LOOP SHOWDOWN - part 1 of 2

[MarkWA1ION@xxxxxxx]

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              LOOP SHOWDOWN: KIWA versus RSM-105  
              (with some comments on the Quantum)

             Mark Connelly - WA1ION - 23 AUG 1995

The Kiwa Medium Wave Loop and the RSM-105 are two of the finest
loop antennas available commercially today to professional monitors 
and serious medium wave DX hobbyists.  They are both excellent 
performers even though they represent two distinctly different 
design philosophies.  A month before I did this study, I purchased 
an RSM-105 from its builder, Ray Moore of Key Largo, FL.  Tom Walsh 
of the "Boston Area DXers" had just acquired a Kiwa Loop which he 
was kind enough to let me borrow.

Before going further, it must be stated that the RSM-105 is a passive
antenna and cannot be expected to have "S-meter gain" comparable to
that obtainable from the amplified Kiwa.  The true measure of a loop's 
weak-signal performance is signal-to-noise ratio.  An S-4 signal masked 
by amplifier noise may be less readable than an S-1 signal from a 
passive antenna as long as the S-1 signal is still above the receiver's 
own noise floor.  The passive 1-meter-square RSM-105, on some very weak 
signals, gave SLIGHTLY better signal-to-noise than the physically-smaller 
(amplified) Kiwa.  But these threshold signals were often fighting 
with the Drake R8's own noise floor.  The best signal-to-noise ratios
were obtained by running the RSM-105's output through a low input 
impedance (inductively-coupled) Mini-MWT-3 regenerative preselector.  
This homebrew tuner - as described in my DL-1 Delay Line Phasing Unit
article - has a lower noise figure than the Drake R8 and can improve
the signal-to-noise performance of the receiver on very weak signals
whether from a wire, the RSM-105, or the output of a passive phasing 
unit.  The Kiwa's signal-to-noise was very impressive considering that
it has substantially less pick-up area than the RSM-105.  The differences  
in weak-station audibility (not S-meter reading) between the loops were 
minimal.  A couple of checks were made with a Quantum Loop.  It, for 
its miniscule size, made a good accounting of itself: as a rule, output 
levels observed on the Drake R8 S-meter were equal to those from the Kiwa 
(slightly greater on some frequencies, slightly less on others, usually 
about the same).  However, the Quantum was noticeably more "hissy" on 
weak daytime stations.  This is to be expected because of the physics
of a small aperture antenna coupled to a high-gain amplifier.  On non-
auroral nights, there would be very little difference noted in DXability 
comparing the RSM-105, Kiwa, and Quantum.  The Kiwa's regeneration might 
give some desired extra selectivity and bandpass tuning in some 
circumstances.  This would be particularly useful with receivers 
having less than optimum IF filter selections.  

Nulling with the Kiwa was a breeze because of its silky smooth mechanical
design.  It is a thing of beauty.  The RSM-105 and the Quantum required
more work to get a null, especially when tilting was required.  Two 
phased wires, or Q-spoiled loop versus wire or amplified whip, can still 
outperform any stand-alone loop, especially for DX in the direction 
opposite to stations to be nulled.  Here in the Boston area, DX from 
Europe and Africa is noticeably better on a properly-constructed phased-
wires system than on any loop that I've ever tested.

The RSM-105 is the ultimate urban loop as it cannot be overloaded.
When either the Kiwa or Quantum is tuned to frequencies other than
the receiver's frequency, intermodulation products can be heard and
inadvertantly peaked up.  This can make peaking a bit dicey when 
operating one of these loops in less than optimum lighting - such 
as in the dark at a beach DXpedition site.  This doesn't happen with 
the RSM-105 unless you're using a very poor receiver that causes 
intermodulation in its own circuitry.

At "nasty" RF-laden sites such as the New Jersey Meadowlands, active
loops are likely to generate intermodulation products even when
they are properly peaked for the receiver-selected frequency.
These unwanted signals can be generated from FM, TV, SW, LW, and 
utility sources as well as from local medium wave AM stations.  
A passive loop is your only choice if you're in an RF "hot spot".

The RSM-105 is touted for its use in precision direction finding.  Its
balance is superb and I have no reason to dispute this claim.  I did not
find the Kiwa lacking in the symmetry of its nulls either.  Both loops
should give very good direction fixes - most likely superior to those 
taken with popular ferrite loops such as the Palomar, Radio West,
SM-2 Space Magnet, Quantum, or McKay-Dymek.

The large size of the RSM-105 and the greater care required in its
transportation rule it out for DXpeditions requiring air travel.  
A Quantum Loop and Sony ICF-2010 combination is probably your best
choice on a plane trip.  

For car-based DXpeditions, the Kiwa would be a better choice than the
RSM-105 (or Quantum) because the control box can be located inside the 
vehicle while the loop is on the car roof.  As one who has used homebrew 
remotely-tuned antennas on most of my car DXpeditions, I am glad to see 
a loop of this design now available commercially.  

In August of 1995, I performed low-signal measurements from a field 
site in Tewksbury, MA near the Tew-Mac Airport / Route 38 / Shawsheen 
River.  The site is away from power lines and it is about 4 miles / 
6.4 km from my home in Billerica.  Measurements were taken between 
12:30 and 1:00 PM local time (1630-1700 UTC).  There was no skip at 
the time: all signals were arriving via groundwave.  
                                                           
Stations selected were considered to be weak signals: at or below the 
noise level of typical consumer portable radios and marginal copy on
many car radios.

The following antenna configurations were compared: Kiwa Loop to R8, 
RSM-105 loop to R8, and RSM-105 loop via (inductively-coupled) Mini-MWT-3
regenerative preamplifier to R8.  The Drake R8, Mini-MWT-3, and Kiwa Loop
were powered from the car battery.  Amplified readings (Kiwa or RSM-105
via Mini-MWT-3) taken represent minimum regeneration.  Add about 15 dB
to get readings taken when the given regenerative amplifier was adjusted 
for maximum level of understandable-audio signal just before oscillation.
It was noted that the ratio of signal to adjacent station "slop" was 
often improved by using regeneration set for intelligible audio coming
from the sideband opposite to the "pest" station channel.  Regeneration 
had little effect (good or bad) on weak station signal-to-noise ratio.

The R8 S-meter showed an S-1 reading for zero signal in.  The empty-
frequency S-meter reading with the Kiwa peaked was about half an S-unit 
(3 dB) over S-1: e.g. S-1.5 .  All Kiwa readings were taken with the 
amp switch on "DX" and no attenuation on the control box.


RECEPTION DATA
==============

Massachusetts Turnpike TIS - 530 / Weston, MA    
Kiwa: S-6; good readability 
RSM-105: S-2.5; fair to good readability, slight receiver noise
RSM-105 via Mini-MWT-3: S-9 + 5 dB; good readability 

WLIX - 540 / Islip, NY    
Kiwa: S-4; fair readability, slight amplifier hiss 
RSM-105: S-2; fair readability, receiver noise floor only 6 dB down
RSM-105 via Mini-MWT-3: S-8; fair to good readability 
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