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Here’s an email from my good friend and cohort in the Central Florida QRP Group. Jim wanted to share his latest mods for the Rockmites and some thoughts on the NEScaf filter. I think you will enjoy his notes too …

Kelly,

I thought I would bring you up to date on my latest project. I just finished a Small Wonder 20+ and used it on the Flight of the Bumblebees. I found my earbuds a little weak on audio output and the IF bandwidth of the SW20+ a bit wide for my liking. I read the specifications on the NEScaf filter offered by the New England QRP group and decided that that would be the best solution to my problem and be usable on other radios as well. I ordered, built and installed the NEScaf board in a TenTec TG-24 enclosure. I tried it on the SW20 and it sounded like a viable solution.

The Rockmite and NEScaf filter in Ten Tec Enclosures

I also have a 40 Meter Rockmite in a TG-24 enclosure and decided I would see how well the RM/NEScaf combination sounded. The Rockmite, of course, uses a DC receiver and the bandwidth is determined by the upper response of your ears… My RM sounds like about 40 khz wide, hears the whole band for me as my response is in the upper tens of kHz. Yes, I know, at my age 69 it shouldn’t be but is. Been tested. At any rate suddenly the Rockmite bandwidth is manageable. I had some audio artifacts, whistles and the RM sidetone would drive the NEScaf into cutoff which only a power cycle would clear. I googled the problem and Charlie KE2SP advised lowering the NEScaf input Z with a 10 to 47 ohm input load. I installed a 27 ohm resistor on the input connector and suddenly all artifacts, whistles and sidetone problems disappeared. WOW, the RM is really sounding great! Except the RX/TX was very low. I measured it at 500 cycles and the NEScaf would not tune down that low…

Closer Look at the Finished Rockmite and NEScaf

After considering several approaches to the problem and considering that the RM crystals don’t oscillate on exactly the QRP frequencies, I settled on completely revamping the RX/TX method used in the RM. Using the RM40 as a test bed, I removed D5, D6, R9 and R10. I purchased 2 Murata trimmers( TZ03 Series) from Skycraft, our local Surplus emporium, and installed them in the holes for D6 and D5, R9 combination of holes. I had to cut a small run on the right side (antenna connector side) to isolate that pad from Vcc and jumper to two trimmers together… I also had to drill out the pads to accept the trimmer leads. The Fet Q2 does a great job in switching to second trimmer in and out for the offset. The alignment was not difficult but I recommend using a freq counter connected thru a times 10 scope probe to the physical top of R5 (base of Q5). Don’t have to key the Tx to see the freq… I set the trimmer in the D6 position for the higher freq (7.030750 Mhz) and the other trimmer for the lower frequency (7.030000 Mhz. The trimmers I use are available at Digikey. I used the Red colored model (4.2 to 20pf, N750) but the Blue colored (2.7 to 10pf, NPO) might have been a better choice. These guys are Digikey p/n 490-1971-ND and are $0.43 each… I also changed the RM40 volume control from an audio control (1 Megohm) to a RF front end attenuator control (1.5 kohm) and there is a noticeable improvement in the overload and broadcaster breakthru problem. I strongly recommend these changes. If you can build the RM you can certainly modify it… If you break it, build another… I plan to make the same modification to my RM80. (CLICK THE THUMBNAILS for larger view)

Rockmite Mode Closeup 1

Rockmite another closeup

The Inner Workings of the NEScaf and Rockmite

How did it work? Well, the RM/Nescaf stack is now a real radio not just a toy. I worked WD8MHT Raul in Waynesville, NC one morning this week and we had a great conversation. He was 569 to me and I was 439 to him. His TS570 was working hard but copied me no problem. The amazing thing for me was that there was a really strong signal at 700 cycles and Raul was about 200 cycles higher. I tuned the Nescaf center freq on Raul and sharpened the bandpass and turned up the volume and he was armchair copy the entire QSO… WOW, not a struggle… I have since used the NEScaf on my SW20+ and yes, it works great…

I have attached a couple of pictures of the stack and the innards of the RM for reference. The switch on the front is for a future expansion.

72

Jim

K4AHO

As a confessed web junkie (how else can you do a site like this) and a ham…… I find many sites that show circuits from decades gone by that are every bit as relevant today as they were when they were originally created.

Then…… there comes a time when you can look at a mere snippet of someones overall body of work and get an interesting glimpse into one of those rare “beautiful minds”.  Such is the case with Sir Douglas Hall.

He started publishing articles nearly two decades after he started tinkering with electronics.  This site has copies of his published works from the early forties to…… here’s the crazy thing….. the new millennium!!!

I can’t even begin to go over the normal lists like I do for many articles here, but there are tons of articles from various methods of volume control, multiple crystal receivers, portable radio kits (transmitters and receivers), tons of theory….. the list goes on.

I think I need to cache a copy of this page for my next trip so I have plenty of neat stuff to read while traveling, truly neat stuff.

