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10 MHz Split vs. Filter Technology


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Poll: Which approach do you prefer? (3 member(s) have cast votes)

Finding more spectrum, or making cheaper, better solid-state filters?

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#1 Ian

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Posted 03 February 2019 - 10:00 PM

At some level, this is a "Why can't we have nice things?" question.  (What nice things, you may ask?  These nice things.)

 

But how hard would it be to find out who's using 452 and 472 MHz frequencies?  Assigning them to GMRS, even on a secondary basis, gives us access to all-eight-channel-at-a-time, all-solid-state, no-oscilloscope-required repeaters.

 

I believe the semiconductor filters used here could be improved to do 5 MHz splits, but that is clearly beyond the current state of the art (though military tech may be capable of it, they have priority access to whatever spectrum they require to do their jobs, so alternately it may simply have never been developed, though their budgets are quite up to the R&D task involved.

 

https://en.wikipedia...Elliptic_filter

 

I think that the Elliptic filter is the best approach to building solid-state GMRS repeaters.  Chebyshev and Butterworth filters have better ripple characteristics, but Elliptic filters offer the sharpest cutoff available, and if the location of the ripple can be controlled adequately, a circuit board with reliable, repeatable and temperature-insensitive performance could be a duplexer.  If Butterworth filters are capable of acceptable performance, however…  A circuit board with reliable, repeatable and temperature-insensitive performance could be a duplexer for every channel, simultaneously.

 

I've also looked into surface acoustic wave filters, which are doing the black magic in LTE base stations.  They're almost certainly capable of the 5 MHz splits we need, but they also cost like black magic ought.  A more compact repeater may be achievable, but a more affordable one?  Not so much.

 

So, I ask you, the brain trust -- what's easier, the bureaucratic burden of finding an underutilized slice of spectrum at least 10 MHz away from our little chunk of the UHF, or the engineering burden of figuring out if this XXXXXXXX is possible, and then making it a product?



#2 Jones

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Posted 03 February 2019 - 11:40 PM

Well, I can help answer some of your questions....

 

The 452 MHz band is fully used by broadcasting, public safety, and general business radio service. (as is the entire 450-470 MHz band, except for our very generous slice)

The 472 MHz band is right in the middle of over-the-air television channel 14, which uses up everything between 470-476 MHz. 476-482 is channel 15, and it goes up every 6 MHz from there until you hit cellular telephone, which is currently 614-890 MHz, and expanding.

 

There is no such thing as an "underutilized slice of spectrum" in the UHF band, or anywhere else for that matter.

 

There is no problem with our current 5 MHz split.  It has been working just fine for decades.  There is already a filtering solution for this.  It is known as cavity filtering.  A set of UHF cavities made to handle 50 watts is about the same size as that little toy shoe-box repeater you linked to in your post, and the tuning on most are loose enough to be used on any 2 adjacent channels.  You seem to want an 8 channel repeater - I still don't know why you need this.  This thing you linked to is only 10 Watts anyway.  Just use Simplex.

 

Interesting note: That Retivis repeater seems to be built into a re-purposed cable TV line amp box.

 

Lastly, please refrain from using non-appropriate language on this site.


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#3 Corey

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Posted 04 February 2019 - 11:08 AM

I dont understand why you are so dead set on solid state filters? If it was possible people would have been doing it long ago, that 10W toy repeater you linked to is no better then a couple HT's in a box. Yes it may use some type of solid state filter but i am guessing the loss and electrical efficiency are what limits it to 10W. I would bet it is just a notch filter, this would be like using a "mobile notch duplexer" vs "4 cavity pass reject duplexer" The mobile duplexer is smaller and cheaper but the performance loss of using such a duplexer reduces the repeaters overall effectiveness. The single largest factor with notch type duplexers is the inability to exceed 70dB isolation between the RX and TX.  BTW I license part 90 frequency in the 452 range, it took months to get a repeater pair because of the waiting list, no such thing as underused or open spectrum. You want a good preforming repeater, purchase a quality pass / reject duplexer.

 

Just my $.02


Just My $.02

 

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#4 berkinet

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Posted 04 February 2019 - 04:51 PM

... that 10W toy repeater you linked to is no better then a couple HT's in a box. Yes it may use some type of solid state filter ...

Or, it may not. I can't find any mention about what technology is used for the duplexer. I couldn't even find a picture or a video of the repeater with the case open.  Also, the specifications do not mention Rx/Tx isolation.  But, at a $400 price point, whatever duplexer they are using, it can't cost much -- though, it is probably the most expensive thing in the box.


