This was the first chance I had to play with Linrad version 028 here at W3SZ. I was most impressed with it; now I can't imagine doing a contest without it!
Click here to go to the bottom of the page where I have a list of png files taken with Linrad of some of the stations I heard during the first weekend of the ARRL EME Contest 2001.
Click here to go to my DSP page, that has links to Linrad pages on this website and elsewhere (with of course links to Leif, SM5BSZ's pages, as he is the creator of Linrad)
I had a great time with the first leg of the ARRL 2001 EME Contest. In fact, I had the most fun I've ever had with any contest. I used Linrad 028 by Leif Asbrink, SM5BSZ with my homebrew receiver for the entire contest. I made png files from Linrad of many of the stations that I worked.
I ran with the following:
2 x 2 M2 2mxp20 array
2 receive chains (one for H polarity, and one for V polarity), each with:
ARR 144 MHz preamp on mast at power divider
LMR600 down the antenna support structure to 7/8 inch hardline to the shack
DCI 2 MHZ bandpass 144 MHz filter at entrance to the shack
KA0RYT cavity preamp between the DCI filter and the homebrew receiver
Homebrew receiver, one for each polarity, as detailed here is a link to the schematic
Delta44 sound card to computer running Linrad.
I used a MACOM T-1000 splitter (1-1000 MHz) to split off part of the Horizontal polarity receive signal after the cavity preamp, to supply simultaneously both Linrad and the SSB Electronics LT2S MK II. The LT2S Mk II transverter fed the Elecraft K2, which I used for both transmit and auxilary receive. I used my FT-1000MP as a shelf to hold the K2, as as I had previously found that the K2 noiseblanker was far superior to that of the FT-1000 MP and I wanted to run the best possible setup. I then fed the output of the K2 to a Pentium Pro 200 running DSP-Blaster, and used mostly 600 Hz center freq 100 Hz wide filter, with LMS noise reduction and occasionally the auto notch, too. This "conventional" system was run in parallel with the Linrad for receive.
I used Linrad both as a primary receiver, and also as a spotting receiver. It had three major advantages that were immediately apparent: (1) being able to see a wide swath of spectrum (30 KHz as configured, but see below), (2) being able to find and receive signals regardless of their polarity at the moment, and (3) showing where stations were transmitting even during the time periods that they were receiving.
Using Linrad as spotting receiver was somewhat limited by the huge number of birdies I have, especially below 144.040. These are essentially all external birdies. They do not mostly come from my shack. My shack is about 100 feet away from the tower, and about 50 feet below it. See the pictures below to see first the spectrum with the antenna connected, and second the spectrum without the antenna connected. Comparing these two pictures allows you to see that the birdies are external. I also turned off the 5 computers in the shack (all except for the one running Linrad) with no effect on the birdies, thus showing that they did not come from these computers.
On the image below you see at the top of the screen Linrad displaying a waterfall of frequency vs time, with most recent times at the top. The "Plaid" pattern is the huge number of birdies I have here from external sources. The frequency range displayed is 10KHz out of a possible 30 KHz. I have zoomed in because of the birdies, to better see what is between them. I have these birdies with all of my receive combinations. They are not unique to Linrad. In fact, because of its capability to run with very narrow bandwidths (I am currently using a 20 Hz filter), Linrad allows you to work stations that would be too close to the birdies to work with other rigs. Another difference is that with Linrad you see all the birdies all at once, and with great sensitivity!
On the next image, below, you see a similar screen, but I have turned off the power to the preamps on the mast, to show that the birdies are not internal to the receiver. The rest of the receiver, and the preamps in the shack are fully powered. You again see the "Plaid" display taken with the antennas functional. But above this you now see a black strip, with just a tiny bit of very faint green trace at the lowest frequencies. This shows an essentially complete absence of signal without input from the antenna/mast mounted preamp. This shows that all birdies are entering by way of the EME array.
