DX prowess of HF receivers

 

Preface

Comparing performance of one receiver to another is quite difficult task. Receiver performance test are described in details in ARRL Handbook in Chapter 26. Short wave DX hunters and contest participants have requested that testing of receiver front ends to be made at conditions representing real on-the-air situations, when extremely weak DX signal from the other end of the world are present, when at the same time, several strong (or extremely strong) local or from the same continent signals are close to that tiny DX station signal. In contrary to standard set by amateur radio community, equipment manufacturers prefer that their product be evaluated at 50 kHz or even 100 kHz “safe distance” measurement spacing where much more “optimistic” results can be achieved.

 

Table 1 illustrates IC-765 receiver front end dynamic range measurements performed by the ARRL Laboratory at various signal spacing:

 

Table 1

 

Signal Spacing         Blocking DR (dB)                               IMD DR (dB)

(KHz)                     IF Shift Off          IF Shift On           IF Shift Off          IF Shift On

  5                           120                           91                         85                           73

10                           130,5                      105                         90                           88

20                           151,5                      139,5                      97                           95

50                           152                         152                         99                           99

 

We can see a big difference between 5 and 50 kHz test results, i.e. BDR and IMD DR measurements for widely spaced signals produce much “better” results than for 5 kHz spacing. That explains why manufacturers are opting for wider spaced measurements.

 

Closer spaced tests can inform us much more realistically about a receiver’s usefulness for DX’ing and contesting on our crowded ham radio HF Bands. The ARRL Laboratory, G3SJX and W8JI have published results of measurements of some HF receivers at close-spaced signals. The ARRL Laboratory and G3SJX used 20kHz for wide-spaced signals and 5kHz for narrow-spaced signals. W8JI has used 10kHz for wide-spaced signals and 2kHz for narrow-spaced signals.

 

Blocking Dynamic Range and Two-Tone 3rd Order Dynamic Range Tests

Blocking Dynamic Range (BDR) is the difference, in dB, between the Noise Floor and a signal that causes 1dB of gain compression in the receiver.

Two-Tone 3rd Order Dynamic Range (IMD DR) is the difference between Minimum Discernible Signal (MDS) and the levels of two interfering signals causing IMD products just equal to the MDS.

The Two-Tone 3rd Order Dynamic Range depicts the strong-signal capabilities of a receiver, i.e. how it behaves under real-world conditions, when strong signals are delivered from the antenna to the receiver input.

 

Receiver IMD immunity is determined by the limits of its linear signal handling capabilities, i.e. by the limiting effects of receiver active circuitry such as the preamplifier, mixer and first IF amplifier. Passive components may also exhibit such limiting effects. For instance, fast RF silicon diodes for receiver input filter selection and for receive/transmit switching or preamplifier/attenuator activation often cause additional IMD in some present-day HF transceiver models. Moreover, overload of varactor (varicap) diodes in automatically tuned pre-selectors, as well as subminiature inexpensive inductors and monolithic two-pole first IF filters placed immediately after the first up-conversion mixer, can have a role in IMD generation and receiver performance degradation.

 

Table 2 demonstrates 20 and 5kHz spacing test results of Blocking Dynamic Range and Two-Tone 3rd Order Dynamic Range for some HF Transceiver Receivers tested in the ARRL Laboratory. Two columns are added for convenience in analysis:

·         in the fourth column, the decrease in Blocking Dynamic Range is calculated between the 20 and 5kHz tests,

·         in the sixth column, the decrease in Two-Tone 3rd Order Dynamic Range is calculated between 20 and 5kHz tests.

Italic numerals distinguishes 5kHz spacing test results.