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#5 Ian

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Posted 04 February 2019 - 10:36 PM

Well, I can help answer some of your questions....

Thank you!   :D

 

Lastly, please refrain from using non-appropriate language on this site.

Apologies.  I'll be operating under "Art's Grammaw" vocabulary from now on.



#6 Ian

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Posted 04 February 2019 - 10:58 PM

Corey, the reason I'd like to play with solid-state filters is to push the state of the art forward.  Cavities are perfectly serviceable, and I intend to use them in my first repeater.  However, at some point I'd like to build a truck-mounted transportable repeater, and it doesn't have room for a 19" equipment rack.  That means compromises in order to achieve acceptable performance and flexibility.  Complexity gets me great performance - imagine a hydrogen balloon carrying the antenna, tethered to the truck bed by some G-line - but drives up the cost something fierce.  It'd be a fun project, though, when I'm independently wealthy.  In the meanwhile, I shall content myself with only a modest improvement in antenna height.

 

Also, if I ever ended up with a lunchbox repeater like that, I'd be using an external linear amp to give it some respectable power output... ideally also a lunchbox form factor with an internal backup power supply.

 

Berkinet, I've looked at tiny fifty-watt duplexers on Amazon (about fifty bucks) that would easily fit in a single-DIN car mounting.  I suspect that's what the lunchbox repeater is using.  Alas, cars these days don't tend to have any DIN mounts, let alone extras.  I just can't afford the equipment to tune them myself, and I'm not sanguine about what I've heard about thermal drift on these things' calibration.  In a perfect world, there'd be community repeaters I could borrow everywhere I go, but most of the time I'm somewhere where I can't reach 'em, though at home I can sometimes barely hear the two in the region.

 

Jones, I do want an eight-channel repeater, and I want it cheap.  Not for me, but for the future of our hobby and the service as a whole.  If everybody could drop no more than $500 on a repeater and a cute little chimney-top tower, suburbia will be blanketed in community repeaters, and the utility and value of having a radio increases exponentially.  Cost, complexity, and colocation will kill budding hobbyists' ambitions, and in the same way you say "just use simplex" hams tell me "just use a cellphone".  I'd prefer not to be beholden to people whose business model includes AI-driven ad tracking and selling personal information; the competition will ultimately limit their options for screwing their customers over.  If people put wi-fi on those community repeaters' cute little towers, many people could get by without any cell plan at all.  I don't want to be the underutilized slice of UHF that gets sold to AT&T next... best way to avoid that, in my opinion, is to increase traffic and use until cell phone companies will look at the spectrum, sigh, and realize that even if they did buy it they'd never in their wildest dreams of enforcement success be able to stop all the people with walkie talkies from causing constant 5G blackouts, and won't be tempted to lobby for this.

 

That's why I want an eight-channel repeater.  Not for me, but for everybody.


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#7 WRAF213

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Posted 06 February 2019 - 06:20 AM

I've said it before and I'll say it again, you can't do a vehicle-installed repeater. Repeaters must not be capable of operating while moving, and cannot have a mobile operation mode. It's written in the Part 95 definition of a repeater station. They must operate at a fixed position. The specifics of what defines a fixed station is a question I'm trying to raise to the FCC's attention.

 

There's no spectrum entering or leaving GMRS. Consider frequency availability in the cities. Public safety is still building out on T-band here around Los Angeles because there is no spectrum available on 700 MHz, 800 MHz, or 450-470 MHz. Frequency reuse on those three segments is intense. Likewise, there's a lot of frequency reuse and congestion on GMRS, and coordination is necessary to protect against interference. In the suburbs out of the coverage areas of the cities, GMRS appears unused. Likewise, T-band and 800 MHz also appear unused. That doesn't mean there aren't any users. You just can't hear them, especially from ground-level. FRS users can exit receive range after a few thousand feet; low-level repeaters typical of private use can exit range in a few miles. Same goes for simplex or building-mounted repeaters in Part 90. The FCC won't deallocate 452.600 because they didn't hear anyone at their Denver field office in the last 25 minutes. For that same reason, they won't deallocate 462.600.

 

Since there's already a lot of Part 95 462/467 MHz equipment out there in a poorly regulated (relative to Part 90) fashion, getting FRS and GMRS users to stay off those bands would be impossible. Removing FRS or GMRS would also kill an industry of unlicensed two-way radio manufacturing (which is a bigger industry than you think), threaten emergency preparedness for what is easily hundreds of thousands of people, and provide very, very little benefit to the FCC. Trying to change the service to illegalize the operating modes of the existing radios is too monumental a task, and is one of the reasons why FRS gained the 8 repeater output channels. The pressures of the license-by-rule system on FRS and fixed channel set of GMRS would actually favor expansion of the band over contraction.

