Radio W4KAZ

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SixPak Online

Finished wiring up the control cables for the Array Solutions SixPak. There’s a mistake on one of the control cable assemblies that were too hastily assembled, a W4KAZ problem, not anything wrong with the SixPak. The work around was easier than re-wiring the cable, so I just re-connected the wires inside the control box to fix the problem.

Using Cat5 cable, the color coding inside the KazShack “Should Be” as follows:

  1. 10m – orange
  2. 15m – white/orange stripe
  3. 20m – green
  4. 40m – white/green stripe
  5. 80m – blue
  6. 160m-white/blue stripe
  7. 12v or gnd – brown and white with brown stripe
  8. (tied to #7)

Note: The SixPak wiring block in the controlboxallows 12v+ to be sent down one cable and 12v gnd on the other cable. I could just have easily sent 12v+ down wire #7 and used wire # 8 for ground. That would allow the SixPak to function using either switch position alone with a single control cable. That’s also probably how most folks would wire the cables, since at leastone radiowill work when wired that way.

For quick connects, I used a set of 8-wire computer power cable extensions(ATX power extensions). Each extension is 8 inches long, and cutting one in half gives a mating connector pair. Easy enough to splice on, but I obviously screwed one up somewhere. When testing, Radio B was found to have 10m/40m and 15m/20m pairs reversed. Again, rather than re work the cable, I just rewired the four appropriate wires inside the control box, and taped a sticky note inside the cover to remind me I dorked up that radio B cable assembly.

Now a bit more cable assembly will have most of the bits in place for some new operating fun.

Outstanding KazShack Issues:

  1. hoisting 15m loop
  2. 10m antenna(???what to try???)(???and why bother???)
  3. 160m antenna on ground
  4. 40m antenna switch
  5. 20m antenna switch
  6. Tuning K2 for 80m/160m
  7. Shack layout redesign
  8. Operate!

Pollenating The Antennas – More 80m Folded Dipole

Whilst toying with some new KazShack toys, I found out an interesting and useful fun fact. The 80m folded dipole does a great job at ignoring some of the AM harmonic mixes. Death to harmonics! W4SAT seems to have the best on-line description of a folded dipole.There is very little written on the internet about this great antenna option. My own original post has a link to a web reprint of the original ARRL design article.

The KazShack is less than a mile from the 50KW WPTF(680) transmitter, and only about two miles from the 10KW WRBZ(850) transmitter. This results in all manner of harmonics and harmonic mixes that I can hear and identify. Pretty much any combination of the numbers 680 and 850 added and subtracted together produce a frequency that have audible audio artifacts that can be identified as from either AM station. Some of the mixes are much worse than others. Some are barely audible. The mixes that fall within the ham bands are obviously the ones of the most pertinent concern.

While playing with some new home brew band pass filters filters and the SixPak, I flipped the radio to 80m. While connected to either 40m antenna or the 20m antenna, I could hear a loud garbled mix centered at about 3570. The WPTF audio was clear. The WBRZ audio was also identifiable, although very garbled. That works out to the 4th harmonic of WPTF mixed with WBRZ’s 850. (i.e., [4*680]+850=3570)

This seemed really curious, since this never seemed to be a problem before. On the 40m antenna broadside to WPTF, the mix was S9+, and at least S7 on the other 40m and 20m antennas. So, it will probably blow my socks off when I switch to the 80m folded dipole, right?

Nope. Switching to the 80m folded dipole, the mix disappeared completely. No more WPTF audio on 3570. No more WBRZ. Both were Gone. Zip, zero, nada. Hmmmm.

Then I rememberated reading that folded dipoles were useful on their primary frequencies and their odd harmonics. So an 80m folded dipole could possibly be pressed into service on 30m, but it is deaf as a dummy load on 40m. That was a fact I have proven experimentally, both on purpose and by accidently flipping to 40m and wondering where all of the signals have gone(duh-uh!). The 80m antenna was rejecting the WPTF fourth harmonic well enough to eliminate the two station mix.

