Radio W4KAZ

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Notes On SSB RFI In Homebrew SO2R Box

Chasing down the RFI caused by inserting all of the home-brewed SO2R components into the station set up was a useful hands-on experiment. Annoying, but certainly educational.  I verged on ordering the ARRL RFI tome, but now the thing is fixed, owning the reference seems less urgent.  Might be worth reading though…..probably quicker than re-inventing each technique personally.

To start, the shack layout resulted in a few less than ideal situations. Both radios are side-by-side, separated by about 300mm. The computer that logs and controls both radios is on a rolling cart normally kept close to the station desk. The computer was also being introduced into the audio chain as the DVK, and I was also working towards routing the mic audio through the sound card full time. The cramped space on the desk is further reduced by the antenna switching controls and an antenna tuner. One set of bandpass filters is built into a relatively large computer case, and that occupies much of the top shelf.

No RF problems were noticed on CW, but on SSB the audio was terrible, and I got many reports to that effect. Apologies to those who were exposed to it.

The unshielded plastic enclosures used may have contributed to the problem, but so far most of the trouble has been corrected by applying the normal RFI kludge, clamp on ferrites.  Shielded enclosures probably can’t make the problem any WORSE though.

The audio stream for the Yaesu FT-920 was relatively easy to clear up. Three or four turns of cable through one or two ferrites seemed to do the trick. The K2 was more difficult to tame, but it was also the furthest from the computer. I expect that the longer audio cables needed to reach the K2 made better antennas for picking up the stray RF. The cables used are a mix of CAT-5 and shielded RCA audio cables. The CAT-5 cables carried the mic audio, PTT and CW from the So2R box to either radio.

In addition to the ferrites, I also routed the audio from the computer through an isolation transformer. That step alone almost completely solved issues with the FT-920.  Using a separate power supply for the SO2R box resulted in acceptable audio –  better, but not BEST.

Two of the issues were a big surprise – and I only discovered them as RFI ingress points because I reached the point where I was determined to cover every base.  The separate power supply was an issue that was unexpected, but should not have been.  NT4D has made that point to me several times over the years.  Unfortunately good advice often falls on deaf ears.  I have heard the gospel now….

The one that I really dd not expect was that the PTT line might be an RFI source.  It became obvious this was a source when I methodically disconnected various cables on the K2 end of the chain.  Low and behold, once the PTT line was disconnected from the SO2R box – no more RFI.

I may now re-visit the entire chain, substituting a better quality cable to see if there is any difference or if fewer ferrites might be required.  It took three ferrites with about five turns on each to subdue the RFI ingress from the PTT line.

Here’s a summary of the mitigation steps taken to end the RFI issues.

Problem #1:
Power supply – One source of RFI problems was sharing the radio power supply with the SO2R box. Putting the SO2R box on a separate wall wart helped a lot.

Problem #2:
SO2R box cabling. Because of the shack layout, the SO2R cables are all pretty long. Putting ferrites on all cables more or less solved the problem with the FT-920. Still had RFI on the K2.

Problem #3:
Added .01 bypass caps across all of the relay coils and DC connections in all switch boxes.

Problem #4:
Didn’t really have ferrites on ALL of the cables. I really didn’t expect the PTT line to be an RFI issue, but solving the RFI problem on the K2 required ferrites on both ends of the PTT line(at the radio mike jack, and at the relay output in the SO2R box), as well as on the foot switch itself(at the SO2R box). While I was at it I added ferrites to the DC power cords too. It took three ferrites right at the mike jack on the radio end, so that may have been the real source of ingress.

Problem #5: Add isolation transformer in line with audio from the computer before going into the SO2R box.  Putting the transformer at the input to the SO2R box was an arbitrary choice – I don’t know if its location in the audio stream is of great consequence in mitigating the RFI.  Its placement there ensured only one transformer was required, since all audio is routed through that location, not being split until later in the SO2R box.  The location was convenient – perhaps not ideal.

Caveat: It is possible I also did something else inadvertently which helped solve the problem, but after a couple of weeks of head scratching and trial and error, I can say that UNdoing any one of #1 thru #5 will re-introduce some amount of RFI.

The problem with the PTT line really has me perplexed, but I guess it is in close proximity to the audio cable at the radio mike jack, so any RF on the PTT line is probably getting into the audio there.

After all of the trial and error, my impulse in the future will be to add ferrites on all control cables on both ends, and immediately after they enter/exit each device.

