Wideband preamp losing distant RX?


Paulie68
 

I bought an Uputronics Wideband preamp to use with my R2 and thought it would improve my reception but it seems to actually diminish it.

I'll describe my situation in detail and hope someone can offer some guidance.

My set-up is a balcony mounted Diamond D-190 discone feeding into 1m of LMR400 then into a Distill FM bandstop filter then the preamp continuing into another 5m of LMR400 and the R2 at the end.  The Antenna is mounted 27m above ground level giving about 30m ASL. Am in Central London so surrounded by a vast amount of commercial DMR TX/TETRA etc.

Listening to the 70cms band without the preamp and 1m coax (but with the bandstop filter) I can decode DMR/D-star from repeaters up to 60kms away.  With the preamp inline I can't.  It does improve the SNR on more local signals (<10km) but loses the distant ones completely.  I'm using SDR# v1807 and DSDplus fastlane  - have played with the software gain controls but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!

Any help would be appreciated.  I can't see any obvious way I'm going wrong but I clearly am!

Many thanks, Paul


Alan G4ZFQ
 

but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!
Paul,

From your description I'd guess the preamp is noisy, maybe overloaded by the strong signals.

73 Alan G4ZFQ


prog
 

On Sun, Feb 28, 2021 at 09:29 AM, Alan G4ZFQ wrote:
but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!
Paul,

From your description I'd guess the preamp is noisy, maybe overloaded by the strong signals.

73 Alan G4ZFQ
Try a good band pass filter with low loss around the signals of interest and place it before the preamp. 


Patrick
 

Hi there !

Speaking of preamp, what about the one that was to be proposed with the Youloop ? Will it soon be available for sale ?

Have a nice sunny Sunday :-)


Le dim. 28 févr. 2021 à 10:49, prog <info@...> a écrit :
On Sun, Feb 28, 2021 at 09:29 AM, Alan G4ZFQ wrote:
but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!
Paul,

From your description I'd guess the preamp is noisy, maybe overloaded by the strong signals.

73 Alan G4ZFQ
Try a good band pass filter with low loss around the signals of interest and place it before the preamp. 


Paulie68
 

Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul


Ken Sejkora
 

I’ve had good luck with the Nooelec Flamingo Plus FM-block filter.  Pretty deep block on 70 to 115 MHz, minimal loss above 120 MHz. 

 

https://www.nooelec.com/store/sdr/sdr-addons/rf-blocks/flamingo-plus-fm.html

 

Ken – WBØOCV

 

From: Paulie68
Sent: Sunday, February 28, 2021 06:11 AM
To: airspy@groups.io
Subject: Re: [airspy] Wideband preamp losing distant RX?

 

Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul

 


--

Ken, WBØOCV East Falmouth, MA USA
41.5997N, 70.5614W  FN41ro


Paulie68
 

On Sun, Feb 28, 2021 at 11:25 AM, Ken Sejkora wrote:

I’ve had good luck with the Nooelec Flamingo Plus FM-block filter.  

 

 
Thanks for the tip.  I already use the regular Nooelec barebones bandstop  but will get the 'plus' version - seem to have 50% more attentuation.

Paul


N8CVW
 

Greetings Paul

I’m supposing your RF environment is quite hot and alongside your signal of interest you’ve got some real bruisers coming at the sdr from the amp

this may require a low loss bandpass filter in front of the amp unless you find it’ll work after

the M/C 5043+ has great specs, low noise, high IP3 so shouldn’t be overloading

the model of your sdr would be helpful and also bias-T

if you can, also sweep the bias-T as some of them can be very poor performers

Paul
N8CVW

On Feb 28, 2021, at 3:29 AM, Alan G4ZFQ <alan4alan@gmail.com> wrote:



but cannot bring these distant signals back unless I detach the preamp. Yes the bias-T is on and working!
Paul,

From your description I'd guess the preamp is noisy, maybe overloaded by the strong signals.

