Radio W4KAZ

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Retroactive Filter Tuning with NanoVNA

NanoVNA

Recently this NanoVNA product popped up on my radar, and like normal, I was a bit behind the group in discovering it. It is relatively newly available, but seems to be rapidly becoming known because it is both inexpensive and very functional. What a wonderful and useful project. There is a group devoted to the project. There is also a secondary product fork recently begun. The second fork will have a larger screen. Software will not be compatible between the two. (update….maybe more than one fork, rapidly changing).

The NanoVNA itself appears to well suited to the home hobbiest, both in price and utility. As an open source hardware design, being produced by various vendors, it is not coming out yet as a mil-spec “ruggedized” product. It is also not $50,000 USD. Also, caveat emptor. The calibration dummy load supplied with the units I obtained measured an intermittent 50K ohms instead of 50 ohms. A second unit supplied a load that measured 43ohms. [I used a known 50 ohm load for the calibrations.]

Dummy loads aside, it appears I wound up with a cheap copy of the cheap copy of the open source hardware. Yet both units appear to function, at least well enough for the 30Mhz of spectrum of interest here at W4KAZ. All images below were captured on a Samsung Galaxy S7, rather than spend a lot of time dorking around with rapidly developing beta software. Truth is – I was in too much a hurry to tinker to take the time for the software, which seems to be FB. Good enough is often “best”. Engineer the Possible. Plenty of time for software later.

Filters Tested

RTL-SDR:  The RTL-SDR 2.6Mhz high pass broadcast band filter.   This filter is quite good.  Its only drawback is that the cut off is above the 160m band.  This helped a lot in testing to find RFI problems, but I needed a filter that allowed 160m.  If 160m is not required this is an inexpensive rx only solution.   AE5X also has posted a quickie scan of this filter recently. My own scans…

W4KAZ Broadcast Band Filter:  While tracking down what I thought were RFI issues on the Red Pitaya CW skimmer, I obtained the RTL BCB filter above.  Because its cutoff is at 2.6Mhz I needed something with a lower cutoff frequency.   As a home brew experimental project I came up with a BCB filter, designed using the AADE filter design program, available from KE5FX.    

The BCB filter design is shown below, along with the projected rejection.  This design is intended to have the cutoff as near to 1 to 1.2Mhz as possible. 680Khz and 850Khz stations are respectively 1.5 and 4 miles from this QTH, and the nearest 680Khz is a 50Kw station.  The intent of the design was to null 680 and 850 as much as possible.   This filter also shows a DC short via the 100uh inductor.  It can be removed if 60hz mains noise is not an issue.

After construction the filter was originally function tested by checking S-meter levels from the AM band up through HF and a few spectrum glimpses using the Red Pitaya with SDR programs.  Using the NanoVNA is a lot quicker. In lieu of computer software for trace captures I just used a field expedient solution – snapping a photo of the teeny NanoVNA screen with my phone. 

It is nice to see the real world corresponds to theory.  The filter  shows a 3db shoulder close to  design at 1300Mhz.  There is a 2:1 SWR shelf across the 160m band.  Beyond 2Mhz it rapidly improves.  Quite good for my purposes.  Also surprisingly good for hand wound coils and ordinary NP0 ceramic caps thrown together without much[i.e., none!] testing of component values.  I am not certain if I want to chance tinkering with the tuning of the 680 and 850 notches.  Both notches came out about 100 KC lower than designed(not yet shown).

Band Pass Filters: 

The following set of band pass filters are all constructed based on the K4VX article “Band Pass Filters For HF Transceivers” .  A good project, but these were originally only swept for SWR and not a lot of effort to properly tune them previously.  NanoVNA to the rescue. 

10m Bandpass Filter  

As originally built, 3:1 SWR range is from about 24Mhz to32Mhz.  Minimum is at 25.7Mhz, and its usable on the lower end of 10m.

after NanoVNA tuning, a slight improvement across 10m:

15m Bandpass Filter

20m Bandpass Filter

40m Bandpass Filter

Able to tweak the 40m filter from -29.4db to -35.7 db, and filter covers 40m band easily.

