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
