Phase Controlled Receiving Array For QRP Low Band Fox Hunting Use And Small Lots

Jeff VE1ZAC

I enjoy a good radio challenge, like many other amateurs. But one that exerts particular adversity on the participants is the QRP fox hunt series that envelops the winter months here in North America. QRP stations hunt for two “Fox” stations that work split (typically) on 40M and 80M each week. Soapbox comments are posted after most hunts on an informal Yahoo group “QFOX”. You do have to join the group to participate in the messaging, but if others find this activity as entertaining as I do, it’s well worth it. BTW, fox hunting is open to everyone. Information and schedules are found at http://www.qrpfoxhunt.org/

This was my third year of fox hunting with this group. There are some regional issues which affect my efforts as well as the usual low sunspot flux numbers which affect everybody lately. The hunts occur at 0100 to 0230 UTC. Let’s look at what that means to me in North East, as compared with someone in the middle of the continent. At my QTH, 40 and 80 start to pickup just before local sundown and aren’t in bad shape to about 2300, typically. Propagation may be off but signal to noise ratios aren’t too bad. The first signals appearing will be from the East on 40M and the South East on 80M. The problems for me really start as 0000 rolls around. The noise levels start climbing on both bands, but especially on 40M. This noise makes it almost impossible to pick out QRP signals which are below the average noise levels on my OCF dipole and vertical antennas. The first two years produced dismal results for me in the hunts. The middle of the continent does a little better since 0100 corresponds to this earlier sundown period with less noise. The noise phenomenon does seem to happen right across the continent however. It also seems worse the more North you are. ( anecdotal evidence).

Propagation Noise reduction tools

I have experimented with two means to aid my fox hunting efforts. Last year I brought some directionality to my 40M vertical with a director wire mounted near the vertical element. This helped a little, but not enough to change my results much. I still had that cloud of noise descending on me at the usual time. The director element did produce a few more contacts, but that might also be due to increasing my ERP in the West direction.

Last summer I started a major rethink of the problem. Typical Beverage antennas are not available to me. I live in an urban area and can just barely get an 80M wire stretched out. In one case I did try a 200 foot BOG (Beverage on Ground) antenna, and it worked wonderfully on 160M, but not so great on 80 and 40. What I need is a way to decrease that noise signal coming from the East by more than a few dB that you get from a director or reflector. I want 3 or 4 S units of noise reduction, maybe -24 dB. If I could do that, the very weak QRP signals should show with an increased signal to noise ratio, which is what I really want.

How to get -24 dB? I have previously studied and implemented techniques from Victor Misek’s book “The Beverage Antenna Handbook” (Available from www.radiobooks.com) I implemented several experimental models of his Steerable Wave Antennas successfully, but mostly for 160M. I built one of the Misek designed phasers, which worked well, but I still wanted a bigger performance increase for 80 and 40. (You can follow along with my experiments on my website)

The “Lankford Files”

During my research, I looked into efforts that some of the keener SWL DXers have been using and came across Dallas Lankford. Dallas is a voluble antenna experimenter and builds nearly everything he writes about. His considerable volume of articles is hosted at http://www.kongsfjord.no/dl/dl.htm . If you loose the URL, just Google the “Lankford Files” and you will arrive at the site. Anyone even remotely interested in phased receiving ( only) antennas should look at these articles ! These keen medium wave and low band DXers routinely pull off some amazing feats with their dedicated receiving systems.

Dallas has experimented with all kinds of receiving antennas but what particularly caught my attention was the similarity of his receiving needs to mine. Dallas also wants to optimize smaller antenna structures for urban lots, and needs a deep steerable null that can be pointed at a noise source or an interfering station. He also employs every practical noise reduction technique he can, and often throws away conventional wisdom and finds out what he can do with cheap and non traditional antenna materials. (He is a great fan of Radio Shack speaker wire as antenna elements and as a parallel transmission lines!)

My phase 1 experiment

Phase one involved experimenting with some of my existing systems to see how this steerable null concept would work on the fox hunts.

I was particularly interested in Dallas’s work with verticals with 60 foot spacing and a Misek MicroSWA that is 60 feet long. Since I became interested in this stuff in January, it was pretty hard to erect anything new, but I did have the ability to re-erect an additional vertical wire about 30 feet high with a simple ground plane and use an existing spare feedline that came into the shack. This new antenna was almost exactly 60 feet from my main 40M vertical. One problem that I couldn’t change was the big difference in feed line length, which would cause problems in phase comparing the signals. Still.. it was fairly simple to route these antennas into the existing Misek style phaser I had in use. Modeling with these antennas shows a null that has two lobes 180 degrees apart. I tried this on the hunts starting in February. Low and behold, this produced results! I could reduce the noise on 40 and 80 (and 160 too when I started testing down there) One issue with these antennas and using a phaser is the low signal outputs. These are inefficient antennas and preamps are necessary. I had already begun experimenting with very low noise Norton style common base amplifiers, but my Misek phaser did not have one of these in place. A more conventional amp was employed with about 10 dB of gain. It worked fine for my winter hunts. During 7 hunts, I used the phased verticals on receive and switched to the transmit antenna to listen every 20 minutes or so. Holy smokes.. there was a world of difference ! The noise was reduced by my desired 30 dB or so, and it was possible to home in on QRP signals that were not detectable on the transmit antennas. This actually works. I forgot to mention that I had volunteered as a fox for the first time this year, and two of my outings were less than stellar. Two more occurred after I rigged up the experimental vertical system and everything went uphill from there. Since I could switch between receive systems and compare, I am convinced that I am on the right track to increase my take on the low band hunts next year.

