DXP-1 DXpedition Phasing Unit Mark Connelly, WA1ION -=- 12 JAN 1999 Phasing units of various types have been in use by medium wave DXers for at least 35 years, since the era of Gordon Nelson and Bill Bailey (and possibly before that). The devices have proved to be very valuable tools in the serious DXer's interference- fighting arsenal. Reprints of many designs may be obtained from the National Radio Club [reference 1]. The object of phasing units, as different as the individual designs may be, has always been to combine the contributions of two antennas at a 180 degree phase relation in order to null out a dominant signal (or noise) and let otherwise-inaudible weaker co-channel and adjacent channel signals come through. The DXP-1 is intended to "pick up where the MFJ-1026 left off", namely coverage of the 100 to 2000 kHz frequency range. DXP-1 provides features, such as tuning, that are not even available on a modified MFJ-1026 [reference 2]. At present (JAN 1999), "homebrewing" a unit such as the DXP-1, or modifying an MFJ-1026 (/ 1025), are the only phasing unit options open to LF and MF DX enthusiasts in the USA and Canada. Al Merriman [reference 3] has been modifying MFJ phasers; contact him if you're interested in going this route. He might even be able to build homebrew design units for DXers as well. The JPS ANC-4 is another option, but it has been noted in some cases to have overload problems and hit-or-miss nulling below 2 MHz. In many countries there is another option: the Wellbrook APU 100, built in the UK (but presently not exported to the USA or Canada). Andy Ikin of Wellbrook [reference 4] can supply additional information. It is sincerely hoped that MFJ, JPS, Kiwa, or some other competent DX-accessory manufacturer will soon produce a good longwave / medium wave phasing unit for US and Canadian DXers. DXP-1 grew out of a need to simplify a larger unit, the Superphaser-1, for easier assembly and uncomplicated operation on DXpeditions where DXing is often done in a vehicle with less- than-optimum lighting, space, and temperature conditions. Keeping the size of the unit compact (chassis: 5 x 4 x 3 in. / 12.7 x 10.16 x 7.62 cm) makes it a good candidate for DXpeditions involving air travel. Phase shifting is wideband in nature: this is similar to the MFJ models, the Sherwood QRM Eliminator, and delay-line d-i-y models like the DL-2 and the (Gerry Thomas) Phase One. Unlike R-L-C shifted models (the Nelson and early Connelly units), a null set up in a given direction is often effective plus or minus 100 kHz or more from the set-up frequency. After the summation point, amplification can be broadband for rapid operation, quick checking of parallel frequencies, etc. Alternately, tuned operation may be used for improved sensitivity and signal-to-noise ratio, Tuned operation is especially beneficial with low to moderate priced receivers, particularly portables from Sony, Sangean, Grundig, et al. There are two tuned modes. Mode TM1 places the varactor tuning capacitance in a series configuration for maximum signal transfer. Mode TM2, shunt (i.e. parallel) variable capacitance provides a bit less signal coupling, but offers somewhat better freedom from spurious responses in high-signal areas such as cities. The TM2 mode also provides sharper tuning (higher "Q"). Frequency ranges are as noted in Table 1. Table 1 S4 to TM1; S3 to position noted ------------------------------- S3 = 1 | 145 to 300 kHz S3 = 2 | 300 to 570 kHz S3 = 3 | 570 to 1070 kHz S3 = 4 | 1070 to 1900 kHz S3 = 5 | 1900 to 3650 kHz S3 = 6 | 3650 to 6300 kHz S4 to TM2; S3 to position noted ------------------------------- S3 = 1 | 95 to 150 kHz S3 = 2 | 150 to 290 kHz S3 = 3 | 290 to 475 kHz S3 = 4 | 475 to 950 kHz S3 = 5 | 950 to 1550 kHz S3 = 6 | 1550 to 3000 kHz OPERATING THE DXP-1 =================== Initial set-up: Connect a passive (unpowered) antenna to J1, or connect an active antenna (such as an MFJ-1024 whip) to J3. Connect the second antenna to J2 (if passive) or J4 (if active). Connect a DC power source of +12 volts (min.) to +18 volts (max.) to J5. Connect the receiver, via coaxial cable, to J6. Set R1 and R2 fully clockwise and set R3 to center. Set S1 to 1. Set S3, initially, to the position for the operating frequency as indicated in the TM1 section of Table 1. If untuned (broadband) operation is desired, set S2 to BB (broadband) and skip over the "Tuning" part of this procedure. Tuning: Set S2 to TUN (tuned). In most areas S4 can be set to TM1 for maximum system sensitivity. In strong signal areas, the TM2 setting of S4 may be preferable. Ensure that S3 is set for the desired frequency range, in accordance with Table 1. Adjust R4 for maximum signal level (peak). Amplitude Balancing: With S1 on 1, observe the level of the dominant signal (or noise) to be nulled. Also do this with S1 on 2. If the signal level had been greater in the S1 = 1 position, adjust R1 so that the signal level measured with S1 on 1 is equal to that obtained with S1 on 2. Conversely, if the signal level had been greater in the S1 = 2 position, adjust R2 so that the signal level measured with S1 on 2 is equal to that obtained with S1 on 1. Nulling: NOTE: To get used to nulling, initially attempt it on steady daytime medium wave signals before trying it at night. Set S1 to Null-a and adjust R3 for the best reduction of the dominant signal to be removed. Also do this R3 adjustment with S1 set to Null-b. Leave S1 on the null position which gave the deeper, better-defined null. Finish up the null by small interactive adjustments of R3, R1, and R2. Occasionally it may help to go back to the "Amplitude Balancing" section and proceed forward from there. ================= References cited: [1] National Radio Club web = http://nrcdxas.org [2] MFJ modication article web = http://www.nordicdx.com/antenna/special/mfj1026.html [3] Al Merriman, K4GLU e-mail #1 = merriman@shore.intercom.net e-mail #2 = k4glu@yahoo.com [4] Andrew Ikin - Wellbrook Communications, UK e-mail #1 = aikin@globalnet.co.uk e-mail #2 = andrew.ikin@baedsl.co.uk [4]