Why can I hear the local police on an FRS frequency?

[diveben]

I'm not really sure where I'm going with this, as I think there may be many possibilities going on here (imaging, bleed over, un-selectivity?). I'm not new to scanning but I am certainly not a veteran either, as you will probably realize by my post.

A little background information, I am an undergrad student located in Newark NJ and I am using a Radioshack Pro-97 and Uniden BC796D.

I was listening to the FRS/GMRS/MURS frequencies with my Pro-97 today when I overheard, quasi-clearly, what sounded like a police dispatcher talking on 462.5875 MHz (FRS Channel 02). I soon recognized the dispatcher's voice because I had been recently listening to the Newark North Precinct (KRI750 460.1250 MHz PL 123.0) The signal was weak, but I could clearly distinguish the transmission. I think I am located close to the KRI750 repeater (within a half-mile). I flipped on my BC796D and set it to the North Precinct. Sure enough the transmissions were in sync, but the one coming in on the FRS channel was certainly less powerful. The Pro-97 has no S-Meter but the Uniden does have a digital 6-bar S-Meter, and police frequency always comes in 100% so I must be very close to the repeater.

I fiddled around with the search range on my Pro-97 and found I can pick up the transmission on:

460.12500 MHz (the center frequency)

+/- 2.38750 MHz @ 462.51250 & 457.73750
+/- 2.45625 MHz @ 462.58125 & 457.66875
+/- 2.46250 MHz @ 462.58750 & 457.66250

Possibly others but I gave up at this point and decided it was time to ask the kind people on RR.com!

When I try 462.5875 MHz on my Uniden, I cannot hear them even with the squelch open...

I am really interested to figure out what is going on here because I have had some doubts about my Pro-97 in the past. I don't have enough experience with receivers to know if this is totally normal or possibly a defect with the Pro-97.

The FCC Database says the power output for the KRI750 repeater is 110.0 (Watts?) and maximum ERP is 220.0 (units?)

Here is some additional information I found in my Pro-97 manual that might assist anybody interested in explaining this to me! =D

Radioshack Pro-97

Sensitivity (20 dB S/N):
FM:
406–512 MHz ....................... 0.5 µV

Selectivity:
All frequencies at AM and FM mode except 25 – 27.995 MHz at AM
–6 dB ............................. ±10 kHz
–50 dB ............................ ±18 kHz

Intermediate Frequencies (IF):
1st ............................... 380.8 MHz
2nd ............................... 21.4 MHz
3rd ............................... 455 kHz

IF Rejection:
380.8 MHz at 154.1 MHz ............ 60 dB
21.4 MHz at 154.1 MHz ............. 100 dB

The Uniden BC796D does not give any of this information in the specifications, nor could I find any information on the web.

Antennas
BC796D - Stock collapsible whip antenna fully extended.
Radioshack Pro-97 - Radioshack Center Loaded Collapsible whip antenna tuned to about 1/4 wave of 450Mhz as recommended in the instructions.

On a side note, I've also had interference between public safety frequencies. For example, the police in Newark are separated on channels such as 460.1250 MHz, 460.1500 MHz and 460.1750 MHz (25kHz between channels). If I tune to ANY of the three, I can hear both the other two (at about half power according to the S-Meter and my ear) on both my Pro-97 and my Uniden BC796D. I can't believe that BOTH could be defective. Thank god for CTCSS tones!

According the my Pro-97 specifications, the receiver attenuates the nearby frequencies by -50dB @ +/- 18kHz. These frequencies are 25kHz and 50kHz away from the one I am tuned to, so why am I still hearing them? Do these police channels really use than much bandwidth?

Ok so I know all of that is a lot of information, and I probably should break it up into more than one post, but my main question here is:

Why can I hear Newark PD on FRS channel 02? And why only with the Pro-97, but not with the Uniden?

 

[landonjensen]

Welcome !

maybe because someone has a frs/gmrs radio on VOX, and whenever that pd they have it VOX'ED on talks they probaley have it transmitted to thier radio, thats what i think it would be, but you should ask a expierenced board member.
Also, Welcome To RR !

 

[16b]

You are pretty much right on in that the reason you are hearing it on other frequencies is that you are very close to the transmitter. 100ish watts is pretty common for repeaters and base stations on UHF. I am not a super electronics geek so I can not explain the gory details of what happens in your receiver; I am sure there are others who can explain that (and also why it doesn't happen on your '796).



Now, an FRS radio on vox? Um, no.

