Category Archives: MF & LF

A Low Drive 630 Meter Amplifier

My first attempt at amplifier building for the new low bands was a disaster. Being low on funds and patience at the time, I tried building a dual band “linear” amplifier that was said to be capable of 25 to 50 watts on both bands. It turned out to be a design plagued by problems which I won’t get into here. I was fretting about what to do next, as there was no budget at all, when Ken K5DNL came to the rescue. He kindly helped with parts and schematic for a modified, low drive version of the popular GW3UEP amplifier. Credit for the design goes to GW3UEP and K5DNL. Where I have made minor changes I will note that in this post.

Schematic of the low drive amplifier

Referring to the schematic, the 2N2222 provides additional gain to fulfill the low drive objective. This amplifier can be driven to full output with 0 dBm (one milliwatt) input. Mine actually produced full power down to -2 dBm but don’t count on every build being exactly the same. The BC550/BC560 pair forms a squarer to ensure we have a nice clean square wave to drive the FET. The FET in the original GW3UEP was a IRF540. Ken sent a couple of 30NF20 FETs which is what I used in mine. C2 and C4 were not on the schematic I received from Ken. After building it I found the gain and power output peak was around 505 kHz. Looking at the original design on GW3UEP’s web site I noted the the originally specified capacitor values were for that frequency, with a notation to add capacitors for 472-479 kHz operation. After I added C2 and C4, my amplifier peaked at 475 kHz. All capacitors in the FET output circuit should be good quality pulse rated film or silver mica. C1, C3, and C5 in my amplifier are WIMA FKP1. C2 and C4 are CM06 size silver mica with a 500 volt rating. The 1 uF DC blocking capacitor is a WIMA MKS4. Watch the capacitor voltage ratings. Theoretically, 100 volt capacitors should be good enough, though marginal if you intend to run on 24 volts. Some digging into spec sheets on the WIMA capacitors reveals voltage ratings are reduced as frequency increases and we are well down the slope on most of them at 475 kHz. If you are going to buy capacitors I suggest going with the highest voltage rating available. The other change I made was to relocate the blocking capacitor to the location shown. It was at the amplifier output on the schematic I received (and yes, if you are eagle-eyed you will see I have it at the output in the photo below. I relocated it later).

The completed amplifier. IMPORTANT NOTE: The 50 ohm shunt input resistor shown on the schematic is brown in color and mostly hidden under the input coax, just to the left of the 0.1 uF input coupling capacitor. The six blue resistors seen in the photo are not on the schematic. They form an input attenuator, needed because I am driving this amplifier with 250 milliwatts. The input attenuator takes that down to 1-2 milliwatts which is a perfect input level for the amplifier as shown in the schematic.

It should be noted that one need not use exactly the parallel combinations of capacitors specified. The important thing is that by whatever method, be it a single capacitor or several in parallel, we arrive at the required total capacitance at each point in the circuit. You will notice my capacitor combination at C3/C4 quite different from the original GW3UEP.

This amplifier should produce 30 watts output at 13 volts, 100 watts on 24 volts. I am running mine on 19 volts and getting about 65 watts out into a 50 ohm load. I have been running this amp on WSPR at 33% duty cycle for several days and it has performed perfectly. One should strive to keep the antenna resonant and matched, but mine has at times wandered off a bit with no ill affect on the amplifier aside from power output variation as the load impedance changes.

Note: This amplifier has a built in low pass filter but it does not meet FCC requirements for spectral purity. If you are subject to FCC regulations, you should use additional low pass filtering after this amplifier.

Building a 630-Meter Transmit Converter

Inside view of completed 630m transmit converter

This is a companion to my earlier 2200-meter transmit converter. Refer to that post for more details. This one produces about 25 dBm output. Not much is different here except for component values in the low pass filter and the trimmer potentiometer. I didn’t have another of the same type and value. It isn’t critical since it is only being used as a voltage divider to set bias on the BS170 FET.





Schematic diagram and parts values for the 630m transmit converter

Building a 2200-Meter Transmit Converter

I recently began running a WSPR beacon on 2200 meters. Beaconing is fine, but I also want to be able to make QSOs on this new band. To that end I needed a very low cost transmit converter so that I could use my FT-2000, which is fully interfaced to the computer for digital modes. Dave, AA1A came to the rescue with an old Anzac MD-143 mixer. The rest I built from stock parts and the junk box. Stock parts are new, current production parts I keep on hand for projects. The junkbox is a collection of old, surplus, used, salvaged and anything else I happen to be able to get my hands on. I don’t need a linear converter or amplifier for this band, and the resulting converter is not linear. It can be used on CW and any mode which does not require linear amplification.

