Create a new Class E (CB/FRS) from part of the Ham 70cm band?

[prcguy]

Yes I only hear a small number of people on MURS but when 100% of them are using illegal radios and running more than 10X the legal power you can assume the majority of users are doing the same.

You want a 50w service that won't get bothered by skip from Texas? Lets see, GMRS is a 50w CB band and as I've pointed out its hardly used. Its requires a no test license that costs $85 for 5yrs, which is $17 a year. If that's the only thing keeping you from using the service then you have other issues and probably can't afford a radio anyway. I let my GMRS license expire years ago but today I'm going to apply for another one and see if I can make use of several GMRS repeaters otherwise rotting away in my garage.

In my opinion there is no need for another license free service until the existing ones are used to capacity and I suspect that will never happen.
prcguy

Overwhelming illegal operation, yet you have never heard anyone but a few locals running high power ham rigs.

That is the entire point of having a VHF/UHF CB band, with a moderate power limit, and enough channels in a single sentence. No mater how illegal they want to go, the affected area is limited. You won’t be squashed by a person in Texas running 10,000 watts. Even the stupidest person is usually able to see that there is little to no point in doing anything extremely illegal like running 10,000 watts. And even if he did try it, the fallout area will be geographically limited. The highest power you will see people running even with no restriction will be about 100 watts. Above 50 watt the point of diminishing returns is quickly reached, So given no limit, that is where people will generally fall. That is the simple fact that is dictated by the characteristics of the band.

That is why you want to set the legal limit to a point reflected by the characteristics of the band you are working with. If you set the legal limit to 100W from the start, then illegal operation (power wise) will not really be an issue because you have selected a reasonable power limit that reflects the inherent upper power usability of the band. People will generally adhere to it even if it’s not put down in law.

That will help prevent the use of poorly designed black market amplifiers because they will have no need to use them when they can buy a 100 watt radio off the shelf. And anyone that does try to use a black market 1000 watt amplifier will soon learn that there is little to nothing gained by the hassle, so they will quickly forgo it’s use, and tell all their friends that it’s not worth the trouble. The lack of aftermarket amplifier use will help prevent out of band interference.

That is where power limits below a specified point will invite more problems than it helps.

Handheld operators will generally stay below 5 watts by sheer technical limitations, unless some super high power battery technology, and ultrahigh efficiency amplifier technology is discovered.

The testament to that is the 2 meter ham band. 1,500 watt limit. How many 1.5KW mobiles, or even base units, and repeaters do you see running around? You don’t. No one wants to put the money, time, and effort into something that gains them nothing.

That is why FRS and MURS, in my opinion have too low of power limit, and too few channels. It temps people to use illegal power, sometimes with spectrally dirty radios, and when they do in densely populated areas, the limited channel set means that there is plenty of other people on the same channel in the affected range that they interfere with.

With enough channels the people wanting to run 100watts to cover a 50 mile range can do so without bugging people talking in smaller areas. And they will do so with spectrally clean radios that don’t bug the people close by using 1W handhelds two channels down.

If there isn’t enough bandwidth to allow all the users to run the maximum practical power that the band dictates is usable, then it’s best not to create an unlicensed band in that area in the first place. Create a 200 channel service with a 100W limit, or don’t create one at all. Creating a 2 watt 5 channel VHF unlicensed service when 50W radios are readily available for that band, is just asking for trouble..

 

[Project25_MASTR]

Only ones I know using murs are places like SAMs,Walmart, and cabelas.

I think another big problem with murs is the lack of mobile radios. Right now I only know of one company making anything resembling a mobile (Ritron) and a Chinese company (Friendcom) making a 95D approved data radio that could be setup as a mobile.

1.25 MHz is a lot to cut out…I have some friends with two VHF frequencies, one is local to the san Angelo area and the other is coordinated in the state. The two pairs are nearly 6 MHz apart …


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[Hatchett]

1.25 MHz is a lot to cut out…I have some friends with two VHF frequencies, one is local to the san Angelo area and the other is coordinated in the state. The two pairs are nearly 6 MHz apart …

The difficulty in getting the bandwidth will depend on how you want to separate it out from the rest of the spectrum on the band. Contiguously, or non contiguously

The most difficult is going for contiguous spectrum. To do that, you would have to migrate users to other frequencies. The best candidate for a frequency block is an area where a lot of users are already at that would possibly be happy using the new service if they could forgo the licensing. Like a chunk of spectrum where there are a lot of itinerant frequencies and users. Ones that are already used to using semi-shared frequencies. The ones that don’t want to operate on non controlled frequencies can be assigned to other frequencies.

