Dumb question about amps for scanner antenna

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Hello everyone,

First of all I know this is probably a really stupid idea but I'm just trying to consider all my potential options. I have a very long cable run from my Scantenna ST-2 to my 3 scanners that I run, total run of about 240'. I'm using tri-shielded RG6 direct burial from the tower to the house. The tower is 60' and the run to the house is about 150' the rest is being ran in the crawl space under the house to my office. I have a preamp out on the mast about 1 foot below the antenna and the power supply is in the crawl space just under the house. Power is supplied via the coax to the preamp.

My question is... Since I've got such a long cable run I'm losing a good portion of my signal strength by the time it gets to the radios. Would it be doable to run double amps? Don't laugh too hard now. What I was thinking was if I could power the preamp on the tower and put the power supply at the bottom of the tower (60') to shove a amped up signal across the yard to the house. Then place a normal amp (no separate preamp box and smaller dB gain like 10dB) in the house to re-boost the weakened signal again? I know of some of the problems that I could run into is that I'm going to amp any interference and noise twice and that it'd be better to replace the cable with a better more suitable cable, but I'm on a tight budget and though if a $10 second amp would help out some of the down falls might be an acceptable trade off for the time being. I was just wondering about the two amps fighting one another and burning themselves out, or possible effecting my radios.

Equipment that I'm using is a Uniden BCT15X, Uniden BC60XT-1, and a Radio Shack Pro-82

Only serious replies please and thanks for your time and help.
 

gmclam

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... I have a very long cable run from my Scantenna ST-2 to my 3 scanners that I run, total run of about 240'. I'm using tri-shielded RG6 direct burial from the tower to the house. The tower is 60' and the run to the house is about 150' the rest is being ran in the crawl space under the house to my office.
One thing you do not mention is the highest frequency you are trying to receive. As you probably know, the higher the frequency, the greater the loss through coax. With that piece of information, we can calculate the loss you are getting through all that coax.

The next issue that POPPED out at me, is the type of coax you are using. I'd run LMR-400 or better for all that distance.

Would it be doable to run double amps? ... I could power the preamp on the tower and put the power supply at the bottom of the tower (60') to shove a amped up signal across the yard to the house. Then place a normal amp ... in the house to re-boost the weakened signal again?
It can be done but you certainly do not want to use low quality (high noise) amplifiers. I suspect these amplifiers you are using are also broad band amplifiers, such as from 25 MHz to 1GHz, and you do realize the lower frequencies are going to come out a LOT stronger on the radio end? With that kind of distance you might need to adjust for the 'tilt' caused by the long cable run. Additionally, if you've got ANY strong signals in the area (FM broadcast, TV, paging, NOAA, etc) you are going to want to filter them before amplifying.

.. and that it'd be better to replace the cable with a better more suitable cable, but I'm on a tight budget and though if a $10 second amp would help out some of the down falls might be an acceptable trade off for the time being.
Tell me about this $10 amp that is going to do you more good than harm? The best thing you can do is replace the coax and get the signal back you are losing.

Equipment that I'm using is a Uniden BCT15X, Uniden BC60XT-1, and a Radio Shack Pro-82.
Have you looked at the sensitivity specifications for these radios? And compared to newer radios? Unless your main interest is VHF band, you're probably better off with newer (more sensitive) radios as well.
 
N

N_Jay

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Unless your amp is extremely low noise, the overall receive sensitivity is being set by that amp, and unless the total loss after that amp exceeds the apms gain, adding a second amplifier will only decrease the system sensitivity.
 
K

kb0nly

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I would upgrade the coax and then just use the mast mount preamp to help overcome the loss.

Just a rough generalization since i don't know the brand of cable your using as they all vary a bit, but looking at the specs for some quad shield RG6 that i used recently..

Loss at 100ft at 450Mhz is 4.23dB.. You have almost 250ft, so 10.57dB of loss. No obviously at lower frequencies the loss is less. At 800Mhz its almost 6dB per 100ft, so you would be loosing close to 15dB over the length of the cable run.

How many dB of gain is the mast mounted amp your using now? I know a lot of weak signal VHF/UHF guys that run mast mount preamps to make up for the cable loss, i always hear them talking about using an amp that is equal to or slightly more than the calculated/measured loss to prevent other problems. So if your loosing 10dB you should only be adding 10dB. I know there is a bunch of guys on here that will argue this to no end, let them, i don't care because i know firsthand what works from experience.

