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End Fed Half Wave Experiment – Part 4

EFHW Experiment Part 1
EFHW Experiment Part 2
EFHW Experiment Part 3
UPDATED 2021-04-08

On a whim I decided to wind a transformer using two FT-140-43 cores.  I went with the same 2:1 ratio as the first transformer.  SWR testing using this second version showed that all transformers are not equal.  The SWR readings taken using the new transformer with the antenna wire as it was trimmed with the larger transformer did not have similar results.  It turns out the new transformer would require re-trimming the wire lengths to bring the 40m band into the same SWR curve.  Since I had the 80m coil&tail attached using .250 quick connects, it was easy to add wire.  Doing that allowed bringing 15m and 10m to good matches.  20m also found a sweet spot but only on the 6.5:1 tap.

New Version?

Without re-tuning the wire no good matches were obtainable on 40m.  Instead of making any permanent changes to the working 40m loaded antenna/transformer combo I am making a different wire based on a 30m wire length.  It will include a quick connect to allow the antenna to cover 20m by detaching the section of wire beyond the ~34 foot 20m wire length.  That will provide an antenna capable of 30m/20m/15m/10m.  This may become a simple way to add a permanent 30m antenna in the w4kaz antenna farm. 

Maybe I get ambitious and add a second link to include 17m, just for grins.  Does an easy tune 30m/20m/17m/15m/10m antenna sound good?  One could just as easily add quick links for each band.  Using the antenna as a full wave on the 2nd harmonic provides radiation patterns that might be more useful, but either choice provides a method to cover each band.  The initial lost opportunity cost is the need for the transformer.  For quick and dirty field construction a simple wire dipole or vertical would still be easiest – but mostly monoband, as fan dipoles can sometimes be difficult to tune.

Future ideas

Thinking about trying a different twist on using the FT-140-43 size cores by pasting three of them together for a transformer, and trying the transformers out with 3:1 turns ratios.  I have enough of the FT-140 size on hand to do this but used my last two of the FT-240 size.  My first test antenna seems to be a success, so it just need to be put on the air some to get an idea of how it fits in.  It is always good to have the option available.

20m/30m Constructed and Trimmed

Using the transformer built from a pair of ft-140-43 toroids sandwiched together, an antenna for 20m/10m is now built.  It includes a simple wire tail that can be attached as a link to extend for use on 30m/15m.  It should be a simple matter to prepare a wire for adding 40m use.  With the 40m wire 15m should be use-able with two configurations, either 30m or 40m.  If 15m is open that might add some interesting changes in radiation pattern when switching from a full wave to a 3/2 wave.  May as well add a jumper for 17m too(later-much later). 

TAP Cheat Sheet, 30m/20m/15m/10m

Taps 20m/30m linked EFHW (jumper to attach 30m/15m)

BAND TAP Best SWR  SWR range to expect
30m(j) 8.5:1 10.25(1.6:1) Entire band, 1.6:1 with wire drooping
30m(j) 8.5:1 10.1(1.3) Entire band, 1.3:1 with wire pulled taut
30m(j) 7.5:1 10.3(1.4) Alternate tap,1.4:1 with wire pulled taut
20m 8.5:1 14.025(1.4) Entire band favoring CW section
15m(j) 4.5:1 21.010(1.4) Entire band under 2:1(wire drooping)
15m(j) 4.5:1 21.010(1.0) Entire band flat with wire pulled taut
10m 6.5:1 28.000-29.25 28.00 thru 28.800 under 2:1(swr 1.2:1 on CW)
10m 8.5:1 28.00(1.4) 28.00 thru 28.75 < 2:1(swr 1.4:1 on CW)

*(j)= jumper attached for 30m/15m

Other ideas and observations

LINKING – Thinking about creating a “linked EFHW” out of curiosity.  If 10m were reliably open I would pursue the idea seriously.  The different patterns of radiation going from 1/2, fullwave, 3/2, 2xfullwave would be fun to work through.