73 Alan G4ZFQ






N8CVW
 

sorry, missed R2 at first

On Feb 28, 2021, at 7:13 AM, N8CVW via groups.io <n8cvw.gbmi=gmail.com@groups.io> wrote:

Greetings Paul

I’m supposing your RF environment is quite hot and alongside your signal of interest you’ve got some real bruisers coming at the sdr from the amp

this may require a low loss bandpass filter in front of the amp unless you find it’ll work after

the M/C 5043+ has great specs, low noise, high IP3 so shouldn’t be overloading

the model of your sdr would be helpful and also bias-T

if you can, also sweep the bias-T as some of them can be very poor performers

Paul
N8CVW

On Feb 28, 2021, at 3:29 AM, Alan G4ZFQ <alan4alan@gmail.com> wrote:



but cannot bring these distant signals back unless I detach the preamp. Yes the bias-T is on and working!
Paul,

From your description I'd guess the preamp is noisy, maybe overloaded by the strong signals.

73 Alan G4ZFQ









jdow
 

Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul


David Eckhardt
 

What frequency are you attempting to receive?  On HF, the AirSpy receivers do not need a preamp.

Dave - WØLEV


On Sun, Feb 28, 2021 at 9:42 PM jdow <jdow@...> wrote:
Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul



--
Dave - WØLEV
Just Let Darwin Work


Bob Dengler
 

At 2/28/2021 01:42 PM, you wrote:
Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.
I don't think it's fair to characterize 5V-supplied preamps as not having good dynamic range. The Mini-Circuits PGA-103 I've been using has an output IP3 of +39 dBm & P1dB of +21 dBm @ 400 MHz. Certainly a lot better than the "good" preamps most hams use on their repeater receivers.

Agreed there is no reason at all to use a 30 dB gain preamp. The old 1970's vintage G.E. UHF NBFM RXs I use are typically 0.5 uV for 12 dB SINAD; that's considered somewhat deaf. 16 dB of gain is enough to get them down to ~0.08 uV, & usually the on-channel antenna noise level you're dealing with is at least that high. Any SDR that needs more than that much gain would be worse than 0.5 uV by itself, and IMO is defective.

Bob NO6B


Eric Oyen
 

Well, actually, band pass filters will also tend to limit your range a bit if it’s skirt falloff is sharp. Better to get some band stop filters. Those will have less of an impact on what you want to receive without much insertion loss. Unfortunately, unless you are using a FET-based receive amplifier or something with a very low noise figure, you will run into noise issues and inter-mod products that will most assuredly get around those band stop filters. The best experience I have had with LNA type amps and band stop filters is to have the filters in a weatherproof container along with the pre-amp located on the radio side of the filters. This way, you have little loss due to the cable and the amp won’t be unduly overloaded.

Below are the pictures (front and back) of a 5v LNA with SMA connectors on it and adaptors to SO-239 female ends. Now, since I am totally blind, I have no idea if these pictures are sharp or not.

DE n7zzt 
Eric Oyen





On Feb 28, 2021, at 2:42 PM, jdow <jdow@...> wrote:

Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul



jdow
 

On 20210228 15:03:06, Bob Dengler wrote:
At 2/28/2021 01:42 PM, you wrote:
Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.
I don't think it's fair to characterize 5V-supplied preamps as not having good dynamic range.  The Mini-Circuits PGA-103 I've been using has an output IP3 of +39 dBm & P1dB of +21 dBm @ 400 MHz.  Certainly a lot better than the "good" preamps most hams use on their repeater receivers.

Agreed there is no reason at all to use a 30 dB gain preamp.  The old 1970's vintage G.E. UHF NBFM RXs I use are typically 0.5 uV for 12 dB SINAD; that's considered somewhat deaf.  16 dB of gain is enough to get them down to ~0.08 uV, & usually the on-channel antenna noise level you're dealing with is at least that high.  Any SDR that needs more than that much gain would be worse than 0.5 uV by itself, and IMO is defective.

