November 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Paul Rockwell wrote a 4-part series on station design for long distance
(DX) communications that covered antenna selection and siting
I), economics and construction(Part
II), station configuration and receiver topics
III), and propagation quirks and operating tips
IV). This is part III, and discusses topics such as receiver
preamplification, preselectivity and matching.
Station Design for DX - Part III
Part III - Station Configuration and Receiver Topics
By Paul D. Rockwell, W3AFM
Fig. 8 - 1. Antenna, Yagi. As long a boom and as high as
possible (W3AFM: 203C up 45 feet); 2. Rotator: heavy duty,
with brake; 3. Low-pass filter, kw. rating; 4. d.p.d.t.
coaxial transfer switch, 110 v.a.c. (DK-2-60B); Coil paralleled
with tx h.v. "on" control line; 5. Transmit/receive electronic
switch (B & W 381); 6. Power amplifier, 1-kw, input,
biased to cutoff. (Pair of 250THs); 7. Exciter, with provision
for transceive operation with item 11 v.f.o. homemade; 8.
Match-box: Johnson 250-30-3; and preamplifier; 9. Bandpass
filters, set of 3 covering 14,000-14,105 kc. not installed.
Would make preamplifier necessary; 10. Main receiver. Calibration
accuracy, ± 0.5 kc. 14,000-04,100. Selectivity: 2 &
0.5 kc. (75A-4). 11. Auxiliary receiver tuned to the station
in QSO with DX, for spotting. (75S-3B); 12. WWV receiver.
On 5, 10 or 15 Mc. Most anything. W3AFM uses a BC-453 with
homemade xtl converter; 13. Digital 24-hour GMT clock. (Tymeter
Numechron); 14. A.c. line-voltage regulator.
The equipment and layout at an amateur station are important
factors in its overall performance. Hundreds of different configurations
are giving good results. Table II summarizes the set-ups of a sampling
of DX stations with outstanding contest achievements, based on returns
from a questionnaire earlier this year.
Several items are worthy of note. Median antenna height is 74
feet and median boom length is 36 feet. Of the antennas, 96% are
Yagis: 4% are quads. There is a preponderance of Eimac tubes and
a preference for 4-1000s in the finals. The tabulation gives a good
approximation of what sort of equipment complement it takes to be
top dog in W/K-land.
Not shown, but evident on the questionnaire responses, is that
less than 10% of the DXCC stations have electronic break-in at present.
Only 20% use preamplifiers, and all of these 20% have antennas below
the median height.
The configuration at W3AFM, plus a few planned improvements not
yet in place, is shown in Fig. 8. Note particularly (a) the use
of a second receiver for spotting, and (b) the T/R bypass. The set-up
for multiple-band or multiple-operator application needs more than
shown here, if it is to do the best possible job. Most of the big,
contest-successful stations use separate finals and separate antennas
for each band. If they use multiple operators, they provide two
or more operating positions. In areas where DX-tip nets exist, such
as the LIDXA 2-meter link, an appropriate standby receiver is a
Receiver Preamplification, Pre selectivity and Matching
There is a line of reasoning which says that on DX bands, like
20 meters, any good modern receiver has sufficient noise figure
to operate effectively without pregain. With respect to sideband
operation, this may be true. However, it has repeatedly been observed
not to be true, even with receivers of very good repute, on c.w.
Apparently the use of sharp-selectivity i.f. filters, accompanied
by the use of a nearby notching filter for further narrowing of
receiver noise bandwidth and reduction of interference, results
in an insertion loss which is more than the receiver can handle.
Also, the 20-meter background noise is a random quantity. For some
percentage of time, however small, a noise figure as low as 2 db.
may permit reception of signals not readable through an n.f. of
10 db. A 10-db. n.f. is typical for many receivers. In any event,
a pregain of 20 db., with n.f. of 2 db. has proven advantageous
on numerous occasions. Convenient means should be provided for switching
the preamplification out during periods when its use aggravates
cross-modulation problems to an extent offsetting its advantages.
TABLE II - Equipment Complements of High DXCC Stations (20
meter ants only)
Particularly in urban areas the subject of preselectivity is
often undertreated in station design. In the first place, the use
of a low-pass filter between receiver input and antenna may result
in a very useful suppression of monkey-chatter due to near-by television
stations, or TV receiver local oscillator radiations. Such intermodulation
products were sufficient seriously to degrade W3AFM's DX capability.
The customary transmitter low-pass filter may of course serve both
receive and transmit purposes.
