L
Admin
A biconical (or flat bowtie) is the workhorse EMI TX/RX antenna from tens of MHz into the low GHz — and one of the easiest ways to publish a field strength that is really “site + balun + mast,” not the EUT. A few habits that keep antenna factor (AF) honest.
**1. What you own is an AF curve, not a free-space gain sticker**
- Compliance work cares about **antenna factor** (dB/m): how you convert analyser dBµV to dBµV/m. Gain and AF are related, but the curve you use is almost always an AF transfer under a **documented geometry**, not a brochure “isotropic gain.”
- Habit: treat the vendor AF PDF as a **starting transfer**. After a drop, a balun rework, a different mast clamp, or a cable change at the feed, re-check against a site reference (comb + known spacing, or a second dated AF) before you argue 1 dB of margin.
**2. The balun is half the antenna**
- Classic bicones are balanced elements behind an unbalanced coax. A weak or resonant balun shows up as: AF walking when you grab the feed cable; horizontal/vertical AF disagreeing more than geometry justifies; nulls filling when a ferrite slides on the coax.
- Clamp-on ferrites are a **sanity check**, not a production balun. If ferrites still move the reading, fix the feed before you recalibrate the site.
- Habit: freeze balun + first λ/4 of cable dress the same way every AF campaign — same spirit as freezing boom clamp on a Yagi.
**3. Element length / flare sets the band edges you actually use**
- Low-frequency AF and VSWR are dominated by element / cone length and the feed region; high-frequency breakup and pattern lobes show up long before S11 looks “bad.”
- A bowtie (planar) and a classic wire/cone bicone are **not** interchangeable AF assets even if both claim 30–300 MHz — log the geometry class with the curve.
- Habit: do not extrapolate an AF curve outside the calibrated band, and do not “extend” it with a smooth spline because VSWR still looks usable.
**4. What usually invents AF ripple (and looks like EUT failure)**
- **Site:** mast height, turntable, absorber gaps, nearby verticals — sliding ±λ/4 at the use band should not move a reference reading outside your uncertainty.
- **Cable / adapter stack:** every saver and polarisation adapter between balun and analyser is part of AF. Swap the antenna, keep the receive chain dress.
- **Polarisation purity / tilt:** a few degrees of element tilt plus a leaky balun fills cross-pol and shifts the apparent AF.
- Habit: when a “failure” is 1–2 dB over limit, re-run the **site reference** (comb or known radiator) at the same marked position before you redesign the product.
**5. A short acceptance gate for a bicone / bowtie asset**
1. Visual: elements straight, feed hardware tight, no cracked balun housing.
2. Match / VSWR spot-check across the *use* band with the production mast and cable.
3. Cable-grab / ferrite smoke test: if level or match moves >~1 dB, stop and fix CM.
4. Overlay today’s site reference against the dated AF / prior campaign at the same height and distance — peaks should agree inside your agreed budget.
5. Log polarisation, height, cable ID, and whether AF is vendor, accredited lab, or in-house transfer.
**Discussion**
On your last radiated EMI or site-survey job with a bicone/bowtie, what moved the reading first — balun/cable common-mode, mast/site multipath, or an AF curve used outside its calibrated band — and do you re-verify AF after every cable change or only on a calendar?
— rfforum 编辑部 / Editorial Team
**1. What you own is an AF curve, not a free-space gain sticker**
- Compliance work cares about **antenna factor** (dB/m): how you convert analyser dBµV to dBµV/m. Gain and AF are related, but the curve you use is almost always an AF transfer under a **documented geometry**, not a brochure “isotropic gain.”
- Habit: treat the vendor AF PDF as a **starting transfer**. After a drop, a balun rework, a different mast clamp, or a cable change at the feed, re-check against a site reference (comb + known spacing, or a second dated AF) before you argue 1 dB of margin.
**2. The balun is half the antenna**
- Classic bicones are balanced elements behind an unbalanced coax. A weak or resonant balun shows up as: AF walking when you grab the feed cable; horizontal/vertical AF disagreeing more than geometry justifies; nulls filling when a ferrite slides on the coax.
- Clamp-on ferrites are a **sanity check**, not a production balun. If ferrites still move the reading, fix the feed before you recalibrate the site.
- Habit: freeze balun + first λ/4 of cable dress the same way every AF campaign — same spirit as freezing boom clamp on a Yagi.
**3. Element length / flare sets the band edges you actually use**
- Low-frequency AF and VSWR are dominated by element / cone length and the feed region; high-frequency breakup and pattern lobes show up long before S11 looks “bad.”
- A bowtie (planar) and a classic wire/cone bicone are **not** interchangeable AF assets even if both claim 30–300 MHz — log the geometry class with the curve.
- Habit: do not extrapolate an AF curve outside the calibrated band, and do not “extend” it with a smooth spline because VSWR still looks usable.
**4. What usually invents AF ripple (and looks like EUT failure)**
- **Site:** mast height, turntable, absorber gaps, nearby verticals — sliding ±λ/4 at the use band should not move a reference reading outside your uncertainty.
- **Cable / adapter stack:** every saver and polarisation adapter between balun and analyser is part of AF. Swap the antenna, keep the receive chain dress.
- **Polarisation purity / tilt:** a few degrees of element tilt plus a leaky balun fills cross-pol and shifts the apparent AF.
- Habit: when a “failure” is 1–2 dB over limit, re-run the **site reference** (comb or known radiator) at the same marked position before you redesign the product.
**5. A short acceptance gate for a bicone / bowtie asset**
1. Visual: elements straight, feed hardware tight, no cracked balun housing.
2. Match / VSWR spot-check across the *use* band with the production mast and cable.
3. Cable-grab / ferrite smoke test: if level or match moves >~1 dB, stop and fix CM.
4. Overlay today’s site reference against the dated AF / prior campaign at the same height and distance — peaks should agree inside your agreed budget.
5. Log polarisation, height, cable ID, and whether AF is vendor, accredited lab, or in-house transfer.
**Discussion**
On your last radiated EMI or site-survey job with a bicone/bowtie, what moved the reading first — balun/cable common-mode, mast/site multipath, or an AF curve used outside its calibrated band — and do you re-verify AF after every cable change or only on a calendar?
— rfforum 编辑部 / Editorial Team
