One man's entertainment is another man's pollution; operating on 160m from my QTH benefited considerably from adding a high pass filter to cut off the energy coming from the AM band.  Every radio I've operated at this QTH has had increased noise level on the low bands (80 and below) without a BCB filter.

So I was naturally curious how the FlexRadio 6700 would work in this RF environment.

Out of the box, the natural antenna choice for initial receiver testing was the 160m vertical - it captures a lot of signal and functions as a receive antenna over a wide frequency range.  I connected the antenna directly to the 6700 - no in-line filters either in the direct antenna path or the RX filter loop.  This is a harsh real world test for any receiver.

Tuning around the different bands with the FlexRadio 6700 was a surprise!  I couldn't detect any BC band induced spurii even without the high pass filters.  160m had its fair share of QRM from local noise sources - I know what causes some of them (the ones on my own QTH) and others I have a fair idea (the variable speed pool pump in a new construction about a half mile away for example).  But that was it!

Another interesting test for any receiver is to see how its LF performance functions. Although we haven't got low frequency allocations at 137 KHz or 497 KHz yet, hopefully we will in the not too distant future.  I was curious to see how the 6700 would perform at those lower frequencies.

At LF, my 160m vertical is a very short antenna!  Despite that, I've been able to receive a lot of LF stations especially aviation non-directional beacons (NDB).  Most of these beacons serve as navigation aids for instrument approaches into airports.  Many of the US NDBs are being de-commissioned as GPS has rendered them obsolete.  But you can still hear multiple beacons in the LF spectrum from 200 to 400 KHz.

All the beacons have a callsign sent in CW or MCW and I quickly found a good directory mapping frequency, callsign etc to location compiled by William Hepburn of the LWCA - you can find the directory here.

Not withstanding the inefficient antenna I can hear beacons all over the Western states (Arizona, Utah, California, Idaha, Oregon, Washington - all logged) as well as many up in Canada.  I used Google maps to find some of the locations like NDB ZP at Sandspit, BC which I can regularly copy.

Here's a capture of SmartSDR tuned to WL (Williams Lake, BC) which Industry Canada shows on 0.385204 MHz running 500 watts into a 0.5 dBi antenna at 6 meters above the ground.  BTW, a couple of notes about this capture...  

1. You can see that the radio is in WIDE mode - no bandpass filters selected in the receiver.
2. The S meter calibration is accurate but ignore the level scale on the panadaptor as its not yet calibrated.

The clip is short - you can only listen to a beacon id so many times!

On the ham bands, the 6700 receiver truly shines!  As I've said before, the radio sounds very clear and its strong signal performance is excellent.  Even with the SteppIR as my primary HF antenna, I can monitor different bands to see if there is activity by using multiple slice receivers - of course a tribander or multi-band HF yagi would allow monitoring with optimum signal levels but wouldn't be as cosmetically friendly!

I'm looking forward to using the 6700 in a contest and also improving my low band receive antennas - the 160m vertical is noisy and this summer I plan to build a receive only 4 square.  That should help my 160m and 80m receiving conditions a lot!

• Alpha hardware delivered ✓
• Alpha software releases ✓
• Production hardware delivered ✓

For anyone with experience in system development and manufacturing, committing the build of production hardware is no small decision – it simply has to be right!  Of course the science is in the “simply” – just like “oh, it’s a simple matter of programming” – wish I had a dollar for every time I’ve heard that!

So when I got the automated notice from FlexRadio that my production version of the 6700 had been shipped, I was even more excited that when the alpha was shipped…  first customer ship was approaching!

As soon as I got the email alert that the package had been delivered, I hustled home and started unpacking.  Here are some pictures…

FlexRadio ships double boxed – this is the inner box

 Inside – Microphone, Quickstart, GPS Antenna, Power & Ethernet cables

Everyone has seen pictures of the front and back of the radio so I won’t consume bits repeating them here…  but when you get a Signature Version of the 6700, be sure to have a look at the BOTTOM of the 6700.

