EQUIPMENT: MFJ-1026 Field Tests

[wa1ion@xxxxxxxxxxxxx (Mark Connelly)]

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MFJ-1026 Field Testing comments - Mark Connelly, WA1ION

Here's a quick "heads up" on field testing of the modified MFJ-1026 
phasing unit done on Saturday, 26 JUL (local) from the Robbins 
Road - Holmes Field beach-DXpedition site located off Route 3A 
in Plymouth, MA (approx. GC= 70.68 W / 41.98 N).  I used the 
Drake R8A receiver.  Both the R8A and the MFJ-1026 were powered 
from the car battery.  Two 90 ft. / 27 m wires lying on the ground 
were used.  The "main" antenna for the MFJ-1026 ran on a slight 
downslope along the side of Robbins Road straight towards the sea 
at a bearing of about 70 degrees.  The "auxiliary" wire ran out 
at a right angle into an open field of grass at an approximate 
160 degree bearing.  I was on site at about 7 p.m. local / 2300 UTC.  
This is about an hour before sunset.  The two wires were laid out,
narrowly missing a patch of poison ivy and some "dog dirt": just
a couple of "the hazards of DXpeditioning".  I felt that one of 
the big challenges would be to null WPLM-1390, located less than 
2 miles / 3 km from the site.  Luckily, its very large signal did 
less overloading damage than WRKO-680 does back at home near the 
Shawsheen River marsh.  Nulling WPLM a good 50 dB was easy !  It 
wasn't too long before evidence of co-channel skip stations from 
ME, NY, and VT started bubbling in behind the nulled WPLM audio.  
Better yet was rather good audio from Netherlands on 1395 heard 
somewhat later !  The stations that the MFJ-1026 had the most 
trouble nulling were those with high-angle skip, especially if some 
groundwave was blended in.  The stations on the top end of the dial, 
such as WNRB-1510, were particularly troublesome in this regard.  
Null control settings required constant adjustment, especially in 
the period from an hour before sunset to an hour after.  The best 
sustained null depth I could manage on stations such as WNRB, WDCD, 
and WQEW was about 15 dB (although momentarily-deeper nulls popped 
in and out).  Shortwave DXers will probably experience similarly
"jumpy" results above 2 MHz.  Pure "groundwavers" like WPLM and 
longer-skip / lower-frequency stations such as WLW-700 nulled more 
deeply and for greater time intervals between required control 
re-adjustments.  These results are consistent with those found for 
any previous-used phasing scheme, whether delay-line, tuned L-C, or 
other.  Physically-large antennas often help to reduce some of the 
problems with jumpiness and shallow null depth on short-skip and
higher-frequency propagation.  There's only so much you can get out 
of physically-small antennas, regardless of the phasing method 
employed.  Theoretically, a computer-controlled system could 
accurately "chase" the optimum null in real time, but no one has 
made this idea a reality yet.

As the evening progressed, the MFJ-1026 / phased wires set-up proved 
its value as numerous Trans-Atlantic stations were logged.  Some of 
these came in fine on the 70-degree "Euro-wire" without the need for 
phasing, but, in a number of instances, phasing the two wires made 
the difference between a slop-plagued DX signal and crystal clarity.  
The two Croatia stations (1125 and 1134) come to mind.  WBBR-1130 New 
York has a VERY strong signal at night here in eastern Massachusetts.  
Indeed, outside the immediate groundwave zones of locals, it's one 
of the five strongest stations night after night.  WQEW-1560 and 
WDCD-1540 would also be on the "King Kongs of medium-wave skip" list.  
When I was tuned to 1134, Croatia was running a good S9+20, but it 
was still trashed by WBBR slop at times - even on the "Euro-wire".  
With a few quick twists of the controls on the MFJ-1026, WBBR was 
reduced by better than 20 dB and Croatia-1134 roared in with 
absolutely beautiful audio.  On peaks, it was stronger than what was 
left of WBBR.  Not only did the phasing accomplish a nice clean-up 
on 1134, but also the much-weaker Croatian on 1125 was brought into 
the clear with just a bit of co-channel flak from Spain.  Prior to 
phasing, it didn't have a ghost of a chance against the barrage of 
WBBR slop.  