The easiest is sniping at frequencies of opportunity until you have a basket of 100 frequencies to use. An unsold block of pager frequencies will be a place to throw 10 channels, A Public service frequency that no one across the country has licenses for would be another one for the basket. A frequency where there are 15 to 20 users licensed for it, and all of them that were contacted would be happy to have their frequency added to the service if it means they can ditch continued licensing for that frequency.

The two methods have operational benefits and detriments too.

The contiguous spectrum will mean less chance of interference to other services, from mistuned and malfunctioning radios. If the radio is off 10khz, then he will just be interfering with the adjacent channel of the same service. It will be easier to organize channels with less confusion because a different brand radio has the frequencies arranged in a different manner. A would be able to use a wideband radio and not worry about interfering with other services. The only one he would bother with would be a narrow band user one channel up, or down from him which will not be a problem in the rural environment.

The non contiguous layout will require stringent frequency and channel layout requirements because they will be interspaces with licensed business, police, ambulance, and other government services. If you are off 10khz, then you may be on a police frequency without even knowing it. If you try to use a wideband radio (which people will try to do) then you will interfere with adjacent services. People trying to program surplus radios to the frequencies may get the wrong frequencies because the article they read had a typo and end up interfering with other services.

If you had a guaranty that only purpose built radios would be put on the service, then the non contiguous option may be viable, but when you get old business band radios, Ham radios, and other radios on the frequencies, then it may get more interesting.

The contiguous option will just create a lot less headaches in the long run.

The contiguous option will be a lot easier in the 460Mhz range than it will be in the 160Mhz range.
And now that I think about it, there is a third option, a semi contiguous option. A 0.5 Meg block of frequencies, here, and a 0.7 Meg block there.

The prime vhf block that I would be looking at is 151 to 152mhz. It already is the heart of MURS, There is a lot of businesses on it though. You would have to send questionnaires out to the users of that block of frequencies and find out their opinion about the possibility of implementing such a service with the frequencies they are on, and if they would want to stay on their frequency if such a service was implemented there. That would give you hard metrics to start calculating how many people would have to be moved if you used that block of bandwidth.

Just because more people use a set of frequencies would not mean that more people will have to be migrated. If their use of the frequencies matches the layout of the new service, very few may want to move when given an option using the existing frequency in the new manner.

 

[Hatchett]

Or I could use the idea that other people have floated for spectrum for other services. That would be to take TV channel 2 for the spectrum. Or go after the 220Mhz band. That is currently unused. But the problem with those will mean that existing business band users would not be able to migrate over to them with their existing equipment. And the existing stock of surplus business band radios would not be usable on the frequencies. So that would limit uptake, and increase the cost of equipment for the service.

The other problem with using channel 2 spectrum is it’s down by 6meters which will mean there is band openings once in a while. Not often, but enough that it would concern me that it may draw the sky wave crowd from time to time. So, in that regard, 220Mhz would be better than channel 2 if you had to pick a total new frequency for the service.

If putting it in amongst the existing land mobile radio frequencies from 150 to 160Mhz is out of question then the other idea would be seeing if you could clear out a chunk in the 160 to 170Mhz range. It would be within the operating range of existing equipment in the field. The problem I have with contemplating a placement in that area, is the government doesn’t publish how much they use the frequencies in that range. So only the government would be able to answer the question of if they could open up a 1.25Mhz chuck for public use.

 

[Hatchett]

The synergies in such a band with the existing business band would be the simplification of itinerant frequency assignment. It would mean that the FCC would no longer have to deal with VHF itinerant frequencies. If you want a set of repeater frequencies for a fixed repeater, and 10 frequencies for itinerant use, then you just have to contact the FCC for the licenses for the repeater frequencies, while you can just select 10 frequencies at will from the unlicensed band to use while your people are traveling away from home base. The two licensed frequencies and the 10 unlicensed frequencies would be programmed on the same piece of radio equipment.

For public service, it would open up the possibility of programming police and fire and ambulance radios with unlicensed frequencies which they can use to communicate amongst each other and other services, or even the public without having to worry about breaking FCC regulations. Or needing another radio in the vehicle. Even a direct connection between community watch, skywarn, and volunteer people and the police on the ground.

