The 'MELITTA Project' 

The ' MELITTA Project ' is the codename for a HF band pass filter which can be used for operating with a 'running' and 'multiplier' stations during contests. The aim was to build a cheap and straightforward compact BPF with parts out of my junk box ... There are sure better concepts and solutions around, like the one using COAX stubs, but here I was specifically looking for something 'portable' to be used during IOTA activations, field days etc.

The idea for a filter came from AF4Z - who designed an alternative design to the classic 'pass-through' filter approach, which is obviously requiring more components. Download the description (PDF 70k) for more details.

For a 5 band filter I used following parts out of my junk box ....



80m 3µH 620pF   2 tores / tune on 3.525 and 3.700
40m 1,6µH 320pF   2 tores / tune on 7.025 and 7.100
20m 730nH 180pF Air wound
15m 500nH 120pF Air wound
10m 330nH 100pF Air wound


For 80m and 40m you will need two resonating sections, as a filter with only one would be  really too sharp (unless you intend to operate on CW or SSB only portions of the band). So one section is tuned to the CW portion of the band, the other to SSB. This can be easily adjusted by adjusting the 'compression' of windings on the tores. For 20m, 15m and 10m, one section is fine to cover the complete band.  Adding a second section does not enhance the attenuation.

When using relays for switching the sections, use decent models capable of handling high currents (8A or so is fine) - this will ensure a high Q of the section and good filtering characteristics (in fact, I have seen one of my relays not performing well - giving up to 15dB attenuation variations - and I had to replace it...) The C's are silver-mIca transmitting  types.

During tests I ascertained that this concept of filter is not useable on TX - the SWR is sky-high, in particular on 15m and 10m...  so I believe it can be used only on RX, where absolutely no adverse effect due to high SWR was noted (no damping of sigs).

Not using the filter on TX will not attenuate phase noise of the transceiver.... if this is no problem for you, you will have to use this filter on RX only (like for a 'spotting station').

Construction of the filter:


(Click on picture to enlarge)

Left and right are the TX/RX relays (black) . By default, when no power is applied to MELITTA, these 2 relays are in pass-through mode (a small PTFE coax interconnects both NC contacts).

When power is applied, signals are re-routed via the filter section (5 white relays) - hereby the 2 outer relays are energized

In TX mode, the filter section is however bypassed,  this is controlled by a small DIL relay (right underside PCB) triggered by a PTT input (cinch).

For the band switching relays,  these are 'closed' when not energized, thus only the 'working band' relay has to be energized (=open, filter section disconnected).

5 LEDs on the front panel show which band is enabled (and if MELITTA is active), one LED lights up  when in TX mode.


The filter band pass, with all 5 sections in line (I forced this in 'test mode', in practice it cannot happen)

 The horizontal line at about -14dB is the reference level of my noise source.

Horizontal scale is 5MHz/div, left end is 0 MHz, center is 25 MHz, and right end is 50MHz - vertical sensitivity is 10 dB/div.

Attenuation is about 20dB (80m) at it's worst to 30 dB (10m) at it's best for 'out of band' signal, 'in band' there is no attenuation at all.

In practice, with my IC7400 rig, a S9 signal is attenuated 'out of band' to S0, a S9+20dB drops down to S3.

With a 3-pole BFP, the attenuation for out of band signals is better (+/- 35 to 40dB) and this type of filter can be used as well in TX mode.

Where to buy the components ? 

It is recommended to use SILVER MICA capacitors, and tores from AMIDON.

I have ordered the components to build a second unit from PROFI ELECTRONICS KUHNA in Germany (see , as alternative several sources on EBAY are available !