At the end of this report, I've included a concise loggings list 
from the MFJ-1026 DX test session in Plymouth.

Earlier on 26 JUL, I had done a few daytime DX tests of the MFJ-1026 
from Harwich, MA on Cape Cod.  The first battery of tests involved
feeding a Quantum Loop into the MFJ-1026 "main" input and using
the 1026's built-in broadband active whip as the "auxiliary".  With 
the loop at normal (i.e. high) Q, audio null depths only reached 
about 20 dB (versus better than 40 dB for carrier).  This is 
consistent with previous nulling scenarios where a high-Q tuned 
source is phased against a broadband one.  You get what sounds like 
a double-sideband suppressed carrier signal.  If the desired DX is 
more than 20 dB below the dominant, you probably won't hear it even 
during stable midday conditions.  Q-spoiling the Quantum Loop (15K 
resistor shunting the L-C tank) increases nullability of "pests" 
maybe to 30 dB, but the loop's usable sensitivity is compromised.  
At night, this is probably a non-issue (except in aurora), but 
during the day you need every bit of signal you can squeeze out 
of the small loop.  The MFJ-1026's active whip, by the way, had 
good sensitivity and low amplifier noise: above 800 kHz its usable 
signal capture was comparable to that of the Quantum Loop at 
normal Q.  Even at 530 kHz, it provided threshold daytime 
audibility of Turks & Caicos on 530 kHz from Harwich: that's just 
about as good as the loop.  On an outdoor sloper to the top of 
a 20 m / 66 ft. pitch pine tree, Turks & Caicos groundwave runs 
about S5 to S6 on the R8A.  

A second battery of tests at Harwich used two wires at a right angle 
(similar to the set-up employed at Plymouth).  Daytime nulls were 
smooth ("like butter" some would say).  WGAN-560 was easily dumped 
to reveal WHYN, near-equal WPRO and CFCY on 630 could each be 
brought up alone, much the same on 740 with WJIB and WGSM, 
WJTO on 730 was nulled a good 30 dB to pull out WACE over CKAC, 
strong WCLZ-900 was phased under the co-channel CKDH/WMVU mix, 
WWNH-930 easily surrendered to CFBC, and so forth.  Nulling with 
two wires was decidedly better than any loop-versus-whip or loop-
versus-wire scheme.

Nulling with the MFJ-1026 was easier than with a JPS ANC-4 and most 
of the homebrew units I've used over the last 30 years.  It must be 
stated that some rather simple, but totally necessary, modifications 
had to be made to the stock MFJ-1026 to get it to up to "world 
class" medium-wave performance.

The first modification is to correct for excessive signal loss at 
frequencies below 2 MHz.  The unit is advertised as covering down 
to VLF, but its stock version has insertion losses ranging from 
5 dB at the top of the medium-wave band (1700 kHz) to more than 
30 dB of loss down at 530 kHz.  By the time you get down to the 
European longwave broadcast frequencies, there is so much loss that 
you might as well be receiving on a dummy load.  Removing six high-
pass filtering components - L5, L6, R27, L3, L4, R26 - totally 
corrected this problem.  Apparently the engineering people decided 
that this was going to be a shortwave-only unit when the advertising 
department thought that writing it up as having coverage down to 
VLF sounded nice ... clearly, somewhere along the line, there was 
a failure to communicate.