Home Brew SO2R Box

This home brew SO2R controller project follows the “old-n-busted” theory, and is based on the design by N6BV in the ARRL Handbook, as well as some input from K4QPL. In summary, it is built around the use of an LPT port for computer control of the CW, PTT, Radio A/B, and band data. As previously outlined, the LPT port is less expensive and easier to accommodate – even if obsolete.  Hence “old-n-busted”.  I expect to be able to bridge the gap to USB at some point by adding a K1EL WinKeyer, and the Piexx SO2Rxlat dongle.

The rig control is still accomplished via a serial port for each radio. The LPT parallel port is used for PTT, CW, transmit focus, and band data for one radio. The K2 band data is a separate option not installed in my K2[another void the Piexx SO2Rxlat dongle will solve].

As it stands now the only parts missing for a conversion to USB device control are the WinKeyer and SO2Rxlat devices. Everything else in the SO2R control chain is home brew.

There are several resources available that block diagram the components needed inside the shack for SO2R[e.g.,see DL1IAOfor the W5XD SO2r Box].   For ease of use and construction, the heart of the SO2R box breaks down into four logical units which were built into three separate boxes.

Band Decoder: One of the peripheral boxes will provide automated band switching driven by the logging program[or directly from a radio] by acting as the band decoder.  Most logging programs provide band data in the “BCD” format, and Yaesu radios provide that format via their hardware dedicated band data outputs.  The binary coded band data make the design of the decoder relatively simple.  In hindsight, it seems like a good idea to expand this component’s abilities by using a set of relays to provide for either positive or sinking switching.  This is a consideration for driving band pass switches or antenna switches, and is also a design factor for home brewing those components.

Inside view of homebre band decoder

The internal view of the band decoder

More band decoder photos.

Band decoder schematic. PNG Image, PDF file

For those looking for an inexpensive band decoder solution, the Unified Microsystems  band decoder looks like a real bargain and could easily be incorporated into a home brew design. To build a band decoder, it is probably better to start off with the Unified Micro unit and build the support hardware around it.

Audio Switch: The second is a simple peripheral to the main SO2R box is a simple remote switch. This device itself is simple, yet it really makes the SO2R a lot easier.  This device was subdivided into two physical component parts.  The actual relay board that switches the audio was built on its own small pc board and mounted within the SO2R box.  The user controls are mounted in a small project box.  The small box sits just above the keyboard on the desktop.

For my own preferences, it seemed better to have one small “remote” user control box for switching in the heat of battle. The remote has a rotary switch to control the headphone audio, and it can choose either radio individually, stereo with one in each ear, stereo with left and right reversed, or it can be set to have the audio follow the transmit focus.  It also has a momentary contact switch for each channel, which can be used in stereo mode to listen to either channel for as long as the switch is depressed. The remote switch control is connected via a Cat-5 cable to the SO2R control box.  The control box and its rats nest of wiring can be placed away from the station controls.

Remote: The momentary contact switch feature will soon be enhanced to correct an original construction oversight.  Parallel connections for the momentary contact switches will be added to allow using a footswitch.  That will provide hands-free audio switching when in stereo mode.  That is important, as I need the hands free to type and deal with CW and radio tuning. Hat tip to K4QPL for the idea.

Remote locating allows both the main SO2R box and the band decoder to be located away from the other major components in the station.  That highlights the single caveat I experienced – RFI on SSB.  After experiencing RFI problems during ARRL SS SSB, both of these  units still need some attention paid to choking RF on the interconnects.  Re-locating them a bit further from the RF hot spots and coax connections should also help with the RFI.  Judicious and liberal use of clamp on RFI chokes seems to abate the problem.

For some reason, the K2 seemed more susceptible to RFI than the Yaesu FT-920.  The RFI source there turned out to be on the PTT line.  A combination of ferrites and a diode in the PTT line finally tamed that hotspot.  Note:  in the PTT line, the PTT hot is at the radio’s mike connector, and it really didn’t sink into the thick head right away.

The completed SO2R control box

W4KAZ SO2R control box

More SO2R box photos

SO2R box schematic  PNG ImagePDF file

Audio switching schematic  PNG ImagePDF file

SO2R Box: The guts of the system all reside in the main SO2R box.  It has inputs for headphone audio from each radio, CW inputs, PTT inputs, and microphone audio inputs.  There is also an LPT Db-25 input for connection to the computer.  The set up is designed to receive control input from the computer LPT port to drive some of the switching and provide band data.

