2-element 40-meter linear loaded reversable beam
I would like to introduce and, in some areas, reintroduce, some valuable knowledge I have gained over 25 years plus, of wire antenna backyard experiments. My QRZ page illustrates the full-size phased beam on 40 that I had been using for years before my friendly side neighbors moved on and I Iost that occasional tree but now I have inherited some new and cranky landlords!
K4VX has a great article about building a shrunken 40 dipole with very close performance to full size.
Now with only 48ft width available in my backyard this article piqued my interest and prompted the thought of why not try using the concepts he provided and put up two of them and phase. The possibility that I could get back the 3db of gain and double my output power in two different chosen directions and get valuable rear rejection up here in the NE was intriguing enough for me venture out in freezing weather.
Yep there is a beam up there!
My backyard is a rain forest with a few good trees up around 50 ft but unfortunately not where I needed four of them, but a prudent use of some para cord catenary ropes helped expedite the process. I followed the K4VX illustration but substituted 16guage insulated wire for the wire line construction laced thru 450 ohm.
I put together 2 dipoles at 45ft long and put both up at 42ft high and then checked each separately for lowest SWR, (see feeding portion later in article.) I used a multiple of half wavelength RG8X to accurately repeat the exact feed point impedance to my radio. Remember to consider the velocity factor, a handy method if you do not own an analyzer. I aimed for 7.150 with the dipoles at 35ft spacing and 42ft high, both came in at 6.998 or so, too long. I will not bore you all with the up and down stuff but realized that whatever I did to the wire length had to be also applied to the 450 linear parts. I ended up about 47 ft overall. I will just say it involved my staggering into the front of my fireplace every 20 minutes to thaw out my 74-year-old bones! When the phasing lines were attached to the dipoles they were now too short due to coupling, I assume. I knew the physical lengths up in the air were very close to 7.150 so they were going to stay despite my; Tear it down instant gratification phobia!
Feeding; I have not owned a tuner or analyzer in years, so the next phase of this adventure began. I am pretty sure this particular antenna experiment is not out there on internet or in my library, lots of yagi and parasitic stuff, but evidently on my own for this one. For convenience, I fed it with RG8X and got a 2.5 to one at 7.150 and worse above and below. When I warmed up my almost frostbit fingers, I figured I was at either around 130 ohms or 20 ohms at the feed point. Every other experiment on phased verticals and dipoles always showed me around 27 ohms. I tried a few methods but ended up using 2 parallel 75-ohm RG6U coaxes cut to a quarter wavelength with soldered shields and center wires and connected to the relay at one end and to the RG8X run to the shack. The final swr was 1.7 to one from 7.128 to 7.250, more than enough for my needs. All my previous feedline attempts resulted in near full power out direct so this tells me a tuner will easily match a 50-ohm feed if more convenient. FYI, both my 40 and 80 loops, at around 100 ohms, at 45ft high, are fed directly with a single 1/4wl piece of rg6u and are flat throughout most of their bands. Got to love ¼ wl transformers!
Phasing: I use the Christman method as detailed in the ON4UN book (Low Band DXing,) my go to for everything! As said, my QRZ page gives a good overlook of the full-size version of this antenna and the phasing aspects are exactly the same on this 30% shorter version.
Pictured; SWR bridge and dummy load for cutting accurate phase lines and quarter w/l feedlines. Radio to transmitter socket on the meter, tested feedline to bottom of T connector and tee connector to out plug on SWR meter. I cut the to be tested coax 2ft longer than velocity factor, shorted out the far end and scanned radio VCO frequency until lowest SWR was indicated on dial. Cut and short test coax until you get the frequency you need. In this case I needed both 75ohm coax to read 7.150 for the parallel stub feedline. FYI, the actual length was 13ins longer than the published velocity factor! The light switch pictured is my direction changing 12-volt supply thingy. For the Christman method, VA7ST has taken my twisted algebra and designed a wonderful plug and play online table a couple of years ago.
His method differs in that I short my ends, no analyzer needed.
About 14$ on Amazon.
Little scruffy but works!
Performance: My 40 loop is an amazing all direction antenna with no gain but perfect for instant comparisons with new beam. It’s out front of my house and approx. 50 or 60ft away from the beam. How a new antenna works is always a subjective experience. You have to use it over a few weeks with different propagation at different times of the day and maybe local and distant weather. I have many DX and Stateside friends who have put up with my assorted backyard antenna experiments over the years and are more than willing to tell me when it stinks!
Observations to date; During the day the loop and beam run neck and neck, however, one late afternoon I worked my first South Korea on the loop, probably gray line; the beam was crickets! After dark, the loop is better to the Caribbean, South America and the far NW and North Pole. The beam really excels out East with a couple of the stations reporting 42db over 9 and 20db less when direction reversed including the UK, Nova Scotia, Greece, Africa, Germany, Ireland, Scotland, France, Poland, Romania, Ukraine, Spain, to name a few. The loop helped with stateside sometimes, but the beam was designed more for DX at night. Majority of all these stations listed reported a dramatic drop in signal when I switched to the loop. Very happy that the concept worked as planned and I really did not see or hear any noticeable difference from the previous full-size version. I am also confident that this linear stuff can be scaled to 80 or 160 meters if so inclined.
Final thoughts and notes: Yes, this antenna is labor intensive but for $75 in total it is well worth it. The stealthy aspect is also a big advantage when you have nosy neighbors! Notes: get your dipoles up and check for your target frequency and trust the length you have in the air. Do not design your antenna for easy match to fifty ohms if you’re trying for a beam/gain design! Design your low loss feed system to match the antenna!!! FYI, the 50ft spyder pole in first photo will soon be used for a vertical dipole for 12 meters, gonna take advantage of this new active cycle, I hope! Also, if phasing is not your cup of tea, try 2 or 3 elements in a parasitic array. If you try three elements, make director 5% shorter and reflector 5% longer than the driven element. Use element spacing at .15 to .25wl.
Thanks for reading, N4JTE
|Thank you Clint it published nice.|