Well, the weather has finally stabilised a bit this week, allowing some thoughts to return to antenna experimentation.
Very much encouraged by the spectacular results over the past year or so at the coast, I decided to experiment with a vertical half rhombic antenna - an antenna type that doesn't get a lot of attention.
The basic expectation is that the real half of the antenna will, at the coast, have an extremely good image in the ground, and thus give near full-rhombic performance - with the added benefit of gain from very low angle radiation.
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| Basic idea of the half rhombic, but using a counterpoise wire. I will start without such a wire, using a direct earth connection at each end instead. |
The rhombic can be bidirectional when non-terminated, or unidirectional when terminated. As it happens, a very ambiguous and incomplete set of results with a Vee-beam last year meant I already have some high power resistors to try with the new half rhombic.
I figured that a wire length of about 34m worked out at roughly 4 half wavelengths total wire at 18MHz, 5 half wavelengths at 21MHz (an odd number, which won't work, according to some texts), and 6 half wavelengths at 24MHz.
A 4:1 balun transition from balanced line to a short coax-to-rig connection should hopefully allow a good match at 18 and 24MHz wavelengths, and also see what happens on the odd wavelength multiple at 21MHz.
The 'outer' of the feedline will be connected to a simple copper ground stake, as will the terminated end. If this fails, I'll have a go at a counterpoise. The near-perfect beach ground should render this redundant, though.
The plan is now to set up the antenna one early morning on 18MHz at the beach, with the terminated end aimed to the East.
We'll see how it goes! Update: here's how it went initially.
3 comments:
Well, that is a large antenna John. I'm very curious about the results. How much gain will it theoretically have over water into the east? Have you ever considered een VDA setup? 73, Bas
Hi Bas. According to the ARRL Antenna Handbook (22nd edition), the gain of a 3-lambda full rhombic is 15.5dBi at 14MHz, but that's for average ground. Some texts say that an odd number of wavelengths won't work. There is probably more uncertainty about the real-world performance of Vee-beams and other such long wires than any other, because they don't seem to be widely used. So it will be interesting to see where I go with this one...
Yes, I have considered the VDA. They seem quite good, though mechanically, and for one-man deployment on exposed coasts, I think they are probably a bit of a handful. That said, I should try and build one one day, as well as a vertical Moxon, which has been on the cards for a long time...
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