[Fourlanders] Question on Receiver Sensitivity

Thu Sep 18 23:25:13 EDT 2008

Scott/Randal/Jim/Ron

You guys have convinced me that I need to go ahead and plan to install the
preamps when putting up my tower and antennas. I have been considering the
GaAsFET MSP (mast mount) series from ARR after I got a look and listen to
Scott's on the two meter station, so it's good to get some positive reviews
on the brand. It looked well built, and the difference it made was
significant. Based on the cost vs. the trouble to retrofit them later, I
think I'll just go ahead and plan on working one in on each band as I get
antennas up. At 160 watts max throughput, RF switched, and with a low NF it
appears to be the best fit. Running a separate supply cable isn't a big
deal, but it's interesting that Icom has the feature to inject the voltage -
I wasn't aware of this. 

Ron, you mention 144-222-432-903-1296, but don't mention 50. Does the
discussion apply to 6 meters as well? 

I appreciate everyone's comments. I'll probably be asking more questions as
I get this project going, so bear with me.

Kos, N4NIA

  _____  

From: Rogers, Ron [mailto:RR124640 at ncr.com] 
Sent: Thursday, September 18, 2008 8:05 PM
To: S Kosmetatos; Fourlanders
Subject: RE: [Fourlanders] Question on Receiver Sensitivity

I see that Scott, Randal, and Jim have already posted their comments so let
me expand the discussion a bit. Here are a few observations based on my
experience over a 35+ year span of VHF-UHF experiments, disappointments, and
lessons learned.

Some History > I have used the best, and less than best, VHF & UHF
transceivers, receiving converters, and transverters over many years during
contests, satellite work, and a bit of EME. I've used Drake, Collins, KLM,
Kenwood, Yaesu, and Icom radios of all flavors through the years of
technical advances in performance married to transverters or receiving
converters made by Lunar, VHF Engineering, SSB, Microwave Modules, and DEMI.

In EACH frequency band situation (144-222-432-903-1296) when trying to
extract extremely weak and barely discernable signals out of the noise
threshold, using the proper preamp made the difference of hearing the signal
comfortably, or not hearing it at all without the preamp !    Notice I said
using "the proper" preamp. 

As you noticed with the 432 contest station, I now prefer the Icom 910H for
432 as it has one of the hottest VHF-UHF receivers on the market, has DSP
capability, and was designed specifically for weak signal satellite work.
It's sensitivity is speced at .11 uv but that is a broadband specification
based on 20 MHz of coverage from 430-450 MHz. I have found that by
optimizing the front end for the 432-435 MHz range and playing with the
internal gain adjustment, I have been able to increase the sensitivity to
around .09 uv for 10db S/N as best as I can measure with my equipment. I
also like the Icom 475H interchangeably and have tweaked it on the bench to
be about as sensitive. 

In addition to the contest station radio, I use the ARR SP432VDG RF switched
preamp with Jim's 4 stage helical filter in front of the preamp. While using
a RF switched preamp near the radio is a compromise as compared to a preamp
located near the antennas, I am convinced that I would not be working the
far distant grids on 432 MHz that we harvest during the contests without the
aid of that preamp. A flip of the switch to remove the preamp from the line
simply makes most all distant station signals simply disappear.

But, the "wrong" preamp for a given situation or equipment environment can
worsen the performance of whatever combination of equipment you are using.
The minimum goal is to obtain 10 to 15 db of realized system gain without
ruining your previous system noise figure or making your receiver's S meter
move upscale 3 "S" units when you turn on the preamp.

I have had less than stellar experience with "bargain priced" preamps or the
"wrong" preamp for a given situation because of the reasons above. I also
like the capability of being able to switch a preamp in and out of the
circuit for situations where a strong signal may overload my receive.  

I also have built a couple of elaborate tower mounted enclosed preamps for
high power switching with bandpass filters in front of the preamp. I have
also used (still do) in-line RF switched preamps between the exciter and
amplifier when I can't be hassled with the tower mounted, synchronized
preamp when running high power in contest conditions. But to keep the system
noise figure as low as possible, the preamp mounted at the antenna is the
best possible implementation.

Having stated the above, and having tested multiple models of pre-amps
through the years, I have 3 preferred suppliers, based on the type of preamp
needed for a given situation, balanced against cost. 

My best experiences >> Advanced Receiver Research (ARR), DEMI, and SSB. 

For FM repeater use (VHF & UHF)  I like the ARR PxxxVDA series of receive
only preamp used in high RF environments.
For weak signal receive only situations, I like the DEMI series of GaAsFET
preamps or the ARR PxxxVDG series.
For in-line RF switched preamps (up to 30 watts), I suggest the ARR SPxxxVDG
low noise GaAsFET version.
For tower mounted RF switched (up to 160 watts) I suggest the ARR MSP series
or the SSB SP series that can handle a bit more power (220+ watts). The SSB
tower mounted preamps are certainly "top shelf" in functionality.

A nice feature about the Icom VHF-UHF multi-mode radios is when you push the
"preamp" button on the front panel, the radio actually injects a DC voltage
onto the feed line to the remote mounted RF switched preamp (ARR type) so
you don't have to run a separate power wire up the tower or buy a DC line
injector. A lot of people get confused by a rumor that the Icom radios are
designed specifically to require a preamp. But in reality, what is being
confused is the DC line injection feature to accommodate an antenna mounted
preamp. ARR also sells a DC line injector if you need it.

Icom also sells overpriced tower mounted preamps (intended for satellite
work) for 144, 432, and 1296 that take advantage of this feature. Do not
waste your money on them. They are junk compared to what I have discussed
above.

So, there's my nickel's worth.

Ron
WW8RR

  _____  

From: fourlanders-bounces at contesting.com on behalf of S Kosmetatos
Sent: Wed 9/17/2008 8:17 PM
To: 'Fourlanders'
Subject: [Fourlanders] Question on Receiver Sensitivity

I'm looking at the specs on my two all-mode radios, the ICOM 706 and the
Yaesu FT-847. What are the receiver sensitivity figures which would indicate
the need for an external preamp? Are the figures shown adequate for the
general WS communications I would be working, or would I benefit from
preamps?

I'm thinking if I do, I want to incorporate them as mast mounted as I
assemble the components for my antennas and tower.

                Icom            Yaesu
 50 MHz .15 uv  .20 uv
144 MHz .11 uv  .125 uv
430 MHz .11 uv  .125 uv

Both are based on 10 dB S/N in the specs.

Kos, N4NIA

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