The converter post-mixer amplification and low pass filter are separate from the mixer

The MD-143 takes a nominal 7 dBm local oscillator signal in the 5 to 500 MHz range. For RF it likes about 0 dBm in the same range. Conversion loss is around 6 dB so we can expect about -6 dBm output at the IF port, which is rated DC to 500 MHz. I wanted to get that up to 24 dBm (250 milliwatts) so it provide the same amplifier drive level as my Ultimate 3S beacon transmitter. The fewer things I have to remember when switching modes, bands, activities or exciters the better! I knew I could use a BS170 FET and low pass filter to duplicate the amplifier section of the Ultimate 3S but I would need something on the order of 10 dBm drive to assure equal output. Wayde, K3MF, suggested using a venerable 2N3904 and kindly sent a long a copy of a schematic. This was good because I have a drawer full of 2N3904 transistors! Some quick and dirty breadboarding and testing showed that I could get 12 dBm out of it with -6 dBm input from the mixer. Perfect!

Inside view of the completed post-mixer amplifier and low pass filter

I set about throwing a circuit together. I was only going to be building the post-mixer amplification and low pass filter stages since I wanted to use the Anzac mixer with this and a similar unit for 630 meters. It’s really very simple: a 2N3904 which produces a nice square wave output followed by a BS170 and finally a low pass filter. There is a 5 volt regulator to power the BS170 in order to hold it to the 24 dBm output level, same as in the Ultimate 3S which runs on 5 volts. After throwing the thing together, it showed exactly 24 dBm output on the first try! Whoa… let me note this on my calendar. It’s a historic occasion!

I am temporarily using my trusty HP 3325B function generator as a 10 MHz local oscillator. When I can afford it I plan to get a dedicated 10 MHz LO for this converter, ore more appropriately one that can be shared between this and its soon-to-be-built 630 meter counterpart. My FT-2000 has been modified for general coverage transmit so I can use it on both bands (10.135.7 to 10.137.8 for 2200 meters and 10.472 to 10.479 for 630 meters.

There is not much else to say about this. I am including a schematic for those who may wish to borrow ideas from this simple gadget. Unfortunately I am reduced to posting photographs of schematics since my printer/scanner finally died after many years of faithful service.

Schematic diagram of the post-mixer amp and LPF stages of the converter


Finally QRV on 2200 Meters!

Results of my second night of WSPR transmissions. W3PM is 1909km from me.

I have been so busy with station building projects that I have failed to write about several of them. Those have resulted in my station being able to transmit on the 2200 meter band after nearly ten months of work toward that goal. In my first two nights running a WSPR beacon at 12 watts transmitter power I have been heard by more than a dozen stations at distances to 1909km (1186 miles). My effective isotropic radiated power (EIRP) is less than 0.1 watt, which is 10 dB down from the legal limit. I am confident my station can span the Atlantic on this band when I reach full legal limit.

My sincere thanks to NI7J/WH2XND for help getting QRV, also to K7PO/WH2XXP for help with an upcoming project to increase transmitter power.

My transmitting antenna is a Marconi in a T configuration with 27m vertical and a top hat consisting of three 30m parallel wires spaced 1.5m apart.

This is the loading coil / variometer for the antenna. Maximum inductance of this coil is 2500 uH. My antenna requires about 2250 uH to resonate at 137.5 kHz. I’m still thinking about a proper replacement for the spring clip.

Low Noise Vertical for LF and MF Receiving

The LNV antenna with my southwest tower (which is the transmitting vertical for 160 meters) behind it

Note 12 May 2018: I moved this antenna to a new location and made a discovery. It had been working in part due to mutual coupling with my transmitting antenna. I should have known this because when I resonated the transmitting antenna on 2200m the LNV noise floor and signal levels came up about 20 dB on that band. Similarly when I resonated the transmitting antenna on 630 meters, output of the LNV came up nearly 10 dB on that band. When I moved it away from the transmitting antenna, output on both bands dropped to within 3 dB of my receiver noise floor even with the preamp (my simple SDRs are not ultra sensitive). Another factor may have been that the new location was surrounded by trees. The antenna has been moved back to its original location. Subsequently, a new receiver allows noise from the LNV to be about 12 dB above the receiver noise floor.