It would appear that a folded dipole also helps to reject the even sub-harmonics as well. This would probably have been obvious, but the thought had never crystalized within my addled gray matter before that moment. That fact could prove very useful for several settings. SO2R. Field Day. Field DXpeditions. IOTA. Sweet.

So, one additional yet seldom documented method to reduce the n/2 sub harmonic is to use a folded dipole. I would expect the folded dipole will also attenuate an interference your transmitter is generating on it’s 2nd harmonic, so switching either antenna to a folded dipole will probably help. This won’t help a 40m/15m problem, but applies to the other common harmonic situations.

Every little bit helps.

BPFF – The Guinea Pigs – Part3

So, a long hiatus between band pass filter musings. In Part 1, I laid out several reference sources for band pass filter projects. Part 2 details the decision process, plus some notes on what happened with initial attempts at reproducing the K4VX Filter and NVARC filter projects.

Here I have a bit more detail on each project. After obtaining a small supply of the capacitors specified by the NVARC design, I’ve completed the 40m and 80m filters.

Measurements on the 40m NVARC filter show about 0.8db loss through the filter, with the SWR pass band covering the entire band easily. Outside the band, the SWR rises rapidly above 7.370 Mc, and below about 6.775Mc. That would seem to indicate the filter is resonant lower in the band, but it actually shows about 2 watts more attenuation at the bottom of the CW segment than it does at top of the SSB area.

The 80m NVARC filter also shows about 0.8db of loss through the filter. The SWR is about 1.5:1 across the entire band, and the filter seens to have its sweet spot right near the SSB DX window at about 3.775 Mc. That should prove fortuitous, since it is also where my 80m folded dipole resonates, but it is completely by chance.

These last two filters probably have slightly higher losses than they should due to construction techniques. The cases I had available for their enclosures were not ideal. Their assembly did not allow easy construction by the NVARC guidelines, and the coils are probably mounted less than optimally inside their cases. Through experimentation I found that slight variations in coil positioning had an effect on their insertion losses.

Additionally, the 15m filter began acting up, showing terrible losses. It turns out that in slinging it around the shield had become dislodged and was either in contact with or too close to one of the coils. Re-securing the shield solved the problem, and put the filter back very close to the NVARC spec’d performance.

By comparison, the K4VX set I have show less attenuation. I have K4VX versions for 20m, 40m, and 80m. Both the 40m and 80m filters show very low losses through the filters, both at about 0.3db of loss, only a couple of watts. The 20m version shows losses similar toits NVARC counterpart, in the vicinity of 0.7db. I expect to rebuild the 20m filter from this series using the ceramic caps rather than silver mica caps. It will be interesting to see if the loss figures change.

I have not yet built the 10m and 15m versions of this design, and may not. The attenuation specified by K4VX on these bandsis not as good as the NVARC spec. It might be worth trying the NVARC filter designwith toroids rather than air wound coils. An excellent experiment idea, and the 10m and 15m NVARC design seems to work well as described and reproduced here.

Given the low losses through the K4VX design, I may use that set for the run station, and the NVARC design for the mult station in an SO2R setting. The K4VX design is also physically much smaller, another practical advantage. The NVARC design has a better set of attenuation figures specified, but it will be nice to get actual measurements on the filters before declaring them a better choice. Some actual on-air testing can’t hurt either.

The coax stub project has been placed on the shelf for the moment. It is worth noting that coaxial stubs are probably better described as notch filters rather than band pass filters, as they are designed to place a notch on the harmonic or sub-harmonic frequencies. The book by W2VJN, “Managing Interstation Interference – Coaxial Stubs and Filters”is a treasure trove of useful information. For anyone with an interest in the subject of coaxial stubs, the book is worth every red penny of its price. Add it to your library and you won’t be displeased.

Previous in series: Band Pass Filter Fever – Untangling The Web – Part 2

Next in Series:Band Pass Filter Fever – The Kludgy Switch Box – Part 4.

