The N2CKH Travel Special 300 Ohm ladder line convertable antenna
An almost all band light weight low loss resonant antenna with gain on certain bands in certain configurations and cheap to boot, to good to be true, not really.
This design supports configuration of the antenna as various 80m 1/2 wave Dipole configurations or an 80m 1/4 wave vertical with counterpoise or a 40m Delta Loop or a 40m Folded Dipole or a V Beam: This antenna is of my own design and is fed with 50 feet of 300 ohm twin lead and can be used through 10 meters (maybe even 6 meters) with an antenna tuner. Where the antenna is resonant a small 4:1 balun can be used with a short length of coax. I have a male banana plug on the end of the twin lead to prevent damage to the fed leads during travel.
The requirements for the design of this antenna were many. First was is a light portable antenna that is relatively easy to transport and erect. A design that can take advantage of various mounting opportunities. A design that could take advantage of the low loss properties of the 300 ohm feed line. Next is to have an antenna that can be resonant on more than one band depending on the configuration without the use of an antenna tuner. A design that will easily handle 100w and much more. A design that could be supported by using 1 1/6 inch dacron rope or nylon fishing line as the center/end supports and tie downs.
As an 80m (about 3.76Mhz) resonant 1/2 wave dipole it can be configured as a Flat Top, Inverted V or Sloping Dipole, or from about the 19 floor of the hotel a Vertical 1/2 wave Dipole.
As an 80m resonant 1/4 wave vertical with counterpoise from a 10th floor hotel room.
On 40m the antenna can become a resonant (7.1Mhz) Delta Loop by connecting the ends together.
On 40m the antenna can become a folded Dipole.
The antenna can be come a resonant a V Beam (Half Rhombic) antenna.
How is all of this done ?
The construction of the antenna uses three (3) Alhpa Delta DELTA-CIN with an extremely rugged UV and RF resistant end-insulators. A very small XXX end insulator as the feed point insulator for attachment of the 300 ohm twin lead. It also uses two #10 wire lugs and stainless stell #10 wing screw and wing nut. The wire used is two pieces of teflon jacketed of #14 to #18 depending on the weather environment, configuration used and length of time it will need to be in the air. You need 62.25 feet of wire is between the center insulator and the end insulator on each leg. I recommend jacketed wire for travel as it wont kink, 14 ga. teflon jacket would be great.
The three DELTA-CIN end insulators are not actually attached to the antenna. One end of each has the jacketed wired fed through the eyelet if you never plan to use it as a folded dipole. (If folded dipole use is planed, the wire is fed through both holes of the DELTA-CINs that are closest to the center feed point of the antenna. When supported as a folded dipole the end tie downs should go around the center of the body of the two DELTA-CIN connectors that are separating the folded dipole halfs) The other end has the 1/16 inch dacron rope or nylon fishing line for tie down attached. Two of the DELTA-CIN insulators are to the left of the center insulator between it and that antenna end wire lug. The other DELTA-CIN is on the other leg of the antenna.
On 80/75m from a hotel balcony all of these configurations are possible with the flat top being the hardest to achieve. I personally prefer the sloping dipole configuration in most cases. The two far end DELTA-CIN are brought out to and in contact with he wire lugs and used as end supports/tie downs. The third (3rd) DELTA-CIN that is immediately to the left of the center insulator feed point can be left handing or can be looped over the center insulator/feed point and attached to the other side of the antenna with a nylon tied or dacron/fishing line to add more physical support at that point in the antenna for the twin lead weight if so desired for that days installation as a Flat Top dipole or Inverted Vee. As a Sloping dipole or Vertical dipole it can be left dangling.
For the 40m Delta Loop the feed point would be at the lower left of the antenna on the bottom horizontal wire with the DELTA-CIN tie down immediately to the left, while the right side DELTA-CIN down down would be some 41.5 feet away. Then you would have 41.5 feet of wire from each end tie down at a 45 degree angle up to the top center support DELTA-CIN to a tree branch or some available object that was preferably non-metallic in nature. If the DELTA-CIN to antenna wire needed securing at all, some of the tie down dacron rope or nylon fishing line could be looped over or a nylon tie could be used in very windy environments.
