Presentation on theme: "The Carolina Windom. A long wire antenna for portable operation. If you want a good argument on HF antennas bring up the Carolina Windom. Craig Williams."— Presentation transcript:
The Carolina Windom. A long wire antenna for portable operation. If you want a good argument on HF antennas bring up the Carolina Windom. Craig Williams W6CAW
Goals An effective multi-band, antenna for portable and emergency HF operations. Can work without an antenna tuner. Easy to construct and deploy. Can be used for local Near-Vertical Incident Skywave (NVIS) or long hall communications depending on frequency. Cheap!
But first - What is NVIS? Near-Vertical Incident Skywave (NVIS) propagation is generally considered to be F- layer ionospheric reflection at angles of 70- 90 degrees. It is skywave propagation without the usual skip zone. The purpose of NVIS is to communicate locally and regionally, out to a few hundred miles, with moderate power, simple antennas, and no skip zone. NVIS is typically used on 160, 80/75, 60, and 40-meter bands by Amateur radio operators using relatively low horizontal wire dipole antennas. From: http://www.athensarc.org/nvis.asp
NVIS or ? 75-meter NVIS antenna at 20 feet high. The -10db ray is at about 38 degrees. The -20db ray is at about 20 degrees. 75-meter antenna at 125 feet high (half-wave). No longer NVIS, but now a "skip" antenna, with most of the power at about 42 degrees. From: http://www.athensarc.org/nvis.asp
NVIS and the F-layer Above shows two of the three mechanisms that combine to attenuate low-angle daytime signals: (1) Compare the radiated power, which is about 2db below peak at 70 degrees, with the 30 degree angle, which is down about 14db. (2) Compare the distance the rays must travel through the absorptive D-Layer (twice) at various angles: the 30 degree ray has about twice as much loss as a very high angle ray. (3) Add the normal attenuation due to path length (not shown). These three factors, plus a little loss in the troposphere, all combine to attenuate low- angle signals in the daytime. As the sun gets higher, D-layer ionization intensifies, and the effective range decreases further. From: http://www.athensarc.org/nvis.asp
What is a Carolina Windom? I found this Radio Works Windom as I was putting this presentation together. $140.00! www.radioworks.com/ ccwcover.html. www.radioworks.com/ ccwcover.html www.radioworks.com/ ccwcover.html Now, lets make one for considerably less.
The 4 to 1 Balun How to at: www.w5fc.org/files/how-to/QRP%20Expressions_version_1.pdf Toroid is a T106-2, Wireman Part # 903
Testing the Balun A 200 Ohm resistor is connected between the balanced leads A1 and B2 to simulate the 200 Ohm feed point of the antenna. The 50 Ohm test device is connected to the unbalanced feed point. VSWR should be 1.0 across frequencies of interest.
4 to 1 built for light weight The throw away putty knife Balun. The material is easy to work with, cheap, and should hold up well for temporary use. I decided to add terminals for easy of assembly and future repair. The coax is attached on the back side with the shield soldered to the ground windings on the balun and the coax center to B2. B2 and A1 go to the wire. The formula to compute the length of the wire is the standard 468/frequency with the 4/1 Balun at 62.2% from one end. The wire is Toughcoat 'Silky' 26 AWG, 19 strand 40% copper-clad steel from the Wireman # 534
4 to 1 Balun Potted Balun “potted” with liquid RTV.
Common Mode On an 80 Meter Carolina Windom the Current Balun is placed around 22 feet down the vertical feed line from the feed point. Using this 22 feet of common mode as a radiating element the antenna pattern is filled in as seen here.
Radiation Pattern At the design frequency of 80M the OCF antenna will have a similar pattern to a center fed antenna. At higher frequency harmonics the OCF will have gain and pronounced lobes towards it’s long end. The pattern in Fig. 4 is for a two wavelength dipole. Keep this in mind when “pointing” your OCF antenna From: http://www.blacksparrowmedia.com/misc_files/ The%20OCF-dipole3col.pdf
Antenna Current distribution Current distribution at center, 90 degree, and OCF fed, 60 degree, feed points. At 80M the center feed point = 70 Ohms. Above 80M it varies between 70 and 3000 Ohms! At 80M the center feed point = 70 Ohms. Above 80M it varies between 70 and 3000 Ohms! The OCF feed point is around 300 Ohms at 80M through 10M. This is the key to multi-band operation with the Off Center Fed (OCF) antenna. From: http://www.blacksparrowmedia.com/ misc_files/The%20OCF- dipole3col.pdf
Antenna Length and Balun Placement? Overall length of the wire. Use the standard formula of 468/ frequency in Mhz for the total length. I find this is usually long which is a good thing. In portable operation I fold back the ends to get a better match for the current environment it finds itself in. ( height, ground Xr, surroundings ) Balun placement. I have been using the figure of 62.2% from one end. On 80M this morning WM6Z from Imperial says he moves his balun around till he gets the best VSWR with his analyzer. Why didn’t I think of that?
The Current Balun The current balun is made from 7 clamp on torroids from the Wireman, part # 916. The RG58 feed line is looped through the torroids 3 times. My current balun’s are somewhat a guess from looking at many articles on the internet. You know they work when your tuner tunes properly. When they don't your tuner does a long mad hunt and never seems to settle in as the common mode current is still on the feed line.
The Mast 33' fiberglass push up pole from The Mast Company. www.tmastco.com www.tmastco.com The pole is mounted in an RV flagpole mount I got at the Del Mar Fair.
Home Made Mast Mount A home made version of the tire mount. The flat part was laying around the barn. It could be duplicated with anything sturdy enough not to flex. Attached to the "tire holder" is a hardware store pipe flange. Screwed into the flange is a piece of pipe screwed to a female plastic PVC connector. Glued to the PVC connector is a length of PVC pipe to drop the push up pole into. Adjust your PVC pipe sizes to accommodate your push up pole diameter.
Measurements VSWR and more for the finished 80M Carolina Windom ( Adjusted for the General phone upper portion of 75M ) F MHzVSWRs 3.81.771 3.91.441 7.1752.420 7.32.520 14.352.731 Measured with an MFJ-269 antenna analyzer
160 Meter Windom Measured with an MFJ-269 antenna analyzer
Questions ? More details on this project, with links to sources, and other projects by W6CAW can be found at: www.craigwilliams.com/radio