Nate Duehr wrote:
KC wrote:
BTW: I have one of the stations that Fred mentioned at one of my sites. It belongs to Jeff DePolo. It will run 200+ watts all day and night, and with a good MASTR II receiver (hand selected) with a Chip Angle preamp, it takes all of the 200 watts to keep up with it. It is matched pretty well with a modern 35 watt mobile.

Now THAT I'd like to see the math on. Just how high does receiver sensitivity have to get to "balance" with a typical say, oh... .15uV "modern" UHF mobile and it sending out 35W... what would the "perfect" numbers be.

Someone who enjoys the math more than I do maybe will feel bored enough to calculate it. I guess you'd need the antenna gain numbers also. What are you running on that system, Kevin?

Hi Nate, et al,

First, the antenna gain doesn't matter, since its "gain" affects BOTH the receiver and the transmitter; not withstanding some slight difference due to frequency or difference due to pattern because of change in frequency. The antenna is a DB-420, so pattern and gain differences between the TX and RX frequencies are minimal.

In a typical mobile installation, the USABLE sensitivity will rarely be the rated sensitivity due to man made noise, noise from the vehicles computer and other electronics operating in or around it. Even the fuel pump can cause serious receiver performance degradation. I use a Kenwood TM-742A Japanese mobile rig for 2M, 220, and 440. This radio is spec'd at .16 uV (-123 dBm) for 12 dB SINAD. I own 3 of these and none of them are any better than -120 dBm connected right to the service monitor. Now, connect it to the mobile antenna.... you will likely never realize the bench sensitivity in a mobile environment - I certainly don't. My 1988 Chevy truck installation allows me to realize about .35 uV (-117 dBm) on UHF.

The repeater site I mentioned above is in the middle of no-where, and there is nothing else around it for miles and miles. At this site it is easy to realize the full potential of receiver sensitivity. The receiver above has a usable sensitivity of -126 dBm (.112 uV) for 12 dB SINAD in full repeat, connected to the repeater antenna. Connected to the service monitor - I really don't know, it's less than any accurate measurement I can make.

The difference in sensitivity between the repeater receiver and my mobile radios advertised sensitivity is 3 dB The difference in sensitivity between the repeater receiver and my mobile radios actual bench sensitivity is 6 dB The difference in sensitivity between the repeater receiver and my mobile radios actual vehicular sensitivity is 9 dB

To be perfectly balanced using my mobile installation in comparison:
The repeater would need to run 70 watts if I was blessed with a mobile rig that really had its rated sensitivity and I could realize all of it operating in my mobile. The repeater would need to run 140 watts if I were to realize the actual measured sensitivity of my rig operated in my mobile - (which I cannot). The repeater would need to run 280 watts to be balanced since my actual vehicular sensitivity is -117 dBm.

From this, it's easy to see that running 200 watts is a necessity, when you have a /real/ good repeater receiver and a quiet site to realize its capabilities - even on UHF. Sometimes we forget that a change of 3 dB is double or half. A repeater that is perfectly balanced only needs a 3 dB betterment on its receive to require the transmitter power to double to remain balanced. There are plenty of repeaters out there that work perfectly well. Many have no preamp. Adding a good preamp can easily add 3, 6 dB, sometimes if the site is clean 9 or more dB can be realized with really good preamps. If you have a 50 watt repeater that is balanced, adding a preamp that gives 9 dB of usable sensitivity suddenly requires you to run 400 watts to operate the same as it did.

Some folks seem to think that running big power is evil, but it's not as long as you do _your homework_ and can benefit from some of these modern preamps that really work well. Your homework might involve running a MASTR II PLL exciter on VHF, a better duplexer, or a Tube Type PA so you don't need to change out your duplexer to isolate this additional receiver sensitivity and BIG power.

Kevin Custer





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