During the initial phase of experimentation I had found that some of my 160 meter Beverage antennas worked reasonably well for receiving on 2200 and 630 meters. I was hearing plenty of Europeans on both bands. Obviously at these frequencies they didn’t work as Beverages. What the actual operating mode was I don’t know. There were a few problems with this arrangement, primarily that I couldn’t run LF and MF receive operations concurrently with 160 (or 80) meter DXing. That was not good!

After reading a number of blog posts and web articles about the “low noise vertical” or LNV, I decided to give it a try. VE7SL has an article on his blog with links to additional information. As is often the case I pulled ideas from several sources, then mixed and matched (and even went off on my own a bit) for my version. My LNV is a 30 foot self supporting vertical made from scrap aluminum tubing I had on hand. It starts out with 1.5 inch OD at the base and tapers to 1/2 inch at the top. The bottom foot of the vertical is slid inside a two foot piece of 1.5 inch schedule 40 PVC pipe and secured in place with set screws (bottom) and a worm clamp (top). Looking at the photo that may be confusing. The set screws (three 1/4-20 stainless tap bolts)  go through the worm clamp and PVC just above the transformer box. You can see that in the photo. What is not visible is the other worm clamp securing the aluminum tubing to the top of the short PVC section. These details are unimportant. Just make sure your vertical is insulated at the base! A piece of wire hanging from a tree would no doubt work just as well, but I wanted to do mine this way.

The base of the LNV antenna with transformer inside the plastic mini box

A couple of words about the overview photo at the top of this post. First, in the thumbnail you may not be able to make out the LNV in the foreground just to the right of the tower behind it. Second, usually the yagi antennas on that tower are lined up with each other! We had a recent storm with wind gusts to 70 mph which skewed things and I haven’t been up to fix it yet. The LNV is only 40 feet from that tower which is my 160 meter transmitting vertical (shunt fed). Thanks to good band pass filters, my 2200 and 630 meter receivers don’t react at all when I transmit with 1500 watts on 160.

In some write ups about the LNV it is said one needs more than one ground rod for best performance. Surprisingly I am using just one four foot “ground rod” which is a section of a stainless steel mobile antenna whip which I pushed into the sandy soil by hand! I plan to try adding real ground rods later but the antenna is working with the minimal ground. As of this writing it has heard 2E0ILY nine consecutive nights on 2200 meters.

LNV base transformer primary winding

My LNV is fed with about 250 feet of WD1A twisted pair wire which is laying on the ground. This is ex military field phone wire and is practically indestructible as long as you keep water out of the ends. It has three steel and four tinned copper strands in each conductor along with incredibly tough insulation that takes a lot of abuse. Any twisted pair wire should work fine in this application.

LNV base transformer secondary

The transformer at the antenna base is wound on a FT-114-JC core, 80 turns primary, 8 turns secondary. First, the 80 turn primary is wound directly on the core using 20 AWG enameled wire. If a non-coated core is used, it would probably be wise to wrap the core with some kind of tape before winding the primary. One end of the primary winding connects to the base of the LNV, the other end to the ground rod.

The primary was wrapped with teflon thread seal tape, then the secondary was wound over the cold (ground) end of the primary using 20 AWG solid plastic coated wire. Another layer of teflon tape keeps everything nice and tight. The secondary winding connects to the twisted pair feed line.

LNV base transformer in its protective box

The finished transformer is mounted in a small plastic mini box of the type that has a rubber gasket to keep out water. The ground and antenna connections are #8-32 stainless machine screws at opposite ends of the box, while the connections for the twisted pair feed line are together on one side of the box.

Twisted pair to coax transformer for the shack end of the feed line. The RCA connector has subsequently been replaced by a BNC

At the shack end of the feed line I use another FT-114-JC core with 8 turn primary (twisted pair from the antenna), 6 turn secondary for 50 ohm coax to the receiver (or in my case two receivers via a splitter).

After using this antenna for 10 days, initial results are very promising, especially on the lower band. I am not able to listen with this and the old antenna at the same time, but I am hearing 2E0ILY on 2200 meters much more often and with stronger signals than I ever did with the short Beverage. To the west it seems about on par with the Beverage. On 630 meters I don’t think it is quite as good, but I am hearing a number of Europeans.

