listenout
Member
Hi peeps, what’s the word on using a pre amp on a handheld scanner , can they damage the scanner ?
Pre amp
- Thread starter listenout
- Start date
- Status
GROL
Member
- Joined
- Aug 29, 2017
- Messages
- 573
Should not damage it and will not help. Modern scanner radios have very sensitive receivers. They do not need help from a pre-amp. The pre-amp will likely cause overload to the RF front end section of the scanner and make it less sensitive due to an issue called desensitization due to the overload. The RF overload also has a high potential to cause RF intermodulation problems affecting reception. Also unless the pre-amp has a lower noise figure than the front end amplifier of the scanner, it will lower the signal to noise ratio. The usual use of an RF amplifier that can be effectively used with modern scanners/receivers is one placed at the antenna end of a long run of coaxial cable to compensate for the feed line loss in the cable. Another application for an RF amp is one placed before a signal divider going to multiple receivers to compensate for the loss of signal in the divider.
Last edited:
Absolute rubbish. You contradicted yourself in this entire post.
GROL
Member
- Joined
- Aug 29, 2017
- Messages
- 573
Hmm. Where is the contradiction? The op has a handheld scanner. He was asking about the use of a pre-amp. Commonly "Pre-amps" for handheld scanners used to be sold that attached at the top connector of the scanner and the portable antenna connected to the pre-amp. Pre-amps are a type of RF amp placed directly at the input to the receiver. The scanner already has a pre-amp. It is the front end amplifier in the scanner. Placing another pre-amp at the input of the scanner is going to do exactly what I stated. Old receivers, especially tube type that have lower sensitivity may benefit from a pre-amp. But newer receivers of at least the last 30 years or more are sensitive enough that they cannot benefit from a pre-amp, especially some of the most recent scanners with 0.25uV or better sensitivity. An RF antenna amp placed at the antenna which is commonly called an antenna amplifier is used to compensate for feed line losses. An RF amplifier before a signal divider (splitter) which are commonly called drop amplifiers are used to compensate for the loss in the signal divider. Usually you need to limit the RF gain of the antenna or drop amplifier to be just enough to compensate for feed line or divider losses in order to prevent overload to the receiver. If he was asking about using an antenna amplifier or a drop amplifier, that was not clear in his question. The terminology does matter. Pre-amps connect to the input of the receiver, antenna amplifiers connect directly at the antenna. Drop amps connect before the signal divider or include the signal divider. Signal dividers always divide the power, so there will be losses you may need to compensate for unless the loss is intended for a particular application.
Last edited:
Antenna signal to noise ratio is determined at the antenna. Any loss after the antenna or increase in system noise figure reduces the signal to noise ratio and you cannot get it back. Every receiver has a particular noise figure from loss in filters and loss before an LNA which degrades the overall noise figure and degrades signal to noise ratio. Its possible to have a low noise preamp or LNA in front of a receiver that has a lower noise figure than the receiver, thus providing a lower overall noise figure and better signal to noise ratio than the receiver by itself.
But that comes with a bunch of caveats like too much preamp gain running the LNA and/or the first mixer in the receiver into generating IMD, etc. In my experience I seldom see any advantage to placing a preamp in line at the receiver unless the receiver is an old tube type job with a wooden front end and poor sensitivity.
But that comes with a bunch of caveats like too much preamp gain running the LNA and/or the first mixer in the receiver into generating IMD, etc. In my experience I seldom see any advantage to placing a preamp in line at the receiver unless the receiver is an old tube type job with a wooden front end and poor sensitivity.
GROL
Member
- Joined
- Aug 29, 2017
- Messages
- 573
Thanks for the expanded explanation about noise figure. I was attempting to help the OP avoid the disappointment of trying a pre-amp. Now, if he is trying to compensate for loss in coaxial cable from an external antenna, he could use an antenna amplifier, but that needs to be a quality amplifier with a low noise figure and just enough gain to compensate the feed line loss.
Last edited:
listenout
Member
G7RUX
Active Member
- Joined
- Jul 14, 2021
- Messages
- 606
You’re unlikely to damage the receiver with a normal preamp.
However, whether it would be of any use varies with the intended application.
A good, *low-noise* preamp can help (when installed at the antenna end of the feeder) with compensating for the loss encountered in a run of coax and also can compensate for the drop in level caused by a splitter to feed several receivers.
A good *low-noise* preamp might be useful with boosting a weak but clean signal that you want to receive but it will always make the signal-to-noise ratio worse because it adds noise of its own…a weak and noisy signal *might* get a little more readable but it’s really hit-and-miss.
A poor or noisy preamp will make things much worse than they were before and can often introduce issues of its own including intermodulation distortion which can be really problematic.
A good preamp will cost a decent chunk of money, maybe a couple of hundred for commercially made or perhaps a hundred or so for the parts to put together a decent one of your own.
A good, high-quality low-noise preamp combined with pre-filtering if you only listen to particular bands can be a wonderful thing…I have a system on a high building which is only for receiving VHF and UHF airband with good band filters and a 10 dB gain to compensate for the 30m of coax leading to the receiver.
I suppose my point is that you probably shouldn’t waste your money with a set-top preamp and a whip or other poor antenna on top of that.
Sorry to be a bearer of unwelcome news!
Piggly
Ubbe
Member
Scanners usually have an internal noise figure of 5dB or higher. If the sensitivity in the receiver are -120dBm then those 5dB of noise will be added and you can only hear signals that are -115dBm or stronger. If you had a preamplifier that had no noise and had a 5dB gain then that 120dBm signal would come in as -115dBm to the receiver and you can now hear it.
Low noise amplifiers could have a 0,5dB or 1dB noise figure so the lowest signal that could be monitored would be -119dBm. The mathematics isn't exactly like this but the principal how it works are very similar.
The $5 PGA103+ E-PHEMT amplifier has a noise figure of 0,6dB up to 1GHz. You can buy a complete amplifier with box and connectors for $20. The 1dB compression point are at +20dBm and IP3 are at +40dBm. What you need to add are a variable attenuator to set the signal level to where a scanner will have it's optimum signal/noise relation without overloading and will differ depending of the scanner; adjust to get the lowest noise while listening to the audio.
/Ubbe
Low noise amplifiers could have a 0,5dB or 1dB noise figure so the lowest signal that could be monitored would be -119dBm. The mathematics isn't exactly like this but the principal how it works are very similar.
The $5 PGA103+ E-PHEMT amplifier has a noise figure of 0,6dB up to 1GHz. You can buy a complete amplifier with box and connectors for $20. The 1dB compression point are at +20dBm and IP3 are at +40dBm. What you need to add are a variable attenuator to set the signal level to where a scanner will have it's optimum signal/noise relation without overloading and will differ depending of the scanner; adjust to get the lowest noise while listening to the audio.
/Ubbe
- Status
