Negating insertion loss of band pass filter with two pre-amps?

[Mikejo]

I have a question, I have built a band pass filter to allow only the 160 MHz band to pass. However, when I insert the band pass filter in line, it's insertion loss is enough to substantially lower the quality of signals of the frequencies I would like to pass through.

My idea would be to place before the band pass filter a pre-amplifier, then place the band pass filter, and then yet again he another pre-amplifier after the band pass filter.

My question is this, will this arrangemen in someway negate the insertion loss of the band pass filter and bring back the signal strength of the frequencies I would like to have passed through?

 

[TampaTyron]

It may be easier to have 2 coax switches where you bypass the filter. Alternatively, you may want to consider a Professionally built filter with less insertion loss. Have you swept the filter to determine its characteristics? Using 2 preamps has the increased likely of noise and covering up signals you actually want to hear. TT

 

[zz0468]

It would negate the loss of the filter, but it would also negate the effectiveness of the filter. Do you have the ability to measure the filter loss? Sweeping it would be better.

Receive front end performance is all about noise figure. The less the better. If you don't know the NF of the preamp, don't use it. The insertion loss of the filter in front of the preamp directly adds to the overall NF of the system, so if you had a 1dB NF amp, and 2 db loss of the filter, the combined NF would be 3 dB.

The NF of the filter/preamp MUST be better than the receivers NF, or it won't improve weak signal performance. Scanner NF is typically 6dB or so, so there's plenty of room for improvement.

Adding a second amp can cause problems because the first amp can overload the second, and the two can overload the receiver.

To summarize, the filter MUST be in front of the preamp. The combined NF MUST be better than the receiver's, and the total gain only needs to be enough to overcome the receiver NF. 10 dB is plenty. If not, something else is wrong.

 

[prcguy]

What is the insertion loss of your filter? A well designed band pass filter should not cause noticeable degradation of your desired receive signals. I use cavity type filters with less than .5dB loss for some things and I have other band pass filters with upwards of 2dB loss and its hard to notice that loss on anything but very weak signals. I suspect your filter has more than 3dB loss, which would be unacceptable.

What others have mentioned about adding preamps is true and I would revisit your filter design before considering a preamp. Even then you would only want to put a good high level preamp with moderate gain after the filter.

 

[Ubbe]

The insertion loss of the filter in front of the preamp directly adds to the overall NF of the system, so if you had a 1dB NF amp, and 2 db loss of the filter, the combined NF would be 3 dB.

You're probably mixing up NF with loss figure. A passive filter have no noise, well maybe some terminal noise at most 0.01dB NF.
Use a good amplifier that can handle any kind of strong signal without barfing and not adding too much NF noise.
A $40 amp with 0.5dB NF and +20dBm capability: Ultra Low Noise Amplifier with ESD+ Gain Stabilization USB cable PGA-103 NF .5dB | eBay

If you have made the bandpass filter extremly narrow, only a couple of MHz wide, it probably attenuate a lot if it also attenuats 20dB at 10MHz outside the bandpass range. Of course you should have the amplifier before the filter to overcome its losses. But do you really need to amplify the signal more than 20dB? A filter always have the same relationship between pass through loss and attenuation outside the passband. If you amplify the signal 20dB and the filter attenuates 20dB of the frequency you want to filter out, then it is useless. You should always use a variable attenuator when dealing with amplifiers. Add attenuation to the signal before the scanners input and dial in where you get the best reception. It will probably be at a total gain of 6dB-10dB in the whole chain, if the filter doesn't have abnormal high pass through loss.

You have used this calculator to see what specification you can expect? RF Tools | LC Filters Design Tool

/Ubbe

 

[kruser]

The first things you should look at are your antenna and coax.
If the coax run is a decent length, using a low loss quality coax can help. The second (and probably most important) item is the antenna. Is the antenna a single band VHF antenna tuned for I assume, the rail band? Both the coax and antenna are passive devices so using lower loss coax and an antenna made for the specific frequency range you want can greatly improve reception as neither will add any noise into the signal being passive devices.
If you are truly working with marginal signals, a band specific antenna with gain plus tuned to the range you want is the way to go.
A bad example would be Discone antennas. Discone's do have a nice and wide range but they offer no gain whatsoever. Some will claim to have gain but that is generally a false claim. Discone's have a purpose in life but if you are only wanting a small range, they are usually not the antenna you want.
If you are already using a quality band specific antenna with gain and quality low loss coax, you probably would not see any further improvement with even better coax or antennas unless you are looking at the signal with test gear.
Once you have the best coax and antenna you can afford, you are done there and the next options are filtering out unwanted frequency ranges and then a pre=amp with a very low noise figure.
The filter you built may also be suspect. Insertion loss of a decent filter should not really be more than 0.5 to 1.0 dB unless the filter is actually filtering part of the freq range you want. If so, a new filter design may be the solution. Or retuning of the current filter. Insertion loss of your filter should not really be noticeable.
If you are adding any splitters for say running more than one radio from a single antenna, each splitter port will add 3.5 dB of signal loss. Two splitters and you can double that reading to 7.0 dB loss. Splitters add up fast! The other things that cause loss are coax fittings or adapters. A good quality connector or adapter can add 0.5 dB of loss at each connection so try and keep adapters to a minimum.

