As I prepare for getting on the birds.. I decided to test out this AG-25 preamp that came with my new radio. Now my experience with preamps is limited, but this one is rather baffling me.
I connected everything in line.. and powered up the preamp and at 1st glance it appeared that it worked fine, however after about 10 seconds I started questioning a few things.
#1 My noise floor went up 10db ( I've never seen this before I guess my experience with preamps is just the good ones )
#2 My Received signal quality was unchanged all it did was make the signal 'appear' stronger on the meter.
Now I know if the preamp was dead.. I would hear nothing.. so I am going to go out on a limb here and assume that its either:
a) an incredibly broad band pre-amp with a completely fictitious ' Low noise floor ' claim or
b) Detuned
I guess what Im going after is a few things with this email .. has any one ever taken one of these AG-25's and given it a proper tuning? Or is this what I should expect with this particular item?
In my previous experience (terrestrial weak sig stuff) a decent preamp may up the noise floor from a s-meter reading of mabey 0 or 1 to a s5, but with a significant increase in received signal gain. and intelligibility.
I really want to do this the correct way.. and if this thing is un-fixable junk.. its going in the junk drawer fast. I don't want to configure rx system around sompthing that wont produce results.
I'd like to thank everyone for their input so far.. Ive learned a lot w/o even Transmitting and R.F. yet!
-Steve Raas
N2JDQ
At 09:30 PM 12/12/2006 -0500, Steve Raas wrote:
As I prepare for getting on the birds.. I decided to test out this AG-25 preamp that came with my new radio. Now my experience with preamps is limited, but this one is rather baffling me.
I connected everything in line.. and powered up the preamp and at 1st glance it appeared that it worked fine, however after about 10 seconds I started questioning a few things.
#1 My noise floor went up 10db ( I've never seen this before I guess my experience with preamps is just the good ones )
#2 My Received signal quality was unchanged all it did was make the signal 'appear' stronger on the meter.
Now I know if the preamp was dead.. I would hear nothing.. so I am going to go out on a limb here and assume that its either:
a) an incredibly broad band pre-amp with a completely fictitious ' Low noise floor ' claim or
b) Detuned
I guess what Im going after is a few things with this email .. has any one ever taken one of these AG-25's and given it a proper tuning? Or is this what I should expect with this particular item?
In my previous experience (terrestrial weak sig stuff) a decent preamp may up the noise floor from a s-meter reading of mabey 0 or 1 to a s5, but with a significant increase in received signal gain. and intelligibility.
Steve,
My guess is that the stock ICOM preamp is not especially low-noise. Typically, they are more like 1.5 to 3 dB NF and that would give the results you see...more gain but little quieting.
Years ago I ran a nuvistor preamp (3-dB NF with 15-dB gain) and all it mainly did was add gain (made signals louder including the noise).
Now if you have the preamp installed in the shack next to the radio with lots of coax up to the antenna, you will get less low-noise performance (even if it is a good low noise amplifier).
Finally, have you checked receiving weak signals? Testing with strong signals will not show much difference other than higher s-meter reading. 73's, Ed - KL7UW ========================================= BP40iq, Nikiski, AK http://www.qsl.net/al7eb Amsat #3212 Modes: V - U - L - S USA Rep. for Dubus Magazine: dubususa@hotmail.com =========================================
Steve,
The preamp is working fine. The earth is at 290 degrees Kelvin. Unless the antenna sees only cold sky and no earth, trees & buildings you will have 3dB of noise going into your preamp. If your radio has a 1 dB noise figure and your preamp has a 1 dB noise figure no improvement of S/N is noticed only the level has changed by the gain of the preamp. Preamps only help to cover cable loss and poor receiver sensitivity. Low noise figures ( less than 3 dB) are only achieved when the antenna is pointed into space and has a good front to back ratio. Frequencies below 70 cm are subject to galaixy noise that becomes larger as frequency goes lower. expect a 3-4 dB increase at 2M, no noise reduction from cold sky @ 2 M and longer wave lengths.
Art, KC6UQH
----- Original Message ----- From: "Edward R. Cole" al7eb@acsalaska.net To: "Steve Raas" sraas@optonline.net; amsat-bb@amsat.org Sent: Tuesday, December 12, 2006 9:37 PM Subject: [amsat-bb] Re: Pre amp question
At 09:30 PM 12/12/2006 -0500, Steve Raas wrote:
As I prepare for getting on the birds.. I decided to test out this AG-25 preamp that came with my new radio. Now my experience with preamps is limited, but this one is rather baffling me.