The main box contains two separate components. Two small perf boards were used to simplify construction.  One board contains switching for the headphone audio.  The other board handles switching for the CW, PTT, focus control, and microphone audio.  The audio is normally switched via the switch remote, but it can also be slaved to follow the logging program’s transmit focus and be controlled from the main SO2R board.

Future Migration to USB:

In order to migrate to a logging computer with USB ports, it should be a minor change to replace the LPT cable with a USB connection via the Piexx SO2Rxlat device.  By making the components LPT port compliant, the SO2R capability can also remain backward compatible with an older computer that has no USB support, running Writelog.  Just in case the only option is an ancient junker from someones junk bin.  The SO2RXlat will also provide band data for both radios via a single LPT DB-25 connector.


The total cost in parts was not large.  The 4401 NPN transistors, relays, connectors, bypass caps and diodes were all generic ‘project part’ items I have been accumulating over the past several years. I have gravitated towards using RCA connectors because of the availability of inexpensive sheilded RCA cables and the low cost of the connectors. The CD2048 IC for the band decoder was a dedicated purchase, and were around $1.98 USD.  The amount of time put into construction, the biggest real expense, amounted to about 15 total hours, spread over a long period in several hour long increments.  I spent more time debugging the RFI issues.

The RFI is probably partly due to using plastic enclosures, but these enclosures were easy to come by.  In hindsight I would add ferrite beads on to all interconnects inside the boxes.  The ferrites on the interconnect cables create a bit of additional clutter and impede quick wire pulls. Kludgy.

The remote switch has since been modified to add a jack for an external switch to concentrate both channels on either the left or right radio.  This will allow for use of a foot switch and will allow hands-free audio switching.

Small Parts List

Some suitable and inexpensive parts for future projects. A continuing aggregated list of Stuff I Use To Play Radio.

last update 2010-04-22, w4kaz

RX Antenna 160m/80m Band Splitter/Switch

edited and amended11/02/2009, kaz

Here is a small project that will work along with the K9AY RX antenna, and solve a minor SO2R problem in the KazShack.

Currently theK9AY feedline comes into the KazShack directly to the RX input for one of the transceivers. I wanted to have a way to share the antenna between the two radios without connecting the RX inputs of each rig directly to one another(RF isolation), or manually swapping the feedline between radios.

There are some comments in various places about using the W3LPL RX bandpass filter design to split the bands to multiple destinations. The NCJ article “Distributing Receive Antennas” by K3NA and W2VJN is a very handy and well explained reference.

This was also desirable here in the KazShack, which sitsin the shadow of the 50KW WPTF on 680kc broadcast transmitter. Rolling the W3LPL filters was done using some T-50-3 toroids and NP0 and high accuracy monolithic ceramic capacitors from the parts bin. The filter is built dead bug style. Each band is on opposite sides of a single piece of copper clad board. Basically, the input is fed to each of the filter banks, and the 160m and 80m bands come out separately, each isolated from the other.

The coils for each band are identical within the band(i.e., L1=L2=L3), so after winding each I used the MFJ-259 to resonate each coil to the same frequency using the same capacitor. After soldering everything together, a quick test with the antenna analyzer into a dummy load showed each section to show minimum SWR right where I wanted it. No other tuning was required. Almost too easy.

Left alone at that point, I could feed either radio from either band, but there needs to be a switch of some sort to eliminate the need for swapping coax feeds during the heat of the action. This appeared to be another ideal application of the small signal relays that were also on hand. Using a single DPDT relay, the filter outputs can be switched between the radios with the flip of a switch. A toggle switch mounted on a remote panel is used for convenience . Simple but effective.

The remote panel is a smallsection cut from 1’5 inch(about 38mm) aluminum angle stock. I pre-drilled pilot holes for future use, and installed the switch for the RX antenna splitter, as well as a control for a planned 40m remote antenna switch. The “panel” is then attached to the inside edge of my home brewed fold out station cabinet. The cabinet is filling up fast – not much room for any more equipment in there.

Using the switch is going to make swapping the low bands from one radio to the other a snap. Literally as easy as flipping a switch. The band pass filters will also help isolate the radios from one another in the SO2R environment, as well as reducing the broadcast band harmonics.