Bob NO6B 
By the time the 5v USB is switched and fed through a long cable it may be as low as 3 volts Your available current is about 100 mA. Since it is typically a 60 mA device, yes, it should be fairly good with an IP3 somewhat below what you cite in actual use. Now, (rhetorically) WTF is SINAD? Review the answer to that and ask yourself if noise figure is a more appropriate measure. Then I don't have to figure extrapolate from thin data the bandwidth in correction and the theoretical  Eb/N0 to make it a more meaningful measure. Alas, all service monitors give you is SINAD to varying levels of accuracy. (That said I'll risk saying something negative about airspy.us. The preamps they sell have quite a bit too much gain. That said, I found a use for a couple of them. And, I'd be delighted to find an RF in line attenuator that passes DC through unattenuated with about 10-12 dB of RF loss above 50 MHz.)

{^_^}


jdow
 

Um, SO-239 connectors are bad++. BNC is better. N is even better. The adapter should be a piece of thin COAX about 6" long or so with the required SMA male on one end and an appropriate female connector on the other. That way you place little or no stress on the Discovery's internal SMA connector to PCB connection.

{o.o}

On 20210228 19:56:01, Eric Oyen wrote:
Well, actually, band pass filters will also tend to limit your range a bit if it’s skirt falloff is sharp. Better to get some band stop filters. Those will have less of an impact on what you want to receive without much insertion loss. Unfortunately, unless you are using a FET-based receive amplifier or something with a very low noise figure, you will run into noise issues and inter-mod products that will most assuredly get around those band stop filters. The best experience I have had with LNA type amps and band stop filters is to have the filters in a weatherproof container along with the pre-amp located on the radio side of the filters. This way, you have little loss due to the cable and the amp won’t be unduly overloaded.

Below are the pictures (front and back) of a 5v LNA with SMA connectors on it and adaptors to SO-239 female ends. Now, since I am totally blind, I have no idea if these pictures are sharp or not.

DE n7zzt 
Eric Oyen






On Feb 28, 2021, at 2:42 PM, jdow <jdow@...> wrote:

Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul




Eric Oyen
 

Ok, thanks for the tip. Those were what I had on hand. SO-239’s are good for HF and below, but above 30 Mhz, they do become an issue. I will have to acquire a couple of SMA male to BNC female pigtails

DE n7zzt Eric


On Feb 28, 2021, at 10:25 PM, jdow <jdow@...> wrote:

Um, SO-239 connectors are bad++. BNC is better. N is even better. The adapter should be a piece of thin COAX about 6" long or so with the required SMA male on one end and an appropriate female connector on the other. That way you place little or no stress on the Discovery's internal SMA connector to PCB connection.

{o.o}

On 20210228 19:56:01, Eric Oyen wrote:
Well, actually, band pass filters will also tend to limit your range a bit if it’s skirt falloff is sharp. Better to get some band stop filters. Those will have less of an impact on what you want to receive without much insertion loss. Unfortunately, unless you are using a FET-based receive amplifier or something with a very low noise figure, you will run into noise issues and inter-mod products that will most assuredly get around those band stop filters. The best experience I have had with LNA type amps and band stop filters is to have the filters in a weatherproof container along with the pre-amp located on the radio side of the filters. This way, you have little loss due to the cable and the amp won’t be unduly overloaded.

Below are the pictures (front and back) of a 5v LNA with SMA connectors on it and adaptors to SO-239 female ends. Now, since I am totally blind, I have no idea if these pictures are sharp or not.

DE n7zzt 
Eric Oyen






On Feb 28, 2021, at 2:42 PM, jdow <jdow@...> wrote:

Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul





Airspy US
 

Did you try powering via the USB port?

---------
Airspy.US
Your USA source for quality SDR products!
www.Airspy.US

NOTE: This email address is NOT MONITORED. If you want to email us use our normal email address. 