Insertion loss of a good low-pass filter is only a fraction of
a db. at 20 meters. However, a KW Match-box can serve this and other
functions, as described below.
Crystal filters at 14 Mc. can pass 30-kc. bandwidths with attenuations
less than 6 db., and reject bandwidths exceeding 80 kc. by more
than 80 db. Figure 9 shows results measured by C-F networks.18
Manufacturers of such filters have not catered directly to the amateur
market because of high engineering costs.
Helical resonators19 invite application. Operating
Qs of the order of 1000 can be obtained in moderate volumes. That
is, in about four cubic feet, using this design technique, it is
possible to construct a tunable preselector having, say 14-kc. nose
bandwidth at 14 Mc.
Urban operating conditions, made less than pleasurable by receiver
overload from nearby signals, can be greatly improved by attention
to preselectivity. Even clean signals, in a radius of a couple of
miles, and offset more than 50 kc. in frequency, can hurt DX reception.
Modern techniques can reduce the trouble-radius from a couple of
miles to a couple of city blocks.
Receiver matching to the antenna has been known to yield as much
as 6-db. improvement in signal-to-noise ratio. Even if the antenna
is matched 1:1 at the feeder connection, there can be (and often
is) a serious mismatch to down-coming energy at the receiver input-terminals.
Energy reflected from this point to a matched antenna never comes
back - it is re-radiated. This may account, in certain situations,
for a part of the effectiveness of low-noise-figure pregain.20
Some amateurs prefer to use a low-loss matching network at the receiver
terminals, omitting preamplification." This is effective if (a)
the receiver happens to need it, and (b) the matching network is
extremely low-loss. An enclosure no smaller than one cubic foot,
and large, high Q coils should be used. Construction as for transmitting
use22 may do.
In c. w. work there is no need to pass audio frequencies outside
the band 300-800 c.p.s. As sharp a roll-off as practical is recommended.
A simple expedient is to put an oil capacitor in series with the
loudspeaker voice-coil. The loudspeaker at W3AFM seems to resonate
around 800 c.p.s. with 10 mf. in series. For earphones, a pair of
old ones with natural resonance (Weco CW 49003) are employed.23
Five-inch trumpets aren't bad.
Fig. 9 - The results measured by crystal front-end filter
For routine c.w. operation, a 500-c.p.s. mechanical filter is
ideal. For special situations, 2-kc. and 200-c.p.s. filters should
be available. The 2-kc. filter is used for wobbly signals, and sometimes
for net standby. The 200-c.p.s. crystal-lattice filter is for QRM
situations and "digging in." The 200- and 500- c.p.s. filters are
used, in the end, about half the time each. use of the 2-kc. filter
is almost negligible, and it could be done without.
On both the 75A4 and 75S3B receivers, it has been observed that
readability of threshold c.w. signals is improved by use of the
"Rejection Tuning" notch filter, accompanied, of course, by careful
optimization of b.f.o. frequency. Careful adjustments of these two
controls call bring in a signal otherwise unreadable. The notch-filter,
in this sense, is not being used in its intended purpose of rejecting
an interfering carrier. Rather, it shades the channel noise-response
and improves both sin ratio and signal readability. This is true
both on 200- and 500- c.p.s. filters.
Filters of 100-c.p.s. bandwidth, 455-kc. center-frequency, are
now available. The 8-crystal, 1/2-db, Tschebycheff (i.e., 1/2-db
ripple) response filter has attractive characteristics but seems
impractical at present because (a) it is not in production; so costs
are high (b) few, if any, receivers have sufficient interstage shielding
to take advantage of the skirt-selectivity of such filters, and
(c) questions of nose shape and ultimately-useful narrowness are
not yet clearly established. It is feasible to make 455-kc. crystal-lattice
filters with 10-c.p.s. bandwidth and steep skirt-selectivity, for
example - but the practical usefulness is very doubtful. Keying
pulses are rounded, making them difficult to copy at bandwidths
approaching F, where F is equivalent frequency of the shortest keying
pulse. For manual telegraphy, F (c.p.s.) = w.p.m. is a useful approximation.
From this, 20 c.p.s. would be near the ultimate. Drift of distant-end
and local oscillators, ease of tuning, and psycho-otological factors
indicate this is too narrow for practical application.
W4KFC finds, with a 75A2 receiver, he gets best results with
tandem use of a 500-cycle mechanical filter and a single-crystal
filter-stage (No. 1 position on the 75A2).
