Simulcast P25 and multi-couplers and Preamps

[KI5IRE]

I am thinking about trying to run a preamp to a scanner to attempt to hear a P25 trunked simulcast system that I can hear just 5-10 miles up the road from my house. Do y’all think it’s worth trying? Does this typically work well with P25 systems? I am also planning to run a multi-coupler from my scanner, 1 scanner all of my close stations and the 2nd scanner for this system I am trying to hear but since I am planning to run a preamp, I was going to put the pre amp after the multi coupler so I don’t over power my other scanners current signals since I am fairly close to those stations and I have read a pre amp can cause overpowering when trying to hear nearby stations. Any advice or thoughts before I spend my money on a 2nd scanner, pre amp and multi-coupler. L


Sent from my iPhone using Tapatalk

 

[Thunderknight]

Pre-amp should ideally go before the multicoupler (assuming it's a passive multicoupler) - the multicoupler would have loss so the pre-amp helps by amplfying the signal going into the multicoupler. Putting it after will only amplify an even weaker signal + noise.
Of course it's a different story if you have an amplified(active) multicoupler.

You might be better off putting the "distant" scanner on a directional antenna pointed in the direction you want to hear, and if that isn't enough, adding the preamp as close to the antenna as possible (to overcome the loss of the cable feed).

 

[kayn1n32008]

Improving your antenna system may negate the need for a preamp. Higher gain antennas and Lower loss coax/hardline eliminate any adaptors you may be using. Replace F connectors with high quality N connectors. (NOT eBay specials, Amphenol or other quality brands only)

Absolutely No RG-6/58/59/213. Especially if you are going more than 50’ on VHF/UHF/700/800/900MHz. LMR-400 at a minimum, if going more than 50’ on 700/800/900MHz LMR600 or 1/2’ hardline.

A preamp should be a last resort device. Cheap ones can make a bad situation worse. If you need a preamp, it should go at the antenna.


Sent from my iPhone using Tapatalk

 

[scanmanmi]

Absolutely No RG-6/58/59/213.

What are you talking about? Everybody and their brother recommends RG-6. I would say absolutely no RG 58, 59 etc, but RG6? LMR400 is great but is a huge step up ($$).

 

[kayn1n32008]

What are you talking about? Everybody and their brother recommends RG-6. I would say absolutely no RG 58, 59 etc, but RG6? LMR400 is great but is a huge step up ($$).

At 870MHz RG-58 has 13.4dB loss at 100’

At 870MHz RG-6/U has over 6dB of loss at 100’

At 870MHz LMR-400 has 3.8dB of loss at 100’.

At 870MHz LMR-600 has 2.5dB of loss at 100’

At 870MHz LDF4-50A has under 2.1 of loss at 100’

Improving feed line and antenna will make better differences than throwing a preamp on crappy cable and a crappy antenna.

At 870MHz, a discone has zero gain. Feed it to a scanner with 100’ of RG-6/U and you have a net loss of over 6dB of signal strength, before you even get to the receiver.

Change that to a 9dB gain yagi, fed with 100’ LDF4-50A and you have a net gain of more than 6dB of signal strength at the radio.

Yea it cost a whole lot more, but the results will be better than a preamp at the radio after losing 6dB of signal strength.


Sent from my iPhone using Tapatalk

 

[kayn1n32008]

What are you talking about? Everybody and their brother recommends RG-6.

Yea because it is cheap. And for short(less than 25’) it does work ok.

It sucks for weak signal work, it sucks for long runs, and it sucks up at 7/800MHz on long runs.



Sent from my iPhone using Tapatalk

 

[bailly2]

are you getting simulcast distortion?

 

[R8000]

are you getting simulcast distortion?

That's my guess. He's making the situation worse. He needs less antenna. Hard to imagine, but to remove simulcast distortion, you need to remove the other transmitters from the equation.

There's no simulcast distortion if your receiver only hears one transmit site Why would you make your receiver hear more sites on purpose ? Makes no sense.

 

[jonwienke]

If you have a scanner that you were planning on dedicating to receiving the simulcast system, forget about amplifiers and multicouplers. Get a Yagi or LPDA antenna, and aim it for best reception of the simulcast system. That will isolate the signal from one simulcast transmitter, so the scanner can receive it clearly. Have a separate coax run from the Yagi/LPDA going directly to the dedicated scanner. Unless you are very careful about matching the phase from both antennas (impossible when the signals are on a variety of frequencies), combining the signal from the two antennas will basically give you simulcast distortion on all signals, because you have signals from 2 antennas that are interfering with each other. Keep the signals from the antennas separated.

If your existing antenna and coax works for receiving everything else, don't mess with it. Just add the directional antenna and separate coax run and scanner for the simulcast system.

 

[Ubbe]

What kind of coax are you using and how long is? I assume it's connected to your discone?
If you manually lock on to the control channel of that P25 system, do you see any signal strenght from it and can you program the frequency as analog and open squelch and hear the data signal?

I guess you have only a weak signal or none at all as you have to go 5-10 miles from your house to hear it. Is that a place that are higher above sea level than your discone?

Putting an amplifier to an antenna should always include a variable attenuator to dial in the correct level to the receiver. You then probably need to feed the P25 scanner without any attenuation and the output from the splitter to the other scanner with local systems need to be attenuated enough to not cause any overload problems.

But it could be that your discone is obstructed from the P25 signal and there's nothing to amplify.

/Ubbe

 

[jonwienke]

Not likely, if it's only 5-10 miles away.