Diagram of the LNV antenna

Update on 2200 Meter Trans-Atlantic Reception

On the better nights I continue to receive WSPR2 signals from across the Atlantic on 137 kHz. Like my early EME days decades ago, the thrill remains high. I get excited beyond reason every time I see one of these pop up in my list of decodes. When I can predict which periods a station will be transmitting, I find myself intently watching the waterfall during those periods to see if I can visually detect anything. There are a large number of periods when the signal can be seen but does not decode. Factors that can prevent a decode on an otherwise OK signal include fading, atmospheric static (which is becoming ever more a factor as Spring approaches), and local interference.

I am convinced that much more frequent Atlantic crossings are possible. There are many nights when no capable stations are transmitting during the peak hours (or at any hour).

Below is an updated list of all 2200 Meter trans-Atlantic decodes to date.

2017-01-29 01:20 DC0DX    0.137469 -27 JO31lk 0.2  N1BUG FN55mf 5399
2017-01-29 02:18 DC0DX    0.137469 -24 JO31lk 0.2  N1BUG FN55mf 5399
2017-01-29 04:14 DC0DX    0.137469 -27 JO31lk 0.2  N1BUG FN55mf 5399
2017-01-29 07:08 DC0DX    0.137468 -27 JO31lk 0.2  N1BUG FN55mf 5399

2017-01-30 06:12 DC0DX    0.137468 -25 JO31lk 0.2  N1BUG FN55mf 5399

2017-01-31 07:20 G8HUH    0.137425 -28 IO81mg 0.1  N1BUG FN55mf 4766

2017-02-01 07:10 DC0DX    0.137469 -27 JO31lk 0.2  N1BUG FN55mf 5399

2017-02-02 05:36 DC0DX    0.137469 -23 JO31lk 0.2  N1BUG FN55mf 5399
2017-02-02 05:58 DC0DX    0.137469 -24 JO31lk 0.2  N1BUG FN55mf 5399
2017-02-02 06:20 DC0DX    0.137469 -25 JO31lk 0.2  N1BUG FN55mf 5399

2017-02-06 04:26 DC0DX    0.137470 -24 JO31lk 0.2  N1BUG FN55mf 5399
2017-02-06 04:48 DC0DX    0.137470 -25 JO31lk 0.2  N1BUG FN55mf 5399
2017-02-06 05:54 DC0DX    0.137469 -26 JO31lk 0.2  N1BUG FN55mf 5399

2017-02-07 02:36 DC0DX    0.137470 -27 JO31lk 0.2  N1BUG FN55mf 5399

2017-02-14 22:32 2E0ILY   0.137551 -29 IO82qv 0.1  N1BUG FN55mf 4731
2017-02-14 23:28 2E0ILY   0.137551 -29 IO82qv 0.1  N1BUG FN55mf 4731
2017-02-15 04:40 G8HUH    0.137422 -24 IO81mg 0.1  N1BUG FN55mf 4766

2017-02-22 04:20 G8HUH    0.137424 -28 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-22 04:30 G8HUH    0.137424 -25 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-22 05:50 G8HUH    0.137424 -28 IO81mg 0.1  N1BUG FN55mf 4766

2017-02-23 05:06 G8HUH    0.137424 -26 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-23 05:10 G8HUH    0.137424 -25 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-23 05:14 G8HUH    0.137424 -24 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-23 05:18 G8HUH    0.137424 -26 IO81mg 0.1  N1BUG FN55mf 4766
2017-02-23 05:22 G8HUH    0.137424 -27 IO81mg 0.1  N1BUG FN55mf 4766

2017-02-23 23:08 2E0ILY   0.137552 -29 IO82qv 0.02 N1BUG FN55mf 4731
2017-02-23 23:56 2E0ILY   0.137552 -27 IO82qv 0.02 N1BUG FN55mf 4731

2017-02-26 00:06 2E0ILY   0.137551 -28 IO82qv 0.02 N1BUG FN55mf 4731
2017-02-26 00:44 2E0ILY   0.137551 -28 IO82qv 0.02 N1BUG FN55mf 4731
2017-02-26 00:54 2E0ILY   0.137552 -28 IO82qv 0.02 N1BUG FN55mf 4731
2017-02-26 00:56 2E0ILY   0.137552 -28 IO82qv 0.02 N1BUG FN55mf 4731

