TECHNICAL PUBLICATION #42
Lowpass Filters: It's What You Have And You Do With It!
Lowpass filters are primarily a passive device used in the transmission and reception of radio signals in the HF frequency range (3-30 MHz). Their intended purpose is to prevent the radiation of signals above 30 MHz that often emanate from transmitters due to the mixing of various signals in the transmitter's internal circuitry. The filters should be thought of as a frequency-selective bypass device. That is, the unit will pass through without attenuation (loss) those transmitted and received signals below 30 MHz and short circuit (between coaxial line center conductor and outer shield conductor those signal products whose frequency is above 30 MHz
The point at which the loss through the filter is measured as -3db (half of the power lost) is called the cutoff frequency. Above this point as frequency increases attenuation also increases, usually at a rate of rapid ascent. Lowpass filters in receiving operations work the same way. They prevent the reception of frequencies above 30 MHz which, generated locally by broadcasters can frequently disturb HF reception.
Many filters produced over the past 30 years or so have been either poor by design or installed by the user in such a way that the filter's ability to work was compromised, or both. The result was the expense of a lowpass filter that did not contribute to enhanced station ability or reduction of interference.
Here's what to look for when selecting a good lowpass filter. First, find a filter whose cutoff frequency is close to 30 MHz Many filters don't reach cutoff below 50 Mhz and their resulting performance is often poor because of the amount of frequency spectrum between 30 and 50 MHz that is allowed to pass through. There's plenty of possibilities for interference and noise to occur in this range. If you're only interested in 30 MHz and below it's best to decide up front to get rid of everything else. Further, a low cutoff point pushes the VHF frequency range above 50 MHz farther into the stopband of frequencies where the attenuation is greatest. Second, be sure that the filter has sturdy housings and is not put together with pop rivets or hardware that will corrode and rust. Third, ask the manufacturer for a typical sweep curve of the filter so you can gauge the performance against other companies' published figures. If the figures are unavailable, shop elsewhere. Ask what insulation material is used and what the expected voltage breakdown of the filter is. If it's not insulated with a modern material such as teflon sheet or thick mica and insulated to 2,000 volts or higher, shop elsewhere. Ask what kind of warranty is offered. If it's not at least one year and unconditional, shop elsewhere. Ask what kind of impedance passivity the filter has. If its VSWR at 50 ohms is greater than 1.2 to 1 anywhere in the passband (DC-30 MHz), shop elsewhere.
Once a filter is selected and purchased it's up to you to install it properly. Most filters are installed by simply connecting coaxial lines and hanging the filter in open space or mounting the unit to the rear frame of radio gear. But try to keep in mind that the filter is used to remove VHF energy above 30 MHz Once the removal is accomplished the VHF signal is applied to the case, and if the case from that point to ground is long (more than several feet) the signal will easily re-radiate or simply not be absorbed and the value of the filter will be lost. Always mount the filter at ground level and as close as possible to a ground rod connection point. Keeping the leads short ensures that high frequency energy will be directly shunted (absorbed) by the earth, and hence removed from the transmission line. Mount the filter outside if you have to and cover with a rainproof enclosure but always keep those leads short - then relax and enjoy!