For c.w. operation only, a recommended combination is to build
in a 500-cycle filter i.f, stage, then precede this with a stage
having options for narrower selectivities. For example, insert a
500-c.p.s. mechanical filter between 1st and 2nd i.f. stages and
a 200-c.p.s. filter between the mixer and 1st i.f. stage. Thus the
limitations of interstage shielding are improved from, say, 50 db.
to 100 db. with respect to skirt rejection.
Receiver Dynamic Range
The exploitation of i.f. and a.f.. selectivity advantages (as
opposed to pre-receiver r.f. selectivity ) is seriously inhibited
by dynamic-range limitations in all present-day receiver designs.
There is no use building in 100-db. rejection to outband signals,
if, as is often the case, a few of them can get together and drop
cross-products only 40 db. down squarely in the passband. Present-day
station-design provisions are (a) pre-selectivity (b) pregain gain-control,
usually by simply switching the preamplifier in/out, and (c) use
of 7360 or equivalent mixers. Naturally, one uses as little r.f.
gain as possible during interference conditions, and the receiver
must have a separate r.f. gain control for this adjustment.
The 75A4 Receiver
Some DXers of proven good judgment hold that the 75A4, suitably
modified, is the best receiver ever made. The simplest modifications
(1) Remove i.f. shunt resistors R46 and R29
(2) Remove a.f. feedback resistors R71 and R109. Substitute 820K
More complicated steps are:
(3) Install 7360 mixers per QST, July, 1964, p. 18.
(4) Install 6GM6 or 6EH7 stage with appropriate cathode and a.g.c.
Reported results are: 12-db. improvement in sensitivity, better
dynamic range (less nearby-signal overload problem), and less hum.
Some experienced 75A4 modifiers (W2JT, K3OKX and W2VCZ) prefer
a 12AT7 first mixer (presumably per 73 Magazine. Oct. 1961, p. 32)
and 6EAS second mixer (presumably per CQ, June, 1960, p. 81, which
is for the earlier 6USA). The 7360 modification is complicated.
Serial numbers of 4200 and over are prized by 75A4 connoisseurs.
These are the latest production version, and include the very-worthwhile
vernier tuning knob. They may be recognized instantly by the lettering,
upper right-hand corner of the front panel, Noise Liiter and AM
CW-SSB all being on the same horizontal line.
A difficulty that occasionally occurs with aging 75A4s is p.t.o.
instability. It is characterized by a lurch of one to five kc. This
is especially noticeable because, when good, the receivers are paragons
of frequency stability. Some steps to correct PTOs:
(1) New 6BA6s, V-14 & V-15; OA2, V-18; and 5Y3, V-17.
(2) Replace C205, 51 pf. This can be done without removing p.t.o.
(3) Loosen p.t.o. mounting screws. Manually wiggle to relieve
stresses. Retighten softly.
(4) Lubricate p.t.o. front bearing.
(5) Wring out 8 holes, 1 inch diameter, on the bottom cover plate
under p.t.o. to ventilate. Replace 5Y3 with silicon plug-in.
(6) Replace the padder, and (especially) the temperature compensators.
If it reaches the point of Step (6), it's worth their fee (currently
$46.00) to send the 70E24 back to Collins Cedar Rapids for turn-around.
They have a temperature-cycling and calibration jig.
18: Meyer, "Front-End Crystal Filters for Amateur
Radio Use, Interadio (annual publication of the International Amateur
Radio Club, (Geneva) 1965, p. 60.
19: McAlpine and Schildknecht, Electronics,
Aug. 12, 1960, p.140.
20: For very-low-noise receivers the input is
customarily mismatched to optimize noise figure. See Vacuum Tube
Amplifiers, Valley and Wallman, McGraw-Hill, 1948, or "Low-Noise
Amplifier," Wallman et al, PIRE, 1948, p. 700. The arguments for
control of input coupling are still valid.
21: From W6AM: "The hams hereabouts find a Johnson
KW Matchbox placed conveniently next to the receiver for receiving-only
improves s/n. The 275-watt Matchbox doesn't work as well. Four attempts
at making smaller receiver-type Matchboxes failed to equal the Johnson.
Apparently the large shielded box and large silver-plated coil do
the job better than anything smaller. The receiver tap is moved
from the 300-ohm position to the transmit 72-ohm position. This
KWMatcbbox has proved far more satisfactory than preamplifiers for
a number of local DX hams,"
22: McCoy, "A Completely Flexible Transmatch
for One Watt to 1000," QST, June 1964, p. 39; and "A Versatile Transmatch,"
QST, July 1965, p. 58.
23: See also W6EUM, 73, July 1962, p. 58.
Posted April 2, 2014