The second modification corrects for an inability, in some cases, to 
obtain enough phase shift to produce nulls.  If the phase adjustment 
covered 0 to 180 degrees, then setting the reverse switch gives us 
180 to 360 degrees (also expressable as -180 to 0 degrees).  But 
what if the phase adjustment range at some frequencies is only 0 to 
120 degrees ?  The inverted setting will be 180 to 300 degrees.  If 
we have the need for a shift of 150 degrees to obtain a given null, 
this condition will not be producible.  Judging by some of the 
medium-wave nulling tests, lack of sufficient phase-shift range 
does appear to be a problem.  Luckily, there is a relatively simple 
solution: reversing the antenna inputs so that what had been the 
"main" antenna during a non-working null scenario becomes the 
"auxiliary" and what had been the "auxiliary" becomes the "main".  
In the example above, we'd change the +150 required shift to -150
degrees, which is equal to +210 degrees (-150+360).  This is now 
within the window of possible adjustments.  As long as the 
adjustment range is better than 90 degrees, it will be possible
to cover all required nulls if the channels are made "swappable".
Tests performed still show a few cases where a null occurs at
either the extreme right or left setting of the phase shift control, 
but at least a null can be produced with the antennas connected 
one way or the other.  A double-pole / double-throw (DPDT) "swap 
switch" was added to facilitate full null coverage.  I mounted it 
on the rear of the unit because of limited space on the front 
panel.  Two lines were cut to allow swap switch use.  The first 
of these was the lead from the SW4B arm to the R20 pot (auxiliary 
level).  The second cut was in the line from C8 to the R9 pot 
(main level).  With the DPDT switch set to "normal", the switch 
completes the previously-wired paths: SW4B arm to R20; C8 to R9.  
In its "swapped" position, the SW4B arm gets connected to R9 and 
C8 gets connected to R20.  A couple of minutes spent studying the 
schematic and board layout should make it obvious how to install 
this modification.  Conceivably increasing the value of the phase 
shift circuit component C12 could also increase the phase-shifting 
range.  I'm thinking of putting a varactor circuit in the box in 
place of SW2A, C12, and C13.  The potentiometer installed (in lieu 
of the SW2 high / low frequency range switch) to adjust the varactor 
capacitance could act as a "vernier" for nailing down deeper nulls.

Once the MFJ-1026 is modified, it makes a very competent phasing
unit that will undoubtedly bring the technology into the hands of 
many DXers who have not previously experienced its value in 
bringing new stations out of "the mud".


APPENDIX: MFJ-1026 Field Test Logs (Drake R8A, MFJ-1026, two wires)
CHRONOLOGICAL LOG (Robbins Road: Plymouth, MA) -=- Mark Connelly  

26 JUL 1997 
(* 2348-2359 UTC *) Algeria-891, Morocco-1044, Libya-1251,
Mauritania-1349, Algeria-1544

27 JUL 1997 
(* 0000-0059 UTC *) Germany-1422 (o/ Algeria), Netherlands-1395,
Spain-Canaries-1179, Croatia-1134, UK-1215 (o/ Spain), Spain
& unID's - 1296, N. Ireland-1341, unID (MidEast ?)-1531.3,
Sao Tome(t)-1530, Kuwait-1548, Spain/Italy-1575, Norway-1314,
Slovakia(/Czech?) o/Spain-1287, probable France/Ireland-1278,
Germany-1269, UK-1197, Spain-1143, Croatia/Spain-1125, Spain-1107, 
Slovakia-1098 

27 JUL 1997 
(* 0100-0115 UTC *) UK-1089, intermittent 1088 het: probably 
Angola, UK-1053, Spain-1044, Spain-1026, Germany-1017, UK-882 
o/ Spain/Canaries, Egypt-864, unID weak het-863.8a, Spain-855 
o/presumed Romania music, Italy-846, Egypt-819, Spain/Egypt-774, 
Switzerland-765, Germany-756, Netherlands-747  

Some Europeans below 747 were also in, but they were not that 
strong compared to the mid-band DX.  Anyway (by 0120 UTC), I had 
to leave the site to go back home to the rest of the family.