Would adding a second slim jim antenna improve my reception?

I just got my station working, and though it is clear that there is a satellite in most cases getting data, eg NOAA APT, isn’t as good-you can kind of make out clouds, but it’s mostly noise.

Would making an additional slim jim and then attaching it to the same coax cable help, or do I need a different antenna style/directional to get any better?

Hi @alex_c-j

Please provide a reference to what you are trying to improve…

I don’t have much experience with this type of antenna, but from the design, I would guess that it has a very poor radiation pattern at high elevations. A simple V-dipole might give you better results.

Combining two or more antennas is not trivial and is the last place I would try to optimize. You need proper impedance and phase matching, otherwise, you end up with the performance of a rusty nail.

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I have absolutely no idea what most of those words meant but, uh, I’ll look into a V-dipole. How would I improve the impedance and phase matching?

Edit: How well would the V-dipole work with non polar satellites?

A Slim Jim antenna is similar to a J-Pole antenna, not only in terms of shape and size, but also technical characteristics: they share a very similar radiation pattern, with the main lobe pointing to the horizon and not the sky (assuming the antenna is physically pointing upwards). Even if you point it to the horizon, your “back” lobe will be exposed to a lot of noise from the ground (due to blackbody thermal radiation emitted by the Earth), so given your application, this is not an antenna you’re looking for.

Before proceeding to build a V-dipole (or any other antenna), can you inform me of your setup (from antenna to SDR)? Do you use an low-noise amplifier (LNA) at all?

How would I improve the impedance and phase matching?

When you electrically combine two antennas together via a coaxial cable (usually rated at 50 Ω), the impedance will change. This will increase your voltage standing wave ratio (VSWR), ultimately reducing the sensitivity of your setup. Even if you use a power combiner to deal with this issue, you will still need to make sure the antennas have a similar radiation pattern (i.e. pointing to the same direction) and the signals are added in-phase. To achieve in-phase addition, you may need to introduce an analog time delay before combining the signals together, by extending the length of one of the transmission lines (coaxial cables) appropriately:


(If the antennas receive the signals at the same time, then there is no need to introduce a phase delay.)

But again, I wouldn’t build a phased array for such application. I would just pick an LNA.

Cheers,
Apostolos

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We have a slim jim wired into a coax cable that goes 2-3 metres into the RTL-SDR. Thanks for the help. I’ll see if we can do a V-dipole as a project. Is a LNA necessary?

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An LNA will definitely help. The most important benefit you’ll get is to account for the transmission line losses which will be a few decibels given a 2-3m coax. You obviously don’t need a first-class LNA: an SPF5189Z-based LNA offers low noise figure and adequate gain to help, at a relatively low cost.

Is your antenna still indoors sitting next to (or near) the raspberry pi? If yes, the very first thing you should do is work on getting the antenna a clear view of the sky (putting it outdoors). Have you been able to figure out what kind of roof you have? You should be able to tell just by looking at it from outside. If it is indeed metal, you have almost no chance of getting good reception - even with an LNA, if you decide to keep the antenna indoors.

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