Truly the Signature Version – K6TU 007!

The home for the production 6700 was ready to go and so I quickly cabled up the unit, installed the latest release of software and was ready for an on air test.

Cut to the chase – How does it sound?

The first real on-air-outing for the Flex 6700 was at Visalia and the N6V demonstration station which I wrote about in the last post.  We had used a Heil PR-781 microphone with no audio equalization and some on-air reports had noted that the audio sounded “muddy”.  Not a total surprise given the voice characteristics of the different operators.

Bear in mind this was alpha software hot off the press!  A scant 3 weeks later, the release I downloaded from FlexRadio for use with the production 6700 has the transmit equalizer operational! Hats off to the software team at FlexRadio – they are a great group of professional engineers!

As a contester, I usually operate with a headset and have been on a seemingly endless quest for one that is comfortable.  I have quite a collection as the real test is a 48 hour contest weekend that doesn’t end with neck ache (from the weight) or a head that feels like its in a vice.

The most comfortable headset I’ve found so far is the Radiosport RS60CF which I purchased from Arlan Communications.  It too has a microphone with a flat response which allows me to choose how I want to sound – BBC quality for casual ragchewing or something with “more punch” for contesting.

With the headset connected to the 6700, I turned on the Monitor, made sure the power level was at zero and hit the PTT.  It sounded very clean with the default profile from my own monitoring.  By the way, did I mention ZERO delay audio?  Testing using the Monitor function on PowerSDR requires a high sample rate and small buffers to minimize the delay between speaking and the audio in the monitor.  There is no delay audible in the 6700 – very nice.

With the 6700 connected to the antenna via my amplifier, I made a test tranmission to set the output power and then started tuning for someone to work. 

The first QSO was with an IT9 in central Italy.  With S8 signals both ways it was a pleasant QSO and got a “nice audio” response to my question about “how does it sound?”

First home QSO with the production 6700 – another milestone to mark DONE!

A very well attended event, the Convention attracts heavy duty DX’ers and Contester’s from around the world so N6V isn’t a light weight station… legal limit and a good antenna are expected – as is really good radio!

N6V is fortunate to be well supported by a number of key vendors and I am personally very grateful to the following folks who helped me put on N6V in 2011 and 2013.

• US Tower brings along a heavy duty mobile tower (55’)
• SteppIR supplies and assembles the antenna on site (DB-18E)
• M² Antenna Systems supplied the rotor (Orion 2800) and cables
• RF Concepts supplied an Alpha 9500
• FlexRadio Systems supplies the radio (2011 was a FlexRadio 5000)

The 2011 station was a great success – it opened a lot of eyes since this was the first opportunity many people had to operate a FlexRadio transceiver.  Everything performed as expected and many contacts made around the world.

So when I was asked to assemble N6V for 2013 late summer last year… you can imagine what I wanted to do.  2013 N6V was just crying out for a FlexRadio 6000 series radio!

I called Gerald and Greg last October and told them what I was thinking – were they game for it?  But of COURSE was the swift reply.

My 2011 experience was good but had highlighted the challenges of assembling a high performance station in half a day.  We had integrated everything together on site for 2011 and this time planned to pre-integrate before the convention.

With the great cooperation of all the vendors we quickly had a plan in place.  Molly and Joe at RFConcepts agreed to ship the 9500 to Austin so that 9500 support could be integrated into SmartSDR.  FlexRadio already had a SteppIR at their offices in Austin together with the SDA100 controller to help integrate support for it into SmartSDR as well.

Think legal limit transceiver with a resonant antenna from 40 through 10m – no configuration required…  change band, change frequency and the amplifier and antenna follow the radio.

Here’s a block diagram of the 2013 N6V configuration.

We knew that timing would be tight – the FlexRadio 6700 is a sophisticated product and Gerald’s team was determined to build the best product possible.  Like making fine wine, building robust software that can be built on for the long term (as new features are added) can’t be rushed.  You have to get it right from the beginning.

