Clean up the airband junk before it hits the receiver. Real FM suppression.
FlatTop is a compact, purpose-built airband bandpass filter for aviation enthusiasts who are tired of scanning aviation frequencies through junk.
This compact 118–136 MHz airband bandpass filter is engineered to keep the aviation band flat while hammering nearby FM junk that can overload real receivers. Available in Standard and hand-screened Premium versions, FlatTop gives SDRs, scanners, handheld radios, and monitoring stations a cleaner front end, lower in-band loss, and stronger real-world airband performance.
In tested assembled units, FlatTop delivered roughly 0.8–2.0 dB insertion loss across the passband, 34–43 dB attenuation at 108 MHz, and a repeatable deep notch near 105 MHz in the 49–54 dB range. The result is simple: more airband, less junk.
FlatTop is offered in two versions
FlatTop Standard
FlatTop Standard is the production version for users who want a serious airband front-end filter without paying for premium screening paperwork. It delivers the same core FlatTop design: broad 118–136 MHz coverage, low loss in-band, and strong built-in FM suppression in a compact package.
This is the right choice for most SDR users, scanner users, handheld aviation radio users, and fixed monitoring stations.
FlatTop Premium
FlatTop Premium is the premium hand-screened version for customers who want the performance plus proof. Premium units are individually measured and selected for tighter screened performance, with serialized verification, for buyers who want greater confidence in exactly what their unit delivered on the bench. This is the right choice for demanding users, avionics troubleshooters, serious monitoring installations, and anyone who simply wants the best FlatTop we can send.
Why FlatTop is Different
Many filters in this category are either too broad, too lossy, too generic, or too narrow to be practical across the whole airband. FlatTop was developed through repeated design, build, VNA measurement, retuning, and relayout to hit a harder target: broad enough to cover the whole civil COM band, low-loss enough to preserve wanted signal, and aggressive enough below the band to keep FM from wrecking the receiver. Even the shield can lid is part of the RF design, improving the finished filter’s FM-notch behavior.
FlatTop is Ideal for
- Airband SDRs
- Aviation scanners
- Handheld aviation radios
- Fixed airband monitoring stations
- RF troubleshooting and front-end cleanup
60-day Guarantee
FlatTop includes a 60-day no-questions guarantee. Try it in your real setup. If it does not improve your airband experience, send it back.
Quick Comparison
Standard - $79.95 - Radiant # RI-2026-4
Best value for most users
Broad airband coverage
Low-loss performance
Strong built-in FM suppression
Premium - upgraded version - $119.95 - Radiant # RI-2026-5
Hand-screened premium units
Measured verification
Tighter screened performance
Best choice for demanding installs and bench-minded buyers
Bottom line
FlatTop was built to do what too many “good enough” filters do not: pass the band you want and punish the band you don’t.
READ JAMES' BLOG ABOUT THE TECHNICAL DETAILS BEHIND THE FLATTOP.
FlatTop Airband Filter
What is FlatTop?
FlatTop is a compact 118–136 MHz airband bandpass filter designed to pass the aviation communications band with low insertion loss while strongly suppressing nearby out-of-band junk, especially broadcast FM.
What problem does FlatTop solve?
A lot of receivers, scanners, and SDRs do not struggle because they are “bad.” They struggle because strong nearby signals — especially FM broadcast stations — overload or desensitize the front end. FlatTop helps clean that up before the junk gets into the receiver.
What frequencies does FlatTop cover?
FlatTop is built for the 118–136 MHz aviation communications band.
Is FlatTop just an FM notch filter?
No. FlatTop is a broad airband bandpass filter with meaningful built-in FM suppression. It is designed to pass the aviation band you want while rejecting a lot of the junk below it.
Does FlatTop really suppress FM?
Yes. In tested assembled units, FlatTop showed strong rejection at 108 MHz, and all tested units also showed a deep notch near 105 MHz. The shield can lid is part of the RF structure and improves that FM-notch behavior.
Is FlatTop a narrow-tuned filter?
No. FlatTop is intended to cover the full civil COM airband, not just one narrow slice.