 

Setting up a bunch of repeaters blindly across all 8 channels just creates a bunch of interference for the other licensed users of the bands. Just because a repeater is open doesn't mean it isn't interference. This applies moreso during emergencies. Packing a bunch of users into GMRS in an attempt to lead the FCC to believe an already alive service is still alive doesn't make any sense. Setting up a bunch of repeaters in that close of proximity both in frequency and in physical space would also create intermodulation problems, further polluting spectrum.

 

If you want to use wide splits and/or tiny mobile repeaters with appreciable output power, use Amateur spectrum in either a wide split on 70cm or crossbanded to 2m/900. That spectrum has already been made available for hobbyist use. GMRS isn't supposed to be a tinkering band, type certification tries to ensure equipment already works when it reaches GMRS spectrum. Amateur is also free from frequency coordination concerns on temporary setups, restriction on mobile duplex operation, and linking concerns.

 

Cavity filtering only costs about $100, and considering there's a transmitter and receiver sharing an antenna, skimping on filtering isn't a good idea. Trying to implement a miniaturized filter would cause greater harm with receiver desense than benefit from running high transmit power, and with the filter order you need to get acceptable isolation even at 10 MHz, you're gonna spend more in component cost and tuning labor than you will buying a sixpack cavity filter. Single-channel repeaters don't cost much more than $400 to build ($100 cavity filtering, $75 transmit radio, $75 receive radio, $100 repeater controller and interface; power supply and antenna fill the balance). Performance isn't great, but it's certainly acceptable. Getting the cavities tuned is about knowing the right people or knowing enough theory to make cheap tools work (such as SDR + noise generator). Knowing a friend with the right tools is the right path to take, since they'll know more about the nuances of making a repeater work well. Building your first repeater alone isn't something I'd recommend, they aren't plug-and-play solutions and there's a reason this stuff costs money.


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#8 kb2ztx

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Posted 06 February 2019 - 07:49 AM

I've said it before and I'll say it again, you can't do a vehicle-installed repeater. Repeaters must not be capable of operating while moving, and cannot have a mobile operation mode. It's written in the Part 95 definition of a repeater station. They must operate at a fixed position. The specifics of what defines a fixed station is a question I'm trying to raise to the FCC's attention.

 

So I'll throw something out on this topic. I agree the rules say not moving, etc. I have a repeater in my motor home. Its not used while i driving down the road because there is no need, nor do i have an antenna system that can stay up. Once i get to the campground or race track I set up my fiberglass stick, fire up the generator and i am on the air. To me this is a fixed location. Back in my early SAR days we also did a similar setup in our incident command post. While that had a mobile antenna for ease it also was only used once we got to a mission. Both my applications allow a decent install and to date i have had no issues with radios, power supplies or duplex failing me. SAR has since moved our repeater to public safety channels but is still used. 

 

So as you said what is the definition of fixed ? A non moving repeater is fixed in my case. 


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#9 berkinet

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Posted 06 February 2019 - 07:55 AM

... the reason I'd like to play with solid-state filters is to push the state of the art forward.... ...Cost, complexity, and colocation will kill budding hobbyists' ambitions ...

95.1 The General Mobile Radio Serv- ice (GMRS).

(a) The GMRS is a land mobile radio

service available to persons for short- 

distance two-way communications to

facilitate the activities of licensees and

their immediate family members. Each

licensee manages a system consisting

of one or more stations.

 

GMRS is not a hobbyist service, it is not for expanding ambitions, it is not a place to push the state of the art forward. However, there is a place for that... Amateur Radio.  Per the FCC, Part of the Basis and Purpose of amateur radio is the: Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.  If those are your interests, get an amateur license.