If a cop worked the beat in the neighborhood and he knew they had a neighborhood watch that used 151.805. He could have the radio dude at the department program it into his radio and it would allow him to monitor it along with his other communications without needing to carry another radio. He could even talk back to them on his 50Watt cruiser radio. All without breaking any FCC law.

It would be a pool of frequencies that all users could use on demand when the need arises while using the existing equipment in the field.

 

[W2MR]

Share the fantasy.

 

[Hatchett]

Share the fantasy.

Never assume anything is outside the bounds of possibility.

About the time you think that something is totally impossible is exactly the time the world decides to F%^#$ with you and it becomes reality. And you are left dumbfounded wondering how the hell that just happened.

 

[DaveNF2G]

With enough money, anything is possible. Even buying public safety frequency blocks.

BTW, there are public safety users in 151 to 152, along with power utilities and others. The old "paging blocks" have already been reallocated.

 

[prcguy]

Another problem I see with a license free VHF service that would allow repeaters is the frequency separation between TX and RX frequencies. A VHF duplexer that will do a 1MHz split is a boat load of money, I would say $1000 average. Narrow that up to .5MHz and you can spend $2000 or more for an adequate duplexer.

If you go 5MHz or more split at VHF you can get away with mobile duplexers which are in the $150 on up range, so any proposal for this service should carve out frequencies at the low and high end of 150MHz. Just my opinion and I am in no way in favor of such a service.
prcguy

 

[Hatchett]

Another problem I see with a license free VHF service that would allow repeaters is the frequency separation between TX and RX frequencies. A VHF duplexer that will do a 1MHz split is a boat load of money, I would say $1000 average. Narrow that up to .5MHz and you can spend $2000 or more for an adequate duplexer.

If you go 5MHz or more split at VHF you can get away with mobile duplexers which are in the $150 on up range, so any proposal for this service should carve out frequencies at the low and high end of 150MHz. Just my opinion and I am in no way in favor of such a service.
prcguy

You are looking at it from the commercial view where antenna real-estate is expensive and sites are forces to use a single antenna for RX and TX.

Low end ad-hoc user installations, have a totally different set of constraining factors. You don’t have a monolithic repeater unit hooked to one antenna. You have two individual off the shelf radios hooked together with a varying length of cable, and a controller. You have a long enough cable to separate them enough so they don’t overload each other’s front end. You use two antennas separated by distance while relying on front end rejection of the receiver. If you can’t get that much distance between the units, then you can use a notch filter on the receiver to help cut down the transmitted energy in it’s front end.

For temporary, emergency repeater installations, that is the quick and dirty method. It works pretty good when you have a radio with a solid front end for the receive. And a clean radio for transmit. If you have 200 feet or so between the units, then 50khz split is workable in a pinch.

Reception range will be degraded but it is workable for the location and job at hand.

 

[Hatchett]

And if everyone has dual band radios, then you can really go simple. A uv8d with cross band repeat. A single frequency in VHF, and one in UHF. Just push a few buttons, and you have a hand held repeater no mater where you are at. All for a low low price of $135 on Amazon.com.

They even have mobile radios with that functionality, all be it at a little higher price.

That is why a dual band service would make in the field repeater setup and operation extremely simple for end users if most of the user base had dual band radios to operate on that service.

 

[quarterwave]

Good technical points. I admin a VHF system for my employer. We have a 6.35MHz split in VHF and I have it setup on our tower with RX at 270 feet, and the TX at 200 feet. We run a MTR2000 at 100 watts with no duplexer at all. We are running a RX filter and preamp setup, part of an older combiner system from when we had VHF paging and another VHF system all working off 2 antennae. Now it's just the VHF repeater. Probably don't need the filters, in fact we tested it with no desense not using the filters, but a local paging co still has a VHF freq near our RX freq, and they have moved it a few times, it's now on a hill about 1/2 mile away, so I configured the cans and preamp just in case. Our talk in and talk out range exceeds our 100 mira license with 50 watt mobiles, so it works to 100% potential no issues. We are licensed to run 300watts / 500Erp if we want...and have the amp...don't need it.

Again, good technical point with VHF and filtering. Local PD was on VHF years ago with a 8+ MHz split...it always worked well. Or course until somewhat recently, only Public Safety, Public Service and Utilities had access to repeater "pairs" in VHF...regular commercial did not.

 

[Hatchett]

The technicalities of two antennas separated by distance, and receiver desense is…

Most business band radios do NOT use an actively tuned front end. They use a band pass filter in the front end ahead of the RF preamp, and mixer stage. Your set receive frequency does not have an affect on the transmitted voltage present in the receive section through the RF preamp and mixer stage.