We had agreed to arrive the day before the Convention opened so that we could assemble the station.  In addition to Greg and Don who manned the booth in the vendor area, Steve Hicks N5AC (VP of Engineering) and Jim Reese WD5IYT came to Visalia to assemble the station and look after the 6700.

FlexRadio delivered!  We had a 6700 with support for CW (semi-break in at that time) and SSB with integrated support for the Alpha 9500 and SteppIR antenna!  The 6700 drove the Alpha 9500 to legal limit with about 40 watts or so of input power.

Some things to keep in mind…  this was alpha level software with support for receive (which I’d had for some weeks prior to Visalia) and transmit (which was hot off the press).  Alpha software in this case means that not all features were yet implemented and that not all the SmartSDR controls were hooked to the radio.  For example, in the release of the radio software we used at Visalia, the MIC Gain control wasn’t hooked into the radio – Steve and Jim preset the audio level and we used the power output control to keep within legal limits driving the amplifier.

We didn’t have any shortage of operators!  Here’s a picture we grabbed of some of them:

Radio Performance

To me as a contester and semi-serious DX’er, the proof of a radio is its receiver and how it is to operate (workflow and usability).  I expect a transmitter to transmit – do it cleanly, not get hot and bothered and sound good.  The receiver I expect to perform miracles – it has to handle weak signals right next to monster signals, have killer filters and have audio that I can listen to for hours on end.

I’ve used every major brand of radio in the 40 years I’ve been licensed and a large number of commercial service radios well beyond my check book.

The 6700 is the best receiver I have ever used.

It’s a FlexRadio Systems SDR so by now, while I don’t take them for granted, I expect the filters to be brick wall, no ringing and all the configurability at my finger tips.  Here’s a screen shot from SmartSDR of adjusting a SSB filter on the fly…

The filters perform!  You can pull signals out of a busy band that are really weak – even when parked next to a very strong station. 

It’s not “just” the filters, it’s also about dynamic range.

FlexRadio hasn’t released the dynamic range figure for the 6700 yet but from real world listening, it’s good – very good.  As an engineer and VC I deal with quantifiable objectives – so you may have some idea about how much it pains me to make a qualitative statement. 

The 6700 is simply the clearest radio I’ve heard.

During the CQ WPX SSB event in late March, I took some time during one of my break periods to compare my 5000 with the 6700.  The 5000 is a great transceiver with an awesome receiver –the 6700 just sounds clearer – same station, same noisy band conditions (40m at 9pm PDT) but clearer.  I don’t yet have a good explanation for this but when you use this radio, you will hear what I mean!

I’m really looking forward to using the 6700 in a contest.  I think it will be less fatiguing over a contest weekend and I’m guessing my QSO rate will benefit significantly.

Another comment about the receiver – it handles local high power stations VERY well!  Greg had a 6700 on the FlexRadio Systems booth that was hooked up to a broadband active antenna on the roof of the convention center… maybe 100’ from the tower with the SteppIR and legal limit from N6V.  Greg and Don often had that 6700 tuned to the same band as N6V was operating and only a few KHz away… monitoring a weak signal.  I’ve been at contest stations with brand X (X something other than FlexRadio) where you couldn’t do this because when the transmitter in brand X keyed up, the whole noise floor rose and obliterated the weak.

Transmit wise at Visalia the radio performed well and did its job quietly, no muss, no fuss.  We even had the GPSDO option installed and with the antenna taped to the sidewalk just outside the door, had solid lock.

For the most part we got good audio reports – we had a lot of different operators and a couple of different microphones.  Some operator’s voices together with a flat response studio microphone could have benefited from the TX equalizer but… that wasn’t implemented at the time of Visalia.

Over the course of the convention we made over 500 contacts – all of the QSOs that got logged will receive an N6V QSL card via the BURO.

Sincere thanks to US Tower, Steppir, M², RF Concepts and especially FlexRadio Systems for their support of N6V.

