# Demo — Agility and Side-lobes
A spot jammer is only powerful if it can find the radar's channel. Frequency agility forces the jammer into a losing choice: a strong spot that the radar simply hops around, or a barrage spread so thin it barely registers. This demo makes that trade quantitative.
## The trade
A jammer that covers $W$ of $N$ channels denies a fixed radar only if it happens to sit on the radar's channel. Against an agile radar it lands a hit with probability $W/N$. If it instead barrages all $N$ channels, its power per channel drops by $10\log_{10}N$ dB.
## Interactive demo
Open in full screen
## Walkthrough
1. **Fixed radar, narrow jammer.** With the radar parked and the jammer off its channel, nothing is denied. Slide the jammer onto the radar's channel and denial jumps to 100% — a fixed radar lives or dies on one frequency.
2. **Switch to agile.** Now the radar hops across all $N$ channels. The denied fraction drops to $W/N$ — the jammer only catches the pulses that happen to land under it.
3. **Widen the jammer.** Increase $W$ and the agile denial rises, but the jammer is spending power to cover more spectrum.
4. **Force the barrage.** Read the barrage-dilution number: spreading across all $N$ channels costs $10\log_{10}N$ dB per channel — at 16 channels that is 12 dB the jammer simply gives away.
## Key observations
- **Agility converts a power contest into a coverage contest** — and coverage is expensive for the jammer.
- **Spot vs barrage is a dilemma, not a choice.** Concentrate and be dodged; spread and be diluted.
- **More channels, more leverage.** The defender's advantage grows as $10\log_{10}N$.
## Source
MATLAB · code/L15_FrequencyAgility.m↓
The in-class script sweeps the number of channels and jammer width, compares fixed against agile denial, and computes the barrage dilution penalty.