There is a bit of signal loss in the filters, but probably not enough to be significant while operating. Hopefully the much lower noise levels on the RX antenna will offset these slight losses. I have not felt a need for an external RX preamplifier before now, but now I am looking at the ARR 1-30. It would be nice to boost the RX signals to parity with the noise on the TX antenna. That might reduce the amount of volume control “riding” needed when looking for the best RX on a contact when toggling between the RX and TX antennas.

Yet another fun little project. It isn’t as much satisfaction as growing an entire rig from scratch, but it is always fun to put a useful bit of home brewed kit into action.

PHOTOS (Full set of photos on external page)

160m/80m Antenna splitter


PNG image of schematic for W4KAZ's "W3LPL RX band pass filters" built as an antenna splitter and switch.

PNG image of schematic for W4KAZ’s version of the “W3LPL RX band pass filters” built as an antenna splitter and switch.

W4KAZ SO2R Collection – Engineer The Possible -SO2R part 2

At the bottom of this page is an accumulation of some of the SO2R resource materials I used in developing my own custom SO2R solution. My first SO2R post hashes out the thought process involved in choosing the homebrew methodology for hacking together a workable SO2R set up via home brew components.

The big issues for customizing an SO2R capability appear to me to be philosophical. It is possible to wear two sets of headphones and manually switch everything, praying you don’t transmit from radio A into radio B. That is a bit TOO minimal, even for me. A minimum SO2R set up for my purpose came down to an audio switch box, a switch for the CW, microphone, and PTT, as well as band pass filters and filter switching(manual and/or automatic).  And something like the Array Solutions SixPak to “keep ’em separated”[cue “The Offspring”].

It turns out none of those components are out of reach for home-brewing – if you are willing to compromise. The audio and radio input functions can coexist, but they could also easily be separated into two discrete units. One unit to handle headphone audio and the second for CW/MIC/PTT switching and route band data(if used). Likewise, the filters, switching, and band decoder for automatic antenna/filter switching can all be discrete units.

The crucial decision is whether to use USB(New Hotness) versus serial/parallel(old-n-Busted) interfacing. It didn’t take very long to determine that Old-N-Busted was going to be much easier to twist up in the KazShack. YMMV, and it is a VERY subjective decision. For my own uses it is just simpler to use the parallel interface, even if it requires milking the life from a few old computers running un-supported OS’s. But I suffer no illusions that “simpler” equates to “better”. That’s a subjective call, and will depend upon the circumstances and resources available.

Building a custom SO2R set-up grew out of my interest in a project by Jim, K4QPL, as well as my interest in filters, both band pass filters and coaxial stub notch filters. Being able to scout a second band will be fun, and it isn’t a great leap from a bit of extra S&P to full two radio operation. I don’t expect to be very proficient at it, but running at low power into mediocre antennas is not terribly productive either. So a full integration of the second radio into the station set-up might be fun.

All of those considerations lead to researching the topic. Others have done a good job of documenting certain things via the internet, so I’m just aggregating a few of the links I found useful. Some are ideas I have incorporated, like the band pass filters. The filters merit their own separate treatment. Some of the other SO2R links discuss ideas that seem to have merit, but did not apply to my situation. Some are just good reading.

The first set of links are station specific information, posted by folks describing their own SO2R set-ups. My own customized designs will be referenced first, simply because I can. But just so it is clear, my own design is an amalgamation of the work of others, including K4QPL, KK1L, and others.

Many thanks to K4QPL. Jim sparked my initial interest in this project via a club program about his own SO2R project, and answered several philosophical questions that led me to my own research and experiments.

The next set of links point to reference materials or other sites aggregating similar links, or some of the commercial sources. Note: There is a lot of duplication and cross linking. K8ND’s site has a good round up of the commercial sources from here in North America.

Hardcopy Reference:

2004 ARRL Handbook, Chapter 22.47, “A Computer Controlled Radio Switch Box”

Last Amended 9/20/2009, w4kaz

BPFF – The Guess-timated Scale and actual Guess-timates – Part 6

Part 6 of the W4KAZ filter project series discusses the actual measured S-meter calibration, and the filter attenuation estimated based on S-meter measurements.

As I meandered through Part 5 of this group of posts, I needed to find a way to calibrate the S-meter scale on the FT-920 to a 6DB reference. Lacking any real test equipment, this will allow me to do relative tests on the band pass filters to measure the filter attenuation on the harmonics and sub harmonic. So I used the attenuator pad(6,12, and 18db) to measure the delta between each S-unit from S-0 to S-9, “10 over S-9” and “20 over S-9”.

