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Re: Kuhne vs DEM preamp?



I liked the tradeoff Ed Cole ( AL7EB)  made below- and thought I might add "
my two cents" for whatever it's  worth.
The DEM 13ULNA is spec'd  at  max .7db NF  and min 16dB gain, whereas the
Kuhne MKU232A2 is
spec'd  at max .7db NF and min 35 dB gain. The Khune MKU232A2 cost $155 from
SSB and the DEM
13ULNA cost $120. What's  one getting for the extra $35 bucks?  First- one's
getting an extra stage of
amplification ( 35 dB vs. 16dB) - but inaddition - the extra stage of
amplification also provides a full-bandwidth
helical bandpass filter - and-  inaddition-  the MKU232A2 provides an L-
band notch filter which ( if you ever decide to use Mode L/S ) will protect
your S-band preamp from being densensed by your up-link L-band signal.  As
you can see
from Ed Cole's noise figure calculations below, the higher G1 the better it
will be in knocking down the overall noise temperature of your system. Thus,
the extra $35 provides at least 15 dB more gain for G1, a bandpass filter
( which will eliminate spurs) and an L-band notch filter. Don't own any
stock or interest in SSB- but spent the extra $35 and bought the Kuhne
MKU232A2 preamp.

All the Best,
Jim, N6MV


----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, September 08, 2001 10:17 AM
Subject: Re: [amsat-bb] Kuhne vs DEM preamp?


> >From: "Greg D." <ko6th_greg@hotmail.com>
> >Ok, time to get a new preamp.  Not enough signal into the headphones,
> >and the preamp is probably the easiest of the pieces to change.
> >
> >Currently I am using the Connifer preamp that came with the MMDS
> >grill antenna and Drake 2880, all as a package deal.  The Confier is
> >supposed to be around a 1.5db nf and 10db gain.
> >
> >My choices:
> >
> >1) replace the Confier with a Kuhne MKU 232 A2 TM
> >Spec is .7db nf and 35 db gain (!).  Sounds good, but the unit appears
> >to be powered through the coax (not easily done in my setup), and
> >also has a design frequency of 2350 mhz, not 2401.xxx.  I don't know
> >how wide the receive bandpass is, and what running it at 2401.xxx
> >does to the nf and gain specs.  Need to adapt the Khune's F-connector
> >output to N, to feed the Drake; it's not really designed to be mounted
> >directly between the antenna and downconverter, as the Confier is.
> >
> >2) replace the Connifer with the DEM 13ULNA
> >Spec is .5db nf and 17db gain.  Better noise, not as much gain.  I'd
> >like to reduce the size of my antenna if possible, to reduce the risk
> >of weather damage, so the higher gain of the Kuhne is tempting.  The
> >DEM's external power feed is good.  Price is a little better too.
> >
> >3) KEEP the Connifer preamp, and stick the DEM 13ULNA in front of
> >it.  Theoretically get .5db nf and 27db gain.  With the original
> >grill and linear feed, this might be the right mix.
>
> I replied earlier directly to Greg, But here is a comparison to provide
> more info for your decision:
> {Assuming the Drake's NF to be 6 dB}
>
> The full math for evaluating noise temp is:
> Trx =  To(L1-1) + T1L1 + To (L2-1)L1/G1 + T2L1L2/G1
> where To(L1-1) is the noise temp added by coax between the ant and the
> first preamp
> and To(L2-1)L1/G1 is the noise temp added by coax between preamp1 and the
> Drake.
>
> To make this easier to calculate, lets assume you are connecting the first
> preamp directly to the antenna and the Drake directly behind so that coax
> losses are zero dB:
> So L1 = 1 and L2 =1, and:
>
> Trx = T1 + T2/G1
>
> #1- The Kuhne NF = 0.7dB, T1 = To[10^(NF/10)-1] = 290*(.17) = 50.7 K
> The Drake NF = 6 dB, T2 = 864.5 K
> Kuhne gain = 35 dB, so gain ratio: G1 = 10^(G/10) = 3162
> Trx = 50.7 + 864.5/3162 = 50.7 + 0.27 = 50.97 K
> converting back to NF = 10Log[Trx/290 + 1] = .703 dB
> The Kuhne clearly overcomes the Drake's NF.
>
> #2- The DEM NF = 0.5 dB, T1 = To[10^(NF/10)-1]  = 290*(.12) = 35.4 K
> Dem gain = 17 dB, gain ratio = 10^(G/10) = 50
> Trx = 35.4 + 864.5/50 = 35.4 + 17.29 = 52.69 K
> converting back to NF = 10Log[Trx/290 + 1] = 0.725 dB
> The DEM doesn't completely rule the NF.
>
> But they come out approximately equal assuming the DEM preamp has a better
> NF.  If both preamps NF were the same then the DEM would not perform as
> well due to the 17.29 K noise from the Drake being added.
>
> BTW my Kuhne preamp came with a bench test sheet indicating NF = 0.6 db
> with 40 dB gain!
>
> #3- Greg's third alternative of two preamps with 25 total gain will be
BEST
> {if the second preamp is not overdriven, which is the risk}.
>
> Trx = T1 + T2/G1 + T3/G1G2 = 35.4 + 119.6/50 + 864.5/500 = 35.4 + 2.4 +
1.7
> = 39.5 K,  and NF = 0.555 dB
> --------------------------------------------------------------------------
--
>
> So practically what do you do?  Well if you build your own DEM preamp, be
> aware that your NF may vary from the spec.  In NF testing at conferences
> like Central States VHF and Microwave Update, individual preamps have not
> done as well as others of the same model.  {know you own building skills}
>
> I'm talking of risk, now.  If you are deciding between buying an assembled
> and tested DEM and a Kuhne then it appears to be a toss-up decision.  If
> you build, there is a little more risk with the DEM preamp {but an
ultimate
> win if the Conifer works with it}.  Greg, the ball is in your court! :-)
>
> Ed
>
> ----
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