Simple Two Position Remote Antenna Switch

The current plan is to incorporate a SixPak into the antenna switching scheme here in the KazShack. The SixPak is to help enable an exploratory foray into SO2R operating.

It would also be useful to have the ability to use more than one resonant antenna on a given band. For example, the station is currently limited to a single dipole on 40 meters. It would possibly be helpful to have another dipole at right angles to the first. This would likely help bring up stations that are off the ends of the current dipole. It might be even more helpful on 20m where the antennas are higher relative to the percentage of the bands wavelength.

So a quickie two antenna remote switch is in order.

Two Position SwitchTwo Position Switch W4KAZ SwitchW4KAZ Switch

The requirements are not critical for my low power application. It is simple enough. The relay is the P&B RTB14012F, a SPDT relay rated at 12A in its normal general purpose AC applications. This series of relays has been in use in my seven position switch for a couple of years now, and is in the same series of relays used in several high power switch project articles from NCJ. In this specific switch there are no losses that are measurable up through 6m. This was tested only with an ordinary watt meter into a dummy load, and applies to either output port of the switch. So far so good.

The only other components are a clamping diode, a 10nf bypass capacitor, a few bits of hook-up wire, and three coax connectors. The diode and the cap go across the coil on the relay to suppress nasty RF side-effects on the DC side of the circuit. The enclosure was easily the most expensive item. Total cost is about $12 USD. The 12v control cables are potted with hot glue. The enclosure is not water tight, but the switch will be mounted in or under a rain shield. The relay itself is sealed, so insects might be the only issue should they infiltrate the enclosure.

The switching controls are going to be in a yet to be determined integrated panel. That will add to the cost by the amount of the switch which will be mounted in the control panel. Cat5e will be used for all control cable runs.

I expect to place this puppy between the SixPak and the 40m antennas. Coax stubs for 40m will connect the SixPak to the two position switch. The default position(switch off) will be the primary antenna facing NE/SW. There will not be a grounded or unused position, as lightning protection will be added at the “radio A/radio B” positions of the SixPak. That arrangement will keep the radios isolated from one another via the SixPak, and unable to simultaneously select antennas resonant on the same band. (maybe….depends on the antennas used for the other bands, aina?)

With the addition of one relay and one extra control wire, it is easy to make a similar three or four position switch.

Sometimes simplicity is good, or as in this case, “good enough”.

Antenna Switch

Obtained a third hand Array Soultions SixPak from N4YDU. The control box needed a wee bit of refurb. The LED indicators were not all working, although the switch itself is functioning well. One LED was cracked, three others blown, along with four of the resistors.

So, heating up the soldering iron and pulling a few parts was needed. This was more trouble than expected. The board is very well done, and it is simple to remove from the box. But I didn’t anticipate the minor fly in the ointment. The holes are ‘plated through’. The LED’s were simple enough to replace, and I had close match replacement LEDs in the parts bin. The four bad resistors were a bit less cooperative. It was difficult to remove enough solder with the solder wick.

The new resistors were difficult to install, because the plated through holes were a close fit even when clean. Downright difficult with a coat of solder in them. I resorted to alternately heating the holes and pushing each lead through a couple of millimeters at a time. Once I had enough fed through, I was able to grab the leads with forceps. Then I was able to hold both leads, apply hot iron, and pull the part down flush with the PCB.

So what should have taken 15 minutes tops probably took almost 90 minutes. Grrrr. Not difficult, just frustrating. The LEDs I had on hand are not exact color matches, but very close when lit. The red matches better than the greens, but I didn’t want to chance messing something up – if it ain’t broke don’t fix it. I’ll replace them if they blow up.

The SixPak is probably overkill here, but will be a good thing if a certain SB-1000 ever migrates into the KazShack. It could happen.

Now I just need to figure out the best way to re-configure the station. The idea is to allow SO2R experimentation at some point. That will require some alternate antennas and a set of filters on each. I’m not there yet. Then maybe a W9XT band decoder board for auto band switching.