As the others mentioned, adding preamps can cause more issues than you already have. Two amps is usually a no-no and you will get unpredictable results especially if one amp overdrives the next. Two preamps can create all kinds of harmonics and other nasty stuff as well as the high chance of overloading the signal at the radio.

My suggestion is take a very good look at your antenna and coax before anything. Once that part of the circuit is as good as can be, start looking at filtering and preamps.
If a decent antenna and coax fixes most of the signal issues, then adding a quality band pass filter may be all you need to get the signal where you want it. Try and do what you want without a preamp.

Some scanner type radios are also known for poor or possibly no filtering of the FM Broadcast band. Mainly the GRE designs can be suspect especially if you live in or close to a larger urban area with several broadcast stations.
An FM trap may be all you need to clean up your signals. I've seen FM stations pretty much wipe out the entire VHF band on some GRE's before. And in that case, the stations that caused the problem were over 10 miles away the way the crow flies.

There are also good and bad FM filters. PAR Electronics sells a good quality FM trap filter that works well. Be suspicious of the 10 dollar filters found on eBay or Amazon. I've seen some of these filters with their center range tuned to 150 MHz before!
The really cheap FM filters that RadioShack sold were famous for being tuned to ranges not even close to the FM band.
If you mess with filters, make sure and buy ones with the proper connectors on them. Many were made for TV problems and will almost always have F connectors on them. Stay away from those.

edit: all kinds of replies while I was typing this. It all still applies though but some of what I typed was already pointed out in the others posts so there is duplicate info here.

 

[zz0468]

You're probably mixing up NF with loss figure. A passive filter have no noise...

No. The noise figure of a passive device equals the passive insertion loss plus the black body noise temperature.

A loss in front of an active device directly adds to the device's noise figure. A preamp with a 1 db noise figure will measure at 1 db. Put a 3 dB pad on it's input, and it will then measure a 4 db noise figure. The loss of a passive filter will behave the same way.

 

[Mikejo]

Thanks Kruser, and everybody Else as well. I've pretty much built my antenna specifically for the 160 MHz range (vertical dipole) I'm using RG 58. Basically the problem is the receiving of distant train signals, but all of your replies pretty much answered my question. Maybe my solution will lie in a better indoor antenna ( outdoor is not an option) with higher gain than the one I am using now.

Does anybody have a suggestion for the highest gain indoor scanner antenna I could possibly either make or buy?

 

[prcguy]

And the first items in the RF chain will have the most contribution to the overall noise figure of the receiver. Change the front end preamp from 1dB NF to 3dB NF and the noise figure of the receiver goes up about 2dB. Add a 3dB attenuator at the receiver input and the noise figure of the receiver goes up by about 3dB. Change the IF amp further back in the receiver from 1dB NF to 6dB NF and the result might only be a fraction of a dB change in the overall noise figure of the receiver.

No. The noise figure of a passive device equals the passive insertion loss plus the black body noise temperature.

A loss in front of an active device directly adds to the device's noise figure. A preamp with a 1 db noise figure will measure at 1 db. Put a 3 dB pad on it's input, and it will then measure a 4 db noise figure. The loss of a passive filter will behave the same way.

 

[zz0468]

I'm using RG 58.

Use better feedline. Unless the filter and preamp are right at the antenna, the line loss contributes directly to the noise figure.

 

[prcguy]

It sounds like the OP is only having issues with the filter he built. Next time I'm in his area I could measure the insertion loss and band pass.

Use better feedline. Unless the filter and preamp are right at the antenna, the line loss contributes directly to the noise figure.

 

[Ubbe]

RG58 are no problem at 160Mhz and have neglectable loss. 0.5dB per 10 feet. A scanner are no high-end receiver and the most productive thing you can do are to add a low noise preamp. Even the $10 sat dish ones with 3dB NF makes a difference when I connect them at the scanner end with a variable attenuator to let the scanner have the exact amount of signal without de-sensening. As mentioned, scanners usually has a high internal noise figure and you easily double it's sensitivity by adding a pre-amp.

No connectors have 0.5dB loss except when they are faulty and have corroded. A good connector have a non measurable loss.

Any scanner user that suffer from weak reception shouldn't be without a good pre-amp for test and a variable attenuator.