Who is FlatTop for?
FlatTop is a strong fit for:
- Airband SDR users - #1 use
- Scanner users
- Handheld aviation radio users
- Fixed monitoring stations
- Avionics troubleshooters
- Anyone dealing with airband overload or front-end junk
Will FlatTop help every receiver?
No filter fixes everything. But if your receiver is suffering from out-of-band overload, broadcast FM energy, or general front-end junk, FlatTop is designed to help exactly that problem.
What is the difference between FlatTop Standard and FlatTop Premium?
FlatTop Standard is the production version for most users. It gives you the core FlatTop design and performance.
FlatTop Premium is the special hand-screened version. Premium units are individually measured and chosen for tighter screened performance. This one is for buyers who want the performance plus proof.
Which version should I buy?
Choose Standard if you want the real FlatTop performance at the best value.
Choose Premium if you want the best screened units, tighter measured selection, and greater confidence in exactly what your unit delivered on the bench.
Is the RF design the same between Standard and Premium?
Yes. Both versions use the same FlatTop design. Premium units are screened and chosen from production for tighter measured performance.
What makes FlatTop different from generic VHF filters?
Generic VHF filters often make ugly compromises: too broad, too lossy, too generic, or not actually optimized for the airband/FM-overload problem. FlatTop was designed specifically around low-loss airband coverage plus real FM suppression.
What makes FlatTop different from a dedicated FM stop filter?
A dedicated FM notch can be great at killing FM, but that is not the same thing as a broad, low-loss airband front-end filter. FlatTop is designed to do both jobs well enough to be very useful in real airband setups.
What makes FlatTop different from a cavity filter?
Cavity filters are often larger, heavier, narrower, and aimed at infrastructure or fixed-site professional use. FlatTop is meant to be a compact, practical front-end filter for real-world airband receivers.
Is the shield can just for shielding?
No. In FlatTop, the can lid is part of the RF structure. It does more than block interference — it also improves the filter response, especially the FM-notch behavior.
Why does the lid matter?
Because at VHF, the final electrical behavior depends on more than just the schematic. Layout, geometry, proximity, and enclosure effects all matter. FlatTop was developed with that reality in mind.
Was FlatTop simulated, or measured?
Measured. FlatTop was developed through repeated design, build, VNA measurement, retuning, and relayout. The published performance discussion is based on measured assembled units.
What kind of insertion loss should I expect?
Measured assembled units landed roughly in the 0.8 to 2.0 dB range across the passband.
Does FlatTop guarantee a specific, exact notch frequency?
No. The deep FM-region notch is real and repeatable, but exact frequency and depth will vary somewhat from unit to unit. That is normal in real RF hardware.
Can I use FlatTop with an SDR?
Yes. SDR front ends are one of the most natural uses for FlatTop.
Can I use FlatTop with a handheld aviation radio?
For receiving, yes. That is another strong use case, especially in RF-dirty environments.
Can I use FlatTop at a fixed monitoring station?
Yes. FlatTop is a good fit for monitoring stations, airport listening posts, and other fixed receive setups. Just don't transmit through flattop: you'll overload it and break it.
Is FlatTop transmit-rated?
FlatTop is intended as a receiver front-end filter unless otherwise stated. It is not presented as a high-power transmit filter. It may cause damage to your transmitter if you try this.
Does FlatTop come with a guarantee?
Yes. FlatTop includes a 60-day no-questions guarantee when ordered from Radiant. Try it in your actual setup. If it does not improve your airband experience, send it back.
Why are you offering a 60-day guarantee?
Because this category has plenty of competitive generic RF accessories and plenty of wishful thinking presented as marketing. We would rather have you try FlatTop in your real environment than argue in theory.
Is FlatTop made for SCARP AirLite Tactical Aviation Scanner only?
No. FlatTop has use far beyond SCARP. It is useful anywhere a receiver needs to hear 118–136 MHz clearly while rejecting nearby junk. (SCARP is James Wiebe's aviation radio KickStarter).
What is the short version of why FlatTop exists?
Because you should be hearing airband, not junk.
