[Elecraft] Advantage of KBPF3 in K3?

[Wayne Burdick]

> if one already has an AM filter in one of the K3's slots what does one need a KBPF3 for (in terms of general coverage SWLing)?
> 
> Does it allow band selection of the popular SWBC bands in addition to the ham bands?


Hi Jeff,

The AM crystal filter is for close-in selectivity (6 kHz) in the I.F. This filter can be used in AM and SSB modes whether in or out of a ham band.

The KBPF3 is at a different point in the receive chain: at the front end. It provides band-pass filters several MHz wide to allow copy of SWL ranges between the ham bands. 

Without a KBPF3, there are a number of places between ham bands where sensitivity would roll off up to 20 dB or so. This is because the K3's normal band-pass filters are very narrow, covering just the ham bands themselves. This is a very good thing from a receive performance standpoint; it removes strong signals that are well out of each ham band that could otherwise cause spurious or image responses. Unlike the K3, most receivers have really wide band-pass filters, leaving them susceptible to such interference in the ham bands. 

By adding a KBPF3, you'll be able to tune virtually the entire range from 0.5 to 30 MHz (plus 6 meters) with your K3 without significant loss of sensitivity. The K3 automatically selects the KBPF3 filters when you tune a certain distance outside a given ham band. This is how we provide MARS coverage (when enabled). 

I said "virtually" the entire range because there is one gap: right around the first I.F. (8.215 MHz). In the vicinity of this gap, sensitivity will be quite a bit lower because the K3 has multiple traps at this frequency designed to ensure excellent first-I.F. rejection. Also, the range from 0.5 to 1.5 MHz is gradually attenuated as you go lower in frequency because of a high-pass filter that protects the PIN diodes used in the T/R switch. To get around this, you can bring the signal into the RX ANT jack on the KXV3 option. You might do this is you were doing low-band (AM broadcast) DXing.

A KBPF3 can be added to the main receiver, or the sub receiver, or both.

73,
Wayne
N6KR