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#10 Elkhunter521

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Posted 06 February 2019 - 09:35 AM

My hunting trailer started life as a 16 x 7 ft enclosed insulated cargo trailer with windows. Power is 4, 6 volt pallet jack batteries connected parallel over series. 4 100 watt solar cells on roof. 4K cummings generator with 40 gal tank. (Mounted on flatbed 1 ton truck ) Radios in trailer, CB and GMRS. GMRS is a $400 dollar repeater made from two Motorola mobiles in a suitcase ( ebay). Every one we hunt with uses Btech gmrs v1 radios or the Betech 82hp. Having a repeater parked on top of a ridge gives solid coverage to both valleys on either side. As this is a trailer it is not used while moving. I belive this makes it a FIXED location while in use. (Hunting elk western Wa. State.)
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#11 berkinet

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Posted 06 February 2019 - 09:46 AM

...  Every one we hunt with uses Btech gmrs v1 radios or the Betech 82hp. Having a repeater parked on top of a ridge gives solid coverage to both valleys on either side. As this is a trailer it is not used while moving. I believe this makes it a FIXED location while in use. (Hunting elk western Wa. State.)

To keep things fair, shouldn't you be giving guns and radios to the elk too?


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#12 Elkhunter521

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Posted 06 February 2019 - 10:08 AM

Im not sure if that would help the elk as they lack opposable thumbs.
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#13 berkinet

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Posted 06 February 2019 - 12:10 PM

Im not sure if that would help the elk as they lack opposable thumbs.


Now that is a place to push the state of the art forward!

Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth.

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#14 RCM

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Posted 06 February 2019 - 12:38 PM

To keep things fair, shouldn't you be giving guns and radios to the elk too?

I thought you weren't going to broach these subjects here. Are you sure you want to go there?


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#15 Elkhunter521

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Posted 06 February 2019 - 02:03 PM

I was describing a legitimate use of gmrs radio. Not necessarily to pursue elk but to coordinate the recovery of a downed animal (up to 750 lbs after field dress. But also to "find" hunting friends who have become CONFUSED about where they are.) ( lost).

I was inquiring if this use of a repeater was legitimate as it is only used at camp.

Is a fixed location temporary or permanent?
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#16 berkinet

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Posted 06 February 2019 - 02:14 PM

...Is a fixed location temporary or permanent?

Neither. the concept of fixed is really orthogonal to the temporary/permanent axis. Of course, philosophically speaking, everything is temporary. However, there is a clear difference between in motion and not-in-motion. Clearly a repeater cannot be used while in motion. In this case I'd take fixed to mean it will remain stable at its location while it is supporting whatever activity it is designed to support. When that ceases, it will move on.

Probably the edge case would be a mobile repeater that was only turned on when the vehicle in which it was mounted was parked. I.e. not in gear and the parking brake set. Such might be the case for some kind of shopping trip, treasure hunt, etc. I'd ague that was also fixed, since it did not move while operating.

In any case. I suspect on the FCC's enforcement priorites list, tracking down borderline "fixed" repeaters ranks slightly below use of Part-90 equipment on GMRS.
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#17 Elkhunter521

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Posted 06 February 2019 - 02:33 PM

Thank you for that. It is a realistic and practical description of the situation. Also, just for the record, I hunt with a recurve bow.
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#18 Ian

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Posted 06 February 2019 - 07:14 PM

Transportable != mobile.  Compliance could be achieved by, for example, powering it from an external 120v circuit via extension cord, or a double-pole double-throw switch that disconnects the battery from the vehicle and prevents vehicle operation while the repeater is in operation.  Tapping the signal line to the "BRAKE (!)" light on the dash to enable the repeater controller is yet a third option.  This is an engineering problem, and it is fairly tractable.  I may not have the patience to hack together the fanciest options, but running the gear off an extension cord should be quite simple, and would in fact allow me to locate the whole kit 'n caboodle in a truck tool box that can be moved, removed, or carried up a freight elevator if a rooftop is available and convenient.

 

Now that is a place to push the state of the art forward!

:lol:

 

Neither. the concept of fixed is really orthogonal to the temporary/permanent axis. Of course, philosophically speaking, everything is temporary. However, there is a clear difference between in motion and not-in-motion. Clearly a repeater cannot be used while in motion. In this case I'd take fixed to mean it will remain stable at its location while it is supporting whatever activity it is designed to support. When that ceases, it will move on.

Probably the edge case would be a mobile repeater that was only turned on when the vehicle in which it was mounted was parked. I.e. not in gear and the parking brake set. Such might be the case for some kind of shopping trip, treasure hunt, etc. I'd ague that was also fixed, since it did not move while operating.

In any case. I suspect on the FCC's enforcement priorites list, tracking down borderline "fixed" repeaters ranks slightly below use of Part-90 equipment on GMRS.