Using taps on the transformer —-After testing with two different tapped transformers, using the taps makes it simple to obtain a good match.  More on the air testing is needed, but at this point trying to obtain good matches using a single transformer ratio seems futile.

Different cores = different wire  With both transformers I found that the length of wire needed was unique to each configuration.  The lesson is to build the transformer and tune the wire to the actual transformer being used.  The ft-240-43 transformer pair would not match the wire cut for the ft-140-43 transformer pair.  Need to build more transformers to see if a pair of identically built transformers(size and number) will both match the same wire.  Best guess is that will work.  But don’t expect transformers with different toroid sizes, or different number of toroids sandwiched together to all work with a set size of wire.  Be prepared to test or trim.

Wire droop not ideal   Pulling the wire as taut as possible not only shifted resonance slightly, but improved the SWR match on every band.



It Is Alive. W4KAZ Skimmer Back In Service

After a couple of months of dead air time, the W4KAZ skimmer returned to service on 2021/4/1.  The basement area housing the skimmer station required overhauling in order to replace the leaky old water heater.  After repairs, the project to return the skimmer system to service was delayed by other issues.  Foremost issue was lack of ambition to tackle the organization project spawned by tearing apart the shack.  Oiy.


Part of the re-org was to add in a new shelf rack, and put all of the shelving on casters.  The area is also used for general storage. Being able to wheel all of the shelving out into the garage will make access easier all-around.

The skimmer SDR was also found to have a dying cooling fan, so there was some delay acquiring the replacement.  Also some of the old input feedline system was replaced. 

This was also a good chance to downsize the computer system used to run the SDR.   A “new” micro form factor system was acquired, a refurbished Dell i5 in a teeny 8x10x2 box.  That’s a huge improvement over the desktop tower sized system previously in use, and the space savings  will be appreciated.  The new system has a slew of USB ports, but little else.  The desktop box will be stored and can be swapped in if needed as a backup.

Skimming System

The skimmer as currently being run still consists of the Red Pitaya used as an SDR, with the 160m inv-L over FCP antenna system.  The new Dell system is a Dell Optiplex 3020 micro form factor equipped with a 4th generation i5-4590T processor, 12gb ram, on a win8.1 pro OS platform.   This processor is 4-core/4-threads and runs somewhat faster than the previous platform.  On a lightly used band the skimmer is humming along using about 5% of capacity.  On a heavy contest this will probably max out at about 20% CPU.  Good enough for CW skimming, but probably not too hot for RTTY.  Other SDR software has not yet been migrated to the new CPU.

The main reason for the CPU  parallel “upgrade” was the micro form factor of the acquired system.  Having the computer be the size of a SMALL cigar box is a huge convenience for a cramped shack location.  It also sports a solid state drive, which has been the single most likely point of failure for previous CPU’s installed in a somewhat humid non-climate-controlled location.  But the size is most important.  (That’s what SHE said….)

Whazzup Wit All Dat?

So now that the system is up and running, more shack clean-up and resurrection is coming.  Organizing mediocrity out of the mayhem is the plan.  But can the plan survive contact with the mayhem?  heh – probably not.


  • Unexpected crash of CW skimmer ~0800Z 20210406
  • Unexected crash of skimmrsrv `0800z 20210407 (???)


End Fed Half Wave Experiment – Part 3

 End Fed Half Wave Experiment – Part 1End Fed Half Wave Experiment – Part 2 
The “Tune-up-ening”

The first attempts to tune up the real antenna did not go well.  I found from the trap dipole project attaching the wires beyond the trap was needed to get the higher bands tuned properly.  This worked against me here.  With the high impedance on the end fed I was better off tuning the 40m antenna segment without the coil and 80m tail attached.  Unfortunately, I wasted a lot of time experimenting/snipping before I decided to detach the inner antenna from the coil.