Big shock(NOT!): The S-meter on the FT-920 is definitely not linear.

Actual Big shock: The S-meter actually IS linear over part of its scale. I was a bit surprised by that.

The S-meter on the FT-920 was “measured” by using the attenuator pad, inserting attenuation and noting the S-meter drop. It came out to something like this:

  • S0-S4 – 6db
  • S4-S5 -6db
  • S5-S6 – 6db
  • S6-S7 – 6db
  • S7-S8 – 6db
  • S8-S9 – ~9db
  • S9-10 over 9 – ~12db
  • 10 over to 20 over – **Not measured**.

It was hard to decide if the drop from 20 over 9 to 10 over 9 was 12db or more, so I didn’t do any testing with any signals that strong. For the sake of an example, when the original signal was reading S4, adding 6db attenuation dropped the reading to S0.

It seems noteworthy that the spread from S0 to S4 is only 6db. I can often work stations that are down around S0, and almost always work anything higher in decent conditions. I guess to me it emphasized how important just a few db difference might be to making a contact. Maybe a 1db loss throught the filters is more siginficant than it appears. To paraphrase the OM’s, “every db counts…”.

Armed with that calibration scale, there now is a way to make an educated guess at the amount of attenuation a filter is providing on its harmonics and sub harmonics. By injecting a signal on the harmonic band, comparing the S-meter readings with and without the filter gives an easy way to approximate the filter’s attenuation on that band. It won’t be surprising to find that the accuracy of the measurements will be poor when compared to lab measurements, but it gives a rule of thumb guideline. Better than nothing.

Amendment, 2009.06.21– Somewhere along the way I misplaced my notes with the measurements made during mid-May. It looks like I won’t have time to re-test the filters for a couple of months, so here are a few notes from an e-mail to N4YDU. The executive summary….

K4VX filters – worst case is about 30 to 35 db of attenuation, through the 20m filter. The best cases are probably 35 to 40 db attenuation on the second harmonics.

NVARC Ugly filters – Woooweee! These puppies may have a bit of loss, but they sure do a great job on the harmonics. All bands showed 6 to 12 db better attenuation on the second harmonics than their K4VX counterparts. An S9 signal is dropped to S0, still readable. An S7 signal becomes barely audible at the noise floor of the F-920. The guestimate here is better than 40db attenuation on the second harmonics. Higher order harmonics were disappeared, so no ideas on the attenuation there, except that is “Enough!”.

Previous in series: Part 5 Guess-timating the filter efficacy

Start from the beginning at the W4KAZ Band Pass Filter series.

Field Day and The K3NG HomeBrew Rain Fly

With Field Day right around the corner the K3NG Home Brew Field Day Tarp Canopy seemed timely. I don’t have(i.e. “will not have”) a google account, so I couldn’t post comments to K3NG’s post. But it’s cool enough to bookmark permanently. Literally. Putting shade on the tent keeps the operating position much cooler.

My initial reaction was that K3NG’s cover would be subject to water pooling. As I kept reading, I saw that he noticed that too. Back in the swamps as a WB5, we used a similar strategery for shade and rain. Rather than risk poking holes in the tarps with center supports, our solution was to make the front posts about 18 inches longer than the rear posts. The slope was sufficient even in a heavy rain.

Our own posts were cut from pine saplings liberated from one of the club member’s farm.

With the front facing north, that also helps throw the shade a bit lower on the tents below the cover. It works pretty well at shedding rain too. Lots of chances for rain on Field Day when you are only 20 miles off the Gulf of Mexico. Ick.

O’course, it rained about three inches here at the NC KazShack Tuesday morning. My front yard becomes a small stream in these conditions, with water flowing over the driveway, down across the yard and over my neighbor’s driveway too. So much for the landscaping. Landscape timbers, mulch, even the grass – whoosh.

I sure hope the wx dries out before FD. Ugh.

Anyway, I like the rain fly solution. Kinda’ labor intensive, but worth the effort if there are enough warm bodies on hand to help throw it up.

BPFF – Guess-timating the Filter Efficacy – Part 5

Part 5 of the W4KAZ filter project series discusses filter losses, an idea for getting a very rough S-meter calibration, and trying to estimate the out of pass band attenuation provided by the filters.