My personal thanks also go to Gerald and FlexRadio Systems – they generously donated a FlexRadio 6700 as the major price for Sunday mornings wrap up event.  Another Gerald, K0JJ, won the certificate for the 6700 in the breakfast raffle draw.  He is going to be a VERY HAPPY Ham when his radio is shipped (after all the pre-orders!).

The FlexRadio 6700 made the day!

I am one of the FlexRadio 6700 alpha testers – these are my revelations.

About 18 months ago, my phone rang and there was Gerald Youngblood, President and CEO of FlexRadio Systems.  After some pleasantries and chatting about ham topics, Gerald asked if I’d be willing to sign a non-disclosure agreement (NDA) to enable us to discuss future product plans.  In my day job as a Venture Capitalist, I NEVER sign NDA agreements – there are good reasons given the number of business plans I see and at the end of the day, professional reputation and integrity are more important than any legal agreement.  Information stays confidential because of people, not because of paper!  Despite my normal reluctance to sign NDAs, and with radio products a LONG way from my investment activities, I readily agreed.

A couple of days later, NDA signed, Gerald, together with Greg Jurrens (FlexRadio VP of Sales & Marketing) and I started a series of long conference calls and email discussions about the next step in the evolution of FlexRadio products – the 6000 series.

As an engineer, business guy, oh and of course a very active Ham, this was heady stuff – getting in at the concept stage of a new radio was a rare opportunity.  As an unpaid consultant, my reward was to see the 6000 series develop from a clean sheet of paper, through development to product.

I also got to be one of the alpha testers of the FlexRadio 6700 providing feedback on usability, the UI and real world testing.

In the next posts I’ll describe my experiences with the radio from the alpha units, the demonstration station at the 2013 International DX Convention at Visalia, through to getting one of the first production units that have been shipped.

I had produced propagation predictions using VOACAP before and so I volunteered to develop predictions for each of the 5 contest bands for a full 24 hours.  Wow!  It took me forever!  VOACAP runs very quickly on a reasonable CPU computer but generating the graphics required a lot of hand manipulation using VOACAP's image generator and Photoshop.  All in all, it took me half a day to produce all the graphics.

The graphics were a a great success and NCCC won the 2012 RTTY Roundup hand down - setting a new record!  The propagation charts were such a success that the club members wanted them for the next contest - that took me about 6 hours to generate.

At that point, I was hell bent on finding a way of automating the entire generation process - that took me almost 2 months of night & weekend coding but by late March I had it done.  Now generating the predictions took less than 10 minutes using the capabilities of my MAC under its UNIX colors!  Then one of the members asked me if I could get the same thing running under Windows...

By the time I looked at all the different packages I had used, I came to the conclusion that what was really needed was a web service.  With a web service, I could remove the learning curve of customizing VOACAP predictions to a level where it would be as simple as filling in a form and clicking a button.  No learning curve, no software to install.

I started work on K6TU.NET - Propagation as a Service web site at the end of March.  I launched the site into a closed beta in September after 6 months of heavy lifting coding and assembling an industrial grade web site.

I'm delighted to announce that the site went live into production this last weekend.  Stop by and take a look what you can achieve at the click of a button - a good way to do that is to visit the Quick Tour page and see the forms and graphics.

Here are a couple of examples to whet your appetite...

This is an example of the regular prediction you can run - this is a screenshot of a single band/hour combination where you can see the predicted propagation by visualizing YOUR signal strength across the world.

And here is an example of the Contest Strategy Prediction - this is perfect for planning which bands to use for either search & pounce or CQ running during a contest.

K6TU.NET supports many different types of propagation predictions and has both free and premium features.  I hope you will stop by and take a look!

Building a product for use by multiple different people is a very different proposition than simply building a project.  A project just has to work and seldom is the cost a significant issue.  A product on the other hand has to meet a set of requirements from functionality all the way through to volume production.  

Kevin K6TD and I had been through this many times before – both in our professional life developing products for companies we worked for as well as our own company.  Not only do you have to produce a product but you also have to sell it!