2017-02-26 23:30 2E0ILY   0.137549 -28 IO82qv 0.02 N1BUG FN55mf 4731

2017-03-05 05:08 2E0ILY   0.137550 -28 IO82qv 0.2  N1BUG FN55mf 4731

2017-03-13 23:54 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-14 00:12 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-14 02:16 2E0ILY   0.137550 -28 IO82qv 0.2  N1BUG FN55mf 4731

2017-03-15 00:32 2E0ILY   0.137550 -28 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 02:48 2E0ILY   0.137550 -29 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 03:00 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 03:32 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 03:48 2E0ILY   0.137550 -25 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 04:16 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731
2017-03-15 04:32 2E0ILY   0.137550 -27 IO82qv 0.2  N1BUG FN55mf 4731

2200m/630m Transatlantic Reception, January 26 to February 2

The past few nights have been very interesting on LF and MF. For starters, January 29 was the first time I heard Europe on 2200 meters. I have decoded at least one transmission from Europe on the band each of five nights since then. This may be in whole or in part due to a change I made to my receiving setup on January 29. However, a number of other curious things emerge when looking at the data.

For one thing, I did not decode any 630m transatlantic signals on the night of January 28/29, but I did decode DC0DX four times on 2200m.

The second thing of note is the decodes on the night of February 1/2. On 630 meters I only decoded EA5DOM, which is quite low latitude for a transatlantic path. The northern stations were completely missing, not only from Europe but also no transcontinental decodes from the VE7 or northern W7 areas. This would be a typical pattern given an upturn in geomagnetic activity from mostly quiet to active/minor storm. Yet this was the second best night for transatlantic on 2200m with three DC0DX decodes, including the best S/N yet observed.

The other thing of interest is that on 630m, excepting an early start by PA0A on the night of January 31/February 1, most of the decodes are about in the middle of common darkness; not near my sunset or near sunrise at the eastern end of the path. There is one late decode of EA5DOM on the night of January 30/31. On 2200 meters we find most of the decodes were late, not too long before sunrise at the eastern end. The only exception is three earlier decodes of DC0DX on the night of January 28/29.

Although not shown here, WH2XND on 2200m was notably weaker on the final night, February 1/2 than on the previous nights in this period.

There is insufficient data to draw any conclusions, but in the early days of hearing transatlantic signals on 2200m it does not seem to correlate well with what is happening on 630m. I wonder if any clear trends will emerge as more data is accumulated.



Transatlantic decodes, 630m, January 26 – February 2:

2017-01-30 02:36 EA5DOM  0.475610 -26 IM98xn N1BUG FN55mf 5578 301 
2017-01-30 02:48 EA5DOM  0.475610 -27 IM98xn N1BUG FN55mf 5578 301 

2017-01-31 00:12 EA5DOM  0.475610 -26 IM98xn N1BUG FN55mf 5578 301 
2017-01-31 01:12 G3KEV   0.475705 -23 IO94sh N1BUG FN55mf 4820 287 
2017-01-31 01:20 G3KEV   0.475705 -25 IO94sh N1BUG FN55mf 4820 287 
2017-01-31 03:16 ON5TA   0.475671 -27 JO20es N1BUG FN55mf 5264 293 
2017-01-31 06:24 EA5DOM  0.475609 -28 IM98xn N1BUG FN55mf 5578 301 

2017-01-31 21:48 PA0A    0.475730 -25 JO33de N1BUG FN55mf 5280 292 
2017-01-31 21:50 PA0A    0.475730 -27 JO33de N1BUG FN55mf 5280 292 
2017-01-31 23:50 PA0A    0.475730 -29 JO33de N1BUG FN55mf 5280 292 
2017-01-31 23:54 PA0A    0.475730 -27 JO33de N1BUG FN55mf 5280 292 
2017-02-01 00:24 G8HUH   0.475782 -26 IO81mg N1BUG FN55mf 4766 289 
2017-02-01 00:24 EA5DOM  0.475610 -27 IM98xn N1BUG FN55mf 5578 301 
2017-02-01 00:44 G8HUH   0.475782 -26 IO81mg N1BUG FN55mf 4766 289 
2017-02-01 00:48 EA5DOM  0.475610 -29 IM98xn N1BUG FN55mf 5578 301 
2017-02-01 00:54 G8HUH   0.475782 -22 IO81mg N1BUG FN55mf 4766 289 
2017-02-01 02:48 EA5DOM  0.475609 -27 IM98xn N1BUG FN55mf 5578 301 
2017-02-01 03:36 EA5DOM  0.475609 -28 IM98xn N1BUG FN55mf 5578 301 
2017-02-01 03:44 F1AFJ   0.475727 -28 JN06ht N1BUG FN55mf 5193 295 
2017-02-01 04:00 EA5DOM  0.475609 -30 IM98xn N1BUG FN55mf 5578 301 