80m Bandpass Filter

????where’d it go?!?!?!?!  This one was misplaced somehow……????

160m Bandpass Filter

Very bad news on 160m filter.  Looks like a new repair project – not even close to “good enough”!

2019 IOTA de W4KAZ From Cape Lookout NC

2019 brought another solo operation for IOTA, but return to Cape Lookout for the event.  Weather was much better than 2018, the only WX challenge being the winds.  Normal for the islands on the NC coast, and very agreeable for comfort, but it caused the only bit of trouble for the antennas.

Antennas were set up on Friday.  This year two folded dipoles were used, one for 40m and the second on 20m.   A hy-gain AV-18VS vertical was set up in the unlikely event 15m or 10 would open. (Neither did for me)

Checking the tuning on the antennas had me chasing shadows for too much of Friday afternoon.  After sorting out the feedlines, there was still an SWR problem on the 40m folded dipole.  These had been tested at home before the contest, but had been tugged on quite a bit trying to straighten the mast in the high winds.  So as a fallback measure I hoisted the 40m/20m trap dipole, and tabled the folded dipole problem.  In the end it turned out to be a problem with the antenna analyzer rather than the antennas.  Since it was already in the air, I just re-positioned the trap dipole to be at right angles to the folded dipoles.   But that was after all too much walking from shack to antenna several times looking for problems.

The remainder of Friday evening was spent relaxing.  It was more of a vacation than an operation, so time for beverages and a nice cigar was available.  A nice spot for stargazing was found on the front steps to the cabin, and several nice meteor streaks crossed the Milky Way.

One of the best moments was a visit by Chris, WX4FLY. Really enjoyed meeting Chris,  learned a lot of info in our chat.  I also got an eyeball QSL card that may be one of the best custom cards I have run across.  Thanks Chris.

Radio conditions at the start of the contest were poor.  Radio conditions in the afternoon were poor.  Radio conditions in the early evening were poor.  Radio conditions overnite were poor.  Interest in operating the contest was low, so it was all done in a series of segments lasting 30 to 60 minutes.   After a decent hour or so on 40m after it opened, I received a final distraction in the form of a phone call from a long lost acquaintance.  Also a cold beverage and a nice cigar.

Call  SO2R Remote CW Qs CW Mults Ph Qs Ph Mults Op Time Score Club
W4KAZ * * 84 20 16 5 10 18,750 PVRC

I included an extra day on the end this year to allow packing up to be more relaxing, allow more relaxing, beach side relaxing, and to allow a bit of relaxed operating in an activation for Parks On The Air.  The parks hunters were a lot more plentiful than IOTA contest QSO’s, so I handed out 60 or 70 Qsos for Cape Lookout Seashore, a successful activation.  For that I used the vertical antenna on SSB, just to give it a shakedown of sorts.  All of the antennas and outdoors kit was packed before sundown, and another enjoyable evening of looking for a good fireball followed.

The island was hit pretty hard by hurricane Florence in 2018, and again by Dorian in September 2019.  It had changed since 2018 IOTA, and I imagine it will have changed again.  Hopefully damage was not as severe as the cabin area to the north, which had a new cut that ran directly through the cabin area, plus lots of other cuts and beach erosion.

The stiff breeze helped keep the islands indigenous obnoxious life forms under control until almost time to leave.  Win. Win. Win.

More interesting than the IOTA contest……The lack of decent 20m conditions had me out experimenting with the camera instead. 

And that, a sprinkling of meteors across the Milky Way,  combined with the mild weather to make the trip a winner..

