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I have the K1JEK Cobra UltraLite and it works exceptionally well. However that was when I had a larger yard. Moved to a house with a smaller lot and now can't use the JEK Cobra (max span between trees is 80 ft, antenna is about 108 ft). I thew up a temporary 40M dipole but it's turning out to be more of a cloud warmer than performer. Could I cut the Cobra UltraLite down to 75 feet or would I have issues with resonance? Besides a vertical, what other options do I have for 75, 40, and 20 meters which have good DX performance?

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  • $\begingroup$ DX performance is primarily about the signal take-off angle, and take-off angle is primarily about height above ground in terms of wavelengths. Higher above ground means lower take-off angle means better long-distance properties (at the expense of short-distance beyond ground wave), and for DX you ideally want the antenna 1/2 - 1 wavelength above ground. Since I doubt a "temporary" antenna would be suspended 20+ meters up: how high above ground is your current antenna? And what frequency are you targetting for DX work? (DX at 4 MHz and DX at 14 MHz are two very different beasts.) $\endgroup$ – a CVn Nov 2 '13 at 17:13
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According to the website, they are 41', 73' and 140' long. Assuming you have the 140' model, do you have more than 2 trees such that you create a hoizontal V? A shallow angle V isn't going to affect the pattern noticably. You could also go with an inverted V especially if you can get the center high enough. Or turn it into the 73' model and accept the loss of 160m (probably).

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Build a set of square full wave loops, one for each band.

Note this antenna, while taking up similar vertical space to a 1/4-3/8 wave vertical takes much less side to side than a dipole (1/4-3/8 wave vs 1/2 wave), and is horizontally polarized when fed from the bottom.

If you want DX, the loop should be in the vertical plane and oriented so that the perpendicular from the plane of the loop points towards two spots (front and back) that are desirable to contact.

Gain has a price and edge on will suffer, a bit more than edge on with a dipole.

For a 80m square loop it is obviously 20m on a side, or about 66 feet. The hard part will be getting the top leg 66 feet above the bottom leg.

If you make it diamond shaped instead of square the top will be higher: 20m * sqrt(2), which is 20m*1.414 = 28m = 92 feet from top apex to bottom and also side apex to side apex. This violates your side to side size limit of 80 feet. So if you want an 80m loop you have room for the square variety if you can figure out how to suspend a wire up high. If you have two trees that are at least 70 feet high and 66 feet apart, it may be worth considering. There are a number of devices that can get wires to height, and as I'll show below a perfect square shape is not required.

Also, 20m and higher frequency versions are proportionally smaller and easily done on the lot you describe.

It is useful to have an antenna analyzer to use the exact length of wire that will resonate at the frequency of interest. However, the resistance component of impedance at resonance is not 50 ohms, so you'll need a balun transformer (try 2:1 or 3:1) or a homemade air wound balun coiled up from an electrical 1/4 wavelength of 75 or 92 ohm coax.

You don't need to get the shape perfectly square, as shown in this picture of a 15m beach loop below I made from 12 gauge solid wire, a jackite kite pole, and a bamboo cross arm. A coil of white 75 ohm coax acting as the transformer/balun can be seen a little below the bottom apex of the loop.

Full wave loop for 15m beach portable use

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