40m Folded Dipole. With 2 inches spacing using two nylon ties on the DELTA-CIN end insulators with the supporting dacron or fishing line being tied around the center of the two DELTA-CIN for end supports The third (3rd) DELTA-CIN that is immediately to the left of the center insulator feed point can be left handing or can be looped over the center insulator and attached to the other side of the antenna with a nylon tied or dacron/fishing line to add more support at that point in the antenna for the twin lead weight if so desired for that days installation. Then adding a jumper length of wire shorting the wire lug ends together to adjust for frequency resonance. This length can be fixed by cutting the wire to length and putting lugs on each end or have a length of non-insulated wire that is attached between the lugs to adjust the frequency. In either case and additional set of wing screw and nut hardware is required. The resonant frequency will be above the 40m band around 7.8Mhz (good for MARS type operation) without this additional length of wire. You could also use the antenna on other bands with the tuner and have an efficient antenna system. The currents on each wire will begin to cancel each other out on even multiples of the cut frequency, so a 40 meter folded dipole should not be used on 14 MHz. On other bands even though the signal may cancel broad side to the antenna, you'll find that there is actually gain! This occurs about 45 degrees off broad side to the antenna. And this might make for interesting portable contacts. It can be mounted as a Flat Top or Sloping.
For the V Beam we are tilting the antenna from its 80m Inverted V configuration so that the legs are now both oriented in the same direction and at a 45 degree angle. A V beam is simply two long wires feed 180 degrees out of phase. Each wire has two lobes. If the apex angle of the V is made two times angle "A", the lobes inside the V add together yielding an extra 3 Db over a single wire. The lobes outside the V cancel each other out so the resulting pattern looks with a large forward lobe and an approximately half size reflected lobe.
Note that the smaller reflected lobe is 180 degrees from the main lobe. This lobe is formed by the signal waves crossing the wire starting at the feed point and sweeping across the wire toward the end point. When the induced voltage hits the end point, it is reflected, and goes back down the wire to the feed point and then into the receiver. This makes the V beam Bi-directional. In real life there will also be some small secondary lobes that appear at various angles off the main lobes. The pattern is similar to that for a rhombic. If you don't want the reflected lobes, you can ground the end points of the V beam through 400-600 ohm non inductive resistors, for travel power levels this can easily be done with carbon resistors. This will provide a path to ground for the signals that come in from behind the feed point, making the antenna essentially unidirectional.
Since the V beam is a non resonant antenna, It really isn't "cut" for any band but the relationship between the length of the legs and the apex angle of the V determine the band on which it will be most "focused". Generally you get useful gain from 1/2 to 2 times the frequency at which it is "focused". If you design for maximum focus on 14 MHz, it will work well from 7 MHz to 28 MHz.
A Rhombic is simply two V beams joined to form a diamond. The two extra legs add another 3 Db to the gain of the V beam or 6 Db over a single long wire. This is similar to stacking 4 yagies to increase the gain over a single yagi by 6 Db. One characteristic that is unique to the rhombic is the ability to design the antenna to produce maximum gain at any vertical wave angle. By changing the relationship between leg length, tilt angle (how fat or skinny the diamond is) and the height above ground you can design for very low angles for radiation with respect to the earth. This is one reason that Rhombics do so well with DX. Like the V beam, if you want a unidirectional antenna, simply connect the far ends of the antenna wires to a 600 ohm non inductive resistor. This will provide very strong front to back rejection. However, basically taking two of these on travel and having the space and time to set up a Rhombic is out of he question, setting up just the V Beam on travel is hard enough. I find that off a balcony, with a piece of dacron rope suspending the feed point 25-40 feet away from the building works quite well. Being on the second or third floor and facing a very little used and grassy side of the hotel is usually best.
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Entire contents Copyright © 1999-2001 by Stephen B. Hajducek, N2CKH. All Rights Reserved Worldwide.