Phase 2 experiments

OK.. I am convinced that this null steering technique is a winner. Dallas has several articles on his favourite low band null steering antennas and has experimented extensively with the SWA methods in Vic Misek’s book, all with great results. What really surprised Dallas was how well the MicroSWA worked, even compared with his short 60 foot spaced verticals. This thing is only 60 feet long. It can be center fed or fed from either end, and can be made from open wire line, twin lead, speaker wire or even coax. Adjustments for all these variations are made in the transformers. I decided to get one of these things ready to put up in the nicer weather with identical feedlines and a new push-pull Norton common base preamp as described by Dallas. Along with those items, I built a new passive phase control box using Dallas described methods. They are all in place now (mid April) . Before describing how they work, here are some construction pictures and features:

Here is a reprint of the MicroSWA from Misek’s book. I pretty much followed this version and used RG6 feedlines.

The Faraday shielded primaries were made with RG174. I used larger -75 ferrite cores, as I had a bunch around. Overkill, I know, but I won’t have to worry about saturating them. I wound the cores with fiberglass tape to provide insulation and the RG174 was fairly easy to use with the Faraday shield method. The transformers are enclosed in ABS plastic pipe housings. I am proud of my simple wire termination methods. You can see how I passed 300 lb. test monofilament trawl line through the housings and crimped an aluminum joiner on the inside to anchor them.

My antenna wire ( which is unusual surplus F18 aircraft cable with Kapton insulation) is then crimped to the monofilament. The two ends are mounted on aluminum poles to shield the vertical piece of the ground wire and the entire system is stayed with a pair of monofilament guys at each end. One of these guys has a bungee cord element, which is all that is needed to keep the system taught through temperature changes and winds. A door spring also works well for this job, and I may replace the bungee cord this summer. I have found bungee’s don’t last very long in our climate. The springs last forever. I took pictures before spray painting all the parts with flat black. I have found that flat black is the best camouflage for antenna poles and components. After painting some of my antenna supports, neighbours have asked me why I took them down, and were very surprised when I pointed to them still standing !

Not shown, but there is an aluminum sleeve shielding the two feedline transformers in the center unit.

The housings and antenna erected, before painting black. They are about 6 feet off the ground.

Here they are after painting flat black. They blend in much better !

Next is the new Lankford style phase control box.

The front panel controls are identical to the Lankford specifications. I have included the 10 dB preamp ( might actually be 11 dB) in the same box and it has a front panel control. Power is taken from my IC7700 acc power output. The two bottom knobs are on vernier controls. They didn’t turn out to be all that necessary. This unit can be used up to 30 MHz, bit is mainly for the low bands. High angle RF doesn’t null very well, which is pretty much everything at higher frequencies.

I decided the back should handle any transmission line I might throw at it. Includes balanced and unbalanced lines, and even a terminal block for power supplies that don’t have the right kind of plug on them ( Field Day, anyone ?)

You can just see the front panel at top. The inductors and caps are mounted on the band switch directly.

Inside you can see the new push-pull Norton amplifier. I made a board with Eagle Cad and my Sherline mill as I intend on making a few more of these fabulous RF pre amps. The IC7700 uses one of these in it’s pre amp stage. Mine use 2N5109’s.

At the lower left on the back panel is a power conditioner… it has a PTC type fuse, reverse blocking diode, choke filters and a 12 volt 3 terminal regulator. NO power line RF welcome here! Copper clad PC board is used liberally through out. Note the lash up parallel lines made from hookup wire and scotch tape, (an act of desperation). Flex braid joins the two panels, floor and case together. This unit sits on feet allowing a slight tilt, and just to the right of the IC7700 in the shack. Very convenient.

How does it work ?

Not shown is a small switch for two sets of parallel feed lines that enter the back of the unit. I can switch between the two verticals or the MicroSWA. I have had this unit together for a few weeks now and have been tuning everything I can find for a good test almost every day. The results favour the Micro SWA so far. Also, Lankford claims that the MicroSWA has a single 360 degree steerable null. That is confirmed by my EZNEC efforts. It produces wonderful nulls as I had before, but there is more range and a nicer feel to the unit. I took Lankfords advice and fussed over the pots used in the unit. His recommended source at www.tedss.com is an excellent tip. I got the 4 pole 6 position switch there as well. I am glad I did, as these pots are superior to the ones I used in my Misek phaser.

Why the fussing over the Norton amp, you might ask ? These amps are capable of producing up to 95 dBm or more at the 3^rd order intercept. In the presence of strong signals, this is what you want. No other design of RF amplifier is as capable, according to experts. That means very little degradation of your weak signals in the busy ham bands.

Last night, April 16, was the last night of the low band fox hunts for the year. The Micro SWA was up to the job and dealt with the usual difficult noise quite nicely. I do not believe I could have worked as many QRP stations without it. Several times I switched to the transmit antennas and lost the QRP signals I was copying handily on the MicroSWA.

I will be using this system extensively this year on 160,80 and 40M. I can’t wait for next winter to see how the fox hunts go with this improved receive antenna system.