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I'm trying to do some basic radio astronomy experiments, and I'm currently working on picking up radio bursts from Jupiter. These are emitted at around 20MHz; for all intents and purposes, Jupiter is a point source, so a directional antenna (with a reasonable bandwidth) would be ideal for this. However, these seem to be, at around 20MHz, large - a Yagi antenna would be something like 5m long. I'd like to be able to take this to observing sites and attach it to a tracking mount, so is there any way to "shrink" the antenna below this sort of size, or is the quarter-wave length going to be a hard lower limit?

edit: forgot to mention; I also have a meter-long telescope that it can be mounted on - so long and narrow is preferred to wide and short.

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    $\begingroup$ There are things you can do, a quarter wave isn't a hard limit, but there are hard limits of a sort. Directionality, efficiency, small size: pick (at most) two. $\endgroup$ Apr 14, 2022 at 19:16
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    $\begingroup$ To give you an idea of antenna size for the 20MHz region, see: radiojove.gsfc.nasa.gov/radio_telescope/ant_manual.pdf It is not going to fit on a tracking mount. $\endgroup$
    – glen_geek
    Apr 14, 2022 at 19:19

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Best alternative for directionality with small (compared to wavelength) antennas is to use more than one, and make a phased array. Perhaps a grid of 4 or more small loop antennas spaced a half wavelength apart, steered by DSP phasing.

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A loaded yagi is a possibility. Elements can be reduced as much as 50% of full size using a loading coil on each side, and still maintain some gain and directionality. At 50% full sized, element lengths would be about 12 ft. at 20 mHz. Normal element spacing needs to be maintained. That way, a three element, loaded yagi would occupy about a 12 x 12 ft. area. Feed with a balun.

Look into horizontal loops close to, or on the ground, and particularly if you are in a high electrical noise environment. Really. They are not gain antennas, but have other desirable attributes for your case. Their radiation pattern is much like a basketball sitting on the ground, with lots of high angle directivity. Coupled with their extremely low noise characteristic, particularly the loop-on-ground, they may work. I use a KK5JY loop-on-ground fed with a balun for my SWLing and it is an amazing antenna. I hear things on it I'd never hear on a suspended antenna due to high noise environment. Noise floor at and above 15 mHz is under -110 dBm. Feed with a non-grounding balun. Use a pre-amp if necessary.

KK5JY loop article: http://kk5jy.net/LoG/

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