2017-02-02 02:36 EA5DOM  0.475610 -28 IM98xn N1BUG FN55mf 5578 301 
2017-02-02 03:24 EA5DOM  0.475609 -25 IM98xn N1BUG FN55mf 5578 301

Transatlantic decodes, 2200m, January 26 – February 2:

2017-01-29 01:20 DC0DX 0.137469 -27 JO31lk N1BUG FN55mf 5399 294 
2017-01-29 02:18 DC0DX 0.137469 -24 JO31lk N1BUG FN55mf 5399 294 
2017-01-29 04:14 DC0DX 0.137469 -27 JO31lk N1BUG FN55mf 5399 294 
2017-01-29 07:08 DC0DX 0.137468 -27 JO31lk N1BUG FN55mf 5399 294 

2017-01-30 06:12 DC0DX 0.137468 -25 JO31lk N1BUG FN55mf 5399 294 

2017-01-31 07:20 G8HUH 0.137425 -28 IO81mg N1BUG FN55mf 4766 289 

2017-02-01 07:10 DC0DX 0.137469 -27 JO31lk N1BUG FN55mf 5399 294 

2017-02-02 05:36 DC0DX 0.137469 -23 JO31lk N1BUG FN55mf 5399 294 
2017-02-02 05:58 DC0DX 0.137469 -24 JO31lk N1BUG FN55mf 5399 294 
2017-02-02 06:20 DC0DX 0.137469 -25 JO31lk N1BUG FN55mf 5399 294

2200 Meter SoftRock Lite II

Any good project starts with some scribbling on paper…

With the 630 meter SoftRock Lite II working well it was time to turn my attention to putting one on 2200 meters. There is no standard SoftRock which covers this range so I would have to make a few component substitutions. The changes are all very minor except for the crystal which cost more than the SoftRock kit! Something around 455 kHz is ideal. I ended up buying a 455 kHz carrier crystal from INRAD. Several capacitors in the oscillator would need to be changed to higher values, namely C10 3900 pF, C11 2700 pF, and C12 680 pF. I also changed R16 to 10k ohms because I find the oscillator never runs properly with the stock value.

The front end was also going to need some changes. C3 becomes .005 uF. C4 should be .16 uF but I used a .15 and it worked well enough. L1 is 267 uH. I used 25 turns on a FT-37-43 core. T1 primary should be 8.23 uH. I used 12 turns on a FT-37-61 core. The secondary is 6 bifilar turns. This input circuit has a bit of loss (about 2 dB) but I considered that acceptable since I was unable to come up with a better design that was also practical. As with the 630 meter unit, additional front end selectivity would be needed. I duplicated the C3, L1, C4, T1 primary once more and placed the circuit on a small prototype board.

Internal view of completed receiver

The finished receiver works well. I have been hearing WH2XND very well every night. I have heard Europe on WSPR-2 mode four consecutive nights, three of those nights DC0DX and the one night G8HUH. There were no problems with out of band signals until I added a preamp, at which time I had to put an outboard bandpass filter in line to keep things clean. LO frequency stability is on par with the 630 meter unit. See my earlier performance evaluation on that for details. I am happy with the performance and this is now my primary 2200 meter receiver.

Final Test of SoftRock Lite II on 630-Meters

Response curve of three section filter

I built a small two stage band pass filter that fits in the box with the SoftRock. Essentially I took the half wave input filter used in the receiver front end and duplicated it twice, including the shunt inductance of the input transformer. Combined with that already on the SoftRock itself, this provides a three section filter which is more than 100 dB down at the second harmonic of the local oscillator (and much more at the third harmonic). Performance of the filter was verified on a spectrum analyzer after construction. It is virtually identical to the modeled plot shown here. It is always good when real world results match the design model!

Return loss of the three stage input filter

Some will argue that this filter is too simplistic. That may be true from a purist standpoint, but results have proved more than adequate. I wouldn’t call it a state of the art design. However it is very inexpensive and fits in a small space. Pass band loss is about 5 dB, mostly due to Q of the inductors. I wasn’t able to come up with a filter that looked better on reasonable size toroids without running into issues with overlapping turns and distributed capacitance causing problems. With any practical antenna (and preamp if needed) I don’t see this loss as a problem. Return loss isn’t great, but exceeds 10 dB across the pass band, which is approximately 420 to 520 kHz. In a receive application I see no problem with this. The input impedance of receivers is all over the place, some much worse than this one.

