Amplifiers mobile scanning

Status
Not open for further replies.

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Has anyone tried using a amplifier in an mobile scanner setup.
I'm in a fringe area and depending on cloud cover my reception varies quite a bit from OK to pretty bad.
I'm wondering if anyone has tried an amplifier and what were the results.
Thanks
Bob

Scanning VHF
 
Last edited:

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
23,618
Location
Hiding in a coffee shop.
Amplifiers can work, however if not done correctly, they will not only amplify the stuff you want to hear, but also the stuff you do not want to hear, like interference.

Often an approach that works well is to look at your overall antenna system.
Using the appropriate antenna for the job:
-You say "Scanning mostly VHF", are you using a high gain VHF antenna, or something like a discone, quarter wave, etc?
-If what you want to listen to is all in the same general direction (distant city, tower site, etc.) you can sometimes do better with a directional antenna.

Antenna height:
-Getting the antenna up high so it can physically "see" what you are trying to listen to. If there is a lot of terrain in the way, buildings, etc. that can impact performance.
-Moving the antenna to a better location. With multipath distortion, sometimes moving the antenna a few feet in either direction can help.

Coax:
-Are you using good coax? All coaxial cable will have some amount of losses associated with it. Those losses go up with length and frequency. Using shorter cables, or higher grade cables, can get more signal to your radio.
-Old, weathered, deteriorated coax might need to be replaced. Coaxial cable will not last forever, and especially true it the connections were not properly weatherproofed.

Amplifiers can increase the amount of signal that gets to your radio, but there's some work that needs to go into it. You need to start with a good signal, so the right antenna will help. Ideally, you want the amplifier to have some filtering so it's only amplifying what you want to listen to, not all the other garbage out there. You also want the amp near the antenna, so it's got a good strong signal to work with, and boost that before sending it down the long coax to your radio.
You also need to power the amplifier, which will require injecting power from the radio end of the cable.

If the coax between the antenna/amplifier and your radio is in poor condition, it's not necessarily going to fix the issues.
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Signal

My signal and reception are good when the weather Gods cooperate . When the clouds clear and the Sun comes out my reception drops due to the fringe area I'm in.
I'm just looking for a little " boost " to help me along and wondering if trying an amplifier would help.
I'm running a 5/8 wave roof mounted antenna cut down to the mid VHF band on my pickup truck.
Decent coax , not the skinny Chinese crap .
 
Last edited:

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
23,618
Location
Hiding in a coffee shop.
Ah, I missed the part about "mobile", tired eyes on my end….

I've never used a preamplifier in a mobile environment. Usually the coaxial cable runs are so short it's not an issue.

Using a gain antenna, like a 5/8 wave is a good idea, so not much you can really improve there. While you can get slightly higher gain co-linear type VHF mobile antennas, they are pretty long and may not be something you want on top of a truck.

You could try an amplifier, but you'd need to be careful not to overload the receiver, so have a way to bypass it.

Since most radio systems are licensed and designed to cover a specific area, you may have issues making this work if you are outside the coverage area. Weather can certainly impact things, but there's not always a lot you can do.
 

popnokick

Member
Premium Subscriber
Joined
Mar 21, 2004
Messages
2,837
Location
Northeast PA
I'm running a 5/8 wave roof mounted antenna cut down to the mid VHF band on my pickup truck. Decent coax , not the skinny Chinese crap .

How much did you cut from the 5/8 wave mobile antenna to get it to mid-VHF? And what was it originally designed to cover frequency-wise?
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
10" or so

5/8 wave 136-174 MHZ

As bought 53.125 " for 136 MHZ

Cut to around 43 " to the 152MHZ area

Cut info supplied by manufacture.

Bob
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
A mobile pre-Amp/LNA is feasible, but the problem is you are MOBILE, which means you move around relative to all other signal sources.

If I was going to try a mobile pre-Amp, this is what I would consider: Low Noise Amplifier 50 MHz - 6 GHz RF LNA; Ultra High Dynamic Range OIP3 +40dBm | eBay

Not too expensive, USB powered, high dynamic range and high OIP3

Depending on where you are drive, you might need a FM Trap on the input to the amplifier, something like this would be a wise idea to add on the input to the Amp:

FM Notch Filter 88-108MHz; 85dB Rejection; Bandstop; 9th order SMA-M + SMA-F | eBay

On many of these Amps, if you can rig up an adjustable 5 Volt power supply to reduce the Amplifier gain, not too tricky, but something you would need to come up with. Usually you can drop these type of Amps down to around 3 Volts, sometimes slightly lower and they will perform well with reduced gain.

You might need to add an input attenuator on the radio as well. I often install a variable attenuator and experiment, then decide if I need to add a fixed attenuator at the input of the radio. Often a 3 or 6 dB attenuator will help the radio from being overloaded. In a mobile application, as mentioned you have a wide varying degree of signals and this might be tricky to get right.
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Why not

That sound like a cheap enough experiment.
I could rig it to a coax switch for when I come out of my " valley " if need be.
Thanks for the idea.
Bob
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
The Amplifier I suggested is not as Low Noise as others offered, but it has better specs for high signal inputs which you may experience while being mobile.