The Losses:

The filters do have losses in the pass band. This is known as the insertion loss, and is reported in db. When discussing the pass band, we want the losses to be as low as possible, or approaching 0.0db of loss. The old rule of thumb is for every 3db of loss you are losing about half of your power. So, 100 watts of RF transmitted through a 3db loss component means there is only 50w coming out the other end.

Run that through the loss formula…. db loss = 10*[log(100/50)] = 10*log(2) = 10*.30103 = 3.01db of loss.

Since loss is defined as a ratio of the actual power levels, a simple watt meter and dummy load can be used to measure the losses of a component in db. That gives a nice yardstick for comparision to known commercial filters. The accuracy of the wattmeter is an issue, but part of the game is to compare the values I come up with against values measured with better test equipment. If I ever manage to hook up with one of the guys who are willing to help with that.

The set up to measure the loss in the pass band is simple.

Transmitter–> filter –> watt meter –> dummy load

By replacing the filter with a barrel connector, you can get the baseline power. The watt meter on hand here is not sensitive or accurate enough to use the same technique for measuring signals outside the passband. Not if the filters are working. 😮

Aside: This is also a good way to test a piece of unknown coaxial cable. Rather than rely on an estimate of what the loss should be for a known length of similar cable, it is pretty easy to measure the loss. A quick computation of the loss into db gives you a yardstick on the quality of the cable by comparing it to known losses specified by cable manufacturers.

The Guess-Estimate:

Anyway…. My set of NVARC filters came in measuring actual losses between 0.6db to 0.8db, and about 1.0db when installed in the integrated switch box. The set of K4VX filters came in at 0.3db to 0.6db.

The problem is that the insertion losses in the pass band tell little about their effectiveness on the 2N or N/2 harmonics.

The only tool available in the KazShack for measuring this type of loss turns out to be the S-meter on the receivers. Receivers are quite good at hearing RF. Kinda their whole purpose in life, right? The new problem is the unknown scale of the S-meter. Is it telling us anything useful?

So: How to calibrate the S-meter?

Okay. I couldn’t solve that one. Is there a possible work-around, or a way to determine the existing calibration of the S-meter?

This puts me off into an area that may eventually turn out to have little real-world validity, but here’s what I came up with. The FT-920 has a three step attenuator pad which is a known quantity. Assuming a simple resistance pad can be easily calculated and implmented by engineers capable of designing such an otherwise comlpetely slick gizmo. For some reason the pad is coincidentally in 6db steps, giving 6,12, and 18db. How convenient. 😮

The unofficial rule of thumb is that an S-unit is supposed to be 6db, with S-9 the 50 microvolt level. So with 18db attenuation, an S-9 signal should be knocked down to S-6. I don’t have a 50? standard, one of countless other things I don’t have, but I am able to generate a signal at various levels. So I decided to use the attenuator pad to calibrate the S-meter markings. Although I may have no idea what level actually causes the meter to read S-9, I CAN use the known values to figure out the values from S-9 down, or S-9 up. I still don’t know the actual signal levels or what signal level corresponds to an S-9 meter reading, but the scale allows measurement of the relative differences in known quantities. In this case, that is exactly what I need.

What this gives me is a round-about way to guess-estimate the effectiveness of the filters where it counts, on the sub harmonics or harmonics. If I know the value of attenuation causing a signal drop from between S-9 to S-2, inserting a filter that causes that same drop will have that amount of attenuation on that frequency.

Nothing is ever THAT easy. S-meters are known far and wide for non-linear behavior, right? Sheesh. But life is full of surprises.

Previous in series: Band Pass Filter Fever – The Kludgy Switch Box – Part 4

Next in Series: BPFF – The Guess-timated Scale and actual Guess-timates – Part 6

Home Brew Projects Page

Time for a centralized list of home brew projects. So all of the following have been listed on their very own page linked in the sidebar to make it easier for me to find them.

SO2R related:

  1. Band Pass Filters(a series of postings)
  2. SO2R Box – Engineer the Possible
  3. Band Decoder


  1. Using PL-259’s on CATV F11
  2. Simple Two Position Remote Antenna Switch
  3. Other Remote Switch Project Ideas
  4. Yet more Remote Switch Project Ideas
  5. Elecraft K2 Cooling Fan
  6. Useful Parts List
  7. Home Brew Station Desk
  8. CW Touch-Keyer kit
  9. G3TXQ BroadBanded hexagonal Beam
  10. Home Brew Knobs
  11. Seven position remote antenna switch
  12. K9AY box
  13. RX antenna 160m/80m band splitter


BPFF – The Kludgy Switch Box – Part4

Part 4 of the W4KAZ filter project series comments on the process leading up to the integrated box full of NVARC Ugly filters for use in the KazShack. The quest continues.

