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DSP vs Filtering for better reception

Jun 25th 2012, 15:45

KD2ADL

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I have a 20-year old IC-725 without many modern bells and whistles. I am constrained to use an indoor antenna (random length doublet with tuner) and my problem is inability to receive on SSB anything but strongly amplified or nearby stations over what I guess is my receiver’s noise floor. Obviously a better, outdoor antenna would help, but that’s not an option where I live.

I’m wondering if I can improve my SSB signal to noise ratio with a more modern rig with DSP and either AF or IF filtering. Is one of these more relevant than the other? I have a sense that DSP is more for transmit than receive, but I may be wrong on this. Or is my poor antenna the kind of “original sin” for which no amount of modern transceiver technology will compensate?
Jun 25th 2012, 19:52

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
DSP is great - particularly for receivers. It allows you to adjust your filter bandpass and do some good noise reduction and other things. But it won't compensate for a bad antenna.

Assuming your '725 is working normally, you should be able to hear lots of signals. (If you can't hear them, they probably can't hear you either.) I'd guess you have one of two problems: 1) a very high local noise level (what does your S meter say?), and/or 2) you are in a metal frame building or room that is keeping the RF out. Do you have the same problem on all HF bands?

You might try a temporary set-up with your rig at some remote location - just to prove that the rig works.

A random length doublet can work (what length?) with a tuner, but it's sometimes possible to set the tuner so your rig thinks it's got a matched load, but most of the power is dissipated in the tuner.

Good luck!

73 Martin AA6E
Jun 25th 2012, 22:11

KD2ADL

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks for the comments. I can respond to a few of your questions.

Shack is on the second floor of a wood-frame building; antenna is 34' long, bent around three walls of a 10' by 17' room, and fed by ladder line into a 4:1 balun connected by 3' of coax to an LDG IT-100.

You asked about what I'm seeing on the S-meter. One of my frustrations is that what I see on the meter and what I hear don't seem to correlate. According to the S-meter, noise is worst on 15 and 40 (S5), lower on 17 and 20 (S2, S3), and nonexistant on 12 and 10 (needle doesn't move). But I am hearing the same rushing sound on all bands, with very little traffic. For example, I would expect to be able to pick up the Maritime Mobile net on 14300 around 1800EDT, but can hear nothing.

I know that the rig works, because I participate in a local 10M net, and get clean copy on various PSK31 hangouts like 7070 and 14070. And I am aware that a tuner might eat much of my transmitting power, but my current issue is reception. It's not only a question of "you can't work 'em if you can't hear 'em," it also inhibits me from calling CQ because for all I know, the channel I am using is occupied and I just can't hear it.
Jun 26th 2012, 01:48

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

Modern receivers use automatic gain control to keep the audio output level of a radio more or less constant, even though the input signal level changes dramatically. Thus, it is quite normal for the noise output level to stay the same, even though the S-meter changes. Some radios allow the AGC to be disabled. Many stations have a much higher noise level than they would like--but it often isn't feasible to do much about it. Even in the best locations, folks with big HF antennas are limited by thunderstorm activity and galactic noise.

The most effective way to identify local noise is to run the receiver off a battery and cut the mains power--most noise sources these days are powered by the AC mains.

A directional receiving antenna can be effective way to null out noise.

Sep 1988 - QST (Pg. 29)
Coplanar-Twin-Loop Antenna, The

Author: Villard, Jr., O.G., W6QYT
Article: QST Archive [PDF]
Keywords: CONSTRUCTION HOMEBREW ANTENNA RECEIVE LOOP INDOOR

Another effective technique, if you have a highly localized noise source, is to use a null cancelling antenna--one can use a sense antenna to pick up noise, and carefully adjust the phase and amplitude to remove much of that noise from your antenna system.


Jul 1994 - QST (Pg. 29)
Null Steerer Revisited, The

Author: Michaels, Charles, W7XC
Article: QST Archive [PDF]
Keywords: CONSTRUCTION NOISE REDUCTION HOMEBREW


Zack Lau W1VT
ARRL Senior Lab Engineer

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