Once taking that step, I found I had probably trimmed too much off.  Tuning for 40m without the coil proved simple enough.  After that the other bands were easier too.  I was also able to re-attach the coil and 80m tail, and find tap settings and tail lengths to allow operation with good matches on 80m and 20m.  With the 20m tails attached 40m operation on the upper edge is possible.  With tails 40m SWR was a bit high on CW segment, although possible with a tuner.  10m seemed unaffected by the coil and tail.  15m provided low SWR only with the coil and tail detached, but SWR of 2:1 up to 3:1 with 80m coil and tail.

Antenna Stuff

The coil as wound measured out to 77.5uh, resonant at 3.85Mhz with 22pf of capacitance.  It is ~63 close-wound turns on a mystery plastic coil form of ~2 inch diameter, wound with 16ga solid insulated wire.  The final wire length in the 40m section is xxft(  The 80m tail is xxft(zz.zzm) in length and the portion for ssb(75m) is xxft(  To make the choices more flexible for POTA or other portable uses, I have a series of jumpers in several key places.

The first of the jumpers is installed near the feed point.  The antenna was trimmed at the feed point for ease-of-access reasons.  Since I ultimately decided I had over-trimmed before detaching the 80m coil&tail, it seemed easier to insert a jumper there.  This will also allow  having an easy method of re-tuning on the fly if it seems necessary in different deployment configurations. 

The next set of jumpers comes at the junction between the 40m antenna and the 80m coil&tail.  There are two more jumpers on the 80m tail itself, positioned to allow choosing between a mid-band SSB section centered at around 3.8Mhz or a CW section centered at about 3.575Mhz.  The lower section will allow use on both CW and in the lower 3.600Mhz SSB segment.

The antenna can be deployed with or without the 80m coil&tail.  I found that the matches in the 40m and higher segments are much with the tail removed/detached. 

All Taps NOT Ideal

As described in previous posts, the transformer brewed up for this project has multiple taps, not the fashionable 49:1 single solution.  It turned out having the choices of taps gave better SWR matches than any single tap.  In fact, only one of the taps worked well for the 15m/10m bands, the lowest impedance 4.5:1 tap.  For 40m and 80m, the 8.5:1 tap proved the best choice.  20m SWR curves favored the 7.5:1 tap, although the 6.5:1 tap proved useful with the 80m cw configuration.  When configured for 80m CW the 6.5:1 tap provides better than a 1.5:1 SWR across all of 20m.  

TAP Cheat Sheet

Taps 40m and up, bare wire,  NO 80m coil&tail

BAND TAP Best SWR  SWR range to expect
40m 8.5:1 7.15Mhz@ 1.2:1swr Entire band, 1.8:1 up to 2:1  centered on 7.15
20m_1 7.5:1 14.15 Entire band good favoring CW section
20m_2 6.5:1 14.275 Entire band good favoring SSB section
15m 4.5:1 21.3 Entire band under 2:1
10m 4.5:1 28.2 28.00 thru 28.800 under 2:1

Taps with 80m SSB section

BAND TAP Best SWR  SWR range to expect
80m 8.5:1 3.825Mhz@ 1.2:1swr 3.7 thru 3.9Mhz , ?tuner at edges?
40m 8.5:1 7.3Mhz, 2:1 7.2 to 7.3Mhz, tuner needed
20m_1 7.5:1 14.1 14 to 14.35 under 2:1
20m_2 6.5:1 14.225 Best choice, entire band good favoring SSB section
15m 4.5:1 21.05 @ 2:1 21.0 to 21.35, SWR over 3:1 above 21.35
10m 4.5:1 28.2 28.00 thru 28.800 under 2:1

Taps with 80m CW section

BAND TAP Best SWR SWR Range to expect
80m 8.5:1   3.525 to 3.75 <2:1
40m 8.5:1   7.2 up under 2:1
40m_2 8.5:1 1.3:1@7.3 40m jumper ins, useable on cw
20m 7.5:1 14.0 14 up to 14.275 < 2:1
20m_2 6.5:1 1.5 40m jumper ins, all band
15m 4.5:1 21.245 @ 2:1  
10m 4.5:1 28.1 28.0 to 28.7 < 2:1

 Test Deploy

All tuning was done with the antenna strung up using two Jackite telescoping poles as supports.  A 31 footer is used for the vertical portion, and the feed point is at about 4 feet off the ground.  The horizontal runs out about 40 feet to the second mast, a 28 footer.  The second mast supports the weight of the wire and the loading coil, probably the heaviest portion of the antenna if used.   I ran a 7 foot long counterpoise wire, and a six foot long coax jumper angles down to ground level from the feedpoint to the first choke, eight turns of coax wound on a ft-240-31 ferrite(another 6 foot jumper for that). 

