FWIW- I am no expert:
How much gain did the preamps have?
There is some discussion in listening for weak signals to try and keep the
front end gain such that the ambient noise is not higher than the AGC
threshold of the receiver, and thereby activating it (as shown by the
S-meter going up). The concern is that if the AGC is reacting on noise, then
some AGC circuits will "intermodulate" the noise with the desired weak
signal and make it fuzzier and more difficult to copy. If the preamp that
worked the best had the least gain, maybe that is in play here.
If the radio has an attenuator, try the other preamps again and then add in
an amount of attenuation that is about 3 dB or so less than their gain, and
see if that makes them sound better.
73
Chet, N8RA
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of Zack Widup
Sent: Friday, August 15, 2008 4:14 PM
To: Les Rayburn
Cc: VHF Contesting Reflector; vhf@w6yx.stanford.edu
Subject: Re: [VHFcontesting] Quick To the Mystery Machine! [The Case of the
Phantom Pre-Amp]
I'm guessing that the devices which performed well were actually
noise-matched and the others weren't; perhaps they were just tuned for
maximum gain? Many component manufacturers supply the gamma (optimum) for
best noise match for the device, which is not the same as the maximum
gain/power out match.
73, Zack W9SZ
On Fri, 15 Aug 2008, Les Rayburn wrote:
> Pardon me if I mix my metaphors but, "Riddle me this, boy wonder"....
>
> Months ago, when I started on this mis-adventure, I purchased an Icom
IC-910H, along with the Icom mast mount pre-amps. Now, I can hear some of
you moaning in despair already in the background, but we all have to make
mistakes in order to learn from them. The decision to purchase the Icom
pre-amps was based mainly on ease of use.
>
> After a few dozen QSO's, it became obvious that engaging the pre-amp
button on the rig served mostly to raise the S-Meter reading on the rig by
bringing up the local noise floor, but did little to improve the all
important signal-to-noise ratio. I sought all manner of advice on an
upgrade, and received a variety of opinions. Most of them reminded me over
and over that simply moving to another neighborhood that would allow for a
nice tower would eliminate the need for pre-amps all together.
>
> Sometimes, I do wonder if any of you gentleman are:
>
> A.) Married (and young enough to worry about your future bedroom
> activities)
> B.) Have checked the housing market in the past few months
>
> But I digress. Ignoring the advice to commit both marital and financial
suicide, I opted instead to accept an offer to borrow a trinity of pre-amps
in the form of the Advanced Receiver Research MSPXXXVDG-160 series. These
are mast mounted GaAsFET preamps rated at 160 watts. They had belonged to a
rather serious DX'er who had long since upgraded to some exotic tuned
cavity, radio astronomy model not suitable for mere mortals. They had been
sitting in his collection of VHF parts for about two years.
>
> Alas, I discovered that they also appeared to have been damaged at some
point...since only one of the three was working. I returned them to AAR,
where they quickly repaired them, and at my request, bench tested all three
to insure performance. With great joy, I installed all three---and quickly
went about trying to conquer the known world with my indoor antenna farm.
>
> I quickly discovered that these amps, while an improvement over the Icom
amps, still mainly brought up the noise floor and increased S-Meter
readings, without a noticeable improvement in signal to noise.
>
> Disappointed, I turned my attention to trying to get a Beko 300watt
> 432 amp that I had also been loaned working properly on 220 volts. I
> was advised to contact Jerry at SSB Electronics for help. I did and
> proceeded to take up about two hours of his time for an amp that he
> hadn't sold, and had no obligation to support. ( Now that's what I
> call customer service)
>
> Out of guilt, mixed with desperation, I also ordered an SSB Electronics
SP-7000 mast mounted pre-amp for the 432 band. Truthfully, I expected
nothing except to sleep better at night haven not taken advantage of a
small-businessman. When I replaced the ARR pre-amp with the SSB Electronics
amp, I made an amazing discovery. When activated (I'm powering it via the
feedline from the IC-910H) the S-Meter barely moved at all. At first, I
thought it must be defective.
>
> A few hours later brought a weak CQ on the calling frequency of
> 432.100---I switched on the pre-amp, and the signal noticeably
> improved. I was easily able to make out the call sign and grid
> square...but the noise hardly changed. A couple of weeks later, I
> verified this over and over again during the ARRL UHF Contest. The
> most hopeful event was that I could also copy a lot of stations who
> couldn't hear me, even in CW. (Once I get the Beko Amp going,
> hopefully that will change)
>
> For those of you would be Nancy-Drews out there still reading along, this
brings us to the mystery.
>
> 1. All three devices use similar (if not identical) active devices
(GaAsFET). Upon internal inspection, construction also seems similar. Why
the marked differences in performance?
>
> 2. I've read some articles that say the impedance match to the antenna can
dramatically affect the S/N ratio, but none of the amps came with
instructions for how to adjust for this. Is this commonly done?
>
> 3. Before I rush out to buy two more SSB Electronics amps (2M and 222), is
there anything I can do to the ARR units to improve their performance?
>
> For those who wonder why this is such a concern for me, I've also
discovered that a db or two difference in performance can make or break 2
Meter meteor scatter contacts...and my system, due to it's indoor location
suffers a bit of deafness. Most MS operators say that good, low noise
pre-amps can make a lot of difference on these bands. In hindsight, I should
have ordered the SP-2000 to start with....
>
> As always, gang, it's free Scooby Snacks to those who help get to the
bottom of this one. Darn those meddling kids!
>
> 73,
>
> Les Rayburn, N1LF
> EM63nf
>
>
> Les Rayburn, director
> High Noon Film
> 100 Centerview Drive Suite 111
> Birmingham, AL 35216-3748
> 205.824.8930
> 205.824.8960 fax
> 205.253.4867 cell
> _______________________________________________
> VHFcontesting mailing list
> VHFcontesting@contesting.com
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>
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