PGA-103+ Low Noise Amplifier 2 GHz; ESD Protection Gain Stabilization;Gain 20 dB | eBay
AIS 162 MHz Band Pass Filter Bandpass 160 MHz 161 MHz 162 MHz | eBay
goobay Variable Attenuator Damping Variable 0-20 dB 2x F-plug 0,1 - 2400 MHz | eBay

/Ubbe

 

[Mikejo]

Thanks Ubbe

I'm just wondering, are they difficult to build? ( The adjustable attenuator) , If it's not too difficult, perhaps I could build one?

can anyone point me in the direction of a circuit diagram?

(If it's not too complicated)

 

[RFI-EMI-GUY]

Indoor antennas are always problematic. You are reducing the desired signal by the amount of shielding your house imparts and you are exposing the antenna directly to ALL the noise inside the house. I will bet you have computers, monitors, modems, routers within direct vicinity of the antenna. You can see why moving it outside will change things drastically. The antenna will get a better signal and receive less noise.

A filter wont reduce any of the computer noise. It is only effective at blocking high power paging or weather transmitters from overloading your RX. Since you like building filters, you can ponder this. Build a slightly simpler filter with low insertion loss and put that between the antenna and a low noise preamp, then follow it with your bigger, more selective filter. As long as you have enough gain in the preamp to overcome the loss of the bigger filter and the noise figure of your preamp and prefilter are reasonable you are in good shape.

Coming full circle on this, you need to get the antenna outside and also prevent common mode noise from your shack from traveling up the coax to the antenna. A center fed BALANCED fed antenna will reduce that noise, or a slew of chokes on the coax. Grounding is a must as well.

Did I mention the ARRL Handbook?

 

[RFI-EMI-GUY]

Thanks Kruser, and everybody Else as well. I've pretty much built my antenna specifically for the 160 MHz range (vertical dipole) I'm using RG 58. Basically the problem is the receiving of distant train signals, but all of your replies pretty much answered my question. Maybe my solution will lie in a better indoor antenna ( outdoor is not an option) with higher gain than the one I am using now.

Does anybody have a suggestion for the highest gain indoor scanner antenna I could possibly either make or buy?

An Outdoor antenna IS ALWAYS an option. That is what Hammer Drills are made for.

 

[Mikejo]

Yea...Did I mention that an (outside antenna is no option), I'm very very very much aware of everything you posted, I'm actually looking for solutions to the situation I have indoors.

 

[RFI-EMI-GUY]

Get one of those center fed 1/2 wave FM antennas they used to sell for stereos that is made from 300 ohm twinlead. Then find the calculations to make such an antenna on line (there are such sites, google is your friend) and input your 162 MHz frequencies to get the new dimensions. Cut the antenna to size, be sure to solder the ends. Get a 300 ohm to 75 ohm transformer and a bunch of RG6 coax. Take that antenna as far away from your computer equipment and close to an outside wall. Tack it up vertically with the 300 ohm twinlead feed going perpendicular to it. After you tack this monstrosity up, connect the coax to your scanner and report back. You might have to try another wall or two. If that does not work, you should find a way to get that antenna outside. It can be pushed under vertical corner of vinyl siding . Or just tack it up and play dumb.

 

[zz0468]

Yea...Did I mention that an (outside antenna is no option), I'm very very very much aware of everything you posted, I'm actually looking for solutions to the situation I have indoors.

Sometimes the indoor solution is acceptance of the fact that distant weak signals are unattainable. There's a good reason that people install antennas outdoors, and very high off the ground. Reset your expectations to match reality.

 

[Mikejo]

Yes again I'm very very very very very did I say very very well aware of all of the things that have been posted here.

Having said that, I've already built myself a J-pole, A 2 m vertical dipole, etc.

I'm just looking to do the best I can with the situation I have here in doors, and improve it in the best possible way; and yes then I will live with that. I just want to make sure I'm getting the most out of the situation.

If you cannot help out with solutions to the indoor problem please please please refrain from the constant preaching about the "OUTDOOR ANTENNA" crap adnausiam!!!!…

If that's all you have to offer, please go bloviate somewhere else!

 

[zz0468]

I offered you a wealth of information when it was about amps and filters. The fact is, an indoor antenna will never, ever, work very well. If it's the best you can do, it's the best you can do. But no one is going to have magic advice that will bring you a miracle.

At RF, everything is lossy. Stucco, wood, drywall, trees, rose bushes, clotheslines. It's all lossy. If your antenna must live inside with and behind all that stuff, you need to match your expectations with that reality. If your antenna is down low, it's not just your drywall and stucco and trees attenuating the signals, it's everyone else's too, for miles around.

I realize you don't want to hear this, but you NEED to hear this.