 

[landonjensen]

You are pretty much right on in that the reason you are hearing it on other frequencies is that you are very close to the transmitter. 100ish watts is pretty common for repeaters and base stations on UHF. I am not a super electronics geek so I can not explain the gory details of what happens in your receiver; I am sure there are others who can explain that (and also why it doesn't happen on your '796).

Now, an FRS radio on vox? Um, no.

____________________________________________________

Now, an FRS radio on vox? Um, no.

Actually it can happen...

Well, there was about 3 people that told me that when this was happening with my local fd that it might be that, so i thought it was the same reason.

 

[Joseph11]

The FCC Database says the power output for the KRI750 repeater is 110.0 (Watts?) and maximum ERP is 220.0 (units?)

ERP stands for Effective radiated power.

 

[Rayjk110]

When I first got into radio stuff I used to use FRS for repeating PD traffic around the block. But, going to a scanner (which are VERY intermod prone) and having a UHF radio TX next to it trying to RX a UHF frequency won't work to well if not at all. That's why I started utilizing 900MHz radios because I don't ever if rarely monitor 900 stuf.

 

[diveben]

Actually it can happen...

Well, there was about 3 people that told me that when this was happening with my local fd that it might be that, so i thought it was the same reason.

I actually read that post earlier today about your fd and someone possibly using vox & and FRS radio to listen from a remote (remote from the scanner) location. I considered that for a while today. After playing around with the frequencies a bit though I realized I could hear the police on multiple frequencies, not just the FRS frequency, so I'm pretty sure that isn't the case in this situation but thanks for your input =)

 

[landonjensen]

Allrite diveben

 

[jhooten]

900 mhz baby monitors are a dime a dozen at garage sales, hmmmmmmmmmmm........

 

[gatekeep]

Ok...listen if he hears it on the Pro97 and NOT on the Uniden, its probably the repeater overloading the Pro97's reciever. I only say this, because if the FRS VOX was the case, he should have heard it on both.

On a side note, I've also had interference between public safety frequencies. For example, the police in Newark are separated on channels such as 460.1250 MHz, 460.1500 MHz and 460.1750 MHz (25kHz between channels). If I tune to ANY of the three, I can hear both the other two (at about half power according to the S-Meter and my ear) on both my Pro-97 and my Uniden BC796D. I can't believe that BOTH could be defective. Thank god for CTCSS tones!

This is probably due to you being so close to the repeater location. It would appear that the repeater is just overloading the scanners.

I have had this happen when I was at the local fire dept., the signal was so strong it was splattering on adjacent frequencies. Mind you this is with a Pro96.

 

[Jay911]

I get overload all the time when near certain towers. Even my professional MT2000 portable overloads on my department frequency, 412.55, when I'm near the tower for the local trunk, on 424.5125. I find the interference worst, in my experiences, in the 400mhz band.

 

[gatekeep]

Jay: Ya I agree with the professional radio overload, my MTS2000 gets overloaded when I'm too close to the FD tower nearby.

 

[kb2vxa]

Hi Dive and all,

Since you're an undergrad in Newark may I assume you're attending NJIT? If so they have an excellent Amateur Radio club or they had last I heard. Your post surprises me since you're surrounded by EEs and more RF energy than a Tesla coil. That should tell you something and that something is that all but the best receivers are blown to hell in and around the city, I know. The worst area of all is the NE Turnpike extension, one of my 2M ham rigs simply died out there. Nothing about Newark surprises me and all is not radio, take care and I hope you survive.

 

[diveben]

how very true it is..

Hi Dive and all,

Since you're an undergrad in Newark may I assume you're attending NJIT? If so they have an excellent Amateur Radio club or they had last I heard. Your post surprises me since you're surrounded by EEs and more RF energy than a Tesla coil. That should tell you something and that something is that all but the best receivers are blown to hell in and around the city, I know. The worst area of all is the NE Turnpike extension, one of my 2M ham rigs simply died out there. Nothing about Newark surprises me and all is not radio, take care and I hope you survive.

haha you are absolutely right kb2vxa, I am attending NJIT. I hear a lot of good things about our amateur radio club here, but I have never had the chance to go and check out their operation. I'm not a licenced ham so I never gave it much of a thought, but I should go and meet them as I am sure they would be able to definitively answer my questions like the one previously in this post (plus they must have an amazing rig).

As for Newark, the hundreds and hundreds of colorful crack vials littering the sidewalk between my apartment and campus pretty much tell it all. But living here does make for some interesting PD/EMS radio!