Coming along, slow but sure. Sure to slow down that is, because the first tuition check for the college bound eldest is due in three months.


One down, one fixed

The 160m-L has been “on the fritz” (i.e., broken) since the ARRL 160m contest. It went crazy when it started raining during the contest. The SWR went high, and it became deaf.

The ground was still covered with snow from our very rare in central NC five inch snowfall. Old XYL tales relate that antennas work best when installed/repaired in inclement weather, so the time seemed right.

It turns out there was a bad connection on one of the jumpers. I took a bit of time to find the problem, and only about 30 seconds to fix. To debug it I plugged the dummy load into the feedline system out at the antenna end. The high SWR into the dummy load told me the problem was in the line and not the antenna. Fixing the line solved the immediate issue.

That whole antenna system needs closer scrutiny. It was intended to be ‘temporary’ when it was hauled up three years back, and the feed line is a hodgepodge of jumpers which were available around the KazShack at the time. If it stays up, the matching network needs to be corrected. At last count, it now has 18 elevated radials. The feed point is about 8 feet off the ground mounted atop the kid’s wooden swing set. The radials are all of different lengths, tied off to trees. The distance of the tree from the antenna center determines the radial length. It seemed to be playing much better with 18 radials than it did in the original four radial configuration.

I suppose I’ll play around in CQ 160 CW, but I expect it will be an all US contest here in the KazShack. 100 watts is effectively QRP when run into a poorly matched inv-L. I’ll find out if I feel like trying to run stations.

The heavy winds from a couple of weeks back knocked loose a rather large branch, which clipped the nested rectangular loop for 15m and 10m. That simplifies the job of fixing it, something that has been much lower on the radio project list.

But without sunspots, I’m still in no rush to get 10m fixed. 15m either, unfortuneatly.

K2 Impressions – Part 7 – Adding 160m

Finally got around to the K2 to assemble and install the 160m module. I’m not sure why they chose to make 160m a separate add-in, unless it seemed superfluous for a QRP rig in its original form. So maybe it was added as an afterthought. Anyway, after a lot of procrastination on my part caused by too many things to do, I decided to squeeze out an hour or two here and there to give it a go.

The 160m board itself has only a handful of components. These went together easily enough, but I would recommend changing the instructions to install C6 after installing the 14 pin header. It was very difficult to solder the end pins on the header with C6 already in place. I may have spent more time on those two pins than on the rest of the board. Maybe it only seemed that way.

Before beginning, my only real concern was the mechanical dis-assembly of the bottom of the K2. That turned out to be less difficult than I anticipated, but it does require a bit of patience. Since I had not done the original assembly of the kit, I didn’t really know what to expect after reading the instructions on disassembling the heat sink. This was fairly tedious, but not as complicated as it first seemed. Someone who had assembled the kit from scratch would not have had a problem.

The project took about three hours from start to final tune-up. I split that over three sessions, so it seemed like less. I have not really been on the air with it yet, but I’m looking forward to trying out the RX antenna input. Thats a bit of an issue, as the RX input uses a BNC jack. This really begs for a BNC to UHF jumper cable, because an adapter adds too much length and puts too much physical stress on the rear of the radio for my preferences.

Even if I were not to operate on 160m, the RX antenna jack is adesirableaddition to the K2 for my methods of operation on 80m. Here in the KAZshack the separate RX antenna is used frequently on 80m, and I expect to add another RX antenna soon.

Maybe today…….

-more- at the K2 Impressions page.

Some Assembly Required

The shack is still in disarray but there is light at the end of the tunnel. The acorns are raining down like hail, so the leaves are not far behind. It is time to wrap up the shack re-assembly before leaf raking(and contesting!) season arrives in earnest. I need to keep the leaves from carpeting the grass, or else the small amount of grass will be croaked by the carpet of leaves.