I connected everything in line.. and powered up the preamp and at 1st glance it appeared that it worked fine, however after about 10 seconds I started questioning a few things.
#1 My noise floor went up 10db ( I've never seen this before I guess my experience with preamps is just the good ones )
#2 My Received signal quality was unchanged all it did was make the signal 'appear' stronger on the meter.
Now I know if the preamp was dead.. I would hear nothing.. so I am going to go out on a limb here and assume that its either:
a) an incredibly broad band pre-amp with a completely fictitious ' Low noise floor ' claim or
b) Detuned
I guess what Im going after is a few things with this email .. has any one ever taken one of these AG-25's and given it a proper tuning? Or is this what I should expect with this particular item?
In my previous experience (terrestrial weak sig stuff) a decent preamp may up the noise floor from a s-meter reading of mabey 0 or 1 to a s5, but with a significant increase in received signal gain. and intelligibility.
Steve,
My guess is that the stock ICOM preamp is not especially low-noise. Typically, they are more like 1.5 to 3 dB NF and that would give the results you see...more gain but little quieting.
Years ago I ran a nuvistor preamp (3-dB NF with 15-dB gain) and all it mainly did was add gain (made signals louder including the noise).
Now if you have the preamp installed in the shack next to the radio with lots of coax up to the antenna, you will get less low-noise performance (even if it is a good low noise amplifier).
Finally, have you checked receiving weak signals? Testing with strong signals will not show much difference other than higher s-meter reading. 73's, Ed - KL7UW ========================================= BP40iq, Nikiski, AK http://www.qsl.net/al7eb Amsat #3212 Modes: V - U - L - S USA Rep. for Dubus Magazine: dubususa@hotmail.com =========================================
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 Steve, N2JDQ
If your receiver has the capability to switch OFF the AGC than the following simple test will tell you if your preamplifier connected to the actual antenna is working or not for you:
1) Switch OFF the AGC, power OFF the preamplifier and tune the receiver in a free frequency to receive only the white noise
2) Connect an AC voltmeter (5 volt f.s) to the high impedance output of your RX audio hadphone jack
3) Switch the mode to CW or SSB wich uses the product detector wich is linear.
4) Reduce the RF gain to minimum
5) Increase the Audio gain to maximum
6) Aim your antenna at 90° elevation to the cold sky and adjust the RF gain until you read 1 volt due to the white noise and call this voltage V1
7) Without changing anything aim your antenna to the horizon at 0° elevation where half of the antenna lobe should now pick up more ground noise and read the new voltage and call this voltage V2
8) Make the ratio V2/V1 wich is your (S+N)/N with preamplifier OFF
Repeat the above steps with the preamplifier ON
If the ratio V2/V1 with preamplifirer ON is a little bit greater than the ratio with the preamplifier OFF than that preamplifier is usefull othervise if the ratio is unchanged it is not usefull because or the noise figure of the preamplifier is too high or your antenna temperature is too high because the antenna pick up too much thermal noise due to side lobes or pick up man made noise particularly in urban or suburban areas particularly if the frequency is 144 MHz or belove.
If you live in an area with high density of FM and TV transmitters located in top of a hill (my situation) you can get the phase noise of the above transmitters as a reference signal because the phase noise of their oscillators extends well in to the amateur band in form of a withe noise ( moving the S meter to me in 70 cm at 10 km distance from the top hill )
If you measure V2 with the antenna aimed toward the source of phase noise (the hill) and than V1 with the antenna aimed to the cold sky 90° elevation than you can better evaluate the ratio V2/V1 with preamplifier ON and preamplifier OFF
I don't mention the possibility to use the sun for the above evaluation because using small gain antennas for satellite communications in to 2 meters and 70 cm the sun noise is not well detectable over the noise of a receiving system.
Greatest is the ratio V2/V1 with preamplifier ON and lower is the overall noise figure of your receiving system or lower is your antenna temperature or both togheter.
For the evaluation of a receiver when is antenna connected is the ratio V2/V1 i.e. the swing between V2 and V1 that count.
73" de
i8CVS Domenico
----- Original Message ----- From: "Steve Raas" sraas@optonline.net To: amsat-bb@AMSAT.Org Sent: Wednesday, December 13, 2006 3:30 AM Subject: [amsat-bb] Pre amp question
As I prepare for getting on the birds.. I decided to test out this AG-25 preamp that came with my new radio. Now my experience with preamps is limited, but this one is rather baffling me.