 

Experimenting With Trap Dipoles- Part Numero Uno

The Project and Situation: After quite a bit of trying over the past 15 years to find the best way to pull dipoles up into the closely packed trees in the yard it is clear the options are limited. Having the dipoles favor the NE/SW directions are the goal, but the arrangement of the best supports make this difficult. To beat this problem a combination of single band and multi band fan dipoles were used. [No, the “chainsaw solution” is not an option – yet.]

The primary supports are now occupied with supporting a 160m inverted L and another with a vee dipole for 80m. These are not high enough for direction to make much difference, but are in convenient locations. So everything else needs to fit around those two primary constraints.

The current problem is that there is really only one support that easily allows stretching out the legs of a 40m dipole in the desired directions while also achieving a good height for 40m(almost 50′). The other high supports will only allow the antenna to be deployed favoring a N/S direction(i.e., legs are stretched out E/W).

Using fan dipoles has come with its own practical problems. The dense tree branch coverage tends to tangle in the multiple wires of the legs. Then the fan legs have become entangled in heavy winds. So it is both a problem deploying the antennas, but also the SWR issues when legs are entangled after bad WX. An ongoing maintenance issue.

Alternate solution: trap dipoles. With dual band trap dipoles, it seems like it may be easier to arrange the antennas in favorable directions AND at good heights. The traps are relatively small compared to the mess of multiple wires on a fan, so also maybe it will be a bit easier to navigate dense branch cover of the biological deciduous antenna support structures. The downside is in the extra effort required in constructing the traps, tuning them to desired frequencies, and tuning the antenna legs for each desired band.

What’s the frequency, Kenneth?!? Using EzNEC 6 I ran models with trap data. Based on those results I initially decided to use traps tuned for just above the top frequencies of any given band(e.g., on 20m tuned for 14.400). I’m willing to live with the trap losses for the advantage of maintenance simplicity. Models showed tuning traps for the top end resulted in wire lengths that are the same as a single band dipole, or slightly longer. I then chose to build antennas with traps above the high end of the band based on the following.

  1. A trap resonant frequency above the band results in the dipole wires being the same length or slightly longer than the single band dipole at the trap frequency
  2. A trap resonant frequency below the band requires the dipole wires to be shorter than a dipole for that band. This might be worth pursuing if trying to reduce the antenna length.
  3. there were already a few spare dipoles laying about, and if the traps project flopped they would still be usable mostly intact if traps designed above band,
  4. I’ll probably be mostly using them on CW so maybe a tad less loss with trap rez at opposite end
  5. the gut feeling that a 20m trap resonated at 13.900 would maybe have more loss than the trap at 14.4 when used at 14.025.
  6. NOTE: SEE Part 2 for notes on how these initial assumptions changed!

Research: The traps will inevitably add unwanted weight to the antennas, and I wished to keep them as lightweight as possible. The reasoning for light weight was to extend the project to portable dipoles deployable on telescoping fiberglass masts. So I ruled out using one of the many coaxial trap designs simply to save weight where possible. For coil forms I chose to use small pieces of 1.5″ plumbing waste pipe cut from small sections of what is sold in the US as a “drain tail piece”. This is thin wall pipe, and much lighter than ordinary schedule 40 PVC. The second form material tried AND ABANDONED is 3.4 inch PVC sched 40.

Excluding the coax trap articles, there are relatively few trap dipole projects written up or documented in places accessible via internet searches. The best[most relevant] source is an ARRL antenna book article on a 2 band trap dipole. W8JI also has some interesting trap info published. Although it does not cover the specifics on the options I chose, it led me to the final result. My choices were made based on materials already on hand(wire, capacitors, and coil form material). Engineer the possible.

Initial Trap Construction: The available values of capacitors also drove the selection of trap resonant frequencies. On this point I made an effort to follow W8JI’s information and make the traps resonant off of the desired operating frequencies to minimize trap losses. Beyond this guideline I could locate nowhere any info to indicate if certain values of inductance vs capacitance were better or worse. A larger inductor will allow the antenna to be shorter overall, but the length of the dipole legs was not a restricting parameter for my project. This was merely about having the dipole resonant on 2 bands. Also, the capacitors are 2KV and 3KV 5% tolerance ceramics from Panasonic that I have used previously in band pass filter projects with great success. (NOTE: MORE ON THIS LATER!!!)