To find these ingenious circuits, go to:

http://www.spontaflex.free-online.co.uk/

**** Update ****

Thanx to Geoff for commenting and letting me know that there are not only updates, but MORE great circuits on the radioconstructors site at:

http://www.radioconstructors.info/

Now into the third week of my sabbatical, I am surprised how much I want to get out and operate my QRP portable gear. Guess I have deprived myself over the years of being a confirmed workaholic and avoiding time off, vacations, and time for my favorite hobby.

Grrrrr! The Polar Bears Are On The Loose!

This should be a good weekend for QRP portable. The Polar Bear QRP Group will be out for another Polar Bear Summer Picnic Event and Polar Bears from Spain to the West Coast of the US will be out looking for BSO’s. Grrrrr! I am PB #173 and we’re over 200 members now. With the new Twitter and APRS connections to QRPSPOTS.COM which also point to the excellent spotting site of K3UK with a section for FISTS/QRP Ops to spot and sked one another, there are plenty of ways to use technology to help find each other. Add in a few other contests and state QSO parties, and there should be some buzzing going on this weekend. Makes me wonder when the FOBB results will be announced! Buzzzz Buzzzzz

I’ll be out and if all goes well I will be field testing a new C Pole antenna based on the suggestion I received from Neil W0LVZ. I added some switchable capacitance to my BLT+ and have rewound the main toroid to see if I can push the range a bit more with it so will have a delta loop and probably my W3EDP in the air too. I love playing with antennas and am still amazed when the ones I build actually make contacts!

Give a listen for me on the QRP watering holes on 40m, 30m and 20m Saturday morning. I’ll be self spotting on QRPSPOTS and the K3UK sites to make it easier for you to find me. Let me know how my newest antenna is workin’.

72,

Kelly K4UPG

PB #173

XTalset society has been around for awhile.  They are a great starting point for anyone either new to homebrewing or anyone teaching someone who is new to radio (and I’m not necessarily talking about ham radio).  Great crystal set projects that truly amaze.

They are also a great resource for purchasing materials to help you on your crystal set journey.

Here are some of the articles I found useful – free for everyone!

How to Read Schematics
The AM Broadcast Band
A Big Ole Hunk of Galena Crystal
Crystal Set FAQs
How Crystal Sets Work
Variable Cap Lineup & Mounting
Equivalent Series and Parallel Circuits
XS-800 Antenna Measurement Bridge
Q of a Coil With Some Unused Turns
Mag-Coupling Selectivity-Sensitivity pdf file.
A Great Teacher: The Crystal Set pdf file, courtesy QEX (ARRL).
Derivation of Step Attenuator R Formulas pdf file.
The Trap Coil Q Measurement Method Revisited (for high Q coils in particular) pdf file.
About Q pdf file, Discusssion on Q Formulas/Measurement.
The AM RF Signal pdf file, Discusssion on makeup of an AM RF Signal.
Anatomy of Crystal Set Modeling: Equivalent Circuit Substitutions pdf file, with spice app.
XS-OB1 Kit Manual (updated 05-01-08) pdf file.
Different OAT BOX COILS pdf file.
XS-801 Kit Drawings pdf file.
Mechanical Dimensions for Mounting Variable Capacitors pdf file.

To take a look at these articles, head over to their site at:

http://www.midnightscience.com/article-index.html

Or their front page at:

http://www.midnightscience.com

After last week’s plea for antenna ideas for my antenna restricted condo, I received a good number of comments both on and off blog. Thanks for those… great food for thought. That is why I enjoy our QRP community. We don’t hesitate to share ideas with one another.

This morning as I took my daily walk around the lake, I met our condo association president. He graciously is allowing me to sink a pipe that will serve as a base to set up my Jackite poles in the backyard. I will be able to pop a cap on the top and keep the dirt and water out and just slide my fiberglass poles into the mount and be on the air in no time. YAHOO… that is a pretty good solution for now.

C Pole Antenna by W0VLZ

But being the antenna tweak that I am, I continue to look for the best alternatives I can find. I’ve started to put together the parts for the C Pole antenna that Niel W0VLZ pointed out to me. He has a fine site with lots of good QRP info too.

So, do you have another idea that can top that one? Or maybe you have a thought for what kind of vertical setup I might use with the new mount?

I’m standing by for my next project assignment.

72,

Kelly K4UPG

The loop and the off-centre fed dipole at Field Day

This is the second article describing the two antennas that were used by the Carleton University Amateur Radio Club (CUARC) at Field Day 2010.  The first post described an off-centre fed dipole with a 200 ohm feedpoint impedance.  This post describes the second antenna a loop antenna that worked on two bands, 15m and 10m.