Notes: Link to photos of the project at bottom of this page. If you want to read about the project from the beginning, go to the”Band Pass Filter Fever” series page.

Part of the project goal is to put all of the NVARC filters into a switched box to allow for SO2R and use at Field Day and on IOTA expedition. The original idea was to use a simple rotary switch. Somewhere along the way the idea morphed into using relays set up to allow control from a band decoder.Toying with the relay switching idea brought up a couple of issues that I chose to avoid. Instead, the individual filters were tied together with a two pole ten position switch.

A previous project resulted in a seven position remote antenna switch.That switch is lossy on 15m and 10m because of the point to point dead-bug style wiring. I didn’t see an easy way to avoid this problem, and I’m not set up for PCB design/manufacture. Using PCB’s and strip line runs would solve the issue. I have an idea for making strip lines that may work, but it is a bit Rube Goldberg-ish, so I chose to shelve that temporarily.

So, back to the rotary switch. I had a 2-pole 10 position switch in the junk box. The contacts are silvered brass, and seem beefy enough for the job, so I tested it out by wiring up the input/output to a bypass position.

Ick. Needless to say, it is not an ideal solution. The loss through the switch alone on 10m is about 0.6db. Losses are lower on 40m and 80m, just barely measurable.

So, WTF. I used it anyway. More suitable switches are a bit pricy if bought new, and this one was already in my sweaty little hands. Impatience, “good enough”, and zero cost won out over quality. Engineer the possible.

After all of that hand wringing was done, some other practical construction choices needed to be thought out. The end goal of constructing a switched filter box could have been implemented in several ways.

Method 1: Use the existing set of filters, switching them in via an external switch box, all connected via a rat’s nest of coax jumpers.

Method Zwei: Build another set of filters into a larger enclosure, and incorporate the switch into the new design.

Method III: Use some less aesthetically pleasing choice that will also benefit from poorly conceived and hacked together engineering practices.

Well the choice was clear – use Method III!

The rationale unfolded as a matter of “least inconvenient compromise” rather than “optimal design”. I was limited in the number of parts available. That was the primary limiting factor for method 2, not enough capacitors of the proper values on hand for a full second set of filters. Keeping the individual filters available was desirable for the sake of future flexibility, so ripping them apart and re-assembling was not considered.

Parts count also played a part in ruling out method 1, as it would use up 20-plus pl-259’s, plus the coax.

My compromised solution was to use the individual filters with a slight modification. Rather than remove the so-239 connectors, I merely tacked on a pigtail of coax for the runs to and from each switch. It is a compromise in every way, electrically, mechanically, and aesthetically. But it sure was simple.

It also seemed to work electrically better than I expected, as none of the loss figures vary substantially from the losses I would expect from the switch plus those of the original individual filters. In other words, the db losses through the filter added to the db losses from the switch in bypass add up to the total loss, when each band is measured separately.

The completed box shows losses on all bands of approximately 1.0 db. This is a bit odd, given the losses of the switch itself vary by band. But the insertion losses of the filters are lowest on 10m, and highest on 80m. Since the losses through the switch component are high on 10m and low on 80m, they all seem to coincidentally hover in the 1.0db range.

It appears that the insertion losses on 40m and 80m filters are a bit higher than the NVARC spec. This is probably because of compromises made in the physical construction, as the coils in those filters are closer to the sides of the enclosure than they should be. They were built last and the enclosure used were more difficult to work with due to their non-standard construction.

The insertion losses in the 1.0 db range are significant enough to be a concern, but everything is a compromise. This is the compromise I’m required to make for SO2R without outlay of more ca$h. The ca$h reserves are currently at less than optimal levels, but there is a lot of that going on. It will be an even larger compromise operating with low power than it would be if the filters were followed by an amplifier, but such is life. Engineer the possible!

In the grand scheme, the finished NVARC box shows about 0.2db more losses than I could expect from the commercial Dunestar series, and maybe 0.4db more loss than is expected from the W3NQN variety. On a positve note, this one cost less than $50.00USD in materials, not to mention everything I learned during the construction. The time required for construction was an educational investment, and was well spent.

Pictures of the KazShack NVARC filter box and K4VX filter sample.

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