Radio W4KAZ

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Red Pitaya SDR as Core of CW Skimmer Station Rebuild.

[Updated 2016-12-23, see text on Compatibility issue]

Recently discovered an interesting,  affordable,  and relatively new product called Red Pitaya, designed as an open source based piece of test equipment.  As a piece of test equipment the Red Pitaya has basic oscilloscope,  spectrum analyzer, and signal generator apps available.  The apps are designed to run as web applications with Red Pitaya board running a custom Linux and acting as a web server.   Currently the apps are quite basic, but useful despite their simplicity.

Possibly more interesting for hams are the SDR receiver and transceiver apps available from Pavel Demin.

With the SDR apps, Pavel has taken this little Red Pitaya board into the areas of interest to many ham ops.  The SDR receiver app has the ability to function with several currently available SDR programs.  The ability to support feeding six channels into CW skimmer server is of particular interest.  There are also transceiver apps which are being used by experimenters to build Red Pitaya based transceivers.

Red Pitaya

The Red Pitaya itself is a board that runs a customized linux OS(their term is ‘ecosystem’) off of an SD memory card.  The board has two RF inputs and two RF outputs for use as the heart of a test system.  5V USB power supply input requires 2A.  The board has a heat sink on the CPU but a small fan helps cooling.  It can connect via ethernet to the network or via a wireless connection.  The OS and apps are downloaded from the Red Pitaya website.   SDR apps are available from both the Red Pitaya site and directly from Pavel Demin’s website.  This little SDR kludge is a viable substitute for the Softrock skimmer system previously being run @W4KAZ.

SDR Uses and v0.96 Compatibility Issue

A couple of issues turned out to be a mix of hardware and software problems.  The largest problem was a software incompatibility issue between the latest OS v0.96 and the SDR software.  This causes problems in the SDR with interference that looks like intermod artifacts.  Too much time was spent here looking for hardware problems before stumbling across the documentation on the issue.  The solution was simple.  It simply required building an SD card with the previous OS version v0.95, and  then configuring(secure password) then re-installing the SDR app.

A couple of the SDR apps available on Pavel’s site originally included an OS that did not allow a persistent password change.  To avoid that security vulnerability, the original SD card here was built with v0.96 of the OS.  As of October 2016, do NOT try to use v0.96 OS with the SDR apps.  V0.95 works with the SDR.  Better to have several SD cards with different OS versions should you need a more recent OS for new apps as they are developed.  [Update 2016-12-23.  Per comment from Pave Demin, the SDR applications have been updated and should now work with v0.96 and v0.97 of the Red Pitaya ecosystem.  Not yet migrated to the updates here in the W4KAZ SDR setup.]

After building the v0.95 system, the SDR app was good to go.   Several different flavors of SDR are available from Pavel.  The SDR app version that supports CW skimmer server also supports PowerSDR, which has a nice spectrum and waterfall display.

There is not a tremendous amount of information available, as folks are just beginning to explore the possibilities.  PA0AER has an interesting post, with a few findings of his summarized in this table.

PA0AER published test data.

PA0AER published test data.

Yeah, -120db floor and 75 db of intermod suppression should work just fine in a CW skimmer application.  Keep in mind, the softrock system being replaced has about 45db of useful dynamic range as implemented here. Plus we get the bonus of using Red Pitaya as a minimal spectrum analyzer and oscilloscope.  Maybe even a VNA app.

SDR Station With Red Pitaya

First of all, one huge thumbs up to Pavel Demin for his open-source work on the SDR apps.  Outstanding!

The time spent here going in circles chasing my non existent hardware issues was not completely wasted.  The power supply  was cleaned up with better filtering.  Very nice.  Using the AADE filter design program we also came up with a simple-to-build design for a high pass BCB filter.  This filter optimizes the nulls at 680am and 850am,  and drops off rapidly below the broadcast band.

This BCB filter exhibits low loss on 160m, with the modeled 3db cut off frequency being at about 1.2mhz.  Ordinary C0G/NP0 capacitors are used in its construction, having had acceptable results with that type with the W3LPL design band pass filters.  The result was good with testing on the base station.  The difficult part was finding good leaded C0G/NP0 capacitors in proper values to use for construction.  Through hole components are becoming rare.  NOTE: As designed this filter is a short circuit at DC.

W4KAZ version of BCB filter. Built using NP0/C0G leaded capacitors, t-80-2 torroids, and a 220uh choke.

