On Sun, 2008-01-20 at 14:11 -0500, W0WOI@aol.com wrote:
> In a message dated 1/20/2008 12:57:49 P.M. Central Standard Time,
> roncasa@verizon.net writes:
>
> Maybe I just stick with my Icom 746Pro.
>
>
>
>
> you mean you are going to base your decision on one reply?
>
> interesting.
>
>
>
>
>
> You bit, Ron .... the key word is Maybe with the intent to prompt replies
> which, by the way, worked.
>
> 73.
>
>
Without test data, I can't judge the radio. But I don't think Tentec
understands weak signal VHF receivers. Their 6N2 has a receiver
sensitivity MDS on the order of -132 dBm. More than 10 dB worse than the
state of the art. Not an unusual value for rice boxes either. 2m rice
boxes have had poor sensitivity for decades. I blame that on lots of
Japanese 2m activity and that backing off the RF gain is the easiest way
to get away from intermod.
I have a Tentec 6m transverter (2m IF) that uses a nice double gate
MOSFET that ought to do better than 1 dB NF, but with their almost PIN
diode (1N4007) switching that loads the incoming signal with the PA all
the time, achieves about 7 dB NF. Yah, in the cities that's good enough
but out here that's not all that good. Tolerable for 10 watts but not
for higher power.
Recently someone on a VHF list commented they had measured a modern 2m
rice box NF at 10 or 12 dB and so needed 20 or 30 dB preamp gain to make
the best use of the fractional dB NF preamp in an EME station. That sure
makes intermod a sure thing. I'm beginning to think its time to toss the
rice box VHF radio and go back to transverters. I had a 3 dB SYSTEM NF
using a 6CW4 2m converter and my 75S3B back in 1965. I'm sure one of the
converter's I've inherited from estate sales using a 417A will do a
little better for NF, though it may not handle the local FM BC station
(at a mile) or the inband APRS stuff at about that distance.
But so far for VHF I'm satisfied that I hear as well as I can on 6 and 2
and maybe 432 with my FT-857D. The DSP isn't all that much help in weak
signals except it rolls off high frequency audio noise on SSB. The
Collins 500 Hz mechanical filter does improve CW weak signal to noise.
And unlike the FT-726 and FT-736 that shifted the receiver frequency
when changing from SSB to CW and back causing loss of contacts from
going off frequency, it doesn't make that dastardly shift. It even works
CW while in SSB with one menu setting so that mixed mode contacts are
trivial. Want CW, start sending with the key. Its that simple. An having
worked WV and Florida last year on 2m, I think the sensitivity is
decent. I copy stations that can't hear me with it's 50 watts on 2m, 100
watts on 6m. I like the first 857 so well, I bought a second one at a
ham fest. I've not used the second one much, the previous owner did a
super master reset that upset the computer settings for alignment and
alarm thresholds. But it does work. I think SSB would be improved with
the optional mechanical filter, might acquire one someday. I've also
used it recently for RTTY PSK-31 with the simplest of interface, a mini
din cable in the radio jack split to stereo plugs to fit the sound card
interface. Having line level ins and outs made the integration super
simple.
A few years back I studied how I was going to be on VHF and up and one
option was a HF rig with transverters or a 736 with 220 and 1296
modules. Turns out the 736 receiver on 2m is junk and has so little
front end gain that there's 30 dB between signal detection and AGC
threshold. That's hard on the ears with the audio gain cranked up to
hear weak signals and a local (within 100 miles) comes on. In the 220
module I put in a MAV11 mixer buffer to give it more front end gain and
to give the mixer a proper termination. The added gain makes it sound a
lot better, but I can't prove with signal or noise generator that I
changed its sensitivity and I still have to use a double 222 cavity to
hear anything but channel 13 hash on the band. Now that module's control
cable has broken again and I've not been successful at fixing the
connector. Same thing has happened to the 1296 module but I got
snookered on it from an epay purchase. The seller claimed it was
perfect. It seems to have a bad output module and when I opened it up,
ALL the RF cables were unplugged. It now receives but I broke its
control cable. Yaesu can't identify the cable assemblies by description
in the technical manual nor in the parts department. It may never work
for me.
Last year I picked up an IC-R7100 "communications receiver." So far I've
heard no signals on 1296, not even from Des Moines. Its deaf too. But
then after being screwed with an IC-211, ICOM is a dirty four letter
word under this roof.
Back on the transverters and HF rig. I was considering Omni V or VI but
I found they copied Icom and shifted to LSB for CW, even on 10m and that
makes cross mode VHF contacts impossible. So I bought a Corsair II that
hears great and doesn't do that stupidity. If the Omni V and VI do that
shift I'm suspicious the VII does it too. Probably can be cured with a
computer doing the control, but I prefer to hear ham signals, not my
computer's hash on VHF and up so I don't use the computer when working
VHF weak signals.
I've not use an IC-746 on 6m but I have for FD a couple times. I like my
857 better, though there are reports the 857 can't handle a strong a
batch of signals as the 746. One year with the stations close together
so we had to use headphones, I kept hearing the rumble of a train, even
though we were a couple miles from the nearest tracks (Isaac Walton). It
was envelope detecting the SSB stations in the shelter.
One year at FD in my barn we used an IC-730 that acted like it had no
AGC. That the 211 have made ICOM not welcome here.
73, Jerry, K0CQ
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