We decided to build the tuning knob with product production in mind, get it working, build enough units to prove out the user requirements and then look for someone who would be interested in producing and selling a product.  Fortunately we had a team of people in Steve K5FR, Kevin and I who knew how to do this and moreover, weren't daunted by the prospect of finding a partner.

It didn't take us long to get the first prototype version designed, built and working!  I guess it's deeply embedded in my genes but the thrill of building something and getting it working never gets old.  Assembling the rev A boards was no exception – we built up a handful and starting giving them to folks we thought would give us good feedback.

We sent one of the boards to Greg Jurrens, the Vice President of Sales & Marketing at FlexRadio Systems.  Greg is an avid VHF operator who also likes to operate contests.  Greg used the tuning knob and really liked it – he liked it so much that he called me and asked if we'd ever thought about turning the tuning knob into a product – BINGO! We had our potential partner!

Over the next few months Greg and the team at FlexRadio gave us great feedback on the knob and we began talking about what be necessary for FlexRadio to take on production and sales.  One thing led to another and eventually FlexRadio assigned Graham KE9H to work with us to turn the tuning knob into its final version.

So today I'm delighted to be able to announce that FlexRadio will be releasing their FlexControl product at the International DX Convention at Visalia, California next week!  FlexControl is the product version of the Tuning Knob designed and built by Kevin K6TD and myself with full software support in DDUTIL provided by Steve K5FR.

Gerald Youngblood CEO of FlexRadio Systems and Greg Jurrens have been kind enough to allow me to provide this preview of the product ahead of its appearance at Visalia.  Greg tells me it will retail for $129.95 and will be available for orders & shipment by the time of the Dayton Hamvention in late May this year.

It's been a great journey and we're delighted at the outcome!

Kevin K6TD and I have worked together for many years – both for the same companies and also with a number of product ideas we'd commercialized ourselves. Kevin is a very successful VP of Engineering and a serious hardware wizard - so I decided to see if I could rope him into a new project. Despite a busy work schedule, Kevin agreed and we started talking about the specification for the new knob.

A simple list of requirements quickly took shape:

• Cost effective
• Simple to use
• Silky smooth tuning (no tuning detents or lag)
• Compact form factor
• USB connected
• Seamless PC side integration

The need for seamless integration on the PC side was a no-brainer – it was a critical element to get that silky smooth tuning experience and part of the system design to eliminate tuning lag. The logical place to provide this was in Steve K5FR's DDUTIL – "the" MUST HAVE for station integration for any FlexRadio owner. You can find Steve's software on his web site.

DDUTIL is an amazing piece of work. Think about all the different elements that make up a modern radio station (amplifiers, power meters, antenna rotators, antenna controllers, switching matrix, filters, multiple software programs…) that need integration. Integration and automation can simplify and streamline operation. Core to this simplification is getting information from the radio to each of the station components – especially frequency information. DDUTIL sits on the CAT control stream provided by PowerSDR and manages the distribution of CAT data to the station components. You can think of DDUTIL as a CAT multiplexor and station component manager.

I got to know Steve shortly after I purchased my Flex 5000 – I'd decided to add an Expert SPE-1K amplifier to the station and wanted to be able to control it via DDUTIL. The Expert amplifier was on the DDUTIL "to do" list and so I emailed Steve to see what he was planning. The reply email came quickly! Steve was ready to go as long as someone could help him debug the implementation. We worked together to get support for the Expert working and during several long phone calls, found we shared a lot of ideas and common experiences – and had a ton of fun working together!

In a quick phone call to Steve I described what I was thinking and got a resounding "I'm in!" – We had our development team up and running!

Kevin started looking for a cost effective replacement for the optical shaft encoder used in the original project. In parallel, Steve and I worked on defining a control protocol between the new knob and DDUTIL. We opted for a simple control set modeled after the ubiquitous CAT format – simple ASCII text commands with a ';' terminator. The spec for the CAT stream quickly took shape and we started thinking about how to prove out the different pieces of the design.

