Rhombic Antenna Patterns Page
I have experimented with the NEC source code in order to model rhombics of various dimensions and design parameters in free space, over real ground and at various frequencies. In order to do this, I have used WinNEC (a windows interface sold by Paragon Technology, 200 Innovation Blvd, State College, PA 16803 tel: 814-234-3335; that allows the user to manipulate the underlying NEC code to design antennas). The interface uses a spreadsheet format to simplify the entry of data. Surprisingly complex antennas can be modelled with remarkable accuracy. Gains are automatically determined and you can model a surprising (almost limitless) variety of antennas. Using WinNEC you can also apply phasing lines, terminations and antennas can be examined over various ground types.
I have modelled three rhombics as examples: the ARRL handbook "compromise rhombic" (210 feet per leg, 67 degree tilt angle and 377 feet end to end), the World War II Signal Corps Rhombic optimized for 3000 mile point to point communications (around 560 feet end to end with around a 70 degree tilt angle) and two of the World War II rhombics fed end to end. All of the designs are terminated.
In my hands, the modelling seems to infer that the maximum useable size of a rhombic is about 5 wavelengths. Much larger than this and the main lobe will split into a "V" pattern and you gain little else. A practical gain of around 16-18 dBd seems to be the peak of gain (substantially better than just about any other antenna for HF). In Harper's rhombic book the inner leaf has a picture of a rhombic with about 20 fed end to end. I haven't yet had the time to enter that into my program!
Patterns (gain is dbi) Here are some examples of antenna patterns for the rhombic antennas above:
I am planning at some point to experiment with either a 440 or 1296Mhz rhombic to test the observations above, especially the issue of maximum/optimum size. It has been said that RCA did some HF modelling and found that there is a maximum limit to the real-world gain of the rhombic (I haven't found a published account of same). I have recently obtained plans for a pair of UHF rhombics fed in parallel (one design is actually a pair of trapezoids and the design is called a dual rhomboid antenna).