Coil Guidelines????: Guidelines for winding the coils are also a bit of guesswork, beyond W8JI’s testing results that show the highest losses occur on the resonant frequency of the trap. I simply started with the inductors, targeting a value of 6uh, initial turns counts generated by a random calculator found via internet search. Then trial and error on actual coil winding. Calculated inductances are based on trap resonant frequency measurements recorded and on the assumption the 5% caps were the most accurate component. Inductances are then calculated from cap face values and resonant frequency.

Test coils for the traps were close wound with #14 THHN stranded housing wire. They were close wound by hand as tightly as possible onto the forms. Coil Q is probably lower than it could be, but the close winding was a compromise accepted for ease of construction and ease of replication. Four inch lengths of 1.5″ waste pipe and three inch lengths of 3/4 inch PVC were tried. The latter were discarded as unsuitable.

Experimenting With the Coils and Caps: The 5% Panasonic caps on hand typically measure very close to the marked nominal values, much better than any 5% or 10% silver mica caps I have used in similar projects. I found that coils wound with similar technique and the same number of turns would reliably resonate within a range of +/-100 to 200hz. Generally the accuracy and reproducibility is better at 7 and 14 Mhz than at 28Mhz. The coils at the higher frequencies have fewer turns, and smaller differences in inductance and capacitance have a larger effect on resonance.

A group of several capacitor values were used along with an MFJ-259C as a grid dip meter to find the resonant frequencies. Pickup coupling coil was a coax jumper terminated on the business end with gator clips and a short length of #14 wire formed into an adjustable sized loop.

Some initial experimenting with the number of turns on the inductors was based on these available values of fixed capacitors. The first inductor was 12 turns on the 1.5″ forms, then resonance was tested with the values of capacitance that were on hand, or able to be easily derived using series and parallel pairs. It was then relatively simple to find the number of turns needed to be able to produce a trap resonant at a given frequency. I also wound coils on the same form material using 7 and 9 turns, and measured resonance for these.

Experimenting With Trap Dipoles- Part 2

 

Telescoping Fiberglass Mast – Variations On A Theme

I have been using a telescoping fiberglass mast of one sort or another since 2005 or so. Most folks seem to be using these masts mostly as designed, i.e. relying on the friction fit, or using tape or hose clamps to keep the mast extended under load. None of those seemed ideal for my plans to use them with dipoles(inverted V config).

The first pole I obtained was from Henry, K4TMC (tmastco.com).(FWIW, I am acquainted with Henry via our membership in PVRC. Henry also sold me a very nice Elecraft K2 when he upgraded to the K3, and other assorted sections of surplus mast.)

This is the 32 foot pole, which results in about 29-30 foot of usable length once extended. Relying on friction fit, I ran into a couple of problems I think common to ALL of these similar type masts. The first problem is the amount of friction required to keep the poles from collapsing was also enough to make them difficult to collapse in very hot or very cold weather.(38C/98F or 0C/32F) Tape and hose clamps are usually enough to resolve that, but bring their own issues.

Tape tends to leave a lot of residue at the joint at 38C(i.e., ‘sticky mess’), which is a problem on sandy beaches. Sand does not enhance the experience of using one of these masts when it sticks to the joints. I also did not like the amount of pressure hose clamps required, nor the amount of time needed to install them in correct order(at 98F oceanside), or to fasten them without crushing the fiberglass accidentally. Because of “spontaneous collapsing” under certain types of pressure, the friction fit is not ideal for use with dipoles, my preferred antenna for portable ops.

The solution I chose was to drill the mast and use 1/8 or 3/32 cotter pins at the bottom of sections just above where they rest when extended. The pin rests on top of the next lower section, so no problems trying to align holes through two sections. Saving another 1000 words……

A section of mast extended showing position of pin, which goes through only the base of the single section.