W4KAZ version of BCB filter. Built using NP0/C0G leaded capacitors, t-80-2 torroids, and a 220uh choke.

w4kaz_bcb_filter_designplot_edit

AADE predicted performance plot of the w4kaz BCB filter from DC to 10Mc. Note the nulls on 680 and 850.

The plot projected by the AADE filter design program above is a best-case prediction.  WPTF, 50kw at 680 and WPTK, 10kw at 850 have transmitters about 3 kilometers and 12 kilometers respectively.  They produced all sorts of intermod in the softrock system.  The design is tailored to place the largest nulls where they might do the most work in the KAZshack.  Getting the 3db cutoff at 1.2Mc was just to try to keep the losses as low as possible on 160m.  To get an idea of its performance, I plugged it into the station and took S-meter readings on the Kenwood TS-590s.  The BCB filter dropped WPTF at 680 from pinning the S-meter down to just another strong S9+ signal.

S meter comparisons on the TS-590s using the W4KAZ BCB filter and built in attenuator

S meter comparisons on the TS-590s using the W4KAZ BCB filter and built in attenuator

Skimming with Red Pitaya SDR

With the software issues corrected, let the CW skimming begin.  Skimming tests seem to have spot signal levels from Red Pitaya SDR slightly better than those from the Softrock skimmer system.  Full system stress test coming during 2016 SS CW.  The Red Pitaya also seems to be very frequency stable, something that was a minor issue with the softrocks.  When running PowerSDR, comparing Red Pitaya by ear shows it to be a bit less sensitive than the main station rig, a Kenwood ts-590s.

Does it work?

Random selected North American spot counts from days with W4KAZ skimmer station under Red Pitaya

Random selected North American spot counts from days with W4KAZ skimmer station under Red Pitaya

System Reconstruction – Permanent New CW Skimmer

After a good shakedown voyage through Sweepstakes CW, it will be a good time to re-arrange the test Red Pitaya SDR system into a more permanent and compact single system.    One of the dead softrock CPU’s will donate a nice clam shell computer case, and all of the components should fit easily.  The plan is to wall mount the completed system near the shack cable entrance.  The softrock system will be raided for its discrete components, the W3LPL style band pass filters as well as the W7IUV pre amps.  New splitters will be built for the new system to allow for antenna options to change in the future.

Unexplored are some more experimenting to find a proper matching transformer and  Red Pitaya input jumper combinations for the best results.  Some research into the transceiver experimenter comments indicate using a step up transformer is best.  Some have also made mods to the front end that are supposed to boost the sensitivity by lowering the noise floor significantly, from 9 to 12 db.  Currently using a 3:1 cascaded into a 4:1 transformer as step up, with the input attenuation pads bypassed via jumpering on input#1.

Block diagram of likely W4KAZ Red Pitaya SDR CW skimmer system

Block diagram of likely W4KAZ Red Pitaya SDR CW skimmer system

Important Notation:

If you choose to experiment with Red Pitaya and the SDR apps,  be sure to create your bootable SD card from a compatible OS.   If you do not, you will be very disappointed at the 25db BDR and the interference and low performance you will experience.  Currently, as of 2016-10-28, the 0.96 ecosystem/OS IS NOT COMPATIBLE with the SDR apps from Pavel Demin.  Either use the ecosystems Pavel has or the last archived version 0.95 from Red Pitaya’s archive.  Do NOT TRY SDR with v0.96 OS!  Been there, done that, have a clean power supply and nice BCB filter to show for it.  [Update 2016-12-23.  Per comment from Pave Demin, the SDR applications have been updated and should now work with v0.96 and v0.97 of the Red Pitaya ecosystem.  Not yet migrated to the updates here in the W4KAZ SDR setup.]

The Red Piytaya ecosystems come with default root passwords.  Reset your password ASAP.  Also, at least one of the ecosystems Pavel provides does not allow a persistent root password change.  I suggest using the 0.95 from the Red Pitaya archive and that the root password be reset to something secure if the Red Pitaya is going to be running on your home network.  With the default root password, your network is open/vulnerable to having a hacked linux system behind your router’s firewall.

Other Useful Red Pitaya SDR links circa 2016-10-28:

PA0AER on a comparison of Red Pitaya to Flex radio, with intermod test result(google translate to english).