I like loop antennas. I have built large loops for amateur bands, as well as small loops for receiving (which are quite different to their bigger cousins) and have found both types perform well.  They are less sensitive to local noise, so often giving good signal to noise ratios. As CUARC was to operate on 10m and 15m at the Field Day site I was considering a loop for one of these bands, however, which one?  10m could be fun if there was an opening, plus the antenna would be smaller than 15m. However, 15m would be more likely to be open.  In searching the internet I came across a potential solution as KT4QW’s website who describes a 17m and 10m hanging loop antenna.  The idea was simple and elegant, two vertical and rectangular loops fed from the same point, similar as to how multi-band dipoles can be fed from the same feedpoint (sometimes called a fan dipole).

I went about calculating the basic design.  A full-wave loop’s total perimeter length is calculated by:

Length (in feet) = 1005 / f,

where f is the intended frequency of operation in MHz.

So for 21.3 MHz that would be a total loop length of 47.2ft and for 28.5MHz it would be 35.3ft.

To attain a 50Ω feedpoint the loop needs to be rectangular, with the longer length vertical.  Studying J. Carr’s Loop Antenna Handbook and KT4QW’s dimensions the sides were close to being 1/3 of the total loop length for a vertical side and 1/6 for the horizontal side.  So this then gave a design as shown below.

Dual band loop antenna for 15m and 10m.

To support the wire I bought two 10ft lengths of PVC piping.  The inner support for the upper part of the 10m antenna was some thin wood.  Both the wood and PVC were drilled with holes prior to Field Day.  The wire was cut from a single 100ft roll and the whole antenna was built by CUARC members at the Field Day site on the day. At the feed point a 1:1 balun was used,  an Elecraft BL2.  The intention was to suspend the antenna from a single central point so the loop(s) could be rotated, using a line tied to one lower corner.  However, we could not get enough height and clearance from tree branches, so we mounted it between two trees and it keep it static with the signal projecting in South-West and North-East directions.

The loops being connected to the balun by Gerry, VA3GLT (left) and Maria, VA3MMI, (right). (Photograph and copyright of Chris Weisner, VA3SM)

After construction the SWR was measured and found to be nicely below 2:1 for both bands (at least the lower part of 10m which is quite a wide band). As mentioned in the earlier post Maria, VA3MMI, brought a portable analyzer to the site on Sunday morning and the trace of the SWR profile is shown below.  You can see the antenna has two nice dips in the SWR within the two bands. In an effort to see the usable bandwidth (below 2:1 SWR) the max SWR on the Y axis has be limited to 2.0:1.

10m and 15m dual band loop antenna SWR plot

To further examine the bandwidth of the two dips in SWR Maria, VA3MMI, set markers at the 2:1 points of the SWR curves.  These are shown in the next two traces (one for each band).  Both show good usable portions of the bands for phone, see the tables on the right of the trace. (Thanks to VA3MMI for the measurements and traces).

Bandwidth for 15m is 467kHz, starting at 21.1MHz

Bandwidth for 10m is 767kHz, starting at 28MHz

So, with the off-centre fed dipole and this dual loop antenna, CUARC constructed two antennas for Field Day that worked on 15m and 10m without the need of an antenna tuner unit.  Performance was good on both antennas and I heard at least one comment that that we had a strong signal (we used 100W).  During Field Day we made over 100 contacts on 15m and 16 contacts on 10m.  Not bad, as for many of the CUARC members this was their first chance at operating on HF.  It was enjoyable for the newly licensed CUARC members to build and then use the two antennas and to find both worked successfully.  For me that was perhaps the highlight of 2010 Field Day.

I still think the tin foil does the magic!

After years of neglect, this analog era ham is willing to admit I only understand a bit about digital electronics. (Yep! Pun intended.) Well into the second week of my sabbatical leave, I have begun to plot out a course of study to get up to speed on the state of the art that I have so long neglected. Should be fun. There seems to be a good bit of open source help online these days and I have my trusty ARRL Handbook, Extra Class study guide, Antenna Handbook and assortment of other aids.

Any suggestions from the veteran techies out there will be greatly appreciated.

I am a learn by doing/hands-on sort of guy and find deep theory puts me into a semi-catatonic state after a few minutes.  For me, it’s a whole lot more fun to take a stab at things and find out if they smoke, and it also makes a lot more sense when I can dink and tweak and see what that actually does. But since that is a bit expensive, perhaps I should learn how to use software like spice to do simulations? Any recommendations for how to learn this software?

Another area of toe dipping will be learning to do antenna modeling. For years I’ve looked at those charts and made little sense out of them. HA… I like to throw wire out and see what happens, but in the summer heat, maybe it is wiser to do a bit of modeling from my air conditioned den?

Thanks WX7S– Your site looks AWESOME for learning this stuff!

I’ll try to share some of the learning curve, but would love to hear from some of you that have gone before. Shortcuts are nice. WX7S your site looks like a great place for me to start! Thanks for the effort it takes to do that.

This weekend (Sat 14 Aug 2010) the Central Florida QRP Group will gather for breakfast at 0730 (Denny’s in Sanford, FL) and off to Sylvan Lake Park in Sanford, FL at 0900 EDT to operate. If you like cw and qrp or are just plain curious… come join us!