The long outstanding antenna switch has been finished. I already had most of the parts ready. The remote switch and inside switch box have been assembled for quite a while. I finished the outside enclosure over the summer. During the last couple of weeks I put connectors on the control cables and ran them from the shack to the location chosen for the switch. One of the first really pretty fall days dropped in on Saturday so final assembly of all of the components got DONE! Sweeeet.

That same beautiful Saturday was perfect for performing maintenance on most of the antennas and their support ropes. Time to adjust the 20m and 40m dipoles. They were both too short, and resonant above the SSB segments. Both are now resonant just above the middle of the CW segments on their bands.

The 15m/10m antenna is a nested rectangular loop. It is the same size or length of wire as a normal quad loop, but instead of square, the vertical sides are just a bit less than 2 times the length of the horizontal sides. It is fed at the bottom(a short side, and closer to the ground) and is thus horizontally polarized. My own antenna differs somewhat from this website description and the original idea goes back at least to K6STI. In my own version, I use a length of aluminum channel as the top horizontal. I’m really happy with this antenna. When the bands open, it works well. Its most endearing characteristic is that it is easily rotatable, as it hangs from a single support. It has tag lines, which are used to rotate and secure the direction.

The rectangle also has the benefits of any quad loop while showing a nice 50 ohm match. Sweet. I had been feeding each loop from a single feedline. Since I’m planning ahead for the use of the band pass filters, it was time to put each on a separate line. Since the antenna needed to be dropped anyway, I also put up a set of fresh ropes.

Each loop required re-pruning after they were separated. Both required about four inches of wire added to bring their resonances down to the CW band segments. They also now show more a broad coverage at less than 2:1 SWR. Using a single feed line is a bit of a trade-off. You trade a bit of bandwidth for the convenience of a single line.

Since they are now using separate feedlines, I could have gone to normal equilateral quad loops, and maybe used a balun or quarter wave matching section. It would have raised the average height above ground. This was a bad idea for my situation given their location, because the increased length of the horizontals would make them more of a snaggle problem with adjacent trees.

I’ve also got the K2 160m module installed, so I’m looking forward to that. Band Pass Filter Fever is on hold until I can scrounge up enough parts.

In a very unusual turn of events, Murphy was off bothering someone else Saturday. 🙂

BPFF – Untangling the Web – Part 2

So, lots of interesting material to read through listed in BPFF Part 1, and much of it is available via the internet. Catch the whole series of w4kaz band pass filter musings.

This project is not really new for me but just something that was recently underscored by recent operations. Deciding which projects to tackle became a matter of choosing those that I thought are ‘possible’. For these purposes ‘possible’ is defined as

  • the parts or substitutes are readily available
  • the construction appears simple enough for clumsy fingers
  • the final tuning is simple

These criteria knocked out the W3NQN filter and the N1AL because I thought tuning the multi-filar toroidal inductors would be enough to exceed my level of patience. The W3NQN design is well recommended functionally, I just thought the tuning procedures were going to prove too difficult. As a side note, the Toroid King sells a kit of the required toroids for the W3NQN filter, and the DL2BNU article describes a method of tuning that is simpler than using a VNA (that I don’t have). The N1AL design bothers me because of the taps on the multi turn inductor and the trial and error tuning. The ARRL sidebar on the N1AL project is interesting, stating that the filters interact to give nulls on the even harmonics. Curious.

A couple of years back, I built the K4VX filters. That project is relatively simple. Tuning can be done with an MFJ antenna analyzer or using a grid dip method. My own analyzer seemed sufficient, and I was able to build filters for 80m, 40m, and 20m fairly simply. The 160m was more difficult to tune and I never achieved a good SWR. It is now being used on the receive antenna to filter out nearby broadcast QRM from 680 and 850 on the AM dial.

The first real problem I ran across came when the 20m filter was mistakenly put into a high SWR load. FUBAR! The damage manifests itself as a high SWR into the dummy load, and the resonances have changed. I suspect the toroids were damaged by overheating. The capacitors used were silver mica’s, and their values seem unchanged.