As long as there is a low enough RF voltage level present to not drive the RF preamp stage, or mixer stage into saturation, then you will not have desense. No mater how close to, or how far away the transmitted signal is from the frequency that is being received. IT can be 100Khz, or 5Mhz from the receive signal, and it won’t make a difference.

As long as it can pass through the RF and mixer stage without causing distortion, or saturation. Once the unwanted signal gets through that and hits the first brick wall filter after the mixer stage, then everything is golden. It no longer exists as far as the radio is concerned.

As long at the first IF brick wall filter has a good cutoff at +/-30Khz or so, then pushing the offset operation down to 100Khz, or even 70Khz should be perfectly workable without noticeable desense. Going down to 50Khz will be the point you may be pushing past the first brick wall and you will just start noticing desense.

It’s basically along the same lines of several people using radios in the same general area. If you are separated by 50 to 100 feet from the other person, you don’t expect his transmitting to interfere with your reception even if he is transmitting only 200Khz from your receive frequency.

So, the key to avoiding desense is positioning your antennas to keep the transmitted RF voltage at the receiver front end below a specified level. Find out how much adjacent injected signal it takes to cause desense to a receiver. Once you have that figure, then work with the antenna locations to keep the measured voltage at the receiver below that level, then your installation should be operating at acceptable levels.

Of course there is exceptions to every rule. If you have a transmitter that has a known spur at 200khz from transmitted signal, then using it for an installation that is going to have a 200Khz split, is going to be a bad idea. You will have to find a transmitter that has a clean area in the transmitted spectrum where the receiver is going to be listening.

 

[DaveNF2G]

How is it that an inexpensive amateur repeater on VHF functions quite well with a 0.6 MHz split if a commercial repeater that can manage "a 1MHz split is a boat load of money"?

 

[kayn1n32008]

How is it that an inexpensive amateur repeater on VHF functions quite well with a 0.6 MHz split if a commercial repeater that can manage "a 1MHz split is a boat load of money"?

Yea that does not make sense to me either. You need a decent duplexer, like a 6 cavity Resloc... Not hard to get to 600KHz or even tighter on VHF with the right equipment.


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[quarterwave]

You can make anything work with good filters. My shop did a backup repeater for an FD once on a 390khz split. Both their main and this warm standby had a huge set of cans.

The backup was a Desktrac with 100 watt amp...then the duplexer was as big as a refrigerator...funny. It worked...about 40 watts out.

For my work, we have an elaborate Phelps Dodge setup that takes up alot of room in the shack at the tower. At one time it ran a constant carrier IMTS (300 watts), a paging TX (300 watts) and repeater (100 Watts)...and it all worked well. I think they bought the PD setup in the early 80's.

I also saw a Low Band repeater once that was 39.18 in and 39.88 out. Talk about interesting.

 

[prcguy]

Duplexing using two antennas spaced far apart can work depending on the antenna isolation and how much signal gets back into the receiver to defense it. I do this all the time with low power handhelds and it works with varying success.

One important thing you did not mention is wide band noise coming from the transmitter and wiping out the receiver, this is one of the things a duplexer is addressing. Lets say for conversation a 50w VHF transmitter has some noise energy 100dB down at 600KHz away from the desired carrier, the common spacing for a 2m repeater frequency. 100dB down is one ten billionth of the main signal unless I calculated wrong.

With a 50w transmitter that 100dB down noise energy would be at a level of 501 microvolts, or 66dB higher than the average specified receiver sensitivity of .25 microvolts for a commercial repeater receive. Or 20dB higher than an S9 signal. Subtract maybe 30dB for some real good antenna separation and your left with interfering noise around 15 microvolts, which will cover up weaker but generally full quieting signals.

Without filters or lots and lots of vertical antenna separation and isolation, the 50w transmitter will certainly saturate the front end or first mixer of the companion receiver and make the transmitter noise problem look easy.

I had an entire carrier assembling, installing and maintaining commercial repeaters and have seen the effects of what I described first hand. My business usually dealt with UHF repeaters but I've built lots of 2m repeaters with 600KHz splits and have measured the improvement in performance and elimination of desense when going from a budget $1,000, 4 can pass/notch duplexer to a $2000+ big daddy Cellwave 6 can pass/notch duplexer.
prcguy

The technicalities of two antennas separated by distance, and receiver desense is…

Most business band radios do NOT use an actively tuned front end. They use a band pass filter in the front end ahead of the RF preamp, and mixer stage. Your set receive frequency does not have an affect on the transmitted voltage present in the receive section through the RF preamp and mixer stage.