 

Table 2

 

Manufacturer      Model                                   BDR (dB)             Decrease               IMD DR (dB)       Decrease in

                                                                                                              in Blocking                                         Two-Tone

Dynamic                                              3rd Order

Range                                                  Dynamic

(dB)                                                      Range (dB)

 

Elecraft                 K2                                          133 and 126         only 7dB               97 and 88             only 9dB

ICOM                    IC-706MkIIG                        120nl and 86         34dB!                     86 and 74              12dB

ICOM                    IC-746                                    113 and 88            25dB!                     92 and 78              14dB

ICOM                    IC-756PRO                            120 and 104          16dB                       88 and 80              only 8dB

ICOM                    IC-775DSP                            132 and 104          28dB!                     103 and 77            26dB!

Kenwood              TS-570S(G) & TS-570D      119 and 87            32dB!                     97nl and 72           25dB!

Kenwood              TS-2000                                 121nl and 99         22dB!                     92 and 67              25dB!

Ten-Tec                OMNI-VI & OMNI-VI+      128nl and 119      only 9dB               100 and 86            14dB

Yaesu                    FT-847                                   109nl and 82         27dB!                     89 and 73              16dB

Yaesu                    Mark-V FT-1000MP            126 and 106          20dB!                     98 and 78              20dB!

 

Table 3 demonstrates 10 and 2kHz spacing tests results of Blocking Dynamic Range and Two-Tone 3rd Order Dynamic Range for some HF transceivers tested by W8JI. The same additional columns as in Table 2 are included. In column 4, the decrease in Blocking Dynamic Range is calculated between the 10 and 2kHz tests. Consequently, in column 6, the decrease in Two-Tone 3rd Order Dynamic Range is calculated between 10 and 2kHz tests. Italic numerals distinguishes 2 kHz test results.

 

Table 3

 

Manufacturer      Model                                   BDR (dB)             Decreasing           IMD DR (dB)       Decrease in

                                                                                                              of Blocking                                         Two-Tone

Dynamic                                              3rd Order

Range                                                  Dynamic

(dB)                                                      Range (dB)

 

ICOM                    IC-751A                 98 and 83.5           14.5dB                   91 and 79              12dB

Drake                     R-4C (stock 1)                      109 and 57            52dB!                     82 and 48              34dB!

Drake                     R-4C (stock 2)                      116 and 80            36dB!                     86 and 68              18dB

Drake                    R-4C (heavy mod)               131 and 127         only 4dB               119 and 118         only 1dB

 

Notes:

·         Stock 1: MOSFET 2nd Mixer,

·         Stock 2: Tube 2nd Mixer,

·         Heavy modifications: rebuilt with solid-state double balanced high-level mixers and Sherwood 600 Hz roofing filter.

 

               The 2 and 5kHz close-spacing receiver tests represent real-world on-the-air DX hunting (split operation) when many strong signals are very close to a very weak DX station signal barely copied in the noise. Less degradation of BDR and IMD DR values means better RX performance for strong close-spaced signals. You can see that some receivers perform better and some are not as good as we want them to be.

 

Considering the decrease in Blocking Dynamic Range and lowering of Two-Tone 3rd Order Dynamic Range between wide- and close-spaced tests, I consider the best receivers for split-operation with DX stations are the receivers of the following HF transceivers:

·        Model K2

·        Model Omni-VI+

·        Heavy modified R-4C receiver

 

The three best results in Table 2 and one result in Table 3 are distinguished by bold lettering.

 

 

K2 and OMNI-VI+ design concepts and features

Elecraft and Ten-Tec manufacture the K2 and OMNI-VI+, respectively. Drake discontinued the manufacture of the R-4C about twenty years ago. For CW oriented DX-hunters, the R-4C is not an impressive receiver when compared to recent year models. But after radical modifications an up-graded R-4C could be a good receiver for weak DX signal CW reception on crowded amateur HF bands, thanks to the low Phase Noises of the R-4C PTO (Permeability Tuned Oscillator). As shown in the ON4UN questionnaire results in second edition of “The Antennas and Techniques for Low-Band DXing” a significant number of questionnaire responders have reported R-4C for DX’ing on 80 and 160 meters.