Another filter project recently bubbled up through the dogpile stream of data. I ran across the NVARC “Ugly” filter project. Besides having a good write up, their project is billed as a “no tune” design.

So far, that seems to be mostly correct. I’ve had success with the two assembled so far. The 20m filter came in with a resonance a bit low, but the loss through it is in the area of about .5db. No tuning required. The 15m filter is actually centered right at 21Mc, and the SWR is dead flat at 1:1 across the entire 15m band. It’s showing insertion losses of about .7db. I have not yet given either of them any on-air trials, so that’s coming later, but the transmit smoke tests into the dummy load(three one minute intervals, 15 seconds apart) showed very little heating in any of the components. After cutting power, the hottest components were just barely warm to the touch. Sweet.

Not so sweet with the first attempt on the 10m version. It originally showed a resonance around 27.500 with an SWR above 2:1 at the lowest. The NVARC document had a discrepancy, which has since been corrected. Re-building the filter as specified in the updated instructions corrected the resonance. It is now good over most of 10 meters – the SWR minimum is around 28.500, and is about 1.4:1 at the bottom of the CW segment, but the insertion loss is low, only about .5db. A look at their VNA sweep shows that it is resonant at the high end of 10m and above. My rendition shows a double dip minimum in the SWR, one centered on 28.500 and a second broader dip at 31.500. Their VNA sweep of the SWR on their filter is very similar to my own SWR plot taken with an MFJ-259, so I declared my reproduction a success.

The NVARC filters are made with air wound coils, using common schedule 40 PVC pipe sections as the coil forms. The guidelines for winding the coils are fairly accurate, and I didn’t really have any problems winding nice, tight coils. The winding process IS a bit hard on the hands, but not impossible. It might be difficult for someone suffering from arthritis. Post winding testing of a set of the coils with K4CZ’s LCR meter showed the inductances on each to be quite close to the value specified on the Ugly Filter schematic.

It occurred to me that the NVARC filter designs might be built just as easily using toroidal cores. It should be simple enough to use an air wound set to come up with resonance values for each section. That will allow for easy rough tuning, similar to the K4VX method. I guess they could then be fine tuned for maximum smoke with an RF probe and a multi-meter. If the air wound design seems to have good performance, I may try one out with toroids. But credit must be given W1XP and the NVARC crew – these filters are easy enough to re-produce, and none of the three required any sort of tuning.

Ultimately I would like to have a pair of each type NVARC and K4VX on 40m and 20m to test forsuitability. For the time being, its a project on hold to gather resources. I gotta get on the 160 mod for the K2, not to mention re-painting thefasciathe roofers had to fix last month when installing the new roof on the QTH.

Note: There will be a bit of a time gap in this “BPFF” series. I have several other projects going on, and of the radio related items I really need to get on the K2 160m option.

First in series: Band Pass Filter Fever – The Tangled Web – Part 1

Next in series: Band Pass Filter Fever – The Guinea Pigs – Part 3.

Band Pass Filter Fever – The Tangled Web – Part 1

Amended 3/25/2012: Again – re-Fixed broken links, added new links for VK4EMM and KG4JJH reference material

———————————— Begin original Post ————————————-

On occasion I’ve noticed I have a tendency to get wrapped up in an idea I can’t shake. Obsessed might be a close description despite the negative connotations. It’s not quite an obsessive mania, nor is it complete fixation. Perhaps fascination is closer to the truth. A “passionate fascination” sure sounds better than “obsession”. Not much difference semantically, but there it is.

Oddly enough, the most recent subject of interest is band pass filters for HF. (Reeks of geek, no?) Conjured by both this year’s(2008) Field Day and the IOTA operations, it is something I became interested in out of need. Commercial filters are available. It also seems that it is still possible to home brew decent band pass filters for relatively low costs. There are several designs widely available. Using coaxial stubs is also an option.