FOSDEM 16 program by open source SDR app designer Pavel Demin (Brussels, January 2016)

Red Pitaya as SDR

Red Pitaya Quick Start Guide

Video 2Khz BDR 107 dbm by Yevgeni Kolganov

**

W4KAZ Softrock Based CW Skimmer Station – 05) Si570 Programmable Oscillator for Softrock CWSkimmers – or for “Whatevah”

Note: More links at bottom of page:  This is a simple-to-build Si570 oscillator for use with the Softrock Lite kits for CW skimmer and input to the Reverse Beacon Network.  It uses the Si570 chip and an AtTiny85-20 programmed with the PE0FKO firmware used for the Softrock Ensemble kits.

Currently a programmed ATTiny chip is available separately from K5NWA.  The firmware is available for download, so programming the AtTiny is also an option.  The PE0FKO site also provides the required USB device driver, software, and guidance on using them.  (links)  The Si570 is available from Digikey(digikey part#:336-2518-ND,  manufacturer part:570CAC000141DG).(SiLabs 570CAC000141DG part )

The oscillator itself is pretty simple, and is the bare essential hardware required for re-programming the oscillator for a needed single frequency to use with a Softrock Lite II rx.  It is based on what I saw in the the schematic of the Softrock Ensemble RX, nothing original, just pared down and hijacked from the original Ensemble design.  The Si570 part itself is the bulk of the expense of the oscillator, and the cost of the Si570 chip is almost as much as the Softrock Lite kit itself.    The oscillator signal is fed into the divider through a 10K voltage divider as in the Softrock RX.

So why an Si570 Programmable Oscillator ?

The RX Ensemble kit is a viable alternative expense wise.  It really depends on the intended usage.  Using separate Softrock Lites as single band CW skimmers leads to the choice of a programmable oscillator for customizing the center frequencies, especially for the high bands.  The method used for 20m using the third harmonic seems to result in a decrease in dynamic range.  That results in an increase in false mirror images being reported to RBN by the CW skimmer as actual spots.

Using the Si570, the oscillators can be set at the frequencies needed by the Softrocks, i.e. 4 times the center frequency.  (for 96Khz bandwidth the oscillator would need to be: 20m=56.188, 15m=84.188, and 10m=112.16).   A programmable oscillator also allows switching from 96Khz to 192 Khz bandwidth(20m=56.38,15m=84.38, and 10m=112.38).  Keeping just the bottom half of a 192Khz bandwidth CW skimmer would at a minimum eliminate at least 50% of bad mirror image spots.  There are also likely to be fewer stations CQ’ing below the “.096” section of a band(e.g., most often there is not so much regular CWactivity above 28.096 as there is below).  That is the idea anyway.

The Si570 Programmable Oscillator Prototype:

The first version is deadbugged on a bit of board scrounged from the parts bin.  Not many parts, but a bit more PCB real estate would have been better.  Functional rather than esthetic.  The USB connection is via the usb cable end clipped from an old computer mouse in the parts bin(unlabeled black coil in left of photo).   “Engineer the possible”.

Si570 Programmable Oscillator board for 10m Softrock CW skimmer

Si570 Programmable Oscillator board for 10m Softrock CW skimmer

Testing the original prototype board pictured resulted in three build mistakes to debug:  a missing 5v connection to the ATTiny and the reversal in polarity on both zener diodes across the USB data pins.  These mistakes prevented function without damage to the components.  After correction of the build errors the software was able to function with the Si570 as needed for both programming the oscillator(‘startup’) frequency and running as a stand-alone oscillator.

The Si570 when programmed for 112.36Mc was found to have an actual oscillation at close to 28.090 exactly from the Softrock divider, as measured with TS-590 and Elecraft K2.  This was with the oscillator inserted in-circuit as the Softrock Lite oscillator via a transformer(5 bifilar turns on a type 43 torroid core) and a 2.2k resistor.  The frequency is very consistent and stable when the power is cycled on/off.

Setting the frequency is accomplished using either the USB-Sync program by DG8SAQ or via the test program Si570_USB_test from the QRP2000 project from www.sdr-kits.net.  DGF8SAQ’s program is easiest.

Easy measurement of the actual frequency in place is good enough for initial setting up of the skimmer software. A few KC either way will make little difference in a CW skimmer set-up, as final adjustments were done in CW skimmer software to put the skimmer signals ‘on frequency’.  In this case the CW skimmer center frequency is nearly identical to the Si570 programmed frequency.  That has not been the case with the versions using ordinary crystal oscillators, those having a bit more drift off their nominal value.

