MFJ-704 (HF Low-Pass TVI/RFI Filter)
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| Frequency | 1.8 - 30 MHz (passband; 40 MHz cutoff) |
| TX power | 1500 W (A passive low-pass filter (not a transmitter). Power handling 1.5 kW (1500 W).) |
| Modes | n/a |
| rf | SO-239 (UHF female) in and out, 50 ohm, inline between the transmitter/amplifier and the antenna/tuner. |
| note | Pass HF (1.8-30 MHz) through with low loss; attenuate harmonics above the 40 MHz cutoff (TV/FM/VHF). |
| power req | None - a passive filter. |
A proper high-power HF low-pass filter for taming harmonic interference: a steep 9-pole Chebyshev design passes 1.8-30 MHz with under 0.5 dB loss and SWR below 1.3:1, while strongly attenuating the VHF harmonics that cause TVI and other interference. It handles a full 1.5 kW, uses Teflon caps and Hi-Q inductors, and lives in an RF-tight shielded cabinet - so it is amplifier-rated and effective.
It only addresses harmonic (out-of-band, above ~40 MHz) interference - it will not fix fundamental-overload problems at the affected device, and it must be installed at the transmitter/amplifier output (with good grounding) to be effective. It is HF-only by design. Note MFJ scaled back operations in 2024, so confirm availability.
The MFJ-704 is a solid 1.5 kW HF low-pass TVI/RFI filter - a 9-pole Chebyshev design with low loss in band and strong harmonic suppression above 40 MHz. The right tool when your station's harmonics are causing TVI; remember it treats harmonics, not fundamental overload.
The MFJ-704 is a high-power HF low-pass filter built to cure television and radio-frequency interference (TVI/RFI) caused by a transmitter's harmonics. It is a nine-pole Chebyshev design with a pass band of 1.8-30 MHz and a 40 MHz cutoff: it lets your HF signal through with very little loss (no more than 0.5 dB, SWR below 1.3:1) while sharply attenuating the harmonic energy above 40 MHz that lands in the TV, FM and VHF bands and bothers the neighbors.
It is rated for a full 1.5 kW, so it can sit at the output of an amplifier, and it is built for the job: Teflon-dielectric capacitors and Hi-Q inductors handle the power with low loss, and ground-plane shielding inside an RF-tight cabinet keeps the harmonics from simply leaking around the filter. It uses SO-239 connectors and is about 8 x 8 x 23 cm with mounting holes for wall fixing.
The important caveat is what a low-pass filter can and cannot do: it suppresses your station's out-of-band harmonics, but it cannot fix interference caused by fundamental overload of a nearby device (that has to be addressed at the affected device, often with chokes/filters there). It also needs to be installed at the transmitter/amplifier output with good grounding to work properly, and it is HF-only by design; note too that MFJ scaled back operations in 2024. As a harmonic-TVI filter, though, it is exactly the right tool. Compare it with the band-pass Dunestar 600 and the Comet CF-706A duplexer.