Over the past 10 years or so I have acquired a few additional masts. Primarily to have the ability to deploy more than a single antenna, but also as redundant or spare masts. [Two is one, one is none.] These additional masts include the 12m Spiderbeam pole, both a 28 and 32 foot mast from Jackite, a 22 foot mast that was marketed as a flagpole, and several Shakespeare 20 foot Wonderpoles. The Wonderpoles are used mostly to elevate the ends of the dipole legs when it seems appropriate(mostly constricted spaces).

The mast from K4TMC has seen the most use over the last decade. It has a good combination of stiffness and flexibility for its length. I had my doubts about drilling holes for the cotter pins, but the mast has been deployed for extended periods with little signs of anything more than minor cosmetic damage. The Spiderbeam mast seems to be much more flexible, which tends to negate is useful length as a center support for dipoles. Both Jackite masts seem to be the most rigid of the group, but I have used these less than any of the others – they are relatively new buys.

The disappointment of the group for me is the Spiderbeam mast. Its flexibility requires guying to keep it from noodling with the weight of a very light weight 40m dipole made of 18ga wire. Best practice seems to be best to attach the feedline to the mast for any of these type masts, but absolutely essential with the Spiderbeam. My Spiderbeam pole also becomes difficult to extend to its full length the more it bends, although that does help keep it from spontaneous collapse. Also more difficult to deploy in heavy wind at the beach due to flexibility, common to all but more pronounced on the Spiderbeam. The other masts are more self supporting when used with the auger bases. This may indeed have more to do with their overall shorter length, and the spiderbeam masts are indeed intended to be guyed by the manufacturer. I would prefer not to use guys to save time, but in several excursions I was unable to use the full length of the Spiderbeam mast sans guy lines. Even with guys the Spiderbeam pole had excessive droop in high winds oceanside, so the additional time required did not seem worth the effort. Taken all together the Spiderbeam mast was not taking my dipole significantly higher than shorter masts.

Auger Bases? Why didn’t I think of that? : The other divergence from the norm is my use of these auger bases. These auger bases are items I have scrounged from different sources. The first pair of them I obtained from Harbor Freight in the early 2000’s, where they were being marketed as beach umbrella stands. That source disappeared soon after my purchase. A second group of smaller augers[NOT pictured below] are marketed as “Aussie Augers”, but needed modification to use with the fiberglass masts(unless you don’t mind removing the end caps from the bottom).

These augers pictured below were available via Amazon in the US in 2019. They work extremely well in sand. They are heavier gauge material than the Harbor Freight versions. The larger tapered base is my first choice for sand and seems to be the strongest. It would also work anywhere with a deep layer of loam or sandy topsoil. The base with the narrow welded on auger is more useful where the soil is less friendly, with stone or tree roots. I use the narrow base in my home yard, which is chock full of quartz stones and tree roots. It sometimes requires multiple placement attempts, but seldom takes more than a few minutes to install. For areas with no topsoil, shallow stone, or mountains, this solution might be less than ideal. The other caveat is leaving a hole in the deployment area.

Both bases are about 60cm in length(22 inches) and have a 60-61mm throat width(approx 2-3/8 inches). This is just barely wide enough for the Spiderbeam mast to fit without removing the base cap. All of the other masts are a bit smaller at the base and fit in easily. The large base has a depth about 178mm(7 inches) and the larger a depth of 127mm(5 inches). FWIW, with the smaller diameter masts I often insert a section of 2″ PVC into the base as a bushing sleeve, and the mast into the PVC bushing section.
Large tapered base at Amazon [American Ground Screw Model 2]
Narrow welded base at Amazon [American Ground Screw Model 1 with Cap ]

I don’t expect I have been the first to go down this less traveled path but have not seen it documented elsewhere. So some photos above for reference. I drilled 9/64 holes in the bottom of each nested section, just ABOVE the joints, and use 1/8 cotter pins.

