Hello
Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
as always-thanks
73 Bob W7LRD
Hello Bob,
Does the preamplifier have a internal gain control ?
If it does, back it off at ~ 6 S units.
Excess gain only reduces the system dynamic range.
The real test is with a SINAD meter or a NF meter, injecting a test signal up at the preamplifier input, and reducing gain until NF or SINAD sensitivity is just barely degraded. That will be the point of best S/N, (best sensitivity).
Most of the S7 reading you have is just noise, not signal.
The preamplifier could be defective and performance is out of spec. Whatever that spec is, I do not recall ever seeing a ICOM AG-25/35 performance spec.
If the preamplifier is a multi stage unit, the first stage which sets the system noise figure could be defective/soft and the second stage just amplifies the noise ahead of it.
Most convincing would be a NF test with professional meter.
Send it to me and I will test for free. I will NOT repair. I will NOT open it. Include return postage if you want it returned (HI).
I will provide you a plot of the NF and gain performance using a HP-8970B Noise Figure Meter and HP-346C Noise source.
Of course, this offer is open to any other AMSAT member. E-mail me for details.
What do you say ?
Stan, W1LE Cape Cod FN41sr
Bob- W7LRD wrote:
Hello
Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
as always-thanks
73 Bob W7LRD _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
----- Original Message ----- From: "Bob- W7LRD" w7lrd@comcast.net To: amsat-bb@amsat.org Sent: Tuesday, March 16, 2010 12:28 AM Subject: [amsat-bb] fun with preamps
Hello
Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise
Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
as always-thanks
73 Bob W7LRD
Hi Bob, W7LRD
No preamp noise Smeter=0, preamp on Smeter=7 means very small. Step 1 without preamplifier tune your receiver in SSB or CW on a very weak carrier like a beacon and rotate your antenna into a null until you can barely hear a bit at about 1000 Hz over the noise. Step 2 now connect your preamplifier and see by hears if the Signal to Noise ratio S/N improves or not. If you can hear better the 1000 Hz bit your preamplier is useful othervise S meter = 7 means only or mostly the amplified preamplifier noise. Your test can be more accurate if instead to use your hears you connect an analog AC voltmeter at the audio output of your receiver.More is the swing of the AC voltmeter and better is the preamplifier i.e. lover is it's Noise Figure.
Have fun.
73" de
i8CVS Domenico
Bob,
I have been running both the Icom pre-amps with my 910 for some time now!
I do not need S/N ratio's or fancy tests to know that without them I would have missed many a contact;
True, they are not brilliant (Icom does not even publish figures for them) but they work and unless I feel like chucking the thick end of a large amount of green at something else, they will continue to work.
And a large plus is that you can tx through them without fear of goobering them up and they operate through the coax so no messy power line runs with the coax!
Just my 2 cents worth of course!
David KG4ZLB
On 3/15/2010 20:31, i8cvs wrote:
----- Original Message ----- From: "Bob- W7LRD"w7lrd@comcast.net To:amsat-bb@amsat.org Sent: Tuesday, March 16, 2010 12:28 AM Subject: [amsat-bb] fun with preamps
Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
Hi Bob, W7LRD
No preamp noise Smeter=0, preamp on Smeter=7 means very small. Step 1 without preamplifier tune your receiver in SSB or CW on a very weak carrier like a beacon and rotate your antenna into a null until you can barely hear a bit at about 1000 Hz over the noise. Step 2 now connect your preamplifier and see by hears if the Signal to Noise ratio S/N improves or not. If you can hear better the 1000 Hz bit your preamplier is useful othervise S meter = 7 means only or mostly the amplified preamplifier noise. Your test can be more accurate if instead to use your hears you connect an analog AC voltmeter at the audio output of your receiver.More is the swing of the AC voltmeter and better is the preamplifier i.e. lover is it's Noise Figure.
Have fun.
73" de
i8CVS Domenico
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Bob,
Here's a method I've used for setting preamp gain. Got it from GM3SEK.
===== Here's a method that requires no test equipment at all. It comes from G4DGU, who designed all the original muTek transverters and outboard preamps to have adjustable gain. This method uses the sharp threshold effect of FM detectors at low S/N ratios, and it allows you to optimize the preamp/transverter gain for your local band noise conditions.
1. Turn the transverter/preamp gain well up.
2. Find a very weak but steady unmodulated carrier (off-air, not from a signal generator or a local birdie). Rotate the antenna until you can just detect the signal in FM mode.
3. Reduce the preamp/transverter gain until you hear the noise increase. The FM threshold is sensitive to a small fraction of a dB in S/N.
4. Increase the gain just a little,to the point where you can't hear the quieting improve much.
5. Switch back to a real DX mode.
Remember that every dB of unnecessary preamp/transverter gain will probably subtract almost 1dB from your system intermod intercept!
The penalty of adjusting the gain correctly is that you're living just above the "knee" where S/N will begin to deteriorate rapidly if something changes. It's worthwhile to repeat this test every few months - especially just before a contest.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book' 'In Practice' columnist for RadCom (RSGB) ====
73, Mike, N1JEZ AMSAT 29649 "A closed mouth gathers no feet"
Hello
Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise
Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack. I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910. Are these real world results for what I have. S7 noise level seems a bit high.
as always-thanks
73 Bob W7LRD
participants (5)
-
Bob- W7LRD -
David - KG4ZLB -
i8cvs -
n1jez＠burlingtontelecom.net -
Stan, W1LE