The first step was to find a development board using the same PIC that Kevin had selected as the core of the system. We opted for a development board sold by Futurelec, a company based in Asia – at less than $50, a good basis for the proof of concept (POC) system shown below.

The POC system is simply the development board with a shaft encoder and LEDs mounted on a daughter board. The daughter board connects to the peripheral connectors on the Futurelec board. This enabled me to start developing the software to handle the shaft encoder and the USB port to connect the board to the PC.

The development of the USB software was greatly accelerated by using Ian Harris' PIC PACK library – I covered this in a previous post which you can find here. I quickly got the development board connected to the PC via USB and began implementing the CAT-like control protocol to talk to DDUTIL.

In short order Steve had the basic support for the knob integrated into DDUTIL and had the POC system working as an active tuning knob with DDUTIL. With a little bit of tweaking in DDUTIL we realized we were pretty close to that "silky smooth tuning" effect we were looking for but if you really spun the knob quickly, there was still a lag.

Kevin suggested I implement acceleration detection – change the tuning rate generated by the knob when it was turned quickly. Some thought and basic calculus later, I had this figured out and suggested to Steve that we modify the CAT stream so that the knob could signal that a faster tuning rate was required.

As we were getting this done, Kevin sent the hardware design to the PCB manufacturer and we built up a handful of tuning knobs. You can see this rev A version of the knob below – you can get an idea of scale as the physical knob itself is a little over 2 inches in diameter.

Acceleration detection got us a long way to eliminating the CAT queue but the lag still showed up with aggressive tuning. The challenge was the chain of commands sent to PowerSDR to change the tuning rate – multiple CAT commands to effect a single tuning step – regardless of the size.

Steve had worked closely in the development of DDUITL with Bob Tracy K5KDN who developed the CAT support in PowerSDR. With Bob's help and support from FlexRadio, we were able to get a couple of new CAT commands implemented in the next development build of PowerSDR. This reduced the number of commands needed to tune a step to one. This nailed the CAT queue problem and we got the coveted "silky smooth tuning" we were looking for!

Feedback on the rev A board was positive – we had used the switch incorporated into the shaft encoder to implement a number of different features. We used switch clicks much like mouse commands – single, double and long click. Clicks are detected by the knob and signaled up to DDUTIL so that it can change different controls on PowerSDR.

At the time, Steve was also working with Lee Crocker W9OY to get DDUTIL as the control center for SO2R operation. Lee really liked the idea of the Tuning Knob but said he wanted some additional control switches that could be used with DDUTIL to automate other aspects of station control.

We had decided to spin the PC board to make some manufacturing changes and so it was a logical decision to add three simple push button switches to the hardware. Kevin re-laid the PCB and sent out for board manufacture. The boards came back and we built the rev B version of new tuning knob.

Between us, we built a fair number of rev B units and got them to folks for "real world" testing. This time the feedback was very strong and very positive – we knew we were getting very close to the final solution!

We started the process of getting the tuning knob turned into a product - more on this in the next post…

Adding the auxiliary switches was easy for both hardware and software but I'd made a decision to detect just a simple on/off operation – allowing another three actions to be automated. By now Lee W9OY had his hands on a rev B unit and just like Oliver Twist wanted "More!" – Lee asked for the same kind of click detection on the auxiliary switches as I'd implemented on the shaft encoder switch.

I groaned when I heard this request… since the auxiliary switches had been an afterthought, I'd grafted support for them into the code rather than taking an integrated approach. Supporting multiple click detection on the auxiliary switches required a fair amount of code re-factoring to avoid an even worse "graft" – ok, kludge!

Over Christmas 2010 I finally found time to re-factor the code, document the changes to the CAT stream to allow for signaling auxiliary clicks to DDUTIL and get new code off to Steve.