My 10+ year old mast from K4TMC has been deployed numerous times. There is still only minor wear to the drilled holes, and zero cracking or vertical splits. YMMV. Caveat Emptor. An additional tip would be to have spare pins, and pins in at least two lengths. The bits of wire are used to keep the pins from vibrating loose in the ocean breeze. The also are used with coax to keep the feedline close to the mast. Generally I tape the feedline when using twinlead.

The choice of feedline is made on deployment depending on the distance from the antenna to the operating position. I use LMR240/RFC240 for the feedline drops when the operating position is close to the antenna, and 300 ohm ladderline from DX engineering for long runs.

FD 2018 with masts deployed

FD 2018 photos

2018 IOTA using single K4TMC mast

2018 IOTA K4Z photos

2018 IOTA K4Z time lapse mast deployment video

Mast Hoisting and IOTA Pages

2015 IOTA W4O at Okracoke with N4YDU

??more??

Field Day 2018 – 1B NC de W4KAZ

The normal group of Field Day scalawags were in the wind for 2018.  N4GU was uncertain if the QTH from 2016 and 2017 would be available.  N4YDU took up N9NB’s offer for FD at Ted’s QTH in VA foothills.  I was also kindly invited, but decided that I didn’t want to drive quite that far, despite the nearly ideal location.  I do love me some VA mountains.

Photos from FD2018: http://w4kaz.com/images/fd2018/

For 2018 I took exit ramp #3, and went with the backup backup plan.  Operated 1B at a campground near Wilmington NC.  A nice easy drive, with a couple of easy on/off stops along the way to stretch out the body parts complaining loudest.  That made the drive tolerable.   Also made it into a mini-escape, leaving home QTH on Thursday with a return on Monday morning.

Weather conditions[i.e., heat] soon had me thinking I’d have been better off in the VA mountains, but after acclimatizing to “swamp butt” conditions, it was fine.  When I sweat enough to remind me of living on the South coast – its pretty darn sticky.  Usually not quite so bad in NC, but it happens enough to know to be prepared.  Lots of water and gatorade.  Thursday afternoon was the worst of it though.

Friday morning was spent doing a bit of unrelated recon.  Friday afternoon I laid out the antennas and supports, and some more unrelated area wide explorations.  WX on Saturday dryed out some, and there was a nice breeze that picked up from the start of operations though early evening.  Never a drop of rain, just temps and humidity in the 90’s.  Just like being back in good ole Bigg Swampy(SE Louisiana).

Antennas:

2018 was a time to test some antenna ideas.  I built a 2 band triangular yagi for 20m/15m, based on article by Herb,N4HA as published in June 2018 QST.  I kept to the published dimensions(mostly) but fashioned the driven element(s) from 300ohm ladder line.  For supports I used a mast from Henry, K4TMC as the support for the drive element/apex.  The tails sloped down to connect to the reflectors, and those ends were supported by 2 masts cobbled together by combining a Shakespeare Wonderpole on top of a section of 4 foot mil surplus mast.   Simple, and easier than I expected.  This antenna was fed with 300 ohm ladder line run to a tuner rather than coax.

40m was a simple inverted Vee supported by a Spiderbeam 12m telescoping mast.  Note: Simple does not mean “easy”.

10m was an afterthought.  After struggling with the 40m dipole-that-wouldn’t, I had a relaxing breakfast and gave some thought to 10m.  Had plenty of time, so may as well.  To get on 10m I made a dipole by cutting a couple of equal 8.5 foot lengths of wire and constructed a “FD style” center insulator from a pair of cable ties taped together.  Used a “composite” feedline – a ladderline drop to a 1:1 unun and a short coax run into the tent.  I had a length of ladder line about 25 feet long so the 10m dipole was up about 23 feet.   At the end of the ladder line at ground level I plugged the ladder line into a 1:1 unun, and ran the last 30 feet or so into the station with coax.  From start to finish this antenna took about 30 minutes to put up, including cutting legs and twisting it all together.