Exactly the edge-case I have in mind.  Besides that, the balloon-lofted antenna in my hobby project simply cannot be deployed in motion, or you won't have a balloon-lofted antenna - or any antenna - very long.  I was planning on something to support a local festival like the Highland games, and people enjoying themselves there.  (PS, really don't use that thing around power lines, and especially high-tension lines.   ;) )

 

 

95.1 The General Mobile Radio Serv- ice (GMRS).

(a) The GMRS is a land mobile radio

service available to persons for short- 

distance two-way communications to

facilitate the activities of licensees and

their immediate family members. Each

licensee manages a system consisting

of one or more stations.

 

GMRS is not a hobbyist service, it is not for expanding ambitions, it is not a place to push the state of the art forward. However, there is a place for that... Amateur Radio.  Per the FCC, Part of the Basis and Purpose of amateur radio is the: Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.  If those are your interests, get an amateur license.

 

I'm working on it.  But mostly this stuff is already available to amateurs, and does me absolutely zero good 'cause nobody else I know has, or is willing to, get an amateur license.  If they give me a justification, it's either due to not being arsed to study, too much hassle, or perceived toxicity in the culture surrounding amateur radio.  "Pushing forward the state of the art" as used here is really about improving the products available to customers via retail channels.  Finding new handies that are repeater-capable is an exercise in scouring eBay for new old stock.  I think the only products available are the TERA TR-505 and the BTECH GMRS-V1; the former requires PC programming and won't cover all the channels in the service, the latter requires … actually the BaoFeng meets spec IMHO.

 

I mean, if nobody makes it yet, there's always Kickstarter…

 

Not that I'll be ready for that until I have a few more years' experience under my belt, but I still really like the idea of a combination satnav and 50 watt GMRS radio.


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#19 WPXM352

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Posted 07 February 2019 - 08:45 PM

I've said it before and I'll say it again, you can't do a vehicle-installed repeater. Repeaters must not be capable of operating while moving, and cannot have a mobile operation mode. It's written in the Part 95 definition of a repeater station. They must operate at a fixed position. The specifics of what defines a fixed station is a question I'm trying to raise to the FCC's attention.

 

(snip)

 

If I read correctly what Ian is describing, it is a repeater to be used at a fixed location on a temporary basis. And using a balloon hoisted antenna. There is no prohibition in the rules for such a station.The fact that it is installed in a vehicle does not preclude lawful operation of the station when deployed at temporary locations. A fixed location is not a specific location. It is simply a stationary one. Further the FCC no longer licenses fixed GMRS stations by site so there is no expectation of a protected service area other than using care to operate without interference. 

 

§ 95.303 Definitions.

Repeater station.

 

A station in a fixed location
used to extend the communications range of
mobile stations, hand-held portable units and
control stations by receiving their signals on one
channel (the input channel) and simultaneously
retransmitting these signals on another channel (the
output channel), typically with higher transmitting
power from a favorable antenna location (typically
high above the surrounding terrain).


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#20 WPXM352

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Posted 07 February 2019 - 09:05 PM

 

95.1 The General Mobile Radio Serv- ice (GMRS).

(a) The GMRS is a land mobile radio

service available to persons for short- 

distance two-way communications to

facilitate the activities of licensees and

their immediate family members. Each

licensee manages a system consisting

of one or more stations.

 

GMRS is not a hobbyist service, it is not for expanding ambitions, it is not a place to push the state of the art forward. However, there is a place for that... Amateur Radio.  Per the FCC, Part of the Basis and Purpose of amateur radio is the: Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.  If those are your interests, get an amateur license.

 

 Oh god, I will have to put my flame suit on. Here it is. I am a licensed ham, have been for years and have pushed the state of the art in many venues. Built an amateur radio satellite station, worked the world, constructed a 98 foot tower etc. Spent a lot of money at AES.

 

However every so often I get the bug to build a repeater. I am building a repeater for GMRS and not for ham as my immediate family will not benefit. This is something my neighbors might benefit from as we are in a hurricane area and power and communications are out at least a few days every year.

 

So I am building a 50 watt (wide band ) GMRS repeater with quadruple receiver diversity. Yes folks I intend to push the state of the art forward. All parts will be Part 95 certified. The four receivers will each have a separate diversity antenna  mounted with separation to exploit the uncorrelated multipath signals. One of the four receive antennas will be a horizontally polarized loop to exploit angle diversity. Why am I doing this? To improve the reception form a 5 watt handheld so that talk back reliability approaches talk in. The heart of this is the repeater shelf I am assembling and the brains are a surplus JPS SNV-4 voter which has DSP S/N voting and DSP noise reduction.  Will it work well? I think so, that is part of the fun. Once the fun is over I will have a powerful GMRS repeater in my town.


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