By late January 2011, we were DONE! Steve added a whole new tab for configuring the knob into DDUTIL and Lee was happy! One of our knob users in Europe took a high place in a major contest and gave the knob the credit for his success.

The journey from concept to product has been very interesting. We started with a distributed development team of three – unique because we've all never met in person. The final version required the collaboration of two more people in different parts of the country… and we've all never met in person or even had a joint conference call together. Email was our primary communication tool and it worked because of a shared vision and mutual respect.

Finally, I'd like to recognize…

• Kevin K6TD and Steve K5FR – they shared the vision and gave their full support to this project. Without them, the tuning knob would likely never have seen the light of day!
• Team FlexRadio Systems for their willingness to incorporate changes into PowerSDR that we requested – there aren't many companies that will take this kind of feedback so willingly to heart from their customers!

In the next post, I'll talk about the final steps of the journey…

My conversion to a radio station without knobs and buttons began in the summer of 2003. I'd changed QTH and finally had enough space to start thinking of serious antennas. However, there was one wrinkle… I wanted to have the antennas located away from the house for aesthetic reasons. Not wanting to pull long lengths of coax, I elected to remote the radio to be close to the antennas. At the time, I chose the black box version of the Kenwood TS-2000 (the TS-B2000) that is completely controlled via a PC – no front panel beyond an on/off switch.

The station setup worked well but I missed the simple process of tuning across the bands – using a mouse wheel didn't cut it with the Kenwood software. So with the encouragement of Kevin K6TD, I set about implementing a PIC based tuning knob. This was my first PIC based project – fortunately both the hardware and PIC software were simple. I built the first generation knob around an optical shaft encoder – I think I paid about $100 just for the encoder itself – a precision unit that produces 1000 steps per revolution. Total overkill but it worked!

The size of the optical encoder drove the selection of the case – for scale, the knob is just over 2" in diameter and the four screws visible in the picture secure the encoder. The case is about 4" wide and 6" or so wide. Looking at it now makes me think of an early 1970's car – big, square and ugly functional.

The PC software wasn't so straightforward – I wrote a Windows program that took input from the knob on a serial port and then faked keystrokes to the Kenwood control program. It worked ok so long as you didn't get too carried away with spinning the knob – the resulting tuning lag was a mess!

I wasn't very active until in the early part of 2009 I decided to upgrade my radio. K6TD was the catalyst for this decision; Kevin had become a serious contester and bent my ear about how much fun he was having especially after a serious station upgrade. Waving a large carrot, Kevin invited me to operate the 2009 ARRL Phone Sweepstakes with him as a Multi/Single station. Years earlier I had been an avid contester and the thought of radio sport got me motivated to improve the station.

By summer of 2009, I'd made two decisions – I applied for a vanity call sign to replace N6TTO (a pain on CW and a mouthful of phonetics) and after much research, ordered a FlexRadio Flex-5000 as the core of a new station. You can read more about that process here on some of the early postings on this blog.

My first serious contest effort with the Flex-5000 was the 2009 California QSO Party – considering low power (100 watts) and limited antennas (an HF vertical and G5RV dipole), I had a blast with about 430 QSOs. It was my first contest using logging software (N1MM) and operating the radio via PowerSDR – so although there were several times I found myself all "fingers and thumbs", I put this down to the learning curve of contest operating.

A few weeks later I found myself at K6TD operating Phone Sweepstakes. Wow! What an experience! Kevin had built an awesome station at his QTH in Cupertino. His home sits on something of a bluff with the ground dropping off in most directions. With a 3 element Steppir at 55 feet, an Elecraft K3 and full legal limit from an Alpha 87a amplifier, we were a serious signal on the band.

Kevin was a very gracious host allowing me first up in the batting order – the initial rates were probably around 100 QSOs/hour and I was having a blast – I was struck by how Kevin had worked to streamline the workflow – the logging program (Writelog) remained in focus the whole time and apart from using the main VFO knob or RIT control on the K3, you didn't have to touch the radio.