Coax switch or variable 5 Volt supply and figure out where the minimum Voltage is that is no attenuating or compromising signals. Maybe just a 2 position switch with 5 Volts and the lower Voltage that the Amp will have very little gain.

Also of note, often as the Voltage goes down on some of these LNA's, the Noise Figure actually improves slightly.

A bit of experimenting and I think you will have positive results.

Post back the good or bad, if there is bad, we can give some suggestions for improvements. Do not throw up your hands if there are negative results on the first try, even with proper test equipment, sometimes there is a need to experiment with attenuators, filters and so forth. Compare performance with and without the engine running, you may have accessories that generate noise and the Amp wiring may need to be isolated, the antenna may need to be moved and so forth. Also pick a location where you can baseline the system, like at home or somewhere you know you have signal problems so you can use this location to verify changes as you make them.
 
Last edited:

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Ok

Yeah that sounds ever better, less coax fittings and cables just a variable 5v feed...
Good thinking.
Will have to start scrounging around on ebay

Thanks again
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
Not sure how handy you are, but once you figure out the lower Voltage requirement, you might be able to just use a resistor to drop the input Voltage to the Amp, then you pretty much have a switch you turn on that shorts or jumps the resistor and causes the full 5 Volts to power the Amp. I think you can envision this? A SPST switch with a resistor across the 2 leads on the switch. +5 Volts on one side of the switch, the Amplifier powered from the other side of the switch. One position the switch is "Open" and the resistor drops the Voltage to the Amplifier, the other position the switch is "Closed" and it feeds the Amplifier 5 Volts. The cigarette lighter 5 Volt adapter should be regulated so whether or not the truck is running the lower Voltage should be reasonably stable.

This eliminates all coax cable problems and the need for a more expensive coaxial antenna switch or switches.

Many of the cell phone cigarette lighter power adapters would be fine, but I have found some that would generate a lot of broadband lower frequency noise.
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Yeah I already though of one of those automobile USB cigarette adapters. Some are fairly long physically which would make room for a small switch and resistor. I'm sure the load from one of those amps is fairly low so a 1-2 watt resistor should handle it.
The wheels are turning !!! Now I have to find the time to fool around with it.
If and when it happens I'll let you know what happens.
Would it be possible to damage the receivers front end or would it just reject the signal.
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
The Amp I linked is not very high gain which is good. Less than 20 dB gain overall.

If you get the Amp, you could experiment with the scanners internal Attenuator which is usually 20 dB or consider a 3-6 dB fixed attenuator. Keep in mind the Noise Figure of the Amp is low so it will likely be lower than the scanner anyway, so an input attenuator may or may not impact the overall receive performance.

I might be worried if you did not put an FM filter on the input to the Amp and drove into an area where the FM transmitters are located.

Keep in mind while the Amp is wideband, the scanner has input filters so it will narrow what part of the spectrum will reach the first stage of the scanner.
 

lmrtek

Active Member
Joined
Feb 11, 2009
Messages
534
The fact you have to keep in mind about using a preamp is that they also amplify the interference and reduce the selectivity and intermod rejection of the receiver

Scanners by design are wide open receivers that allow everything to come in so preamps can easily make things worse

If you opt to try one you should use the lowest gain preamp you can find

A gain of 6 to 10db is all that is needed so any more and you are asking for interference and desense
 

zz0468

QRT
Banned
Joined
Feb 6, 2007
Messages
6,034
Also of note, often as the Voltage goes down on some of these LNA's, the Noise Figure actually improves slightly.

But the IP3 can get worse, too. Operating the preamp at other than rated voltage will yield unpredictable results. If the gain is too high, then attenuation on the output side is the way to handle that.
 

zz0468

QRT
Banned
Joined
Feb 6, 2007
Messages
6,034
Has anyone tried using a amplifier in an mobile scanner setup.

Not in a mobile scanner setup, I'd like to say I know better, but a certain someone will attempt to rain on my parade.

I do run preamps in mobile environments, but only behind bandpass filters appropriate to the task at hand.

I'm in a fringe area and depending on cloud cover my reception varies quite a bit from OK to pretty bad.

Take a close look at the frequencies you're interested and see if it makes sense to order a bandpass filter for the frequencies of interest. Another option, if you're strictly listening to VHF on a few specific systems, is get a commercial grade mobile radio, program for receive only, and THEN put the bandpass filter and preamp on it.

The results can be spectacular.
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
$$$$$

Not in a mobile scanner setup, I'd like to say I know better, but a certain someone will attempt to rain on my parade.

I do run preamps in mobile environments, but only behind bandpass filters appropriate to the task at hand.



Take a close look at the frequencies you're interested and see if it makes sense to order a bandpass filter for the frequencies of interest. Another option, if you're strictly listening to VHF on a few specific systems, is get a commercial grade mobile radio, program for receive only, and THEN put the bandpass filter and preamp on it.

The results can be spectacular.