A new Softrock Lite II is the 10m test bed, with 15m revision to follow.  These two bands suffer the most from poor dynamic range and false mirror images.  The 15m oscillator also has a nasty tendency to drift with temperature changes.  If the modified softrocks perform as desired it will be time to pair these two bands with the best of the sound cards available.  That will be a separate game of trial and error.  The 20m softrock skimmer may also be retrofit, as using the third harmonic for the softrock center frequencies seems to adversely impact the dynamic range.

Photo of 10m Skimmer

Photo of 10m Skimmer at W4KAZ

As an aside, the first 10m center frequency chosen was 28.060 into a 192Kc bandwidth sound card. Horrible choice, as it was close enough to the 15m harmonic that interference spikes were present on both bands every 900hz.  Resetting the Si570 oscillator to place the center Fo for 10m at 28.080 greatly reduced(but not eliminate) the problem.  Currently set on 28.090 as of 20150414.  More tinkering required, and migrating the 15m Softrock over to an Si570 oscillator may help.

The current Skimmer package for 20m, 15m and 10m. 20m and 15m will likely be re-worked to use Si570 Programmable Oscillators.

The current Skimmer package for 20m, 15m and 10m. 20m and 15m will likely be re-worked to use Si570 Programmable Oscillator.

Si570 Programmable Oscillator UPDATE, 2016-11-08

The Si570 oscillator as described was perfectly usable in this application.  However 10m and 15m performance was was poor on the softrocks, the primary difficulty being a low dynamic range.  This is indicated by mirror images that appear when SNR values on the actual signals were higher than 35dbSNR.

The most useful work around for this problem is to scan at 192Khz sample rate, and only use the lower half of the sample for the CW skimmer.  Using the upper 96kc might be easier, as the center frequency could be set at 28.0Mc and 21.0Mc.  The latter may ulimately be the best approach.  There are unlikely to be any useful signals below the bottom of the bands, and those could be readily discarded as false or otherwise unusable(i.e., out of band).

LINK LIST, Si570 Programmable Oscillator :

  1. W4KAZ Schematic

    Schematic for W4KAZ version of Si570 Programmable Oscillator

    Schematic for W4KAZ version of Si570 Programmable Oscillator

  2. W4KAZ BareBones Parts List (PDF) (HTML with links)
  3. KB9YIG Five Dash softrock products page
  4. K5NWA ATTiny85 page
  5. PE0FKO Firmware page
  6. Document:

 

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…

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.

Re-purposed HyperDawg as Antenna Launcher

Ran across the hyper dog ball launcher a couple of years ago, and the potential for re-purposed applications for hanging antenna supports seemed obvious.  It is not as much fun as a pneumatic launcher, but it sure is easily understood by any boy of 8.  No air pump required.

The modified hyper dog 
The hyper dog ball launcer modified to launc lines for antennas Photo of tennis ball modified for use as line launcher

The normal slingshot type Wrist-Rocket/Crossman slingshot launcher has served the purpose for years, but not always without problems.  A 1-oz(28g) lead weight works, but not without a relatively high rate of mis-fires, line tangles, and “Oh S**t!” moments.  The hyper dog is a lot less likely to draw whining complaints from those inclined to wring their hands and moan about things that don’t really concern them..”See, its just a tennis ball.  Now p**s off!”

The hyper dog has a much larger pouch designed for use with tennis balls.  A slight bit of hacking to the hardware gives a nice re-purposed tool for lofting lines into all of those beautiful deciduous biological antenna supports lining the back yard.  So far it has been a lot more reliable in actual usage than the ole trusty Crossman, although Field Day proved its not impossible to Dork Up.  [You Know Who You Are….lol]

The reel deal:

Here the body was altered by adding a cheap spin-cast zebco reel picked up for $2 at a yard sale.  A spinning reel or open faced casting reel might be better, but I have used the zebco’s since I was 6yo.   Being more familiar with the Zebco quirks and limitations is useful. For most, a spinning reel is probably the best option.  10 or 12lb test line has proven the best choice over the years – light enough to fly, strong enough to pull, and not impossible to break if it becomes hopelessly snarled at altitude.

The reel is simply attached below the ball carrier with a couple of hose clamps.  That was later wrapped with an ugly mess of electrical tape just to reduce the number of exposed sharp edges.

Yes, the tennis balls work FB.

To modify the tennis balls, they were  just drilled with a 9/64 bit.  A loop of 1/8th braided nylon cord is secured to a small hardware store drywall toggle bolt/spring bolt. Then just cram the bolt/cord through the hole, reaming the hole out slightly if needed[leaving most of the loop of cord hanging out!].    The base of the cord is sealed at the hole with a goop of liquid nails or hot glue or some-such.  The loop of cord is about 6 inches long(~150mm), and the spring bolt serves the same purpose it normally does by providing a large area preventing pull-out.  After drying completely – good to go.

