“Gain” is not a PIM cure...it’s a power discount coupon!

(Context) Low-PIM hardware is real and worth having in a repeater build. But PIM is a system problem...and “more gain” is not a magic linearity button.

A repeater site is a brutal RF environment: your receiver is trying to hear microvolts while your transmitter is blasting tens of watts (sometimes more). Anything that turns a little bit of that TX power into in-band garbage can look like a raised noise floor and steal real coverage.

One of the most misunderstood claims in the repeater world is: “Get more antenna gain and your PIM issues will fade away.”

That’s not how PIM works. But here’s the twist: higher antenna gain can absolutely help your repeater’s PIM performance...just not for the reason most people think.

Why PIM is a repeater killer

Passive Intermodulation (PIM) is unwanted signals created when two or more strong RF signals mix in a non-linear passive junction. Think loose or corroded connectors, dissimilar metals, questionable adapters, oxidized hardware, or anything that behaves like a tiny diode under RF stress.

For repeaters, the part that matters is simple: PIM products can land where your receiver is listening. That can desensitize the receiver (or just make it “feel deaf”) even when your duplexer and frequencies look “correct” on paper.

The myth: “More antenna gain inherently means less PIM”

False.

Antenna gain is a far-field property (directionality and pattern shaping). PIM is born at the ugly stuff: junctions and materials that behave non-linearly under high RF current and voltage.

If you change from (say) a 6 dBd to a 12 dBd antenna but keep the same transmitter power at the connector, you did not automatically make your RF path more linear. The same watts still stress the same connectors, jumpers, duplexer ports, and antenna feed network.

• Gain does not magically reduce PIM generation.
• Keeping TX power the same means the “PIM stress” is mostly unchanged.

What that “-153 dBc” number really tells you

Most “low-PIM” specs come from a two-tone test (two strong carriers). The result is reported in dBc...how far below the carrier the PIM product is.

A very common benchmark is two +43 dBm (20 W) tones per tone. So when you see -153 dBc (often for IM3), that points to a very low level of intermod relative to those carriers.

(Important) This is meaningful for a component. It does not magically describe your installed repeater site...because the site includes the duplexer, jumpers, adapters, mounting, nearby metal, weathering, torque, corrosion, and everything else that can become a PIM generator.

The truth: gain helps only if you “spend it” on less transmitter power

Here’s the repeater hack that actually moves the needle:

For third-order products (IM3), the classic idealized rule is: 1 dB TX power change can move IM3 by ~3 dB (real life varies, but the trend is real).

The 60-second math that makes this click

Let’s say your antenna system gain improves by +6 dB (example: 6 dBd → 12 dBd) and you want the same ERP in the direction you care about.

Very simplified:

ERP(dB) ≈ TX Power(dB) − losses(dB) + antenna gain(dB)

To keep ERP constant, a +6 dB gain increase means you can drop TX power by about 6 dB.

• 50 W ≈ 47 dBm
• 47 dBm − 6 dB = 41 dBm ≈ 12.5 W

Now apply IM3 scaling:

• ΔPIM ≈ 3 × (−6 dB) = −18 dB (absolute PIM reduction)

That’s huge. Even if your datasheet still says “-153 dBc,” the absolute interference power your receiver is suffering from can drop a lot when you turn the transmitter down.

So does the “PIM noise floor” go down with more gain?

Not automatically.

If you bolt on a higher-gain antenna and keep blasting the same TX power:

• PIM generation in your RF path is not guaranteed to improve.
• In some geometries you may illuminate nearby metal harder in your main lobe, changing external PIM coupling.

But if you use gain to reduce TX power for the same coverage, then yes...often dramatically. You reduced the single biggest driver: carrier power at the non-linear junctions.

In a repeater where the receiver is “on” all the time, this can show up as:

• less desense,
• a cleaner in-band noise floor,
• better weak-signal access at the edge of coverage.

A repeater-friendly way to think about gain & PIM

Gain is not a PIM cure. Gain is a power discount coupon.

If you cash it in by turning the transmitter down, you often get:

• lower PIM,
• lower thermal stress,
• lower “mystery IM” risk in marginal components,
• and sometimes better overall balance because the receive path benefits from the same antenna pattern/gain.

Practical repeater checklist: how to use the “gain → less power” trick

• Don’t ruin your coverage shape. Higher-gain antennas often narrow the vertical pattern. That can be great for flat terrain...or terrible if you need fill-in close to the site.
• Actually reduce transmitter power. If you keep power unchanged, you threw away the main PIM benefit.
• Keep the RF path boring. PIM loves adapter stacks, questionable jumpers, loose connectors, weathered seals, corrosion, and “rusty bolt” mounting hardware.
• Validate like a repeater owner, not like a brochure. Watch RX noise/sensitivity while keying the transmitter. If the site is shared, observe behavior while other nearby transmitters are active too.

Bottom line

A higher-gain antenna does not inherently reduce PIM. But higher gain can let you run less TX power for the same ERP...and that’s where the PIM improvement can become very real, very fast.

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