All i can say is if your worried about it upgrade the coax! For example, LMR400 though not direct burial type has a loss of 2.7dB per 100ft at 450Mhz. That means 6.75dB of loss over 250ft compared to your RG6 loosing 10.57dB.

You might also want to look around for some surplus CATV hardline as a cheap low loss option. Or some 1/2" heliax, etc... But the underground run will need to be in conduit for most of your other options.
 
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One thing you do not mention is the highest frequency you are trying to receive. As you probably know, the higher the frequency, the greater the loss through coax. With that piece of information, we can calculate the loss you are getting through all that coax.

The next issue that POPPED out at me, is the type of coax you are using. I'd run LMR-400 or better for all that distance.

It can be done but you certainly do not want to use low quality (high noise) amplifiers. I suspect these amplifiers you are using are also broad band amplifiers, such as from 25 MHz to 1GHz, and you do realize the lower frequencies are going to come out a LOT stronger on the radio end? With that kind of distance you might need to adjust for the 'tilt' caused by the long cable run. Additionally, if you've got ANY strong signals in the area (FM broadcast, TV, paging, NOAA, etc) you are going to want to filter them before amplifying.

Tell me about this $10 amp that is going to do you more good than harm? The best thing you can do is replace the coax and get the signal back you are losing.

Have you looked at the sensitivity specifications for these radios? And compared to newer radios? Unless your main interest is VHF band, you're probably better off with newer (more sensitive) radios as well.
Hi gmclam,

The bulk of what I listen to would be 55% in the 140-160's, 35% in the 400's, and the remaining 10% is the rest. I don't just listen to one thing really. I listen to anything and everything I can hear for the most part. I'm a HAM operator so I monitor the local repeaters and the SkyWarn network, I also listen to PS, fire, EMS, Air traffic etc. I don't really spend too much time monitoring local business traffic.

The amps are broadband amps, I'm not sure that I'm following you a 100% on the lower freqs. coming in a lot stronger... Do you mean strictly signal strength or signal volume or both? Because I have noticed different town dispatch have a WIDE variance in volume. My main town is 7 miles away, they are always loud and clear. The secondary town that I like to listen to in my local area is about 15 miles away. The second towns dispatch is extremely quite and you about have to crank the radio to hear them even though they are all within the same freq band. 156.180 (close town) and 159.390 (far town) to be exact. Wasn't sure if this was something I'm doing wrong or if that's just the way they dispatch over there or what exactly. Cause I also listen to a 3rd town about 25 miles away and they are fine and for the most part match my closest town for volume. Is there something they can do to kind of block the volume or something? You pretty much have to decide to listen or not to them and them only. Otherwise you blow your ears out on everyone else or you can't hear them.

How do I adjust for the "tilt" as you mentioned? I don't have any strong signals, like FM, TV or anything in my area so that shouldn't be an issue. As for as the sensitivity on the radios, I don't know what or how to look for that I guess. The BCT15X is brand new only had it a few months and and it hasn't been available much longer than that.

Thanks for your input and reply, gives me some more to think about and work through.
 
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Unless your amp is extremely low noise, the overall receive sensitivity is being set by that amp, and unless the total loss after that amp exceeds the apms gain, adding a second amplifier will only decrease the system sensitivity.
I'm sure that the amp isn't the best that I could have. Would there be any suggestions on brand or type of amp that would be a good choice. As budget friendly as possible. I don't know what the price ranges are so I can't give a ballpark to shoot for. So whatever will be fine. I listen to a broad range of freqs. 140's clear up through some 800Mhz for the most part. Thanks for your reply
 

gmclam

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The bulk of what I listen to would be 55% in the 140-160's, 35% in the 400's, and the remaining 10% is the rest.
The issue is: what is the HIGHEST frequency you want to recieve. It does not matter if that is 1% of what you monitor, or 99%.

The amps are broadband amps, I'm not sure that I'm following you a 100% on the lower freqs. coming in a lot stronger... Do you mean strictly signal strength or signal volume or both?
We're not talking about "volume" (modulation level) here, this is strictly about (RF) signal strenth. If your receiver does not get enough signal, you won't hear much.

