Repeater spacing, and using input and output tones to prevent repeater interference??

[kc2zpk]

Looking to put up a GMRS repeater in my area as there is nothing really in my area that handhelds can reach, and with a few hiking trails and wooded preserves, I thought it would be a good idea.

With only eight available pairs for repeaters, how far should 2 repeaters on the same pair be? I realize that more distance is better, and that there are other factors, but there are only so many pairs. Repeater input tones will keep unwanted transmissions from being repeated, but how effective are output tones? I know users without the receive tone set on their handhelds 'may' receive both repeater transmissions. How bad is it if the fringe areas of more than one repeater overlap?

[LeoG]

Output tones work well.  They will fully block any transmission that doesn't have the correct tone/code.  What they won't do is block those transmissions if someone else is on the frequency using the correct code.  If that happens then the strongest signal will come though as FM locks onto the stronger signal.  If they are close in strength you will get the double key warble.

Transmit tones are almost as good.  I have a repeater on the same frequency as mine that was one tone above what I chose.  And on occasion I would trigger that repeater.  Don't know if his tone rejections was bad or my tone frequency is off enough to trigger his.  Either way I changed my tone so that no longer happens.

The frequencies are far enough apart so they don't interfere with each other unless someone has a really crappy radio and causes spurious interference.  Also possible if you are sitting on top of each other which doesn't sound like your case.

 

 

[Radioguy7268]

You would need to read up & understand the Capture Effect on FM communications to truly answer the question of "how far apart is enough?"

The factors other than distance are usually much more important than the distance. It comes down to signal saturation and the signal level from a given repeater in a given area.  If my high power, high elevation mountain top repeater is 40 miles away, you may hear me all day long in your valley on your same frequency, but yet when your low power garage roof repeater cranks up, everyone in the valley within 4 or 5 miles of your system hears your low power repeater because their receiver will "capture" your stronger local signal level. Once they travel outside the core coverage of your local repeater, the stronger signal level of the big mountain top repeater re-gains the advantage.  

Generally, 3dB of signal strength is enough to create capture with a decent receiver design in a static setting, but 6dB difference is more than enough for the real world.

Is it bad that signals overlap? Not really.  It already happens every day in the real world, it's just that we consider certain signal levels to be so low as to become a part of the noise floor.  Then it starts to become a question of how high of a noise floor we are willing to tolerate, but that's a whole 'nother discussion.

[kc2zpk]

Great answers so far.

The other repeaters signal drops significantly after going over a hill, but is faintly there, My 'guess' at this point is my signal would be stronger and likely capture.

 

I have been looking for a decent "FM" book, any suggestions?  Was going to look for a used copy of the ARRL's "FM & Repeaters" book.

[SteveShannon]

34 minutes ago, kc2zpk said:

Great answers so far.

The other repeaters signal drops significantly after going over a hill, but is faintly there, My 'guess' at this point is my signal would be stronger and likely capture.

 

I have been looking for a decent "FM" book, any suggestions?  Was going to look for a used copy of the ARRL's "FM & Repeaters" book.

I would go to the local library and see if they have a copy of the ARRL Handbook and the ARRL Antenna Book.  
Pay particular attention to UHF propagation (for GMRS).

The book you mentioned might also be there.  I see Thriftbooks has a copy:

https://www.thriftbooks.com/w/fm-and-repeaters-for-the-radio-amateur/11974728/item/43401626/?utm_source=google&utm_medium=cpc&utm_campaign=us_shopping_zombies_lvbl_22798529178&utm_adgroup=&utm_term=&utm_content=764515355699&gad_source=1&gad_campaignid=22798529178&gbraid=0AAAAADwY45htYSrRgOpHt-GaFpO1fdeOd&gclid=Cj0KCQiAxonKBhC1ARIsAIHq_lsH5t_w3mFANgsM3rYYVPOfmAVgaitAW5QWtUkIH88KsiiDkespcgoaArLVEALw_wcB#idiq=43401626&edition=11614353

[nokones]

For one signal to capture the other signal, there needs to be 6 dB of signal strength over the other signal. That is how far you need to be.

[HHD1]

All 8 repeater pairs are being used in your area, yet no HT's can use them?  Then use whatever pair you want.  Just make sure your tones are unique. 

[AdmiralCochrane]

15 hours ago, HHD1 said:

All 8 repeater pairs are being used in your area, yet no HT's can use them?  Then use whatever pair you want.  Just make sure your tones are unique. 

I think he is saying there is little possibility of HT to HT simplex and that a repeater is the only HT to HT comm