend a fortune to begin utilizing this technique. If you have a dipole and a vertical and a primary radio and maybe an older standby radio, you have the rudiments of an SO2R contest station. This is well described in multiple sources on the internet.

Now let's walk up the ladder a bit and say you now have two nice transceivers and multiple antennas that can be switched to either radio to work multiple bands during a contest. There will come a point when you realize that you can't hear on the second radio because of interference from Radio 1. :shock: The radio you are listening for multipliers simply de-senses and you hear nothing. There are two circumstances that this happens - either the antennas are in close proximity to each other, or you are running higher power. If both are the case, you run the risk of damaging the front end of the receiver. In fact, it doesn't have to be both for this to happen. Enter the item in the subject line - band pass filters (BPF's).

Band pass filters are designed to eliminate as much as possible out of band signals from the band you are operating on. They are designed to be inserted after your transceiver, but before an amplifier. It is a low power unit. Additionally, it expects that the antenna load it sees is resonant on the band of operation. There are several commercial units available - the ICE 419B unit and the Dunestar 600 unit are both two stage filter designs. They are relatively inexpensive. The W3NQN filters are a three stage design and therefore have much better filtering capacity. These are sold by Array Solutions and their new Filter Max III is a 6 band design and is quite expensive. You need a set for each radio.

Recently I had an epiphany ! :o During the last Field Day operation, I had a set of ICE 419B filters setup at the RTTY station which consisted of my Elecraft K3 and a low ladder-fed dipole doublet antenna. This is NOT a resonant antenna. What was utilized was the internal antenna tuner of the radio to find a suitable match ... for the radio ! The operator of that station and others were coming to me saying that they were smelling something acrid. Uh-oh ! :? I had presumed that the switching power supply was getting too hot and was causing the plastic on the table cloth to heat and smell. This was NOT the case. When I got home from Field Day, I found the power supply to be in perfect working order and it was returned to service at my station. What I discovered is that the ICE 419B unit had several blown filters. :x As I came to realize, The SWR after the internal tuner was still, of course, not resonant. And that is where the band pass filters were - experiencing non-resonant high currents and voltage spikes. :oops: These units will be repaired at my bench soon. Lesson learned - use an external antenna tuner after the bandpass filters. :roll:

A very good article - easy to understand - is written by KG4JJH that is titled Contest Bandpass Filters - check it out here - http://www.kg4jjh.com/pdf/Contest%20Bandpass%20Filters.pdf. In order to understand what's at risk, you need to know about receivers and specifically the blocking dynamic range. :geek: It took me a few readings to understand it all but it should be required reading - and that's Leif SM5BSZ's article in the Mar/Apr 2006 QST titled Blocking Dynamic Range in Receivers. The best source of information about all this is George W2VJN's book titled Managing Interstation Interference. It goes into great detail about bandpass filters and coaxial stubs. Stubs are filters that can be inserted on the high power end of a station - after the amplifier and often at the tower.

My plan is to fix the two stage bandpass filters that I have but I also have plans to build my own set of W3NQN 3-stage filters for far better performance - and safety. The Yahoo Group TXBPF is a group managed by a UK ham that kits parts, boards, and enclosures for this very fine set of filters. The design is by 5B4AGN and you can check out his web site at http://www.5b4agn.net/.

Next year at Field Day I should have a better plan and one that won't smoke up the room ! :D

73, Jamie
WB4YDL
Jamie_WB4YDL
 
Posts: 46
Joined: Sun Nov 14, 2010 8:45 am
Location: Union City, TN

Re: Bandpass Filters

Postby Jamie_WB4YDL » Sun Jul 31, 2011 3:45 am

I thought I would report that I got the bandpass filters repaired. Industrial Communications Engineers or ICE is not presently operating due to the untimely death of their CEO. So it was left up to me to figure out what was needed and repair them myself. After opening both boxes, I could see no obvious problem. There were no cracked capacitors. They were all silver mica capacitors and some of them were rated for 2000 volts ! :shock: It's not hard to understand that you could easily exceed this value if there was a great impedance mismatch. At any rate, I was able to identify which relays and components were for which bands and I set up a table to make an order for mica capacitors. The first thing I needed was a way to measure capacitance in the picofarad range. The best and easiest solution is an LC meter sold by AADE http://www.aade.com/lcmeter.htm. This little gem came in the mail and was just the thing I needed to proceed. :D

What I discovered is that the capacitor of each stage (not the coupling capacitor) was grossly out of range. For example, after de-soldering a 2200 pf capacitor on the affected 40M stage, I discovered that it only measured as a bit over 400 pf ! :o The highest rating I could find for these capacitors was 1000 volts, so I made the order from Mouser. After getting the affected capacitors installed, I connected my antenna analyzer to the radio port and a dummy load to the antenna port and found that things were very nearly back to normal on all bands. An occasional band needed a little tweaking by simply squeezing or spreading the inductor coils of each stage.

They're back in service now and as long as I maintain a good antenna match, they should operate as they should to protect the front end of the radios and eliminate interference. 8-)

73, Jamie
WB4YDL
Jamie_WB4YDL
 
Posts: 46
Joined: Sun Nov 14, 2010 8:45 am
Location: Union City, TN


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