On Feb 27, 2021, at 10:07 PM, Paulie68 <mail@...> wrote:

I bought an Uputronics Wideband preamp to use with my R2 and thought it would improve my reception but it seems to actually diminish it.

I'll describe my situation in detail and hope someone can offer some guidance.

My set-up is a balcony mounted Diamond D-190 discone feeding into 1m of LMR400 then into a Distill FM bandstop filter then the preamp continuing into another 5m of LMR400 and the R2 at the end.  The Antenna is mounted 27m above ground level giving about 30m ASL. Am in Central London so surrounded by a vast amount of commercial DMR TX/TETRA etc.

Listening to the 70cms band without the preamp and 1m coax (but with the bandstop filter) I can decode DMR/D-star from repeaters up to 60kms away.  With the preamp inline I can't.  It does improve the SNR on more local signals (<10km) but loses the distant ones completely.  I'm using SDR# v1807 and DSDplus fastlane  - have played with the software gain controls but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!

Any help would be appreciated.  I can't see any obvious way I'm going wrong but I clearly am!

Many thanks, Paul


Airspy US
 

Also check to make sure the side with the USB port is facing the receiver. 

---------
Airspy.US
Your USA source for quality SDR products!
www.Airspy.US

NOTE: This email address is NOT MONITORED. If you want to email us use our normal email address. 

On Feb 27, 2021, at 10:07 PM, Paulie68 <mail@...> wrote:

I bought an Uputronics Wideband preamp to use with my R2 and thought it would improve my reception but it seems to actually diminish it.

I'll describe my situation in detail and hope someone can offer some guidance.

My set-up is a balcony mounted Diamond D-190 discone feeding into 1m of LMR400 then into a Distill FM bandstop filter then the preamp continuing into another 5m of LMR400 and the R2 at the end.  The Antenna is mounted 27m above ground level giving about 30m ASL. Am in Central London so surrounded by a vast amount of commercial DMR TX/TETRA etc.

Listening to the 70cms band without the preamp and 1m coax (but with the bandstop filter) I can decode DMR/D-star from repeaters up to 60kms away.  With the preamp inline I can't.  It does improve the SNR on more local signals (<10km) but loses the distant ones completely.  I'm using SDR# v1807 and DSDplus fastlane  - have played with the software gain controls but cannot bring these distant signals back unless I detach the preamp.  Yes the bias-T is on and working!

Any help would be appreciated.  I can't see any obvious way I'm going wrong but I clearly am!

Many thanks, Paul


jdow
 

For me at least the "UHF" connectors are all rather poor connectors for the shield. Getting them tight enough they do not become problems requires some lubricant and carefully applied vice-grips.  Otherwise a very little bit of wiggling demonstrates they are not really tight unless you are half large silver back gorilla.

{o.o}

On 20210228 23:07:09, Eric Oyen wrote:
Ok, thanks for the tip. Those were what I had on hand. SO-239’s are good for HF and below, but above 30 Mhz, they do become an issue. I will have to acquire a couple of SMA male to BNC female pigtails

DE n7zzt Eric


On Feb 28, 2021, at 10:25 PM, jdow <jdow@...> wrote:

Um, SO-239 connectors are bad++. BNC is better. N is even better. The adapter should be a piece of thin COAX about 6" long or so with the required SMA male on one end and an appropriate female connector on the other. That way you place little or no stress on the Discovery's internal SMA connector to PCB connection.

{o.o}

On 20210228 19:56:01, Eric Oyen wrote:
Well, actually, band pass filters will also tend to limit your range a bit if it’s skirt falloff is sharp. Better to get some band stop filters. Those will have less of an impact on what you want to receive without much insertion loss. Unfortunately, unless you are using a FET-based receive amplifier or something with a very low noise figure, you will run into noise issues and inter-mod products that will most assuredly get around those band stop filters. The best experience I have had with LNA type amps and band stop filters is to have the filters in a weatherproof container along with the pre-amp located on the radio side of the filters. This way, you have little loss due to the cable and the amp won’t be unduly overloaded.