The tennis balls seem to be a good compromise between weight and a non-destructive & non-threatening projectile. [Just don’t try to pull them back up through the tree-too fast!].   The ‘trick’ to success with it seems to be making sure the cord on the tennis balls clear the end of the slingshot.  It seems to work best when the corded end of the ball is facing  up(i.e., at the top of the pouch when pulled back for a shot).

What’s the catch?

The only genuine problem I have with it is that it has a “long draw”.  Being impishly short my arms are not long enough to get the maximum performance out of the rig.  But despite that it works much better than the regular slingshot with fewer snags and mis-fires. It easily sends the tennis balls up to about 90 feet(~30m).  The canopy here prevents anything higher, so no real top-end found yet.

I suspect golf balls would be the ultimate high-flying projectile for rural locations.  Too much window glass and nervous-Nelly neighbors around the home QTH for me to try golf balls here.   A day-break early morning experiment for the future…. 😉

There is somebody here on the east coast marketing these re-branded as antenna launchers, and asking $80.  See Radiowavz Hyper Hanger, now $90USD….

Too easy to homebrew from the $22 Amazon original to peel out 80 samolies 90 GreenStamps, but it is there as an option. (!~yikes~!)

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.

Open Sleeve Dipole, Tribander Plus

Tribander Plus?  Plus what?  Friends with benefits maybe?

Back in 2002 or so, references to the “open sleeve dipole” sparked a curiosity in the topic.  Very few web references were available back then(in the internet dark ages).  The article that initially tweaked the curiosity was N6LF’s article “A Wideband 80-m Dipole”, which built upon an earlier article by K9AY from 1995. The open sleeve idea is also mentioned by Bill Orr, W6SAI.  Current ARRL Antenna books have information on this topic in Chapter 10, but that was not available in 2002.  Not much else, but there are a few engineering references going back at least as far as 1945.  None of which were readily available a decade ago.  Time marches…..

Recently Joel Hallas, W1ZR, ran a couple of articles on the subject in QST. This article on adding 6m to a tribander duplicated an idea I had myself, although I was more interested in the possibility of easily adding 17m to a tribander. W1ZR uses the term favored by K9AY, the “coupled-resonator”, from K9AY’s article “The Coupled-Resonator Principle: A Flexible Method for Multiband Antennas” in the ARRL Antenna Compendium #5.  K9AY’s “coupled-resonator” terminology is more precise and accurate as a general description of the principle for amateur uses.  Yet my own fuzzy gray matter remains wed to the term “open-sleeve”, which is really an example of the general principle in a narrow usage.

More references are available today: NQ6K with a well written treatment used as a sloper, and DK7ZB has a good page briefly explaining the concept. Plus a new ARRL Antenna book(Chapter 10.4-10.5) entry. This is the idea in general terms.  Placing another wire in parallel and “close” to another dipole allows you to feed the antenna at two different resonant frequencies.  The feedline is attached to the longer dipole only, the second(and/or third) wires are excited parasitically.  By carefully spacing the two wires, it is possible to get a 50 ohm match for both resonances.

Pretty handy idea….but….

O’course, the trick is in the details.  Getting the spacing correct can be tricky if you are intent on that 50 ohm match.  EZNEC modeling is simple enough for this gray cat, and it is easy enough to fiddle the dimensions of the second dipole and its length to get that 50 ohm match.  It turns out you can also add a third dipole too, but the spacing dimensions become a lot more critical.  Much like a fan dipole.

Generally, it seems that the higher frequency dipole winds up being slightly shorter than it would if it were a single dipole, and that is more pronounced as additional resonators are added.  Harmonic combinations also seem more sensitive to dimension changes affecting feed point impedance.  [NOTE: ARRL antenna book, chapter 10.5 contradicts my note of “slightly shorter”, indicating the dipoles might need to be slightly LONGER due to the capacitive coupling.  Hmmmmmmm.  Mayhaps wire spacing….?]

Outside the box of conventional

But what if we do NOT concentrate on the 50ohm match? What if we wish to use a balanced line feeder and a tuner, like for, oh, maybe a multi-band wire antenna for FIELD DAY?  Something with improved and predictable radiation patterns over a non-resonant doublet?