 

               The K2 and OMNI-VI+ Blocking Dynamic Range for 5 kHz spacing between strong signals is 22 dB and 13 dB, respectively, greater than for the third-ranked FT-1000MP Mark-V. Accordingly, the Two-Tone 3rd Order Dynamic Range of the K2 and OMNI-VI+ for 5kHz spacing from two strong signals is (respectively) 8dB and 6dB better than for the third-ranked IC-756PRO. This advantage is especially useful for DX pile-up oriented operators.

 

Such good receiver front-end parameters prove the design concepts implemented by Elecraft and Ten-Tec in K2 and OMNI-VI+ models. Both makers have abandoned ideas commonly exploited during last twenty years by most other makers of HF Transceivers and returned to proven designs used previously but with implementation of modern components.

 

The K2 and OMNI-VI+ crucial design ideas of the receiver front end are as below:

·         dedicated only for the ham radio HF bands, therefore no general coverage capability,

·         only single (K2) or double (OMNI-VI+) mixing is used instead of a chain of several mixers commonly used by other makers,

·         both models have excluded the first up-conversion IF into the 50–90MHz range with the associated wide bandwidth of first IF roofing filter (with its pass-band bandwidth wide enough for narrow FM transmission and adequate for Noise Blanker operation),

·         both models use a first IF low enough to allow installation of narrow SSB / CW crystal filters with good shape factors and high attenuation of out-of-band IF signals just at the front of the IF amplifier,

·         the main IF selectivity of the crystal filters is very close to the receiver front end, which helps substantially to obtain high Blocking Dynamic Range and wide Two-Tone 3rd Order Dynamic Range even for close-spaced strong signals,

·         both models implement only ham-band receiver input selective band-pass filters that substantially suppress strong signals outside of the ham bands and prevent receiver front-end overload and IMD.

 

The main goal of Elecraft designing its model K2 has been to construct an HF transceiver devoted only for the ham bands to be useful for DX hunting, mainly CW, with SSB as an option. As Table 2 indicates, this has been done successfully.

 

The K2 HF Transceiver implements single conversion superhet receiver concept:

·        double-balanced diode mixer offers excellent Dynamic Range. Narrow and only for ham-bands double tuned band-pass RX input filters are switched by relays and therefore Receiver Front End offer much better IMD response than one achieved when fast diodes are used in RX input band–pass filters switching circuitry (by the way: only a few manufacturers implements PIN diodes for that purpose. PIN diodes used for RX input filters switching are less prone and contribute to produce IMD effects in comparison with “fast” diodes),

·        switch-able HF pre-amplifier and switch-able attenuator increase the range of sensitivity adjustments of the receiver, which in turn allow the operator to adjust the receiver to particular band propagation conditions and receiving antenna actually in use,

·        AGC is derived from IF signal probe. AGC offers fast attack time and smooth operations (without popping effect on strong signals) for fast and slow settings. There is even the possibility to switch-off AGC, which is sometimes (experienced DX’ers know it) the last chance to copy extremely weak DX surrounded by strong signals,

·        sharp IF crystal filter is close to the mixer and because of relatively low IF frequency (4.915MHz), the crystal filter has great attenuation of out-of-IF filter’s pass-band signals. That helps to prevent receiver overload by strong signals outside IF crystal filter pass-band. The IF crystal filter offers adjustable pass-band for CW from wide 2000Hz to narrow 200Hz,

·         low Phase Noise PLL synthesized local oscillator.

 

Implemented microprocessor control offers:

·         split operation with 2 VFO’s,

·         dual range RIT and XIT,

·         memory operation for:

-          Mode (CW or SSB),

-          Dual VFO A/B, Split operation,

-          RX IF crystal filter pass-band selection,

-          RX CW side-band selection (allows to cancel one side interference from strong nearby station when switching to opposite received side-band),

·         direct keypad entry of frequencies and memory channels,

·         three tuning rates: 1, 10 and 100kHz per main knob revolution,

·         10Hz tuning resolution,

·         adjustable RX CW offset with tracking TX side tone,

·         auxiliary I/O RS-232 Interface for computer logging and remote control purposes.