Homebrew is more fun, and in this case, possibly even less expensive. The difficult part of the problem for me is a simple method for tuning the filters. I don’t have access to a lot of test equipment. Also, some of the designs are easier to build than others. Selecting appropriate parts is also a bit of a problem in some cases.

Ideally, a good band pass filter will have a low insertion loss over the pass band, and a high level of rejection outside the pass band. I’m no Werner Von Einsteeen electronics whiz, but there are going to clearly be some trade-offs there somewhere. So knowing your application can guide you to deciding which trade-offs are appropriate.

A look at commercial filters is also helpful. It i$ certainly ea$ier to buy off the $helf filter$. Filters are available from Dunestar, I.C.E., and the W3NQN filters from Array Solutions. I’ve also run across a set of high power filters, but can’t find the link. EU I think. 4O3A also has a version of high power filters.

In an article by Peter Pfann, DL2BNU (German language only)of the Bavarian Contest Club, there is a chart of test results as conducted by DL2BNU. Peter’s chart, page 3, shows the measured insertion loss for most of the filters on each band, lacking only measurements for the I.C.E. filters on 80m or 160m. His chart highlights some of the trade-offs. The Dunestars have better attenuation outside the pass band, but at the cost of higher insertion losses. The I.C.E. filters are least expensive and have low insertion losses in the pass band, but generally show slightly less useful attenuation in the adjacent bands.

I expect to need two sets of filters. I’d like to have them for exploring SO2R as well as for use at Field Day and IOTA. All of that operating is at low power. So – a low pass band insertion loss is desirable.The other significant considerations include total cost, availability of suitable components and materials, and ease of assembly. Finally – they need to add value by !FILTERING! That’s the whole point, right?

Proceed to Part 2 -Band Pass Filter Fever – Untangling The Web, or follow some of these additional references below.



Primary web references:

The Nashoba Valley Amateur Radio Club, the NVARC “Ugly” filter project, by W1FP and KD1LE.


Article by Peter Pfann, DL2BNU of the Bavarian Contest Club, a construction project write up on building the W3NQN filters.

RF Filters, ARRL Technical Information service page with filter definitions and article references

Band-Pass Filters for HF Transceivers by Lou Gordon K4VX QST September 1988

W3LPL Receive Only Band Pass filters via K1TTT technical references site.

K2TR Coax Stub Filters via K1TTT technical references site.

K3NA Coaxial stubs via the K1TTT technical reference site

Clean Up Your Signal with Band-Pass Filters, By Ed Wetherhold, W3NQN 1998 ,[ARRL Members only] 2 parts [ –Part 1– , –Part 2– ].

Additional references:

Excellent advice from KG4JJH about building and tuning W3NQN filters.  (added 2012/03/03)

Band Pass Filters for Contesting by Brian Bartlett, VK4UM and John Loftus, VK4EMM

QST Product Review of Dunestar commercial filters Dunestar Model 600 Multiband Bandpass Filter, QST March 1995

Dunestar bandpass filters

Array Solutions commercial filters based on W3NQN QST article June 1998

Industrial Communication Engineers LTD, ICE commercial bandpass

Inexpensive Interference Filters by Alan Bloom N1AL, QST June 1994 (ARRL members only)

Narrow Bandpass Filter [ARRL Members only] article in QEX by William E Sabin W0IYH

Band Reject Filters by AF4Z

amended, 9/15/2008(tnx K9ZW!) :

“Managing Interstation Interference – Coaxial Stubs and Filters” by George Cutsogeorge, W2VJN, available in hardcopy from International Radio for only $15 USD.

amended, 3/31/2010:

K9YC, Some Q&A About Coax and Stubs for your HF Station. Good stuff.

Amended 9/30/2010: Fixed links broken by ARRL website re-design

Circular Link-amended, 7/06/2009:

This excursion into filters turned into a series of musings, so there is more on the process andmethods of discovery as I re-invented the wheel. It is a circular link because it also takes you back here. Oh No!