No antenna at all on 80m.  Decided I’d have enough business on 20m & 40m to keep occupied, and figured on sleep rather than a night of 80m T-storm QRN.  😮

Now, about that 40m Vee.  The antenna that would NOT.  Still not certain where the problem was, but it had an issue in one of the feedlines somewhere[update-think one of the legs has broken wire].  Far too much time was wasted raising and lowering the antenna trying to debug the issue.  Lesson1: Always have an alternative.   Lesson2:  Don’t dick around debugging when you have the alternative at hand ready to go.  Lesson3: Save debugging for down time.  Lesson4:  Read Lesson1 and Lesson2 until they really sink in.

Operating:

Once the CQ’s started, there were plenty of QSO’s to log.  Saturday was a bit slow at first, but I got a better rhythm in the evening.  Was tired though, and sacked early, including a 45 minute nap at 5pm in the nice cool breeze that came up.  Also got up early Sunday, 5am-ish.   Sunday morning was quite a bit of fun, right up until my keyer interface died around 11am.  So I finished the event with a bit of lackadaisical SSB, mostly S&P.

Camp:

The Cabelas tent goes up easily.  My only regret is not getting one of the larger sizes.  It has room for setting up a table for the station and also for a cot along the opposite side.  But it is a bit cramped.  Next time I do this I think I will use a screen tent for the station and the tent just for snoozing/bad wx.  Also, the ideal site would allow for the tent to use an overhead spike support and avoid the need for the center pole.

Overall a big win.  Keeping the ants away…..the real challenge!

 

 

CW Skimmer Errors and RBN spots

After several contests, monitoring of the softrock skimmers has turned up a bit of a problem with using softrocks as the skimmer platform.  Very strong signals are producing a mirror image that is often reported as a spot to the RBN.  Certain to be annoying for the S&P packet crowd during a contest.  Annoying enough that a few flame mails have arrived.

The volume of the bad spots is relatively low on the lower bands, and more common on the higher bands.  40m is somewhere in the middle, with most of the bad spots being sent for domestic USA stations.

The problem is a combination of the hardware and software, both contributing to the problem.  A software fix could potentially be made to CW skimmer or to the RBN aggregator to correct for the problem.  Will inquire to the authors…..

In the meantime the best solution available is to throttle the RBN aggregator to allow only spots below the center frequency to be reported.  For example, the 15m skimmer is based on a softrock with a center frequency at approximately 21044.5Mc.  So for the duration of the ARRL DX CW contest, an entry in the “Notched Frequencies” will be active to not report 21044.5-21100 to the RBN.

That solution does nothing to correct for half of the possible bad spots(i.e., a strong signal above the center frequency whose mirror image is being spotted below the softrock center frequency).  But it should alleviate many/most of the actual bad spots, since most run stations prefer to operate as low in the band as they are able.

Open to other suggestions short of replacing the softrocks with better (yet unaffordable) hardware.

Update 20130217, 2140Z:  There are new versions of both skimmer and aggregator.  Perhaps upgrade will help.

More on Xonar DX Experiments

A thread over on the softrock user group list spurred the curiosity…..: http://groups.yahoo.com/group/softrock40/message/68324

G4ZFQ has  RightMark test data for a high end Xonar D2X card, as well as several others.  An internet search found other RightMark tests of several other Xonar cards, all of whose test data show curve trends remarkably similar to those of the D2X, albeit with somewhat worse IMD, spurious, and noise figures.

The curiosity is the test data shows a roll off on the frequencies above 50khz.  The nature of the loopback test is an issue, but it also seems likely that using a sound card as the source may be having an effect on the test results at the higher edges of the sound card frequency response.  But signal generators as input to the tests shows the same general trend.  SDR at wider bandwidths pushes at these edges of a ‘sound’ card’s ability….So perhaps the SDR software is compensating for the expected performance drop-off at frequencies above audible levels?