I'm sure your right and the Bill would also be spectacular....$$$$$$$

I'm a poor retired Government employee just eating beans from a can...

Bob
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
The funny thing is people are quick to shoot all of this down without ever trying it.

Sure it may not work out well, sure it may have problems in some areas, but it has a good chance in solving the problem you are trying to address. This is not commercial or public safety radio, this is hobby radio. Give it a try, see what the results are, of there are problems, see of they can be cheaply and easily resolved.

I understand there are a lot of "Professionals" or "Retired Professionals" on this forum and some are spec crazy. So I have not "measured" specs for the Amplifier in question and while dropping the Voltage does typically reduce the overall gain and IP3 figure, keep in mind the IP3 valve may only degrade by a small amount as well, maybe by 3 dB, but if the Amplifier gain is dropped by 6-10 dB, the reduction in IP3 performance may be a moot issue anyway.

I have used plenty of consumer/hobby grade products in a professional systems over the years with very good success, But have run into the same problem of people within my organization claiming it would not work, would not work properly, was not accurate enough and so forth. I was involve in global satellite communications and set up a rather complex monitoring system at a large satellite earth station. I used consumer grade satellite receivers for the monitoring and measurements. I used TVRO hardware and amplifiers to feed these receivers. I build my own noise source out of a cheap consumer grade LNB and then used splitters and variable attenuators to create specific Carrier/Noise levels for each service I wanted to monitor.

I was told how this was all invalid, not "calibrated", would not work, we could not relay on this operationally and we could only use very expensive spectrum analyzers to do what I wanted to do. The problem is we only had 2 spectrum analyzers and they could not be dedicated and it was a PITA to attempt to remotely communicate with them, much less have them "push" out data.

Well to say the least, it became VERY apparent that my home brew/consumer grade monitoring system was a key operational monitoring tool. I was able to quickly identify problems with services and hardware within the earth station and even narrow down exactly where the problem occurred far faster than chasing it from end to end. \

Once interesting case was how I even found one morning that the earth station levels had drifted down by about 1 dB due to an overnight Air Conditioner failure for the 10,000 square foot facility where there was a 24 hour Network Operations Center, but nobody ever walked out into the comms area unless they were notified of a problem. Typical Operations Support staff!!! These clowns sat at terminals and if and only if there was an alarm or the phone would ring would they pay attention to anything.

I had not had time to set threshold alarms for my equipment as I had just implemented and installed the hardware, but a quick look at the trend graphs it was clear there was a problem. As I walked into the comms are, within 2 seconds I knew the facility was too hot and this was the cause of the drop in levels. Needless to say, my consumer grade gear even had on on board temperature sensor that could be queried and this was quickly added as a parameter to monitor and alarm on after this situation.

Until you actually try something, you never really know if it will work or not. As long as this is within your budget and financial means, buy all means why not try it. Play with it and see if you can get the end solution you are looking for. I find there are always far more cases when someone tells you why something will not work when in reality more often than not you can get the majority of what you need by just rolling up your sleeves and trying to solve the problem and forget about everyone telling you it will never work.
 

bobsav21

Member
Joined
Feb 22, 2009
Messages
78
Location
Maine / Florida
Well said but I do feel that the majority of posters were positive in their comments and others were simply trying to put out their concerns as to why there may be problems with my idea. I dont think anyone actually said it wont work just that there would be many variables.
As you said this is a hobby, and sometimes some people take everything to seriously .
I ordered an Amp from ebay and unfortunately I thought it was coming from Canada but it's coming from
China via Canadian post whatever that means. So I may be in for a long wait for delivery.
Meantime I'll try to put together the connecting cables and power supply so when it arrives I'll be ready to try it.
Will let you know what happens.
Bob
 

JamesO

Member
Joined
Jan 22, 2003
Messages
1,814
Location
McLean, VA
Did you order the Amplifier I linked above from the user iseeabluewhale??

I have bought from them before and I recall everything was shipped from Canada.

If might be possible they have an Asian fabrication house that will direct ship?

If you did not purchase the item I linked, please provide a link to the item you purchased as it may not have the performance and specs of the item I recommend. While many of these Amplifiers look alike, they do not all perform alike.

Depending on how the initial performance of the system is, you might need to add this on the front of the Amplifier because the amplifier is a rather wide band device. http://www.ebay.com/itm/1-GHz-Low-P...151414?hash=item41cc481976:g:yy8AAOSwqYdZhpfu

Again, without really understanding if you have much 1 GHz+ signals in your area, something like this may be something to add to the set up.

While the majority of the posters may have been positive in this thread, but I have had a bunch of push back on many other threads and I feel this push back tends to scare people of from even trying different things. There are some very good low cost Amplifiers available these days and in many cases they will work will if a small amount of time is spent configuring the system. Granted you cannot just drop an Amplifier in every system and every environment and have a successful outcome,

Sometimes fine tuning and filtering is required, but each case is different. I have had more success than failures overall with everything I have ever implemented. Maybe because I am lucky, maybe because I am smart about how I go about setting up my receive systems. Maybe both.
 
Last edited:
Status
Not open for further replies.
Top