Below are the pictures (front and back) of a 5v LNA with SMA connectors on it and adaptors to SO-239 female ends. Now, since I am totally blind, I have no idea if these pictures are sharp or not.

DE n7zzt 
Eric Oyen






On Feb 28, 2021, at 2:42 PM, jdow <jdow@...> wrote:

Not obviously wrong what?

What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

A band pass filter will increase noise figure if placed in front of the preamp. And if placed after the preamp it is not much help for overload.

Consider this as an incentive to learn about how noise figure and overload work in analog (sub)systems. For the most part simple algebra level math works unless you become motivated to learn "why" in greater detail.

{^_^}

On 20210228 03:11:14, Paulie68 wrote:
Thanks for the replies, glad there doesn't seem to be anything obviously wrong with my setup.  I'll investigate band pass filters though I also listen to Civil Airband and 2m so will need more than one?  Any recommendations are appreciated.

Also appreciated is prog's work with the SDR# improvements and the R2 hardware which has always impressed me.  Many thanks for your hard work.

(As a note that might help others - I 'upgraded' from RG213 to LMR400; with only a 6m cable length I didn't expect much improvement but was surprised how much of a difference it made.)

Thanks again,
Paul






Graham Dawson
 

As I have mentioned in other scanning groups, Pre amplifiers are a fallacy in many antenna systems. Firstly if the receive signal is below the noise floor you won’t get it back (unless CDMA), I assume we are talking conventional AM/FM modulation. You need a better antenna. If the signal is above the ambient noise floor, the signal to noise is what it is from the antenna, most LNAs add noise!. The feeder cable will have losses which will reduce the signal to noise and yes a pre amp as close to the antenna is a solution. However, it needs to be low noise and enough gain to overcome feeder losses (but not too much gain as it amplifies the noise floor too). It also needs to be high IP3 to prevent strong signal overload and IMD.

 

I believe that the issue being seen here is that the amplifier is wideband, seeing 390MHz Tetra and FM radio signals which will be amplified and potentially causing IMD in the external amplifier.

 

However, an even bigger issue is the Airspy which is based upon the wide receiver R820T below. As you can see the input has a variable gain amplifier and ALC which is wideband and frequency independent, the harder you hit the input the more the gain is reduced to keep in the linear range without overload. Effectively, the amplified external signal is probably overloading the airspy (especially if high gain). The only solution would be to filter the unwanted high level signals before hitting the Airspy input. This could be done after the pre amp without affecting sensitivity, a filter before the preamp will add to input losses and reduce the signal relative to the noise floor.

 

 

Sent from Mail for Windows 10

 

From: Bob Dengler
Sent: 28 February 2021 23:03
To: airspy@groups.io
Subject: Re: [airspy] Wideband preamp losing distant RX?

 

At 2/28/2021 01:42 PM, you wrote:

>Not obviously wrong what?

>What is the gain of the preamplifier? If it is more than 10 dB you are running excessive gain. That leads to overload. USB derived bias-T voltage is typically too low for a good dynamic range on a preamplifier, particularly one with 30 dB gain which seems to be too typical.

 

I don't think it's fair to characterize 5V-supplied preamps as not having good dynamic range.  The Mini-Circuits PGA-103 I've been using has an output IP3 of +39 dBm & P1dB of +21 dBm @ 400 MHz.  Certainly a lot better than the "good" preamps most hams use on their repeater receivers.

 

Agreed there is no reason at all to use a 30 dB gain preamp.  The old 1970's vintage G.E. UHF NBFM RXs I use are typically 0.5 uV for 12 dB SINAD; that's considered somewhat deaf.  16 dB of gain is enough to get them down to ~0.08 uV, & usually the on-channel antenna noise level you're dealing with is at least that high.  Any SDR that needs more than that much gain would be worse than 0.5 uV by itself, and IMO is defective.

 

Bob NO6B