I connected everything in line.. and powered up the preamp and at 1st
glance
it appeared that it worked fine, however after about 10 seconds I started questioning a few things.
#1 My noise floor went up 10db ( I've never seen this before I guess my experience with preamps is just the good ones )
#2 My Received signal quality was unchanged all it did was make the signal 'appear' stronger on the meter.
Now I know if the preamp was dead.. I would hear nothing.. so I am going
to
go out on a limb here and assume that its either:
a) an incredibly broad band pre-amp with a completely fictitious '
Low
noise floor ' claim or
b) Detuned
I guess what Im going after is a few things with this email .. has any
one
ever taken one of these AG-25's and given it a proper tuning? Or is this what I should expect with this particular item?
In my previous experience (terrestrial weak sig stuff) a decent preamp may up the noise floor from a s-meter reading of mabey 0 or 1 to a s5, but
with
a significant increase in received signal gain. and intelligibility.
I really want to do this the correct way.. and if this thing is un-fixable junk.. its going in the junk drawer fast. I don't want to configure rx system around sompthing that wont produce results.
I'd like to thank everyone for their input so far.. Ive learned a lot w/o even Transmitting and R.F. yet!
-Steve Raas
N2JDQ
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
Is this test useful on a receiver where the AGC cannot be turned off? My FT-736 has only fast/medium/slow. No off. I would expect a reduction in accuracy, but is it even worth doing?
Greg KO6TH
----Original Message Follows---- From: "i8cvs" domenico.i8cvs@tin.it To: "Steve Raas" sraas@optonline.net, "AMSAT-BB" amsat-bb@amsat.org Subject: [amsat-bb] Re: Pre amp question Date: Wed, 13 Dec 2006 20:55:50 +0100
Hi Steve, N2JDQ
If your receiver has the capability to switch OFF the AGC than the following simple test will tell you if your preamplifier connected to the actual antenna is working or not for you:
{snip}
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Greg,
Yes, you can do it by reducing the RF gain to minimum which generally opens the AGC to max gain. Since the noise floor is not a large signal, it will not drive the AGC very much this way. Note, do not use your s-meter for these readings. Connect a good audio voltmeter (ideally with a good analog scale).
Another approach is to use a step attenuator in front of the receiver: You set you reference on cold sky the same, then point at horizon and adjust attenuation until the audio meter reads the same reading as cold sky. The amount of attenuation in dB give the result. AGC does not affect this method.
73's Ed - KL7UW
At 07:48 PM 12/13/2006 -0800, Greg D. wrote:
Is this test useful on a receiver where the AGC cannot be turned off? My FT-736 has only fast/medium/slow. No off. I would expect a reduction in accuracy, but is it even worth doing?
Greg KO6TH
----Original Message Follows---- From: "i8cvs" domenico.i8cvs@tin.it To: "Steve Raas" sraas@optonline.net, "AMSAT-BB" amsat-bb@amsat.org Subject: [amsat-bb] Re: Pre amp question Date: Wed, 13 Dec 2006 20:55:50 +0100
Hi Steve, N2JDQ
If your receiver has the capability to switch OFF the AGC than the following simple test will tell you if your preamplifier connected to the actual antenna is working or not for you:
{snip}
Get the latest Windows Live Messenger 8.1 Beta version. Join now. http://ideas.live.com
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 Greg, KO6TH
If the AGC cannot be switched OFF put it to slow and reduce the RF gain to a minimum necessary to read about 100 mV in the voltmeter connected to the audio jack and this because you don't want to drive the AGC
For this type of measurement with AGC ON the IF of your receiver must work with a gain wich is well below the gain necessary to drive the AGC
In this situation the AC voltage across the AC voltmeter is low but it can be elevated to about 3 volt using an audio transformer with impedance ratio 5/5000 ohm recovered for example from an old receiver using a 6V6 for the audio amplifier.
Connect the 5 ohm secondary winding to the low impedance audio of your receiver and connect the 5000 ohm primary winding to your voltmeter via a 10 k resistor with a 100 n capacitor across the AC voltmeter to damp the pointer indication.
If the above transformer is not available then a small power supply transformer 9/220 volt works equally well.
