...and that's on UHF. Let's talk low-band!
Kevin, thanks for the thoughtful math.
73,
Paul, AE4KR
----- Original Message -----
From: Kevin Custer
To: Repeater-Builder@yahoogroups.com
Sent: Thursday, September 04, 2008 4:08 PM
Subject: Re: [Repeater-Builder] Re: GE uhf high power
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
