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Re: 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

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! :-)


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