L
Admin
A “standard gain horn” (SGH) is the cheapest absolute-gain transfer most benches own — and one of the easiest ways to invent a 1–2 dB lie that then infects every antenna you certify against it. A few habits that keep the curve honest.
**1. What the manufacturer curve assumes**
- Free-space, far-field, boresight, stated polarisation, and usually a **well-matched** feed.
- It is not a warranty that your chamber + mast + cable + adapter stack will reproduce that number.
- Habit: treat the SGH curve as a **transfer reference under documented geometry**, not as an absolute field strength you can paste into a compliance report without a site check.
**2. Three-antenna vs substitution — pick deliberately**
- **Three-antenna:** three unknown (or two unknown + one SGH) antennas measured pairwise; solve for three gains. Catches a bad SGH curve and a bad AUT in the same campaign — but needs stable geometry and three pairings.
- **Substitution (comparison):** SGH in, AUT in, same distance / height / polarisation / cable. Faster; **transfers every SGH + site error straight into the AUT**.
- Habit: if you only ever substitute, run a three-antenna (or at least SGH-vs-SGH) sanity on the same range when the SGH is new, after a drop, or after a flange rework.
**3. What usually moves the reading more than the AUT**
- **Mismatch:** SGH VSWR is not zero. Use a good coupler / VNA-derived mismatch factor, or at least the same pad in both legs, before you argue about 0.3 dB of AUT gain.
- **Cable / adapter / polarisation:** swap the SGH for the AUT **without** changing the receive chain dress. A different adapter on the AUT path is a different measurement.
- **Proximity / multipath:** small chambers put the SGH in the near-to-far transition and light up absorber seams. If sliding the mast ±λ/4 moves the SGH reading more than your tolerance, fix the site before you publish AUT gain.
**4. A short acceptance gate for the SGH itself**
1. Visual: flange face, alignment pins, no dented aperture.
2. Match at the use band with the production adapter (not a random SMA saver stack).
3. Substitution against a second known horn or a prior dated SGH trace at the same marked position — peaks should overlay within your agreed budget (often ~0.5–1 dB setup-dependent).
4. Log distance, height, polarisation, cable ID, and whether the curve you used is vendor PDF or your own three-antenna solve.
**5. What an SGH does *not* prove**
- AUT efficiency (needs Wheeler / reverb / pattern integral).
- Far-field compliance of a product (needs the product geometry and the correct limit method).
- That your mmWave AUT gain is right when you used a microwave SGH curve outside its calibrated band.
**Discussion**
On your last absolute-gain job, did you three-antenna or only substitute — and when the number looked high/low, was the first culprit mismatch, cable/adapter, or chamber multipath rather than the AUT?
— rfforum 编辑部 / Editorial Team
**1. What the manufacturer curve assumes**
- Free-space, far-field, boresight, stated polarisation, and usually a **well-matched** feed.
- It is not a warranty that your chamber + mast + cable + adapter stack will reproduce that number.
- Habit: treat the SGH curve as a **transfer reference under documented geometry**, not as an absolute field strength you can paste into a compliance report without a site check.
**2. Three-antenna vs substitution — pick deliberately**
- **Three-antenna:** three unknown (or two unknown + one SGH) antennas measured pairwise; solve for three gains. Catches a bad SGH curve and a bad AUT in the same campaign — but needs stable geometry and three pairings.
- **Substitution (comparison):** SGH in, AUT in, same distance / height / polarisation / cable. Faster; **transfers every SGH + site error straight into the AUT**.
- Habit: if you only ever substitute, run a three-antenna (or at least SGH-vs-SGH) sanity on the same range when the SGH is new, after a drop, or after a flange rework.
**3. What usually moves the reading more than the AUT**
- **Mismatch:** SGH VSWR is not zero. Use a good coupler / VNA-derived mismatch factor, or at least the same pad in both legs, before you argue about 0.3 dB of AUT gain.
- **Cable / adapter / polarisation:** swap the SGH for the AUT **without** changing the receive chain dress. A different adapter on the AUT path is a different measurement.
- **Proximity / multipath:** small chambers put the SGH in the near-to-far transition and light up absorber seams. If sliding the mast ±λ/4 moves the SGH reading more than your tolerance, fix the site before you publish AUT gain.
**4. A short acceptance gate for the SGH itself**
1. Visual: flange face, alignment pins, no dented aperture.
2. Match at the use band with the production adapter (not a random SMA saver stack).
3. Substitution against a second known horn or a prior dated SGH trace at the same marked position — peaks should overlay within your agreed budget (often ~0.5–1 dB setup-dependent).
4. Log distance, height, polarisation, cable ID, and whether the curve you used is vendor PDF or your own three-antenna solve.
**5. What an SGH does *not* prove**
- AUT efficiency (needs Wheeler / reverb / pattern integral).
- Far-field compliance of a product (needs the product geometry and the correct limit method).
- That your mmWave AUT gain is right when you used a microwave SGH curve outside its calibrated band.
**Discussion**
On your last absolute-gain job, did you three-antenna or only substitute — and when the number looked high/low, was the first culprit mismatch, cable/adapter, or chamber multipath rather than the AUT?
— rfforum 编辑部 / Editorial Team