 

The K2 itself is devoted for CW QRP enthusiasts but could be “tailored” for other preferences adding following options:

·         the SSB option offers an adjustable Speech Compressor and optimized 7-pole 2,2kHz wide IF crystal filter,

·         100 watts PA Module (will be offered soon),

·         160 meters band with second RX antenna,

·         an Automatic Antenna Tuner,

·         Noise Blanker,

·         auxiliary I/O RS-232 Interface,

·         and Audio Filter eliminating residual noises outside the desired pass-band.

 

It is sold in a kit form. Assembly instruction is well written. Anyone can complete the kit and buy what one really prefers.

K2 “Product review” has been published in March 2000 QST by Larry Wolfgang, WR1B and “Impressions of the Elecraft K2 Transceiver” by Rich Arland, K7SZ has been published in April 2001 QST.

 

Ten-Tec designing OMNI VI+ has also departed from the prevailing General Coverage Receiver concept and returned to ideas used 20 years ago. Ten-Tec have abandoned:

·         wide semi-octave noisy first local oscillator generated by synthesizers,

·         first IF up-conversion into 50 – 90MHz region,

·         wide first IF roofing filter.

OMNI VI+ HF transceiver is designed for ham bands only, from 160 to 10 meters. There are only two mixers in receiver chain: first IF = 9MHz, second IF = 6,3MHz. All ham bands are covered in twelve 500 kHz segments each having 30 kHz margins at lower and upper band edges. This model is a successful comeback to already proven concepts but with implementation of present-day components:

·        first local oscillator signal is produced with bands crystal oscillators mixed with low noise 4,97 – 5,53 MHz PLL. Therefore all synthesis noise problems causing reciprocal mixing has been avoided,

·        first IF frequency is low enough to implement a narrow IF crystal filter with a good shape factor (having band-pass adequate for SSB and CW) offering high attenuation of out-of-pass-band signals.

 

The first IF is 9MHz and can be fitted with following pass-band IF crystal filters:

·         SSB: 1,8kHz or 2,4kHz,

·         CW: 250Hz or 500Hz,

·         or special 500Hz, 6 pole IF crystal filter centered for digital modes.

 

Second IF is 6,3MHz and can be equipped with following pass-band crystal filters:

·         SSB: 1,8kHz,

·         CW: 250Hz or 500Hz,

Such a mixing concept allows installation of narrow crystal filters in both IF chains right at the beginning of first and second IF receiver amplifiers. Therefore the receiver main selectivity filters are close to the mixers (where they should be according to DX’ers demand and where they has not been put consequently by other makers of Ham Radio HF Transceivers the last twenty years).

Depending on chosen crystal filter combinations resultant good shape factors could be achieved:

·         1,3 for 2,4kHz wide first and second IF crystal filters for SSB reception,

·         1,4 for 1,8kHz wide first and second IF crystal filters for SSB reception,

·         2,6 for 500Hz wide first and second IF crystal filters for CW reception,

·         2,9 for 250Hz wide first and second IF crystal filters for CW reception.

Other pass-bands of first and second IF crystal filters combinations are possible (as indicated above). All installed IF crystal filters can be selected on demand (independently of mode actually used). Superior receiver selectivity significantly decreases interference even from very close signals.

DSP broadband Noise Reduction (5 to 15dB), DSP Auto Notch elimination of interfering carriers, and DSP Low Pass (5 choices) help to customize receiver selectivity (additionally to selectivity already offered by IF crystal filters) and reducing of accompanying noises for somebody’s preferences and depending on actual occupancy of particular received channel.

 

Influence of Phase Noises

The main limiting factor of modern receivers performance is local oscillator Phase Noise. Although DDS has greatly simplified computer frequency tuning capabilities, but at the same time, introduced a close-carrier Phase Noise. High Phase Noise contributes to poor Receiver Blocking Dynamic Range (desensitization by nearby strong signals due to reciprocal mixing).

