LNA Placement and Coax Loss at 1420 MHz

Most of the email I answer about weak results comes down to one of four things, and all four are about where the amplifier sits relative to the cable. I’ve made two of them myself. So this paper is less about theory than about the specific numbers that come out when you put real cable and a real LNA in the wrong order, worked through the receive-chain noise calculator so you can redo them with your own parts.

The rule, and why

Put the LNA at the antenna. Put the long cable after it.

The reason is the Friis formula. Every stage in a chain adds some noise, but the noise a stage adds gets divided by all the gain in front of it:

T_rx = T₁ + T₂ / G₁ + T₃ / (G₁ G₂) + …

A length of coax is a stage with gain less than one. Loss L, at physical temperature T, adds (L − 1) × T of noise, and it also reduces the gain that every later stage gets divided by. In front of the LNA, cable loss hurts you twice. After an LNA with 30 dB of gain, a thousand-fold, the same cable is nearly invisible.

At 1420 MHz cable loss is large enough that this isn’t a refinement. LMR-400-equivalent cable loses about 4.9 dB per 100 feet here. RG-8X loses about 10 dB per 100 feet. Those are the figures from LAB-006 and the calculator’s presets, converted to about 16 and 33 dB per 100 m.

The reference chain

Bench 1 is the calculator’s default and what we’d call well built: 30 cm of good cable and 0.1 dB of adapters between the feed and an LNA with 0.6 dB noise figure and 30 dB gain, then 30 m (about 100 ft) of LMR-400 equivalent, then an RTL-class SDR with a 6 dB noise figure.

Receiver temperature: about 58 K. The LNA’s own noise is about 44 K of that. The short cable and adapters in front add about 10 K. The entire 30 m run and the SDR together add about 3 K, even though the run is nearly 5 dB of loss. That’s what you’re aiming for: the LNA sets the floor and nothing after it matters much.

The 0.6 dB is what we measure on our SAWbird+ H1. The 30 dB gain is a deliberately round placeholder; use your own LNA’s datasheet figure. A higher gain only makes the rest of the chain matter less.

Mistake 1: spending money on the wrong cable

Bench 2 swaps the 30 m run for RG-8X. Receiver temperature goes from 58 K to about 66 K. That’s it. Integration time goes from about 1.2 s to 1.3 s in the calculator’s idealised case.

Now look at the bottom rows of Bench 2, which move the LNA to the receiver end of that same RG-8X: about 3,000 K. Integration time goes from a second or so to about sixteen minutes.

The lesson isn’t “RG-8X is fine.” It’s that cable quality matters enormously before the LNA and barely at all after it, so the place to put good cable is the short jumper at the feed and the place to save money is the long run. If you can only afford one length of LMR-400 equivalent, it’s better spent on a short, clean connection at the antenna than on the hundred feet to the house.

Mistake 2: “just a short cable” in front of the LNA

People mount the LNA indoors, or at the bottom of the mount where it’s easy to get to, with a “short” cable up to the feed. Bench 3 puts three metres of LMR-400 equivalent there instead of thirty centimetres.

Receiver temperature: about 95 K, up from 58 K. That three metres is only about half a dB of loss, and it costs you about 36 K, more than half again on the receiver. Integration time nearly doubles.

Mount the LNA on the feed. Bolt it to the feed support, tape it, put it in a small weatherproof box, but put it there. For a horn, screw it directly onto the probe connector.

Mistake 3: no LNA

It does happen: someone reads that the SDR has its own amplifier and assumes that’s enough. Bench 4 takes the LNA out entirely. Receiver temperature: over 3,300 K. That’s every decibel of the 30 m cable plus the SDR’s 6 dB noise figure, unrelieved.

You may still see the line with no LNA, given a big enough dish and long enough integrations. But the calculator’s idealised integration time goes from about one second to about nineteen minutes, and the real one scales with it. An LNA is the cheapest sensitivity you will ever buy.

Mistake 4: a long run with one LNA

Some sites need the receiver a long way from the antenna. Bench 5 is 60 m of RG-8X with one LNA at the feed. That’s about 20 dB of cable loss, and 30 dB of LNA gain is no longer enough to hide it: receiver temperature is about 165 K, almost three times the reference chain.

Bench 6 adds a second 20 dB stage at the feed, right after the first LNA. Receiver temperature goes back to about 56 K, slightly better than the reference chain, because now there are 50 dB of gain in front of the long run. The second stage’s own noise figure hardly matters; it’s divided by the first LNA’s gain.

So when your run is long, the answer is more gain at the feed, not better cable. A wideband amplifier such as the Nooelec LaNA is a reasonable second stage behind the filtered SAWbird+ H1. Put the filtered amplifier first so the second stage never sees the strong out-of-band signals, and check that your total gain doesn’t overload the SDR: if the noise floor is high and the spectrum looks flattened, back off the dongle’s gain.

Powering two stages over one coax needs a bias tee that can supply both, or a separate supply at the feed. Check current draw before relying on the dongle’s bias tee for both.

A note on temperature

The calculator has a field for the cable’s physical temperature, default 290 K. Lossy cable makes noise in proportion to how warm it is. In midsummer, coax lying on a sunny roof can sit well above that, and in winter well below. With the LNA at the feed this hardly matters. With it at the wrong end, the day-to-night temperature swing of a long cable shows up as a slow drift in your baseline, which is another reason the reference chain is easier to observe with.

Checking your own chain

Open the noise calculator and enter what you actually have: the lengths you’d measure with a tape, the cable type, the LNA’s datasheet gain and noise figure, and your SDR. Look at the “where the noise comes from” table. If any stage other than the LNA is more than about a quarter of the total, it’s worth fixing before you spend money on anything else, including the antenna.

Then read LAB-012 for what the improvement looks like on a real spectrum.

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