the 45mhz channel is going to be a new blue-2 channel from the chppers chp enhanced radio system
in their infinite wisdom - the state pursued licenses for an extra blue channel on 45mhz instead of finding a channel on 42 mhz - so now the vehicles (when in place) are supposed to operate 3 mhz wide
a nice feat for a low band mobile antenna
Avtarsingh,
I still maintain that, at least from a theoretical and practical design perspective, a properly designed commercial public safety oriented quality radio should handle the SWR you will likely see from that spacing with little problems. Now, I cannot speak for the actual radios being used as i do not know what their internal design specifics are, however, I do know that a design for a final amp able to handle high or medium high reflected RF energy is quite possible and one need only specify the constraints and go from there. I simply cannot imagine that the radios used in the CHP vehicles are weak in this design area.
Also, from an earlier post, it was confirmed that the low band antennas used on the cruisers in at least one area are designed to yield a high usable bandwidth. I can't recall what the post said exactly and am currently too lazy to go looking for it but I think the band width was something like +/-5MHz. That's pretty good for a low band loaded whip and is certainly adequate for the spread of frequencies used in the current (new) CHP radio system assuming we are talking about an SWR of no worse than, say, 2:1 at the limits. I cannot imagine that the radios are so weakly designed so as not to be able to handle at least a 3:1 VSWR at their rated power outputs and rated duty cycle. Concerning the whip design, if you are working with a loaded whip shortened relative to the center design frequency wavelength, one thing you can do to improve the bandwidth is to use very high quality conductive material for the loading coil construction and make the coil as large as practically possible (in terms of the diameter of the coil wire). Doing so will increase the effective usable bandwidth of the antenna around the center frequency design point (Fc).
Again, if you know your system operational characteristics, nearly anything (within practical limits) can be designed to handle normal usage within those constraints and usually a good error margin is employed to allow for considerably harsher constraints that may be encountered. Know the normal power output of the transmitter, know the normal duty cycle, and know the expected worse case VSWR to be encountered and simply design accordingly. I am not saying it is done, in the CHP radio system sense, since I don't know but I am saying it certainly can be done and is quite practical!
As for the frequency spread of the new low band CHP system and the overall approach to using it, I rather think it is cleverly designed. Consider that the system as a whole, like nearly all LMR systems employing dual frequency base/mobile designs, is optimized for the mobile transmit-to-base frequencies' best efficiency. This is only logical as it gives best performance where needed - for the base to hear the mobiles. The new CHP system has at the center the 42MHz frequencies which, it looks to me like, are being made into the mobile-only portion of the spread. I say this because It looks to me like their ultimate goal is to move all base transmitters off of the 42MHz frequencies to reduce high power base transmitter interference to much lower power mobile sourced signals. Again, a very logical choice from an RF system design perspective! Now consider that the newly acquired base transmission frequencies are spread out at a nearly equal offset from the middle mobile-only (at least future mobile-only) band. The 39MHz channels are as far in frequency spread from the middle mobile band as are the 45MHz frequencies. Obviously the 44MHz channels are closer but not enough to cause difficult interference issues and are still more than enough in terms of spread from the center mobile band to ease duplexer and isolator design at transmission sites. Once again, very logical and well thought out. Next, consider the problem of acquiring frequencies from a limited pool and maximizing their effective usage. There are only so many available channels within a 1MHz spread given a standard 20KHz channel spacing (which it is in the US LMR low VHF range for non-federal part 90 operation). So, adding in more high power bases in amongst relatively much lower effective power mobiles would be very problematic. Now you could argue that they could have placed the mobiles at the ends and the bases in the middle but you then encounter two difficult issues. One, you end up having more mobile frequencies than base frequencies which means less base output choices; it's far harder to design for effective geographical frequency reuse when dealing with high power fixed stations (most especially considering the very long effective coverage when dealing with frequencies in the low VHF range) than it is to design for the same issue when dealing with relatively low (effective) power mobile transmitters! The second problem with this is that the VSWR issue for the mobiles becomes more difficult in that mobiles would be limited to the extreme end of the system band for the area they are in and would be forced to deal with a worst case situation in terms of reflected RF in potentially more numerous circumstances given that they may need to communicate on channels with mobile transmit frequencies in either the 39MHz band or the 45MHz and to do so for the most important design element of the system - the stations' need to hear the mobiles under the worst case conditions! Now you have high VSWR and lower effective transmit energy efficiency on the mobile transmit channels when a mobile is forced to transmit on a channel on the opposite side of the system band from the one they normally use while attempting to communicate with a base station - the most important element of the system design!
By devoting the middle 1MHz wide 42MHz to mobile only use and giving base transmitters three 1MHz wide bands of channels nearly equally spaced out from the mobile band you have made the best possible compromise in terms of giving the most channels to the high power base transmitters while allowing effective reuse of the more limited number of mobile channels. Also, you have done the best practically possible to limit the VSWR issue for the mobiles as much as possible given that they need maximum transmit efficiency when transmitting to the base station on only one contiguous 1MHz band as opposed to being forced to do so on two bands located 6MHz apart in the worst case! In addition, you have made life far more easy for the base transmission site designers and maintainers because they can place mobile receivers and base transmitters much more effectively (they can be closely spaced now, even co-located, if need be) given that they no longer have to deal with high power transmitter frequencies being within the same 1MHz segment as receivers needing to receive very low level and dynamically varying level transmissions from distant mobile units!
Personally, unless they were to leave everything as is or was in terms of simply leaving bases and mobiles all on one 1MHz wide swath of channels, I think they (the state radio system designers responsible for the new CHP system) did a very commendable job of making the right design choices and limiting the overall number of compromises! And if they were to keep everything on the 42MHz band then they would be right back to where they started in dealing with the limited number of expansion capacity in a geographical area that is growing rapidly in terms of the population! Even considering narrowbanding which is not required on the US VHF low band (as far as I know) yet, that would only give you so much in terms of more channels and would not solve at all the problem of geographical co-location interference issues between base transmitters and mobile receivers nor the problem of interference issues between bases in geographical frequency re-use terms.
Sorry avtarsingh, I simply do not agree with you on this. All manmade engineering designs contain compromises; effectively limiting the compromises to the lowest number and lowest impact is the best we can do. This is all the more true for a very large statewide radio system! Again, I personally think that the state radio system designers did a good job on designing a new system given the constraints they had (staying on VHF low band, getting three new 1MHz bands of channels (though down here in San Diego County we have had 39MHz full-on repeat for several years now) centered around the original 42MHz band, and dealing with a very large number of mobile units that may need to effectively operate well outside of their normal day-to-day operating area) and applaud them for it!
Now I am only speculating when I say that the 42MHz band will become mobile only but I see strong evidence that that is what the system designers are at least shooting for, at least eventually. It sounds to me like the most logical choice to make with the frequencies and band they have to work with.
From the content of your posts it seems to most of us that you are in some way connected this effort from a technical standpoint. I can understand why you may be reluctant to say if this is true one way or the other, of course, but if you are close to the technical elements of the system design (either in the maintaining of system elements or its design) you would, therefor, be likely to have access to design documents as well as access to the designers themselves and other related technicians and engineers. If true, I strongly suggest that you read those documents and speak to those other technical folks who are involved and ask them about your concerns and criticisms. See what they have to say for themselves! I rather expect that you may find answers which reflect much of my speculation and opinion as I so wrote above.
-Mike
They did not have 24 hour patrol and in fact only got that THIS last month 2009. Just 30 years later from when I monitored them.