In the OMNI VI+ Phase Noise are –122dBc for 1kHz spacing, –123dBc for 10kHz spacing and –138dBc for 20kHz spacing.

In the K2 Phase Noise are –120dBc for 4kHz spacing and –126dBc for 10kHz spacing.

Therefore, both OMNI VI+ and K2 have superb RX ham-bands performance with an extremely high close-in dynamic selectivity. That enables reception of very weak signals from DX stations when strong signals are few kHz apart from DX station frequency. Several on-the-air A/B reception comparisons (using the same switch-able RX antenna) of HF Transceivers made by other makers against OMNI VI+ and K2 has been made recently. Generally, these comparisons were in favor of OMNI VI+ and K2, especially in case of CW reception on 160 meter band.

The Figure below explains better performance of OMNI VI+ and K2. That Figure demonstrates a typical situation when barely heard DX station (only a few dB above Receiver Noise Floor – gray dotted line) is operating SSB on 14.195kHz. That DX station is operating split and listening up. Pile-up of strong stations is calling him up from DX frequency. For simplicity only four signals are shown on below graph. Also for illustration - let say - that someone is in QSO just 3 kHz higher on neighboring frequency of 14.198kHz.

 

The ability to copy such a weak DX station in presence of many nearby strong signals will depend on receiver:

·         Selectivity,

·         Blocking Dynamic Range,

·         Two-Tone 3rd Order Dynamic Range,

·         amount of Phase Noise in LO signal.

 

We can presume that almost any modern HF receiver has enough sensitivity and selectivity to copy weak DX station, if there would not be any other signal than that of the DX station. But, for real on-the-air situations, when plenty of strong signals are present on nearby DX station frequencies, some receivers will do better than the others. That will depend on:

·         how great is Blocking Dynamic Range,

·         how wide is Two-Tone 3rd Order Dynamic Range and

·         how much Phase Noises accompanies the LO for any particular HF transceiver.

 

If the receiver has only average Blocking Dynamic Range, even a single signal (strong enough) on adjacent channel (for instance on 14.198kHz) will desensitize that receiver and weak DX station will not be copied in presence of strong station.

 

When many strong stations are calling spread-out 3 to 20 kHz up from weak DX signal, the Two-Tone 3rd Order Dynamic Range performance play a big role in performance of the receiver. We can find in pile-up situations that many combinations of 2F1 – F2 and 2F2 – F1 are present, which will produce intermodulation products also on weak DX frequency and these IMD products will interfere with or distort weak DX signal or they can even completely bury DX signal in noise and hiss. As you can see from tables above some receivers are more and some are less prone on IMD.

 

Most of present day HF transceivers implement synthesizers to produce LO signals for mixing. Analyzing BDR and IMD DR results you can judge yourself (look for noise limited remarks in test results) which makers do better and which one not as good as we want them to be. Some synthesis designs produces more Phase Noises than one can obtain using methods implemented by Elecraft in K2 and by Ten-Tec in OMNI VI+ models to produce LO signals. Therefore K2 and OMNI VI+ models are better predisposed to deal with nowadays pile-ups on crowded ham-bands.

 

The gray dotted line on figure above indicates Receiver Noise Floor. Noise Floor levels of OMNI VI+ and K2 do not change in presence of strong signals in close proximity of weak DX signals due to their Phase Noise because OMNI VI+ and K2 Phase Noise are lower than Phase Noise of most HF receivers utilizing synthesis to produce LO signals. The general situation for synthesis produced LO is illustrated by red line. The presence of many strong signals nearby a weak DX station frequency leads to the appearance of reciprocal mixing signals on DX frequency that will interfere with that weak signal on the DX station frequency. When LO Phase Noises and calling stations signals are high enough, then reciprocal mixing products can bury DX station signal completely in noises. That case is illustrated by yellow bar around 14.195kHz.

