Alright, I believe your numbers. A repeater receiver with a high-end
preamp vs. a ham grade mobile receiver averages just under 6 dB better.
I still contend that in a mobile environment, under motion, that the
user will not detect the 6 dB difference. It will be barely
distinguishable most of the time.
Is it enough to justify 140 watts (or more) transmit power? I'm not so sure.
I guess I'll modify the challenge. Drop the 200 watt transmitter by 6 dB
and see if it gets noticed. Bench tests are a good yardstick. Real world
applications can be different.
Chuck
WB2EDV
Jeff DePolo WN3A wrote:
>OK, here are the results of my quick bench measurements for whatever it's
>worth. All receivers were on the same frequency (448.800 MHz). Signal
>source was a Fluke/Philips 6060A sig gen locked to a rubidium reference
>oscillator, modulated by 1 kHz AF at +/- 3 kHz peak deviation. The output
>of the sig gen had a 10 dB pad on it for isolation -- all sensitivity values
>below have already been corrected for this 10 dB offset. SINAD measurement
>done on an HP 8920B using the speaker output of each radio (standard EIA
>test method), AF PA terminated with an 8 ohm speaker, with a transformer in
>parallel with the speaker to provide isolation to the test equipment. I
>used the averaging function on the 8920B to help smooth out the inherent
>variation in the SINAD measurement.
>
>The commercial radios (Micor, M2, Delta) were tuned using the factory tuneup
>procedure without a preamp in line, i.e. no optimization of the front end
>tune was done to improve sensitivity when the preamp was added. RF patch
>cables for all tests included a 4' piece of RG-400 with type N connectors on
>each end connected to the sig gen. A short secondary patch cable (less than
>2') with the appropriate connector on the other end (RCA for Micor and M2
>receivers, UHF for Delta-S and Kenwoods, N for Icom, etc.) was connected to
>the main 4' cable, either with the preamp serving as the connecting point of
>the two cables, or a type N "barrel" was used when the preamp was out of
>line. All commercial radios were the standard 450-470 MHz split models,
>with no mods done to them. All sensitivity measurements are for 12 dB
>SINAD. In other words, I did everything I could to make the test setup as
>close to identical for all of the scenarios.
>
>My goal here wasn't to determine which receiver/preamp was more sensitive
>than another - even though I've shown resolution down to a tenth of a dB,
>there is at least a few tenths of a dB of inherent uncertainty in the
>measurements. SINAD readings fluctuate on a signal that noisy, even with
>averaging, so you have to take the absolute values with a grain of salt
>(i.e. any of the readings that are within maybe 0.5 dB of each other should
>be considered "too close to call").
>
>REPEATER RECEIVERS
>------------------
>
>GE Mastr II Rx
>--------------
>-116.3 dBm (0.34 uV) without preamp
>-127.7 dBm (0.092 uV) with TE Systems model 4420N GaAsFET
>-127.5 dBm (0.094 uV) with Angle Linear 448GNT PHEMT
>
>Motorola Micor Rx
>-----------------
>-116.2 dBm (0.35 uV) without preamp
>-126.3 dBm (0.11 uV) with TE Systems preamp
>-126.6 dBm (0.10 uV)with Angle Linear preamp
>
>GE Delta-S
>----------
>-117.9 dBm (0.28 uV) without preamp
>-127.5 dBm (0.094 uV) with TE Systems preamp
>-127.6 dBm (0.093 uV) with Angle Linear preamp
>
>The simple average of the linear (microvolt) sensitivities WITHOUT a preamp
>for the repeater receivers is 0.32 uV (-116.6 dBm).
>
>The simple average of the sensitivies WITH a preamp is 0.097 uV (-127.3
>dBm).
>
>
>MOBILE RECEIVERS
>----------------
>
>Kenwood TM-732A (my bench radio): -121.1 dBm (0.20 uV)
>
>Kenwood TM-731A (retired, collecting dust for the last few years): -123.5
>dBm (0.15 uV)
>
>Icom IC-45A (really old, but a workhorse in its day): -115.6 dBm (0.37 uV)
>
>Kenwood TM-V7A (my most-hated radio): -125.0 dBm (0.13 uV)
>
>Syntor X9000 (with internal preamp): -122.3 (0.17 uV)
>
>As you can see, the repeater receivers with low-noise preamps out-performed
>all of the mobile radios. Even if you throw out the relatively-deaf Icom
>IC-45A, the repeater receivers beat the mobiles by a margin of anywhere from
>1.3 dB to 6.6 dB. Personally I'd argue that the TM-V7A should be
>disqualified too; it has to have the most intermod-prone receiver of any UHF
>radio I've ever used.
>
>Side note: for the heck of it, I also tried using the sig gen in the 8920B
>when I had the Delta-S on the bench, and I got SINAD measurements within a
>couple of tenths of a dB difference. I also wish I could have found an ARR
>preamp and thrown that into the mix, I know I have one around here
>somewhere.
>
>I don't claim any of these tests to be indisputible either in terms of
>absolute accuracy of the values, nor reproducibility by others. I'm just
>giving you what I came up with. Run your own tests and publish the results
>if you don't like my methods (or results :-).
>
> --- Jeff
>
>--------------------------------------------
>Jeff DePolo WN3A - [EMAIL PROTECTED]
>Broadcast and Communications Consultant
>
>
>
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