The short section of straight coax jumper will likely be a second counterpoise for the antenna.  The first choke plugs into a second choke constructed of 10 turns of a 14ga wire pair wound on a second ft-240-31 ferrite and mounted in an enclosure.  From the chokes a fifty foot long lmr 240 feedline is attached, and all testing was done at the end of the feedline.  That simulates the most likely portable deployment.

FWIW, SWR results did not change when the fifty foot section of coax was removed.  Will be a curiosity to find out if altering the shape of the deployed antenna changes SWR. 

Additionally, I toyed around with inserting pieces of wire as jumpers to lengthen the 40m section.  These jumpers are inserted close to the feedpoint on the vertical section.  I inserted them to try to improve 40m SWR when the 80m loading coil&tail were linked in, as 40m SWR was too high for CW operation without tuner with tail attached.  This turns out to be a useful compromise solution.  It will allow using either 40m or 80m without adjustment, and 20m/10m can be used with tap change.  15m is the odd man out in this configuration, but could maybe be used with a tuner.

Stress Test Awaiting

Following are some key down stress tests to gauge how well the RBN can hear and to see if there will be any heating in the ferrite cores when at 100w levels.  Perhaps a POTA operation would be better.   No “SWR creep” was found when I did 60 second key down tests on the dead bands at 1800Z.  Both extreme ends were tested at 60 second key down, 80m and 10m.  Then a series of test transmissions that consisted of three repetitions of the test string: “V V V TEST W4KAZ W4KAZ W4KAZ V V V”. 

RBN Spots from 40m EFHW at 1800Z on 2021-03-11

RBN Spots from 40m EFHW at 1800Z on 2021-03-11

The RBN spots from 1800Z show decent results, not too dissimilar from what I am accustomed to seeing from the permanent wire dipole antennas.  The test antenna is deployed in a much-less-than-ideal location.  The test EFHW horizontal section is running parallel to the horizontal tail of my permanent 160m inverted L, and it is only about 20 feet away.  So I’m not too concerned with the locations of spotting stations.  I expect to try a POTA activation soon, which will maybe be more interesting.

RBN Spots from 40m EFHW at 2345Z on 2021-03-11

RBN Spots from 40m EFHW at 2345Z on 2021-03-11 

Likewise the above set of RBN tests run later, near to and just after local sunset  @2330Z, show fairly typical results. 

80m RBN Spots from 40m EFHW at 2345Z on 2021-03-11

RBN Spots from 40m EFHW at 2345Z on 2021-03-11   *

I expected 80m results to be poor but they were maybe better than my low expectations.  Nothing great, but certainly good enough for an easy to deploy portable antenna.  Better to have some 80m capability than zero capability. 

Next Question:  What happens if I use a different transformer?

see part4   EFHW Experiment – Part 4


End Fed Half Wave Experiment – Part 2

End Fed Half Wave Experiment – Part 1

The Antenna as constructed before tuning.

The Initial cut of the antenna and the completed feedpoint enclosure.

Having looked at many of the articles available on the WWW, I wound up favoring the design well documented by G0KYA, Steve Nichols.  When modeling the antenna I started out using a coil value of 67.5uh.  Then I ran the model using the appropriate value of impedance at each given frequency.  The coil as shown above is about 63 turns of 16ga solid insulated wire on a plastic coil form of just over 2 inch diameter.  That was right at the limit of the amount of wire I could lay down on the available form.  The completed coil resonated with a handy 22pf capacitor at 3.855Mhz, which calculates out to an actual inductance of close to 77.5uh.  With the coil value known, I re-ran the model to tweak the initial guestimates with actual values of XL for the model’s load.    Then cut wire for the 40m leg as well as the 80m tag end.  BEFORE tuning wire lengths are long, 71 feet and 15 feet(about 21.6 meters and 4.5 meters). 