The Test- (Pertinent Excerpt from list post):

Having not yet thought of a better way to do a meaningful real-world test on the sound card with what is available in the KazShack, I fired up the 80m softrock on the xonar DX.

Test condx:
Transmitting a cw signal(a string of dashes at about 18wpm) at 5w into a dummy load on separate radio, noting the SNR readings obtained by CW skimmer from the SoftRock center frequency(353395x) to its upper limit. With the xonar DX set to 192khz scan rate, the actual upper limit on the readings was 3629.60. SoftRock connected to normal antenna system, a NE facing K9AY with W7IUV pre-amp. In summary, a sound card test using the SoftRock system as input source.

fq….—-SNR(dB)
3534.5—-42

3543-3593-42-40

3603——37
3613——35
3623——32
3629.6—-36

After CW skimmer collected a bit of data, the SNR readings above 3600 improved to 37-39.

So the worst case for CW skimmer(as currently configured) using a Xonar DX is being 6db less sensitive at the upper edge of the 192khz bandwidth than it is at the center. That is actually a lot better than I expected for an audio device pressed into service outside normal audio ranges (and I already liked the Xonar DX).

My curiosity is now nagging me to run the same tests on all of the other in-shack cards more methodically at their maximum scan rates(mostly 96khz), and to find a lower level outside signal source. I’ll try to recruit a fellow in the near field who will better be able to generate a low level test signal.  It would be useful to see what happens at the band edges when the best copy close to the center of the SoftRock’s scan range starts out at 20dB, 10dB, or 6dB SNR.

But with the WX here improving, all of that might not happen for several months.
😉

Engineer the Possible…

Xonar DX es SoftRock CW Skimmer

Yesterday, having not yet thought of a better way to do a meaningful real-world test on the sound card with what is available in the KazShack, I fired up the 80m softrock on the ASUS Xonar DX for a bit of putzing around.

Test condx:
Transmitting a cw signal(a string of dashes at about 18wpm) at 5w into a dummy load on separate radio, noting the SNR readings obtained by CW skimmer from the SoftRock center frequency(353395x) to its upper limit. With the xonar DX set to 192khz scan rate, the actual upper limit on the readings was 3629.60. SoftRock connected to normal antenna system, a NE facing K9AY with W7IUV pre-amp. In summary, a sound card test using the SoftRock system as input source.

fq….—-SNR(dB)
3534.5—-42

3543-3593-42-40

3603——37
3613——35
3623——32
3629.6—-36

After CW skimmer collected a bit of data, the SNR readings above 3600 improved to 37-39.

So the worst case for CW skimmer(as currently configured) using a Xonar DX is being 6db less sensitive at the upper edge of the 192khz bandwidth than it is at the center. That is actually a lot better than I expected for an audio device pressed into service outside normal audio ranges (and I already liked the Xonar DX).

My curiosity is now nagging me to run the same tests on all of the other in-shack cards more methodically at their maximum scan rates(mostly 96khz), and to find a lower level outside signal source. I’ll try to recruit a fellow in the near field who will better be able to generate a low level test signal.

But with the WX here improving, all of that might not happen for several months. 😉

engineer the possible….
73 de w4kaz

Skimmer Station

Added a new page to the skimmer station fun facts list.

The new page describes observations from using several different sound cards for both music and as the interface for SoftRock software defined radios and the CW Skimmer software.

Skimmer Notes

Created a page of links and several related pages of information on the ongoing construction of the CW Skimmer station at W4KAZ.

Because of the nature of the blog package used for this website, it is easier to save this skimmer related info on ‘pages’ rather than as a “post” because it seems like a project that I’d like to have semi-permanently documented, and have the documentation easily found.  Whooop…there it iz….Incomplete, but slowly growing, and probably to be frequently edited in the short term.