That seems to be a pretty good option.  The dipoles are resonant, even if they show an impedance that is not 50 ohms.  Resonance does not imply 50 ohms….that is just a happy convenient choice since radios are designed to have input impedances coincidental with the general impedance of a half-wave dipole fed in the center. But an end fed half wave is just as resonant even though the impedance is nowhere near 50ohms.

With resonance the radiation patterns are more predictable, and there are less likely to be odd lobes as might be the case with a generic doublet.  In the past we have used several different options for loading up a doublet on multiple bands, with mixed results.  So in April I decided to cobble an open sleeve antenna together and try it out for a few days.

Models?  We don’ neeed no steeeenkeeeng models…

No, but in this case the model was interesting.  Going for a decent compromise of performance versus convenience, a bit of EZNEC tinkering showed that a 40m/20m/10m  combination of elements might produce decent  radiation pattern results for four bands. Input impedance?  Maybe not so ideal.

The plan modeled was using 18ga ladderline for the mid-section, and attach legs on the ends for the 40m resonator.  Spacing the 10m wire at about 1.5 inches in the model produced a more stable impedance curve for 10m, so rather than just taping the 10m wire to the ladderline, small spacers are used.  Radiation patterns for 40m and 20m resemble normal dipole patterns.  The 15m pattern is normal for a 40m wire pressed into 15m service – a dipole pattern that is  breaking into two lobes with a nulled lobe perpendicular to the dipole.  The 10m pattern without the 10m element would show the same butterfly pattern.  Adding the 10m element makes a huge difference on both the radiation pattern and the feedpoint impedance in this instance.

Actual antenna

The antenna was constructed from a segment of 18ga 450 ohm ladder line and some scraps for 14ga thhn laying about.  The ladder line section was from a 20m folded dipole.  The parasitic radiators modeled best if the were slightly shorter than they would be in a single wire dipole.  So the ladderline was shortened to the model’s 20m dimensions.  Enough wire to complete the 40m legs to model dimensions was added to the fed wire. Dimensions were pulled directly from the model for all legs – no other trimming, and the pieces stitched together.  [No point in trimming since the intent is to feed the antenna with ladder line to a tuner, rather than obtain a 50 ohm match directly.] The 10m radiator was attached to the ladder line via a few sections of PVC pipe cut into 2 inch lengths for use as spacers.  These spacers were drilled and attached to the ladderline via cable ties and tape.  The 10m element then is attached to the outside of the spacers, away from the ladderline.

In hindsight, adding the 10m element with spacers is problematic for a “portable” antenna.  The spacers complicate storing and deploying the antenna.   Just taping the 10m element to the ladderline is probably worth the efficiency trade off – unless you have real reason to expect 10m to be significantly better than it has been of late.

The end result is a dipole that exhibits the radiation pattern of resonant dipoles on those three bands, and also is relatively easy to load on 15m.  The model show the 15m pattern is “butterfly” shaped, which is normal for a 40m dipole pressed into service on 15m.

Workee workee

It is now hung at almost exactly 30 feet height, and shows the expected performance on those four bands.  One of the major lobes on 15m must be favoring Europe, as the EU stations are better on this antenna than anything else in the yard.  20m and 40m are equivalent to the other dipoles.  Not much 10m activity heard yet, so no idea there.

I expect a similar fan dipole could be used in the same manner, where all of the dipoles are physically attached.  My experiments with fan dipoles was in an attempt to match to 50 ohms.  Very difficult to do for more than 2 bands.

Another caveat to be aware of is that combinations of dipoles at widely separated frequencies, i.e., more than a harmonic, will tend to have “unusual” patterns.

There were too many outside issues to allow much butt-in-chair time in one of my favorite contests, CQ WPX CW. But enough of a chance to try the antenna out, and I’ happy with the results.  The butterfly lobes on 15m must be in very favorable directions.  During the contest Europeans had huge signals, and I worked three JA stations on Sunday afternoon….which are certainly the only JA’s worked from this QTH this decade.

I’ll not leave it up permanently, because running the ladder line into the shack always raises RFI issues.  This antenna is RFI “cranky” on both 40m and 15m inside the shack, but thats probably more an issue related to the kludged feed-thru into  shack and the general in-shack rat’s nest of wiring rather than the antenna.  It is worth testing out for a bit longer though.

Maybe a few RFI issues will find resolutions during the testing.

Interesting open sleeve(coupled resonator!) ideas….

