What if any is potential benefit for adding filtering and amplification at antenna in meteor scatter detection arrangement at 143.050 MHz?
Andrew Thornett
Short answer: yes—put a low-noise preamp at the antenna and (usually) a filter right after it. You’ll lower the system noise figure by multiple dB and make weak pings at 143.050 MHz (e.g., GRAVES) much easier to detect. Numbers below.
What changes, quantitatively
Let’s compare a typical setup at 143 MHz. Assumptions (all very common with SDRs):
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Receiver noise figure (NF): 5 dB
-
Feedline loss from mast to shack: 2 dB
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LNA at the antenna: NF = 0.6 dB, Gain = 20 dB
-
Optional band-filter insertion loss: 2 dB
Using Friis’ formula:
A) No LNA (coax → receiver):
Overall NF = coax loss × receiver NF = 2 dB loss × 5 dB NF ⇒ ≈ 7.0 dB
B) LNA at the antenna, then coax, then receiver:
Overall NF ≈ 0.75 dB
→ ~6.3 dB improvement vs. no LNA (that’s roughly a factor of 4 in noise power).
C) Add a 2 dB filter
before the LNA:
Overall NF ≈ 2.75 dB
→ Still far better than no LNA, but you “spend” ~2 dB by filtering before the LNA.
D) Put that same 2 dB filter
after the LNA:
Overall NF ≈ 0.86 dB
→ Almost as good as case B, but with the interference protection of the filter.
Takeaway:
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The single biggest win is a low-NF LNA mounted at the antenna.
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If you need a filter, put it after the LNA whenever possible to avoid burning precious SNR.
-
Only put a filter ahead of the LNA if you’re fighting very strong out-of-band signals that would drive the LNA or SDR into compression.
Why it helps for meteor scatter
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Meteor echoes are often near the noise floor; cutting the system NF from ~7 dB to ~0.8–1 dB yields ~6 dB SNR headroom, turning marginal, short “pings” into clearly detectable events.
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A post-LNA band-pass (or FM/airband notches) prevents desense and spurs from strong 88–108 MHz FM, 118–137 MHz airband, and other VHF services without penalizing NF much.
Practical picks (what to aim for)
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LNA: NF ≤ 1 dB (0.5–0.8 dB if you can), Gain 15–25 dB, high IP3 (≥ +15 dBm) so it won’t fold under strong locals. Weatherproof, mast-mounted, with ESD/lightning protection and a bias-T.
-
Filters:
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Best: a narrow 143.05 MHz band-pass (helical/cavity).
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Or: a wide VHF pass plus deep FM/airband notches.
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Keep insertion loss low; if it’s >1–2 dB, place it after the LNA.
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Feedline: good coax (e.g., LMR-400/Ultraflex) to keep loss low, though the LNA largely “cancels” downstream loss for NF purposes.
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Gain staging: Total front-end gain (LNA + any post-LNA filter loss) of ~15–25 dB is usually enough before the SDR; more can invite overload.
Rule-of-thumb improvements you can expect
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SNR on weak pings: +4–7 dB vs. an un-amplified, long-coax setup.
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Dynamic range / clean spectra: markedly better with post-LNA filtering (desense and intermod products drop dramatically).
Richard Flagg
The analysis is incomplete because it neglects the relatively high level of noise at 2 meters due to the cosmic background noise.
Richard
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Andrew Thornett
Sent: Wednesday, November 5, 2025 7:32:32 PM
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] What if any is potential benefit for adding filtering and amplification at antenna in meteor scatter detection arrangement at 143.050 MHz?
Richard Flagg
The galactic background (due to electrons spiraling the the galactic magnetic field) adds noise to your receiving system. The amount of noise is a function of frequency.
At 20 MHz the equivalent temperature is close to 50,000 kelvin while at 150 MHz or so it is in the range of several hundred to a thousand degrees kelvin.
The signal to noise ratio of the meteor scatter signal is degraded at the antenna terminals due to this added galactic noise, and a low noise preamp will not be as beneficial as one might expect.
A preamp will usually help - just not as much as it would if the galactic background noise were not present.
Additional noise may be present at the antenna - particularly if you live in an urban area. In Honolulu the background noise level at 150 MHz is over 10,000 kelvin.
Installing a good preamp say with an equivalent temperature of a couple of hundred kelvin wont help much.
Another concern with using a broadband preamp is overload from strong FM stations.
A good FM band reject filter say with at least 50 dB of FM band rejection ahead of the preamp may be necessary depending on your location.
Richard
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Marcus D. Leech
So what do that do to change the analysis, Richard?
noise picture is dominated by the galactic background, and NOT your system noise.
As you move down in frequency, the galactic background noise becomes even *hotter*, which means that attempts to lower your Tsys aren't going to improve your
sensitivity to any great extent.
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Eduard Mol
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andrew....@googlemail.com
Thanks Marcus & Eduard
So, basically, no point in filtering or amplifying signals at 143.050 MHz………
Andy
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andrew....@googlemail.com
Thanks Richard
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Marcus D. Leech
Thanks Marcus & Eduard
So, basically, no point in filtering or amplifying signals at 143.050 MHz………
Andy
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Jeremy Waller
One cannot fault that analysis. Galactic noise etc. is added to the reflected signal from the meteor trail and this will constitute the input signal and this will have an SNR ... SNR_in. Added noise from the receive chain will only degrade this received signal (SNR_in).
The problem detecting this meteor scatter signal is one of detecting the meteor signal in noise. One may attempt to model the reflected signal ie. form a signature and use this to aid in the detection of the signal. Example: If the reflected signal is from the meteor trail then as the trail grows would we not expect to see multiple reflections in time? Multiple delayed reflections with each reflection coming from a different place in the meteor trail? etc. etc. etc.
The issue of interference mitigation is a different matter. One may adapt to the interference prior to the meteor signal. etc. etc. etc
73
Jeremy.