 

Summary

                K2 made by Elecraft and OMNI VI+ by Ten-Tec are relatively new models on the market. American makers have manufactured both. As far as I know (written: October, 2001), there is not response from other makers offering HF Transceivers yet. DX Hunters can optimistically expect that “good time has came at last” for them and other makers will offer their new models designed appropriately for DX Hunting and contesting. But, this is still a market economy and next step of other makers will depend how much popularity and admire K2 and OMNI VI+ will achieve among DX Hunters Community.

 

I’ve analyzed Equipment reviews articles published in QST and some articles devoted to Receiver Front Ends published in QEX for same time now. At the same time I was gathering components to build my own home-made “dream-receiver” to perform better in extreme DX hunting situation than equipment offered commercially on the market (European QRM on low HF bands is much, much stronger than in other parts of the World). I’ve planned to begin construction when going on retirement. But recently I’ve noticed that there are models on the market performing almost as well as I need. Additionally, K2 is offered by Elecraft as a kit to build yourself (and have a fun building it) and – many options could be purchased and tailor according to someone preferences, without unnecessary bells and whistles packed in excess in general coverage / multi purpose machines.

According to W8JI there is also a challenge for ambitious constructors up-grading an old R-4C with narrow 600Hz Sherwood roofing crystal filter in first IF, replacing poor second mixer with high level input double-balanced low-noise mixer and adding more gain after the narrow IF filters following the second mixer (solid-state IF amplifier instead of tube version). R-4C up-graded that way with gain properly distributed in receive chain could offer better performance for extreme DX situations than most of nowadays HF transceivers.

Perhaps I am old fashion man but my motto is: if equipment is designed properly to achieve best performance in some specific and narrow area (this case solely for reception of tiny CW and SSB DX signals only on crowded HF ham-bands) you can expect better performance from it than from general coverage / multi-band / multi-mode and multi purpose machines.

Therefore, if HF Transceiver is used mainly for CW & SSB DX’ing, and evidently only inside ham-bands such a case receiver general coverage concept with associated up-conversion and its first IF wide roofing filter (wide enough for narrow FM and for proper Noise Blanker operation) is not the best way to reach the main goal. Adversely to concept used in general coverage / multi-band / multi-mode and multi purpose machines the main bandwidth selection should take place at a point as close to the front end as possible. That will enable us to achieve the greatest immunity against strong adjacent signals.

Unfortunately, that crucial demand is not acted upon in most of HF Transceivers offered on Ham Radio market at the present time. Being myself devoted DX Hunter I recognize concepts implemented by Elecraft in K2 and Ten-Tec in OMNI VI+ models as a step forward in right direction to meet demands of DX Hunters expecting – first of all - very good Receiver performance and its immunity to strong signals adjacent to frequency of very weak DX station. In my opinion Elecraft in K2 and Ten-Tec in OMNI VI+ have properly designed Receiver Front Ends for DX oriented Hams.

                                                                                                                             Tadeusz Raczek, SP7HT

                                                                                                                             Autumn 2001

 

References:

1.         Product Review articles in QST,

2.         W8JI home page,

3.         Elecraft and Ten-Tec specifications of K2 and OMNI VI+,

4.         ON4UN second edition of “The Antennas and Techniques for Low-Band DXing”,

5.         WWW reflectors.

 

Some words about SP7HT.

SP7HT has been involved in DX hunting for last 45 years. During first 25 years of activities all HF equipment was home-made (including SSB crystal filters production). These past years a home-made rig has been the only way to be on-the-air from this part of the World. During the last twenty years SP7HT has used several ICOM and Kenwood HF Transceivers and has not experience with other makers.

SP7HT has been the very first DX-er from Poland reaching DXCC Honor Roll (1981) and DXCC Honor Roll #1 (1984). For last twenty-eight years SP7HT occupations has been associated with microwave satellite telecommunication at Polish Telecom, Satellite Services Centre “TP SAT” in Psary, Poland.

 

E-mail: sp7ht@wp.pl

Address: Tadeusz Raczek

skrytka pocztowa 728

25-324 Kielce 25

Poland

Telephone: +48-41-342 8731