NQ6K  slopers…. http://findatlantis.com/wiki/index.php/20-15-10m_Triband_Sloper

N6LF Wideband 80m dipole:  http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf

Dan Levin, N6BZA and Marty Levin, W6BDN: Notes on phase delays when stacking: http://www.k6if.com/c3_stack_article.html

 

Skimming During WPX

“Don’t get cocky kid….”

Just when everything seems to be going smoothly, Murphy arrives.  Two days before the WPX contest, it became clear the 80m softrock had developed an issue and was useless.  It lost half of the signal, and so was useless(on 5/27) and required repairs.  Sideline that issue for now….

The 40m skimmer session kept right on chugging.  A couple of problems became clear.

  • The softrock will need better band pass filtering if they remain active during operations, or switching to shut them down during transmit
  • running CW skimmer at 192khz sample rate used a great deal of CPU time, averaged 45-50% CPU utilization.
  • Limiting the number of decoders helped somewhat
  • Reducing the scan rate to allow a 96Khz bandwidth drastically reduced the CPU usage.  Running at 96khz required an average of 15-20% of the CPU, even with the limit on the number of decoders increased.
  • The K9AY rx antenna worked well for the 40m SR skimmer.  The problem is it will need to be split and amplified for 80m and 160m as well.

All together, it looks like the skimmer posted about 8K spots while it was active during WPX.  Trying to figure out a way to check for errors.  Low priority – spot checking the spots showed most to be valid.

The 80m SR problem was resolved on Monday evening.  After having made three passes over the op-amp section, I made a fourth pass, concentrating on components in the “ring” side of the op-amp output, as well as the path back to the QSD section.  Also made a first pass over the transformer solder joints, using a higher wattage iron.  80m issue resolved.  Odds are it was in the op-amp chain(seems to be the most common cause of the symptoms), but I suspect a cold solder on the transformer was the real culprit.

Looking at the case from a dead home audio system as a permanent enclosure.  If I can fit everything in, it should be a good choice.  Time for a block diagram…

 

 

Skimming CW at W4KAZ

Note….Post dated this material to original date written…superseded by new reality…..

“All is proceeding as I have foreseen it…………..”

Been just about four years since the CW skimmer stuff really hit the contest rotary impeller. A bit of review…..

So in hindsight – I’m glad contest sponsors read my blog.  😉

Since the rule-parsing panned out to my immense satisfaction, it is easier to concentrate on the toy itself.  Since then, skimmer stations sprouted, and the Reverse Beacon Network was born.  That is a really interesting project.  Its a great tool for checking propagation, comparing station signals, and getting impartial signal reports.  Outstanding resource.

Unfortunately there is not a local skimmer station that is feeding the RBN.  Spots from MD are not always useful in Central NC.

So it with the moon in phase and the planets approaching the grand alignment, it seemed like time to look into the subject of skimming.

Options

The Skimmer software comes with a component that is designed to work with the QS1R SDR.  That combination is likely the ideal solution.  So all I need is a fast Pentium i7 Quad core, and a thousand samolies……  Great idea, just not possible.

Much more possible….Combine a few Softrocks with some cast off circa 2008 computers.  Yup, that’s the ticket.  Rather than sit on my thumbs….Engineer the Possible.  O’course, the possible is not always completely practical.  Everything is relative.  What tradeoffs are reasonable?  Engineer the Possible.

It has been done before…AC0C has documented the challenges and his solutions.  Despite the validity of his conclusions, it is now possible to cobble together a scaled down version using scrounged computer hardware.  Using softrocks, it is now very practical to put together a skimmer package for 160/80/40/20 meter bands with obsolescent computer hardware.  The price/performance ratio of the softrock is a huge factor.  If they were a mass production commodity, they would probably cost under $10.

15m and 10m may be more of a challenge, so that has been shelved for the moment.  So the softrock solution is not perfect.  But there are solutions to that too.  Future project….

The current project direction

So the game plan is to skim on 160m thru 20m using softrocks.  40m and 80m softrocks are done.  Its not going to be as professional a finished product as AC0C’s, but it should function.  Reclaimed from the off-lease refuse stream are an Dell Optiplex 745sff and a Dell optiplex 360 SDT.  Both of these boxes require low-form-factor cards.  The on-board sound of the 745 leaves something to be desired, but the 360 has an on-board sound card capable of 192khz bandwidth.  So small form factor add-in cards are needed to run skimmer on multiple bands.  Four bands on two computers.

Other Naughty Tidbits….