If your AC voltmeter has a scale directly calibrated in dB you can directly read the ratio V2/V1 expressed in dB as I do with my popular analog Volt- Ohm-Milliammeter model ICE 680R calibrated for -10dB ---0dB---+22dB with 0 dB = 1mW over 600 ohm
Before to make any measurement be sure that having reduced the RF gain to a minimum necessary your S meter do not move at all tuning the band particularly when the preamplifier is ON
Good references to better understand about this type of measurements are:
Don Lund WA0IQN, Using Sun Noise, QST 4/1968
D.W.Bray K2LMG and P.H. Kirchner W2YBP "Antenna Pattern from the Sun QST 7/1960
P.Schuch N6TX and P.Wilson W4HHK,"Calibrating the Signal Generator in the Sky" QST 11/1992 , pag-42
W.Atchison, Calculating System Performance Using Solar Flux Data" Proceedings of Microwave Update 1991, pag 61
D.Shaffer W8MIF ,Microwave System Calibration Using the Sun and the Moon, ARRL UHF/Microwave Experimenter Manual, pag-60
J.Kraus W8JK, Radio Astronomy, Cap. VIII Mc Graw-Hill Book Company 1966
73" de
i8CVS Domenico
----- Original Message ----- From: "Greg D." ko6th_greg@hotmail.com To: amsat-bb@amsat.org Sent: Thursday, December 14, 2006 4:48 AM Subject: [amsat-bb] Re: Pre amp question
Is this test useful on a receiver where the AGC cannot be turned off? My FT-736 has only fast/medium/slow. No off. I would expect a reduction in accuracy, but is it even worth doing?
Greg KO6TH
----Original Message Follows---- From: "i8cvs" domenico.i8cvs@tin.it To: "Steve Raas" sraas@optonline.net, "AMSAT-BB" amsat-bb@amsat.org Subject: [amsat-bb] Re: Pre amp question Date: Wed, 13 Dec 2006 20:55:50 +0100
Hi Steve, N2JDQ
If your receiver has the capability to switch OFF the AGC than the
following
simple test will tell you if your preamplifier connected to the actual antenna is working or not for you:
{snip}
Does anyone have a link or other resource regarding the operational status of non amateur satellites in general? I'm interested in how the current solar activity is affecting the space environment. I've got links to the standard SOHO measurements, but I was wondering if there was information such as if satellites are being put into intentional spins because of charge buildup, or if any have had glitches, etc.
73,
Jason N1XBP
Somewhat related questions: Are magnetorquers useable when there is a geomagnetic storm? Do the changes in symmetry of the magnetic field during a storm disrupt the orbits of satellites containing ferrous or magnetic materials by exerting force on them?
Jason, Not sure how much it'll help, but try the following: www.nasa.gov/centers/goddard/missions/index.html
73 de Doug KA8QCU
----- Original Message ----- From: "Jason White" jason@jason.white.name To: "AMSAT-BB" amsat-bb@amsat.org Sent: Thursday, December 14, 2006 10:37 Subject: [amsat-bb] General satellite status?
Does anyone have a link or other resource regarding the operational status of non amateur satellites in general? I'm interested in how the current solar activity is affecting the space environment. I've got links to the standard SOHO measurements, but I was wondering if there was information such as if satellites are being put into intentional spins because of charge buildup, or if any have had glitches, etc.
73,
Jason N1XBP
Somewhat related questions: Are magnetorquers useable when there is a geomagnetic storm? Do the changes in symmetry of the magnetic field during a storm disrupt the orbits of satellites containing ferrous or magnetic materials by exerting force on them?
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
Jason, Not sure how much it'll help, but try the following: www.nasa.gov/centers/goddard/missions/index.html
73 de Doug KA8QCU
----- Original Message ----- From: "Jason White" jason@jason.white.name To: "AMSAT-BB" amsat-bb@amsat.org Sent: Thursday, December 14, 2006 10:37 Subject: [amsat-bb] General satellite status?
Does anyone have a link or other resource regarding the operational status of non amateur satellites in general? I'm interested in how the current solar activity is affecting the space environment. I've got links to the standard SOHO measurements, but I was wondering if there was information such as if satellites are being put into intentional spins because of charge buildup, or if any have had glitches, etc.
73,
Jason N1XBP
Somewhat related questions: Are magnetorquers useable when there is a geomagnetic storm? Do the changes in symmetry of the magnetic field during a storm disrupt the orbits of satellites containing ferrous or magnetic materials by exerting force on them?
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
participants (7)
-
Doug Kuitula -
Edward R. Cole -
Greg D. -
i8cvs -
Jason White -
kc6uqh -
Steve Raas