An achieved article that might be of interest to you.
| January, 1998
Hello all CCA AM night fans,
I always look forward to the CCA "special events" and this one was not disappointing. Since I don't have a Collins AM transmitter, I stayed off the air and enjoyed the event from another perspective. Originally, I was going to evaluate several different receivers to see which ones produced the best sounding audio from ham transmitters, but it was obvious from the start that idea was out of the question. I had serious QRM here on the east coast. It was especially bad from the 3778 group when I opened up the receiver bandwidth to 6 or 8 KHz. This, combined with signals that were somewhat hard to copy due to low signal strength and the lack of punch, which is typical of AM rigs, meant that another strategy was in order. I was looking forward to the melodious tones of good AM rigs but all I was getting at my QTH was the ragged edge of ear-splitting QRM. However, it was a good opportunity to evaluate a different set of receivers under the less than ideal conditions which prevailed.
First, let me preface this report by saying that the point of the exercise was to take advantage of the multitude of AM stations, both strong and weak, to determine the best receiver and configuration for optimizing my own station for AM operation. Of course, we all have our own opinions as to which is the best receiver for a particular mode, so just consider my remarks for what they are - simply my opinions.
My original plan was to compare my three main AM receivers used here for Short Wave monitoring with a 75A-4, SX-115, and an RME-6900. The R-390A is the AM receiver standard as far as I'm concerned, especially when the audio is recovered from the diode output and applied to a full-range amplifier and quality speaker. It was obvious from the start that my R-390A, RACAL 6117, and the SP-600 could not be used in this evaluation. All of these receivers are excellent choices as high performance SWL receivers, but the lack of QRM fighting features sent me off to quickly select another group of receivers. The receivers used in this test were:
I'll submit my observations in reverse order from worst to best. I will not include the FT-1000MP because it is neither Collins nor a boat anchor. However, it does have all available Collins filters. The FT-1000MP has a synchronous detector which produces wonderful copy and really pulls a station out of the crud, but it was difficult to get it to lock onto the stations since most of the rigs were drifting and the synchronous detector had to capture each station as it came on the air. With the synchronous detector turned off, the FT-1000MP did an acceptable job. Suffice it to say that it is indeed a first class rig. In 30 years it could be a classic.
R-4B - Due mainly to their reputation, I expected this receiver to really shine, but I have to say that IMHO it was the worst of the bunch. I even tried a second R-4B just in case there was a problem with the first one. While these receivers are OK on SSB and CW, I didn't care for the audio quality. The passband tuning was helpful but due to the poor skirt selectivity, it was not up to the performance provided by other receivers used in this evaluation. The speaker was a Optimus Pro X7 high quality bookshelf speaker, which is my personal choice for a small speaker. There are a couple of more expensive models in the Optimus line, but I have found them to have too much tweeter and hence, highly accentuated highs. This model is a good compromise and most rigs will drive it just fine. The only caveat is that they cost $80 unless you can find them on sale.
75S-3C - When conditions permitted full DSB AM, I was amazed at how good this receiver sounded. I've always liked them on SSB and CW, but AM is a nice surprise. I'm sure this is not news to anyone. Under good conditions, this would have been a perfectly acceptable AM receiver. However the bandwidth is a bit wide to be useful under actual 75 meter conditions. On the other hand, if we get something going on 10 meters in a couple of years, this may be a good receiver choice due to the S-Line's excellent sensitivity on 10 meters. I was using the standard 312B-4 speaker. A higher quality speaker would have improved the sound quality further.
After evaluating the R-4B and 75S-3C (I started out on the FT-1000MP) it was obvious that extreme conditions required an extreme receiver. SSB stations above and below 3775 were especially strong at my QTH so I had to be able to select whatever sideband of the AM signal was the clearest the any give time. Needless to say, this technique worked superbly and best results were produced by the 75A-4's and the Sherwood/Drake R-4C.
Both 75A-4's came out on top with no major discernable differences. I'm sure that under some conditions the highly modified "HM" receiver would have produced better results if overload was a problem or there was a need for high sensitivity. This was not the case last Thursday - high selectivity was the key to good copy at my QTH. The two cascaded 3 KHz filters with their nearly vertical combined "skirts" eliminated the QRM to the point that with the judicious use of the passband tuning control, I could copy just about everyone who check in with W0CXX. I also heard a bunch of station who didn't make it. Further, the 3 KHz - 6 dB bandwidth still permitted acceptable audio quality. I was not able to evaluate the added distortion caused by eliminating one sideband. This subject was covered in detail in ER, so no comments are necessary. I did find that turning on the BFO and followed by very careful adjustment of the passband control produced very good results on some signals. However, this was only useful on signals that were fairly stable.
To sum up the comparisons to this point, simply put, the difference between the 75S-3C in the wide AM mode and the A-4's was amazing. Like I said earlier, the S-3C sounded great when the QRM was low and the station's signal strength high, but in really heavy QRM, the A-4's came to the rescue. Incidently, neither of the 75A-4's could be opened up to wider bandwidths than 3.1 KHz due to the installation of the second filter in the i.f. strip. Next time, I'll include in the mix one of the 75A-4's I'm optimizing for AM. I plan to publish the info on installing the Kiwa filters in the A-4. It's totally non-destructive and can be removed in just minutes. I have also modified a 75S-3 with a 2.3 KHz second filter with amazing results. Some of you have heard me taking about it on the weeknight nets.
Just to complete the information on the 75A-4's, 75A-4 #1 was feeding a 312A-1 speaker. 75A-4 #2 uses a 270G-3 speaker.
While this is a Collins reflector, I know that I lot of you have Drake gear, so I'll give you my observations of the Drake receivers. First of all, the highly modified R-4C was excellent in this application. The results are comparable to the 75A-4's but I think the A-4's had the advantage with the stations most difficult to copy. The A-4's also sounded better. One problem with the R-4C is that the notch moves when the passband tuning is adjusted. This make heterodyne removal difficult. The results of the R-4B was disappointing as I indicated earlier. The speaker used on the R-4C was a slightly modified standard MS-4.
There are many of you who have a great deal more experience with AM than I do. I will gratefully accept any suggestion on the subject of AM reception under high QRM, weak signal conditions. My conclusion from Thursday's event, which is no surprise, is that under normal band conditions, the secret to digging the weak AM stations out of the mud was the passband tuning which permitted selecting the best sideband and carefully tailoring the receiver i.f curve to minimize distortion and adjacent QRM. This, combined with very steep skirted double filters and the occasional use of the notch were real weapons in the QRM fighting arsenal. While fidelity was somewhat limited, solid copy was possible and in an operating event like this, it made all the difference. Until I have a better suggestion, I'm going to equip both of my AM stations (a B&W 5100 and a Valiant II) with 75A-4's equipped with dual 3 KHz filters and dual 6 KHz filters.
On Thursday night, I'm supposed to be the Net Control Station for the CCA net and our topic for the night will be your comments on this subject and general comments on AM operation under high QRM situations.
73 to all. CU on the nets.
The RADIO WORKS
ARRL Life Member
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