is an asphalt ground plane reflector equivalent to a grassy field ground plane reflector? (lemmy.world)

submitted 7 months ago* (last edited 7 months ago) by [email protected] to c/[email protected]

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I’ve been working on improving performance and portability of my V-dipole for NOAA weather satellite reception.

Previously, I had the antenna mounted to a PVC pipe stuck in the dirt in my backyard at the appropriate 0.44~ish meters.

Today, I tried it in the front street with a new free standing mount. Despite being at the appropriate height, SNR was terrible unless I physically touched the antenna rod connected to the center conductor. Also, there’s already a ferrite choke on the cable close to the antenna.

I’m trying to work out how much could be different between my front road and backyard. Is it possible that the layer of asphalt is not acting like an appropriate ground plane mirror? Should I create a physical ground plane? For the record, grass was wet and asphalt was dry.

I’ve ordered a cheap NanoVNR to investigate, but in the meantime, I thought I’d ask since I only get so many chances to test the antenna.

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[-] [email protected] 3 points 7 months ago

A ground plane needs to be conductive, wet soil with grass usually has ~1Ω per meter, asphalt is closer to 10,000Ω per meter.

[-] [email protected] 1 points 7 months ago* (last edited 7 months ago)

Good to know, thanks!

I tried it again today with a section of chicken wire spread out beneath, but results were similar. I suspect my antenna's measurements are off somewhere.

Edit: Actually, looking back at the images, performance was definitely improved. I'm just kind of annoyed that touching the center conductor seems to vastly improve performance. It's a very simple antenna, and I've double checked all my measurements.

[-] [email protected] 1 points 7 months ago* (last edited 7 months ago)

I’m just passing through via all but i happen to be a ham and enjoy antenna design. Im not 100% sure on what you’re doing but I believe dipoles do not need a ground plane.

Where does the .44m come from? Typically dipoles are mounted at least one wavelength above the ground which would be about 2m for 137mhz.

[-] [email protected] 1 points 7 months ago

I think the goal is to sacrifice signal for directionality. If you go a full wavelength, you introduce lobes in the radiation pattern that will kill your signal as the satellite passes overhead. The "rule of thumb" for NOAA V-dipoles is anywhere from 0.4 to 0.6m with 0.44m being best. Not sure where that exact number comes from, but it's close to 1/4 wavelength.

this post was submitted on 14 Feb 2024

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