As I switched off with Kevin for a break, I realized that I need at least ONE knob to operate my Flex-5000 in a contest and optimize the workflow between radio control and the logging software. The seeds for the next generation tuning knob were planted…

From my location it peaks towards the North – I listened to one QSO where it was clear the folks on the frequency could hear the signal as well so it's not a local source of RFI.

I've uploaded a WAV file of the signal - click to listen.

You can hear a distinct chirp at the start of each transmission as though this was an oscillator being keyed by turning the power on and off. There are three distinct frequencies that repeat – you can see them in the following waterfall I captured from the Flex 5000.

This is one of the strangest RFI sources I've seen in quite a while… it's a ways away from Woodside I suspect so I'll probably never know what it was…

As station enhancements have progressed, my 80m antenna has gone from a G5RV (a total compromise on 80m with tuner use being mandatory), to a homebrew 80/40 "shorty" dipole (good for about 80 KHz on 80).

After hauling the 80/40 dipole up and down for a year to tune it for specific contests, I started looking for a broader band antenna for 80m that wouldn't require a tuner and at the same time was simple to make.

Unusually, I didn't find the solution on the Internet but found a reference in ON4UN's Low Band Dxing handbook (just issued in the 5^th revision and a "must have" for the low band operator) to the "80m Dx-Special" designed by Frank Witt AI1H.

The coverage of this antenna in the ON4UN book is sparse but did have a reference to a couple of articles in QST back to 1986 and 1989. Fortunately, as an ARRL life-member, I have access to the online archive of QST so I quickly dug out the articles.

The AI1H DX Special is an 80m half wave dipole that uses a coax resonator as part of the dipole legs. Here's a diagram with the correct lengths (the lengths in some editions of the ON4UN book are incorrect):

From the diagram, you can see three separate lengths of coax are used. The shortest length is used to establish an off center feed for the dipole as part of an impedance match and then crosses-over (coax center->shield, shield->center) at the connection to the longest length which acts as a capacitive section. The remaining length is a shorted stub which acts as an inductive section.

The remaining length of the dipole sections is made from 14ga copper wire. The antenna is fed as a T – in my case using a standard SO239 T connector via a sleeve balun (a length of coax cable that passes through a number of ferrite cores) to choke off common mode currents.

If you have access to the QST archives, you can find AI1H's excellent analysis of the underlying design of this antenna in:

1. Broadband Dipoles – Some new insights, Frank Witt AI1H, ARRL QST Magazine October 196, pp27-37.
2. The Coaxial Resonator Match and the Broadband Dipole, Frank Witt, AI1H, ARRL QST Magazine April 1989, pp22-27.

The three sections of the coaxial resonator in this antenna act as a combination of an impedance matching transformer coupled with a resonant LC circuit which broadens the range of operation of the DX Special to an SWR match of better than 1.6:1 across the range of 3.50 – 3.85 MHz. Using the coax as the resonator provides an efficient way to broadband the antenna with minimal added loss – the broadband operation incurs a 1 dB penalty at the band edges, less elsewhere.

Here's the SWR plot captured from the version of the antenna I built and put up.

From the first contacts I have made on 80m using the antenna, it radiates at least as well as the 80/40 shorty it replaced and the amplifier operates happily all the way to about 3.9 MHz.

A couple of construction notes:

• I built the cross over section of the two coax stubs by simply soldering the coax together crossing over the shields and center conductors. I used 18 ga copper wire to wrap the sections together and then a high wattage soldering iron to make the soldered joints. I used a couple of pieces of a large tie wrap as a mechanical brace across the joint.
• Don't substitute anything other than RG8U for the RG213. The lengths of the coax stubs are calculated based on the dielectric properties and velocity factors of these cables.
• Be sure to waterproof the ends of the coax stubs. Water ingress will destroy the antenna in short order.
• The coax sections add significant weight to the legs of the dipole so provide strain relief on the connectors at the center T. I opted to use crimp PL239 connectors because of their stronger mechanical properties than the solder-on type.