77.5uh inductor, ~63 turns of 16ga insulated wire wound on a 2 inch plastic form.

77.5uh inductor, ~63 turns of 16ga insulated wire wound on a 2 inch plastic form.

The coil form used seems better for RF than PVC pipe, and is a one-off that happened to be in the parts bin, original source un-remembered.  I gave it a run in the microwave, and it showed little heating.  It’s other quality is it is 1/16 wall thickness, so most of the weight is the winding.  The coil is just over 6 inches in length.  The second choice for a coil form was going to be fabricated with a few layers of fiberglass and epoxy laid up on some sort of removable former.    The measured  value is only about 2/3 of the specified 110uh from the G0KYA document(original designer-????), but it comes down to the trade-off question again.  This 77.5uh coil should provide 3400 ohms XL at 7Mhz.  Hopefully that will be sufficient to isolate the tail.  Actual testing should be interesting.  

Step-up transformer in terminal box

This is the completed transformer installed in terminal box. 

It has two ft-240-43 toroids together with a two layer cover of teflon tape to make it more slippery for winding.  It is wound with a 2 turn primary and 20 turn secondary.  Taps are placed on turns #9, #11, #13, #15, #17 and #20.  For the capacitor I used three TDK 330pf 3kV 5% ceramic caps in series.  I went that route simply because I had more of that value cap handy than either 220pf or an actual 100pf.  There are no crossover turns – that seemed counter productive for multi taps.  I would have wound the secondary turns more tightly but needed to loosen the turns up some just to fit it in the box and attach the taps.

Closed box showing tap wire.

Closed box showing tap wire on top and counterpoise on lower right.

The feed point box is an ordinary Carlon 4x4x2 terminal box. Feed thru studs are all 1/4 stainless hex bolts.  I expect to attach the antenna to the 10:1 tap(located top center) and use a jumper to short down to the lower impedance taps as needed for best match rather than have the high impedance taps float.  The stud on the bottom right is the ground/counterpoise attach point.  If nothing else, the multiple taps provide handy places for extra hardware to replace wing nuts fumbled into the sand.


End Fed Half Wave-Experiment du’Jour : Part 1

Just about the only wire antenna in common use I had not experimented with is the End Fed Half Wave(EFHW).  So WTF.  May as well give it a go. 

2021 planning had me booking a stay on Cape Lookout National Seashore(CALO) for the end of June rather than the end of July.  I tried unsuccessfully to make the reservation in 2020, but could not.  Success for 2021.  So my FD 2021 will be from Cape Lookout. 

Given the layout of the cabin I prefer at CALO campground I decided to tinker with the EFHW as a possible solution for allowing multi bands with least effort.  The “least effort” factor is growing more important with each passing year.  But the EFHW itself piqued my curiosity as well, and it seems it my provide a solution to the “least effort” vs. “works well”  conundrum.   

So, here lies the experimental portion.   NUMEROUS versions of the classic 49:1 transformer are documented on WWW and in the now ubiquitous scrootoobe video.  Version guidelines exist for either 80m or 40m multi band versions.  Quite typically I chucked some of that and decided to re-invent the wheel – because what is the fun of experimenting if you are just going to follow the cookbook?  Wellll…..not quite that either.

Where to start

Looking for a 40m size and decided to try modeling out a 40m EFHW fit out with a loading coil and tag end to allow 80m or 75m.  The idea is to have an antenna that the floppy fiberglass masts would be able to support easily in the normal 20 knot winds typical on CALO.  The feed point will be at about 4 feet high as I expect to deploy it.  The mast will support the wire vertically to about 30 or 35 feet, depending on which mast is used(Spiderpole or Jackite or K4TMC).  The rest of the wire will be stretched out horizontally with the coil supported by a second mast.

