Rhombic Antenna, 30m-6m
Rhombics are high gain directive wire antennas. A rhombic is shaped like a diamond, if you’re looking down on it. I.e., there are 4 ‘legs’, 2 end ‘points’, and 2 side ‘corners’.
This one was designed for 10 meters and 6 meters, but it’s proven useful all the way down to 30 meters. Gain figures are:
- 6m 11db about equal to a 7el yagi
- 10m 10db about equal to a 5el yagi
- 15m 8db about equal to a 4el yagi
- 20m 6db about equal to a 3el yagi
- 30m 4db about equal to a 2el yagi
(gain figures are referenced to a horizontal dipole at the same height)
This particular rhombic is 120′ long, 70′ on a leg, 25′ high, and it’s a “terminated” rhombic, which means:
- it is not a resonant antenna
- it is inherently broadband, 2 octaves or even more
- it does not require a tuner…if you can match it to the xmitter properly!
Rhombics have a high input impedance of about 600-800 ohms, so it’s a non-trivial matter to achieve a match to 50 ohm coax while still retaining the very broadband nature inherent to the terminated rhombic.
Matching the Terminated Rhombic:
After much wailing and gnashing of teeth (i.e. experimentation with many different balun transformer designs), I came to the conclusion that the books are right; you just can’t get a ferrite balun xformer to work well at both high frequencies and high impedance. About 200-300 ohms is the limit for “broadband” work.
This led me to do some research on “tapered lines”, the result of which is shown in the pictures here. After several prototypes, I settled on the tapered 2-wire transmission line shown here. I used a modified exponential taper, from 10.00″ to .300″, over a length of 33′, creating a broadband linear transformer from 750 ohms to 200 ohms. The 200 ohm end feeds into a ‘normal’ 4:1 ferrite balun, which completes the 16:1 ratio required to match to 50 ohm coax.
Results with Tapered Line:
With this setup, I’ve finally achieved a good SWR over the entire bandwidth of the antenna; less than 1.3:1 from 10-60 Mc. At the important (to me) 15, 10, and 6m bands, the SWR is under 1.2:1. I was never able to get less than 2.5:1 when using two 4:1 baluns in series, so to me this represents a big success.
Tapered line also has the interesting property, unlike ferrite xformers, of being able to handle lots and lots of power…. Now I need to work on a higher power terminator; and then I’ll be able to run an amp on the rhombic if I choose. The current terminator is rated 50w continuous; enough for a 100w transceiver, but not for a linear.
The picture above is a shot looking up the feed-end tree; showing the tapered line and the wire-supports. The entire antenna “floats” in its supports. I.e., it is only firmly attached down at the terminator end. At every other corner, the wires slide freely through their “pulleys”, which are poly egg-insulators. This design automatically equalizes tension on all legs, and has proven to work very well. The antenna is always flat, and it handles high winds and tree-branch hits with ease.
The rhombic itself is made from #18 enameled copper wire, and the tapered-line is made from #18 stranded tinned-copper PVC insulated wire.
The 1-gallon jugs are the counterweights used to tension the antenna. The amount of water was adjusted to produce the correct tension in the wire.
It came as an unpleasant surprise to me that it’s very difficult to take good pictures of thin wires against the sky and trees! In any case, below is another shot of the tapered line (red vertical wires). You can see some of the “spreaders”, which were cut from IC-tubes.
The extra ropes in the picture are the halyards for the ends of the 80m and 40m dipoles, which also use this tree for support.
I’m going to try re-shooting these pictures when the sky is a ‘white’ overcast, in hopes that the wires will be more visible. I also hope to get a better focus. Usually this Oly D500-L shoots outstandingly sharp pics. Maybe I was vibrating… Stay tuned…
It took a lot of clearing to put up this antenna! Three hundred and fifty feet of this….but hey, we needed the firewood anyway.