Live results are gratifying. Even with one of my 22 dB gain W1VD preamps in line, there was no sign of broadcast stations appearing in the useful range of the receiver. (Note: I don’t need the preamp with my receive antennas. I only used it as an acid test to evaluate filter performance, since it makes the signals we want to reject that much stronger.)

Inside the completed receiver

The receiver and filter fit nicely into a Hammond 1590BBK diecast box which gives it a nice look in a compact package. I probably didn’t need to use miniature coax (in this case RG-188) for the very short runs interconnecting the BNC input connector, filter board, and SoftRock. At this frequency and with this layout, the very small capacitance between unshielded wire leads would probably not cause harmful leakage around the filter. But I have a bunch of this stuff, so why not use it? Besides, the teflon cable is a joy to work with. The insulation on most of the small hookup wire I have melts easily while soldering. Not this stuff!

Building this receiver was a pleasant and worthwhile experience. I finally understand how simple SDRs work. Studying the schematic, reading the descriptions of individual stage function, building it and probing various locations in the receiver with an oscilloscope and spectrum analyzer was very enlightening. I learned more about filters. Perhaps most importantly, I am no longer afraid of projects involving surface mount components. Performance of the completed receiver is good enough that I can recommend it to anyone wanting a simple, inexpensive receiver for the 630 meter band or anything else in the 420 to 520 kHz range (with a sound card sampling at 192 kHz the receiver will tune 368 to 560 kHz, but the filter cuts off at 420 and 520 kHz). It outperforms my FT-2000 in every measurable way. If there is one caveat it would be that if one wished to use it for serious reception of QRSS or DFCW signals it either needs to be well insulated or have a crystal heater added. For everything else its frequency stability is excellent. To my knowledge there is no QRSS or DFCW activity on 630 meters at the present time.

Update 19 January 2018: Adding schematic for the two additional stages of front end filtering. Coils are the same as those in the Softrock Lite II 455 kit.

External band pass filter schematic

More Evaluating the SoftRock Lite on 630-Meters

Last night I decided to run a few tests. I don’t have an elaborate lab, but I do have two calibrated signal generators and some other test equipment.

What I had read about the SoftRock not responding to signals on even harmonics of the LO frequency is not true according to my tests. First I looked at MDS (minimum discernible signal) at 474 kHz (LO frequency + 10 kHz) with a signal generator connected to the receiver. I could still clearly see the signal in HDSDR at -135 dBm. Next I measured at 938 kHz (2 x LO frequency + 10 kHz) and got -57 dBm. At 1402 kHz (3 x LO + 10 kHz) I found it to be -53 dBm. There are some things I don’t understand about this result. Theoretically at 1402 kHz it should be down no more than the SoftRock front end filter attenuation at that frequency plus a few dB because the LO third harmonic is 10 dB down from the fundamental. According to my modeling of the filter and spectrum analyzer sweep of the same filter outside the receiver, it attenuates 41 dB at this frequency. 41 plus 10 is only 51 dB down according to theory. Yet I was seeing 82 dB difference in MDS. That doesn’t make sense to me. What is clear both from this test and from those broadcast stations I was able to identify while listening on the SoftRock is that both the second and third harmonic response is considerable.

I spent several hours carefully examining the strength and number of broadcast signals making it through the front end selectivity. My external bandpass filter removed all traces of broadcast stations being heard with the receiver. Its calculated and measured response is -60 dB at 900 kHz, -84 dB at 1400 kHz. The filter I plan to build for the SoftRock should be -66 and -88 dB respectively.

I attempted to run a close spaced IMD test on the receiver. With signals on 473 and 474 kHz, I detected no IMD products when both of these signals were 90 dB above the noise floor. This is probably sound card dependent but it indicates to me that the SoftRock hardware has good strong signal performance. I reduced one signal to barely audible and left the other at 90 dB above noise floor. Switching the strong signal on and off did not noticeably affect my ability to hear the weak signal. I need a different test setup to go beyond 90 dB in these tests, though I am satisfied it is as good as it needs to be.

This continues to look like it should be a relatively low cost, high performance 630 meter receiver once the added front end filtering is in place.