Input ?

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dave3825

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How does one determine an input freq? I ask because a fed freq I receive is listed in the database. I noticed in sdrsharp, that when that freq is active, there's another one not far away that's active at the exact same time with the same nac.

The one not listed is much stronger than the one listed in the db.

Thanks
 

ecps92

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There is no VHF standard even in Part 90, altho they are moving to low VHF as input and 166+ mhz output.
https://wiki.radioreference.com/index.php/Federal_VHF/UHF_Channel_Plans
Federal UHF can vary, there is a NEW standard for + 9 Mhz for an input, however
a. That can be waived due to interference and b. Legacy systems.

AS to hearing the same traffic on more than freq at a time, it could be a Vote-Scan or Networked Repeater system.

What Two Freq's and NAC's ??
How does one determine an input freq? I ask because a fed freq I receive is listed in the database. I noticed in sdrsharp, that when that freq is active, there's another one not far away that's active at the exact same time with the same nac.

The one not listed is much stronger than the one listed in the db.

Thanks
 

sflmonitor

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Also, are both frequencies always active at the same time? If you are hearing an actual input frequency, you will more than likely not always hear it, at least not with the same signal strength every time. But if what you're hearing are the output frequencies of a networked repeater system, the received signal strength should generally always stay the same.
 

ko6jw_2

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Federal systems can be different from anything else you might listen to. As an example, the Los Padres National Forest has multiple mountain top repeaters. There is only one input frequency and one output frequency. (Actually there are two separate nets) Which repeater you want use is determined by the input PL tone. All the outputs have the same PL. The repeaters are not linked. If you want to listen to the dispatcher and you cannot hear the repeater, you can listen on the input which comes from a mountain near the headquarters. The dispatchers have microwave links to all mountain tops so they can hear the traffic coming back to them. They also have a backup link system in the 406 Mhz area.

Thus, the system you are listening to may have a common input frequency and multiple non-linked sites. The thing to listen for is if you can hear the input, but not the repeater. If there are distant repeater sites that you cannot hear, you will still hear the input. Naturally, there will be no mobiles that you can hear on the input unless they are close.

The use of linking repeaters by federal systems in the 406-420Mhz band was more common 30-40 years ago. The USFS had four separate ones at one time in this area and the FBI had one that relayed radio traffic from Los Angeles into this area. No encryption then either.
 

Squad10

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Most Chicago area federal P25 VHF and UHF repeater output frequencies have about a three second long (single frequency sounding tone) right before every RF transmission ends. Input and simplex-only use frequencies have about a one second long (single frequency sounding tone) right before every RF transmission ends.

I use two analog-only receivers to attempt to pair input/output frequency transmissions and to hear the different solid sounding tone lengths.

Except for the different tone lengths described above, a repeater's beginning and end of output frequency transmission will coincide with the repeater's beginning and end of input frequency transmission. P25 input/output transmissions in the clear are easy to pair. You'll need to use the tone length difference for P25 to pair transmissions in the code.

Assuming federal repeater outputs in your listening area use a longer end-of-transmission solid sounding tone, identify a repeater input and output frequency pair in your area and you'll hear the difference.

There are a few federal repeater outputs in my area whose solid sounding output frequency tone is the same duration as the input frequency. In this case, I use Radio Signal Strength Indication and Direction Finding.
 

RFI-EMI-GUY

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A waterfall display might help ferreting out the input frequencies, over a long period of time.

Sent from my SM-T350 using Tapatalk
 
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