This deployment will have 80m or 40m basically functioning as half wave in an L configuration.  20m will probably have a larger horizontal component than the vertical section, and if it works there 15m/10m will provide who-knows-what.  i.e., “PERFECT!” – for values of “perfect” where whatever happens is perfect.

Modeling showed best result with the wire between the feed and the load at a longer length than I expected at 70+ feet.   Using insulated wire for actual construction, I expect that to be shorter in real life.   I plan to test with a .05wave counterpoise, and use a 6 foot coax jumper at feedpoint.  The real feedpoint will be a current choke at the end of the coax jumper.  It seems likely the shield of the coax jumper will act as another counterpoise.  Maybe another choke at the radio end.

Transformer Conundrums

The item I had more questions about was the step-up transformer.  A lot of conventional wizzdom surrounding the 7:1 turns ratio versions.  I chose to give myself more options and provide multiple taps, and use the larger ft-240-43 size toroid to allow 100w.   Hopefully this will give less core heating.  Then two turns vs. three turns on the primary was considered.  Initially I favored using three turns on the primary.   I expect 80m to be more useful than 10m going forward.   Physical reality – two turns seemed more practical with the 16ga wire I used. 

The other wildcard I threw into the construction detail was about sticking to the “norms” of putting the taps on nice-and-easy turns ratios.     While waiting on my toroid order to arrive, I stumbled across N8NK’s videos.  I found the playlist on EFHW and UNUN’s interesting viewing.  The main idea I fortified was to tap the toroid in several different places.  That would have been more versatile using 3:1 windings.  Even with 2:1 windings I still liked the range of impedances available by placing “irregular” taps, i.e not on the even multiple windings.  With 3:1 I had expected to place the taps on every 4th turn.  With 2:1 windings I thought maybe every 3rd turn but with 6 taps every 2nd turn was “good enough”.

The only genuine benefit that conventional “even numbered taps” provides is ease in predicting  the transformation factor(i.e., 7:1 turns gives 49:1 step up).  To switch the flip I decided to tap the transformer at 4.5:1, 5.5:1, 6.5:1, 7.5:1, 8.5:1 and 10:1.  (FWIW, that should be stepping 50 ohms up to about: 1013, 1513, 2113, 2813, 3613 or 5000 ohms.)  Just for grins, it is easy to test with the ordinary antenna analyzers and a resistor test box(or well stocked junk box).  

If it is important to your thought process, measure the damn unun to be sure.  To simplify, the secondary is actually tapped on turn #9,#11,#13,#15,#17, and #20.  With 2 primary turns and just 5 taps on secondary, tapping every 3 turns at #8, #11, #14, #17 and #20 should allow matching impedances of 750, 1513, 2450, 3616, or 5000 ohms.  That includes the obligatory 7:1 ratio for purists.  I only chose to use the odd taps as an appeasement to my increasingly contrarian curmudgeonly nature.

Precision Optional

 I figure to just use the antenna with the tap that provides the best SWR. I can then produce a cheat sheet for each band.  I would like to have a best choice for each band.  If it turns out the same tap works best for all bands – so much the better.   If they are NOT all the same tap I also want compromise choices that will allow one tap to allow operation on several bands without moving the tap.  Another tradeoff option – sometimes moving the tap will be easy, sometimes inconvenient.

Functionally we can call it good if some doing RBN compares to permanent dipoles prove it to perform acceptably.  The truth is I don’t care what the actual step up ratio might be.  I just want the antenna to function, and experimentally finding a good tap is “good enough”.  We are hoping that the taps provide enough options for multi-band operation with a good match.  It will be nice if a single tap is good on all bands.  It is not a deal breaker if changing bands requires moving a tap that can be reached at ground level.

Engineer the possible. Mind the trade-offs, because “best” can be the enemy of “good enough”.

Related Links(updated 2021/03)

End Fed Half Wave Experiment – Part 2
End Fed Half Wave Experiment – Part 3 Multi-Band End Fed Antenna

G0KYA, Steve Nichols.