The Asus Xonar DG is a small form factor sound card that turned out to be a fabulous bargain. It allows only 96khz bandwidth, but has excellent dynamic range for its cost.  Sounds great with music too.  Also had an Asus Xonar DX, which is higher fidelity than the Xonar Dg, and offers 192khz bandwidth with a softrock.  The sound card issue is the real sticking point in this design, but the Xonar cards are able to coexist with the onboard SoundMax devices in the Dell boxes.

Not so much luck with a Soundblaster Live 24.  Experiments installing and using the Soundblaster were problematic.  Compatibility issues with the other sound devices and SDR software crashes. The Asus cards are much higher quality, but attempts to pair either with the soundblaster caused problems.  Attempts to install both Xonar cards in the same system were also buggy.  So the Soundblaster is sidelined for later rainy day experimentation,  the ASUS cards are each on a different host system, and it is fortunate that the onboard sound cards are useable.

The final compromise chosen was to install the 192khz Xonar DX in the Optiplex 745 that has 48khz onboard SoundMax.  The Xonar DG is installed in the Optiplex 360 that has 192khz SoundMax onboard sound.  In testing the Xonar cards work very well with all of the SDR software tested.  The SoundMax cards are noticeably less capable, but not terrible.

Mix and Match

The skimmer sessions sound card pairings in daily usage are likely to be:

  • 160m…48khz on Optiplex 745 onboard sound
  • ……………..Center@????????,Covers ?
  • 80m…..96khz on Optiplex 360 Asus Xonar DG
  • …………….Center@3533950, covers ~3485 thru 3581
  • 40m…..192 or 96khz on Optiplex 745  Asus Xonar DX
  • …………….Center@7055015, covers ~6959 thru 7151@192Khz, 7007 thru 7103@96Khz
  • 20m…..192 or 96 Khz on Optiplex 360 onboard sound
  • ……………..Center@????????,Covers ?

Those pairings should spread the CPU load somewhat.  A live test on 40m and 80m during CQ WPX should give me a benchmark for CPU loading.  CW Skimmer allows the definition of the maximum number of active decoders, and I expect to get some insight on setting those values to help moderate the load.  Currently, allowing 500-600 decoders seems workable.

During 160m contests, the 160m skimmer will likely switch to a wider bandwidth card, at least 96Khz.  The fall contest season will allow more testing to determine if the 192khz skimmers will need to be narrowed during contests or throttled by limiting the max number of decoders – maybe both.  Also, the nature of any given contest may also make temporary changes to the line-up appropriate.  But that’s the basic setup.

Using the spots

The Reverse Beacon Network Blog has info on connecting to the RBN telnet server.  Highly recommend taking their advice on filtering!  The RBN server uses DXSpider documented on DXSpider Wiki.

Softrock Lite II

After some large amount of initial interest, I quit paying attention to the Softrock.  As the years trickled by, the Softrock project kept moving.  Lots of projects, mods, versions, and changes.

Here in the present, I had an older Softrock v6.2 sitting on the ‘ToooDooo” batting lineup since around December.  It had originally been built as a 9Mhz IF kit, to be used as a panadapter.  It was a gift from W3DQ.  When I saw the NorCal group had a run of kits available, I ordered a pair.  Wish it had been three….

But….it seemed like a good point in time too examine the IF kit, with an eye on re-working it for one of the bands of interest.  As it was built, it required only four changes to put it on 40m.  The Softrock Lite II kits come with components for building any band from 160m-20m, so the needed crystal was available from one of the kits.  The mods took only a few minutes.  That got done first.

On a roll, it was time to sift through one of the kits to see what the build was going to take. One thing leads to another….build it!  The smell of solder smoke was soon wafting about.  The “most difficult” surface mount parts were the first on the plate.  As it turns out, these are not the smallest of surface mount parts.  An ordinary 15w RatShack iron with a fine tip was sufficient for the task.  The difficult part turned ot to be simply identifying the other parts.  The numbers on the capacitors were difficult to read, and the color bands on the resistors all look like brown.

Lots of light and magnification?  Better, but still some confusion.  Most of the issue is progressive myopia, but I had not realized that color-blindness might also be progressive.  Not so Fast!  In order to get a second opinion, NumberTwoSon took a second look.  Even with his 17 year old eyes and 20/13 vision, he also had difficulty.   So, after rolling out the ToolTimeTim’s XL 2550Super’scope, the parts were sorted.

After sorting, building was trivial.

Ran first skimmer test on both units on night of May 10th.  Its interesting to see the spots a local skimmer finds versus thoses several hundred miles away.  A whole project in itself….