Even with the birdies, I found Linrad very useful for spotting. There were many stations that I quite simply probably wouldn't have found without Linrad, as I could be "working" one part of the band and see when activity popped up 20-30 KHz away. I could also see when stations I needed and was going to QSY to had signed off, so I didn't waste my time chasing them. As a weak signal receiver, Linrad was fully the equal of, and sometimes better than the transverter/K2/DSP-Blaster combination. This is the first alternative to the "standard" configuration that has achieved this distinction here. There were one or two weak signals the first day that I couldn't copy with DSP Blaster that were complete, adequate copy with Linrad. The second day, after daylight broke, nearly all signals were vertically polarized, and I couldn't hear the other stations at all on the conventional setup, as it was limited to Horizontal polarity. Thus I operated entirely with the Linrad for receive. I will eventually get things set up so I can go H and V with both receive systems.
It was a major advantage that Linrad was able to see all activity regardless of polarity; no stations were missed because I forgot to check the other polarity, and at least theoretically, I had the advantage of always being aligned with the received polarity of the other station. With Linrad you have all polarities avaiable. You ARE NOT LIMITED JUST TO ZERO AND NINETY DEGREES!!!
I was of course able to monitor one frequency audibly with Linrad in addition to watching a wide range visually, while working another frequency with the K2/DSP-BLaster combo. The sound characteristics of the two systems are different, and I have the K2 pitch at 600 Hz and the Linrad pitch at 400 Hz. I fed both systems into both ears of a pair of Sennheiser Noise Gard Mobile noise-cancelling headphones. I found that I was easily able to copy both systems independently because of the different audio characteristics. What was really nice was listening to the same station with both systems. The Linrad audio was delayed, and I found I could often fill in what I had missed from the K2 audio by catching it as it came by several seconds later in the Linrad Audio.
I was not able to hear or see my own echoes with Linrad, as my configuration just doesn't recover fast enough from the effects of transmit. On some fast exchanges, I was not able to receive quickly enough to get my own call when the other station just sent it at the very beginning of his transmission. I believe this cost me two contacts.
But this is the first contest where I (1) didn't run out of stations I could hear and (2) had a relatively large number of stations that I heard Q5 that never heard me well. This has never happened before; if I could hear them in the past, they could work me. And I know my transmitter has not gotten worse, so the receiver must be better ;)
I had a few dislikes or 'wishes':
It would be nice if there was a key to numerically set the frequency display. I run the band in 10-30 KHz segments, and if there were keys to set the frequency shown at the top of the waterfall it would be nice; this would just need to be redone when I retuned the receiver by changing the 1st LO frequency. The frequency could be set by for example typing "F" which would bring up a question "Set actual zero frequency to" and if I was for example using my setup so that my LO2 was 10.727 and my center frequency was 144.032 I would say 144.005, or 5000 perhaps. If I was using the same LO2 (I don't change it, of course) and my center frequency was 144.062, then I would say 144.035, or 5000 perhaps. This is important as in the throes of a contest, especially when sleep deprived, it is difficult to do the calculations. IT would also be nice if it were possible to type a character to just move the waterfall display and associated spectrum display once screenwidth to the left or right.
A few (5) times when I clicked on the signal in the high resolution window to center it the audio and the high resolution display signals disappeared, and I couldn't bring them back without going "X" to the last screen and then "B" to reenter the weak signal CW mode.
I set my transceiver to the correct frequency for transmitting on the same frequency that the Linrad receiver is receiving by audibly centering a 144 MHz synthesizer in the filter window on Linrad, and then bringing the same synthesizer signal to the center of the passband of the K2/DSP-Blaster combination, with the doppler offset already dialed into RIT on the K2. This works fine, even on those signals where I couldn't hear the other station on the K2 combo (due to only having H polarity for the K2), and by the second day of the contest it only took 5-10 seconds to complete.
Copyright © 1997-2007 COPYRIGHT Roger Rehr W3SZ. All Rights Reserved
Brought to you by the folks at W3SZ