As long as there is a low enough RF voltage level present to not drive the RF preamp stage, or mixer stage into saturation, then you will not have desense. No mater how close to, or how far away the transmitted signal is from the frequency that is being received. IT can be 100Khz, or 5Mhz from the receive signal, and it won’t make a difference.

As long as it can pass through the RF and mixer stage without causing distortion, or saturation. Once the unwanted signal gets through that and hits the first brick wall filter after the mixer stage, then everything is golden. It no longer exists as far as the radio is concerned.

As long at the first IF brick wall filter has a good cutoff at +/-30Khz or so, then pushing the offset operation down to 100Khz, or even 70Khz should be perfectly workable without noticeable desense. Going down to 50Khz will be the point you may be pushing past the first brick wall and you will just start noticing desense.

It’s basically along the same lines of several people using radios in the same general area. If you are separated by 50 to 100 feet from the other person, you don’t expect his transmitting to interfere with your reception even if he is transmitting only 200Khz from your receive frequency.

So, the key to avoiding desense is positioning your antennas to keep the transmitted RF voltage at the receiver front end below a specified level. Find out how much adjacent injected signal it takes to cause desense to a receiver. Once you have that figure, then work with the antenna locations to keep the measured voltage at the receiver below that level, then your installation should be operating at acceptable levels.

Of course there is exceptions to every rule. If you have a transmitter that has a known spur at 200khz from transmitted signal, then using it for an installation that is going to have a 200Khz split, is going to be a bad idea. You will have to find a transmitter that has a clean area in the transmitted spectrum where the receiver is going to be listening.

 

[Hatchett]

You are not taking into account antenna patterns and other stuff.

If you are using any vertical offset, the antennas will not be in each others gain lobes.

A 50W TX at 47dbm. With a -110dbm wideband noise floor.

A receive with .25uv sensitivity at -120dbm.

If you hooked the transmitter directly to the receiver you would have a -63dbm noise level at the receiver on the specified receive channel.

If you hook the RX/TX to two vertical dipole antennas spaced 100feet vertically apart. Receive at the top of the tower. Transmit at the base.

Free space path loss between two isotropic radiators at 100 feet will be around 45db. That will push it down to around -105 to -110dbm.

When you include antenna gain patterns, you will see that each antenna is setting in each other’s null at over 20db attenuation below isotropic. For over a 40db cut in signal level. That brings the noise level down to -150dbm or lower.

The transmit carrier will be about -40dbm at the receive. Which lands it at around 80db above the minimum usable desired signal which is right at most receivers 80dbm adjacent signal rejection level.

Now you will have conducted energy along the support structure. And I have found that coax leakage will show up big. (the receive coax going past the transmit antenna). But if you use proper isolation then signal level below lowest receiver sensitivity is obtainable.

The easiest way I have found to get around coax leakage is to separate the two antennas horizontally, and to use two horizontal dipoles with each one pointed so that the other antenna is in it’s null. With the transmitter running, you can rotate the antennas and hear in real time when you hit the null and the desired receive signal comes cranking in. Lock the antennas down, and the repeater is on the air. From my own experience 100 foot spacing at 600khz offset is usually enough to produce good results.

Granted, you want to pick the horizontal offset of the antennas so that it’s 90 degrees from the intended coverage area. So that the target area is in the main lobes of the receive and transmit.


If you push the RX/TX offset closer, then you start getting into the noise skirt of the TX a little more. That is why I stated 200 feet at 50Khz spacing will be usable with a bit of degradation.

 

[KF5YDR]

It doesn't look like anyone has actually presented a use case for these 160 extra channels.

When have you ever been in a place where all 14 FRS channels were so saturated that even with PL tones you couldn't communicate? The only situation I can think of is at a 24 Hours of LeMons race, and that's why my team now uses MURS, and a lot of teams use CB or ham frequencies.
Now when have you ever heard even four GMRS channels in use at the same time? Let alone all fifteen?
Why do we need more channels than the pile we already never use all of?

And yeah, just get a ham license. It costs $15, you'll get to meet other radio hobbyists, you get to use all kinds of different modes, can do whatever you want with your antennas, can run higher power, can talk to satellites, etc etc etc.