Let's look at some BASIC calculations here; you are running 240 feet of RG-6. The VHF low band stuff I monitor is at about 42 MHz, and the 800 MHz stuff I monitor is about 868 MHz, so I am doing some rounding here. But remember the loss is higher as the frequency goes up. That means your signal loss will be the following:
50 MHz = 4.5dB
500 MHz = 15.3dB
900 MHz = 21.2dB
Do you see that you are losing about 1/4 of the signal at 50 MHz that you are at 900 MHz?

Now let's use LMR-400 at 240 feet:
50 MHz = 2.1dB
500 MHz = 6.9dB
900 MHz = 9.4dB
Not only do you lose less than 1/2 of what you do with good RG-6, but the signals from VHF low to UHF are 'closer to the same amount'.

I think if this was my installation, I'd actually run LMR-600 for the lion's share of the distance, and then smaller coax cables nearer the radios. If the entire run is LMR-600, the losses would be:
50 MHz = 1.3dB
500 MHz = 4.4dB
900 MHz = 6.0dB

Look at this another way, by simply using a better coax, you get more than 10dB of "free amplification"; or more accurately, you don't lose so much of your signal at higher frequencies.

Because I have noticed different town dispatch have a WIDE variance in volume. My main town is 7 miles away, they are always loud and clear. The secondary town that I like to listen to in my local area is about 15 miles away. The second towns dispatch is extremely quite and you about have to crank the radio to hear them even though they are all within the same freq band.
I deal with this problem too. There is a local agency that literally whispers into their radios. My solution was to use a separate scanner just for that agency and crank the volume wide open on that scanner. There are some scanners out there with automatic volume adjustments, but they can only do so well.

How do I adjust for the "tilt" as you mentioned? I don't have any strong signals, like FM, TV or anything in my area so that shouldn't be an issue.
My first approach in dealing with tilt is to not create it in the first place. Running that length of RG-6 creates quite the mismatch of levels from the lowest to the highest frequencies (about 16dB in my calcs above). By changing to good coax, even with the LMR-400 the difference is only about 7dB, or less than half of what it was). There are devices used to alter the tilt, or amplifiers with separate settings per band, or equalizers built into some amplifiers. Not amplifiers you will buy for $10. As pointed out, you are better off spending the money on coax.

kb0nly said:
I know a lot of weak signal VHF/UHF guys that run mast mount preamps to make up for the cable loss, i always hear them talking about using an amp that is equal to or slightly more than the calculated/measured loss to prevent other problems. So if your loosing 10dB you should only be adding 10dB. I know there is a bunch of guys on here that will argue this to no end, let them, i don't care because i know firsthand what works from experience.
This does work for people who are in a rural area and/or totally away from high power signals. It can work if you use a good quality amplifier, not a cheap thing you get someplace like Radio Shack. While logic says you should amplify the signal 10dB before you lose that 10dB through some coax, you are better off to not lose it at all. Amplifiers are not magic and bring their own issues. Aside from noise, there is also the issue of any strong signals you don't want getting amplified with the stuff you do want. That's why I asked about local cell towers, FM broadcasters, paging, etc. You have to be in a very rural place anymore to avoid all of these things. People want their cell phones to work out in the middle of nowhere, and those are the type of signals that can screw up an otherwise good installation.

As for as the sensitivity on the radios, I don't know what or how to look for that I guess. The BCT15X is brand new only had it a few months and and it hasn't been available much longer than that.
It is a specification usually provided in the User's Manual for each band that the radio receives. A lower number means the radio is more sensitive. So 0.5uV would be more sensitive than a radio with 1.0uV of sensitivity. In my example, the better radio is TWICE as sensitive as the other radio. Some of those older radios have some high numbers by today's standards. Regardless, there still has to be a signal there for the radio to receive, and you don't want extra noise added by amplifiers either.
 
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Thanks for all the feed back everyone especially gmclam, that was a very detailed and in depth explanation, and it was very helpful! Probably the best break down of things I've ever gotten on here! Not to belittle anyone else answers cause they all have helped in one way or another. It just seems that my resounding answer is the expensive one. Replace the coax, replace the amps and the splitters too! I'm beginning to believe that I can't manage to pick a hobby that I can actually afford to have. I mean 200-300 dollars for a "good" splitter versus a $5 splitter is a HUGE jump! I know you get what you pay for, but holy cow! Where is the middle ground on this stuff? Oh well I guess maybe some day I'll be able to. Thanks again for everybody's help, it's greatly appreciated!
 
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