Skimmer Outage – Temporary

The W4KAZ CW skimmer station reporting to the RBN is down, due to work in the basement in the area of the station.  The station is going to be down for at least another two weeks, it needed to be moved to allow for replacing the water heater.  Yes, hot water is more important to XYL than CW spots.  Go figure.

Station replacement may also be delayed by migrating the skimmer software to a newer computer running Windoze 10.  That may turn into a temporary situation, just a test run to verify the backup system is good-to-go.

Been a while since any activity of any sort, website updates included.  Been paying attention to other things, have not been active with radio much.  Maybe that changes – gonna take it as it comes.

Outage began 2020-12-29.  Anticipated end of outage 2021-01-15 at earliest.

Ghosts In The Machine-Red Pitaya CW Skimmer-Debugging BCB Interference

Many thanks to Guy, K2AV for tips, suggestions, and testing assistance to help resolve the following problems.

An unanticipated problem with fixing broken antenna systems is that it begins to work better.  Well – “duh”, right? 

After making repairs to the antenna switch and the 160m inv-L over the past couple of months,  the antenna systems are nearing the end of a minor overhaul.  Since about May, the CW Skimmer has been using a K9AY as the main RX antenna.  For the period of 2 years prior it has been using the 160m Inv L as the RX antenna, until problems cropped up in May 2019[a broken connection on the feedline at the switch box.]

After  correcting the broken connection and re-assembling the skimmer station, K2AV reported having spurs on harmonics when testing on 160m.  Upon a closer I found several other stations for which the SDR was also generating harmonic spurs on higher bands for signals of 40-43 DB SNR into the skimmer station SDR.   This was resulting in bad spots to the RBN, and as is sometimes noted “results may be unpredictable”.

All of the spurs appear to be caused by overload mixing from nearby BCB stations on 680, 850, 1360, and 1510.  Some of the problem was previously handled with the BCB filter constructed with notches tuned for 680 and 850.  Upon re-assemble, the BCB QRM was still sneaking in.

The bulk of the issue has been resolved by bonding the BCB filter enclosure directly to the SDR enclosure.  In a belt and suspenders approach, I constructed a second filter that has a higher cut off frequency.   It appears to function as designed, notching 1360, 850, and 680.  It adds about 20db attenuation at 1510, and rolls off at about 1650 while applying less than a db attenuation at 1800 to allow 160m into the system.  This second filter was placed at the input to the skimmer station preamplifier in the hopes of reducing the interference further.

Currently the result appears promising, as the signal SNR numbers from the skimmer seem to have benefited from the dual filtering scheme.  Perhaps the stations at 1360 and 1510 were causing more overload than I originally thought.  While neither is as strong as 680 or 850 into this QTH, both are over S-9 on the base radio using the same antenna for RX.

So now the curiosity about BCB filtering has been tweaked, and experiments modeling and building alternate filter choices are continuing.

If you are seeing bad RBN spots originating from the W4KAZ skimmer, please email the info so I can attempt to remediate the problems.


Retroactive Filter Tuning with 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 2

see: Experimenting With Trap Dipoles – Part 1

Those coil and cap experiments described in part #1 eventually led me to the ‘best’ compromise solution for my situation. In the end I chose to build traps that were resonant below the higher band. I also chose to use cap values on the smaller end of the capacitance value range.

Sadly the Panasonic capacitors are no longer available. Possible TDK replacement are being tested. These TDK caps are physically smaller, and only 2Kv rated. I intend to use these in series/parallel groups once I determine the best values to stockpile. (TIP#x: leaning to several caps in series to extend the voltage rating) (TIP#x: Also decided to mount the caps in slivers of PCB, and use generous solder on the pads as well as not clipping the leads short, all in a hope to have that function as heat sinking).

Antenna Experiments:   In testing, these capacitors worked well on xmit for the first 20m/40m trap dipole, but I ran into problems with a 10m/15m trap. Using 33pf with an inductor of about .92uH I had failure of the capacitors while testing the antenna. 

