[Fourlanders] Question on Receiver Sensitivity

[Rogers, Ron]

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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