As an alternative on 10m I used a bit larger inductance and a piece of rg8x coax as the tuning capacitor(low value approx 8-9pf). No final verdict on this solution yet, but the antenna functioned for light usage in 2019 WPX cw contest. Antennas will be used at 100w levels, so these cap variations should also prove suitable for this project. If weight is not an issue, gimmick caps from coax are viable choices, though I’d not use them with the traps resonant close to the operating frequency.

Alternatively the 20m/40m dipole has now been used in two different contests with success. A second 20m/40m dipole was constructed, using smaller coils and increased capacitance. This second experiment was less stable than the first, with the 20m SWR increasing slowly. Presumably the capacitors were heating and having the same problems as the 15m/10m model.

What Did NOT Work:  In the end, the experiments using larger values of capacitance proved to be poor choices for practical reasons. In the case of 10m, the caps failed outright. In the 20m case, the caps showed instability in the form of rising SWR, likely because they were heating. A revised 20m dipole with traps using a larger coil and smaller values of capacitance proved to be stable. Tip#1….Marginal caps can maybe stand the abuse in a trap if the coil is larger.

At this time I also chose to move the resonant frequency of traps a bit farther away(lower) from the operating frequency on new construction. This resulted in :

  1. the dipole legs being shortened,
  2. the impedance on 20m seemed more stable,
  3. The bandwidth on the lower band(40m) was decreased
  4. Precision in component selection becomes less critical

That set of compromises suit me, as the capacitance values are readily available, and the overall antenna length is reduced slightly, without any serious performance compromises as compared to an ordinary single band inverted V.  The antennas will be tuned to favor the CW segments, and if needed I will use the radio internal antenna tuner(at the home station) to find a match for SSB if required for 40m.  The tuner would likely only be required at the upper band edge if at all.

What worked well enough – Final Versions Constructed:  RBN testing of the trap dipole versus a normal 20m dipole showed enough uniformity in results that I am not concerned with trap losses.  The end result was a set of dipoles both for permanent use at home and several variations made as light weight as possible for portable operating.  The final 20m/40m dipole for the home station was constructed with the traps resonant at 12.650Mhz.  A coil with 14 turns close wound on the 1.5″ form was used with a capacitor constructed of several ceramics in series giving a value of 23pf. 

A 40m/80m antenna was also constructed.  These traps used a coil of 12 turns on the 1.5″ form and capacitors in series parallel for a value of 100pf.  Resonant frequency was 6.65Mhz.  For future construction this design will likely be modified to move the trap resonant frequency down to the 6Mhz range by increasing the number of turns on the coils while using the same 100pf capacitance value.

A practical benefit of having the resonant frequency away from the operating frequency is that component selection becomes less critical.  By selecting a resonant point below the band rather than on or near the band, it is not required to have values to resonate at an exact frequency.  Instead, it is only required that each trap resonates at close to the same frequency.   This is easier to tweak by adjusting the coil, and it becomes fairly simple to have traps that can be adjusted to within 100hz of one another.  The caveat here is that the dipole legs are different based on the trap frequency – but these need to be trimmed to length anyway.  [It is possible to easily replicate antennas if the traps are easy to replicate. ]  So for a 20m/40m dipole, it makes little difference whether the trap is resonant at 12.5Mhz, 12Mhz, or 13Mhz, so long as the antenna legs are trimmed properly for each.

My experiments show that having the capacitive reactance at the higher operating frequency be in the order of 400-750 ohms seems to reduce the current flow through the caps.   Probably this is enough to allow otherwise marginal caps to survive without heating and/or exhibiting SWR variations on xmit.  The voltage rating needs to be sufficient, and this can be aided by using several caps in series.  The caps in use here are all 2kv or 3kv, and used in series to increase voltage ratings.  This is sufficient for 100w levels, but unlikely to survive at 1kw or 1.5kw.   I have also mounted them on bits of circuit board to keep the leads short and provide a tiny bit of additional heat sinking.  Again, good enough for 100w, but QRO – probably not.