[amsat-bb] Re: LMR400 or 400UF?

i8cvs domenico.i8cvs at tin.it
Mon Aug 18 20:56:18 PDT 2008

----- Original Message -----
From: "Michael Tondee" <mat_62 at netcommander.com>
To: <amsat-bb at amsat.org>
Sent: Tuesday, August 19, 2008 1:05 AM
Subject: [amsat-bb] LMR400 or 400UF?

> I need to rework my 70cm antenna system and was thinking about going with
LMR 400. I have about a 40 foot run. Is there any real advantage to using
the ultra flex for the entire run or can I get by with a short section of
the UF between the preamp and antenna to accomodate the rotor turning? How
about just using a short section of RG8U there instead of the LMR400UF? I'm
trying to do this on a budget.
> Tnx and 73,
> Michael, W4HIJ
> _______________________________________________

Hi Michael, W4HIJ

The attenuation in dB of any transmission line connected between the antenna
and the preamplifier add directly to the Noise Figure NF of the preamplifier
and the power of the signal lost between the antenna and the preamplifier
cannot be recovered.

The attenuation of RG8U at 420 MHz is 4.8 dB /100 ft while the attenuation
of LMR400UF at 400 MHz is 3 dB / 100 ft

You don't mention how many ft of cable you plan to use between the antenna
and the preamplifier wich is antenna mounted by the way it is obvious that
the LMR400UF has less losses in comparison of the RG8U and it is preferred.

Suppose that the run of cable between the antenna and the preamplifier is
only 10 ft and the Noise Figure of your 70 cm preamplifier is NF= 0.5 dB

Using RG8U the NF of your preamplifier is like to be  0.98 dB while using
LMR400UF the NF is like to be 0.8 dB

About the real advantage of using the ultra flex LMR400UF instead of LMR400
for the entire run of 40 ft between the output of the preamplifier and the
RX input from the receiving point of view there is not any advantage because
the loss for 40 ft of LMR400UF is 1.2 dB while the loss of 40 ft of LMR 400
is 1.08 dB

By the way the gain of any good preamplifier for 70 is in the order of 20 dB
minimum so that the loss of 1.2 dB for the entire run of LMR400UF versus
1.08 dB for the entire run of LMR400 subtracts directly to the gain of the
preamplifier but the loss of gain is so low in both situations that the
overall Noise Figure of the receiving system is not deteriorated using one
cable or the other one as calculation shows.

>From the transmission point of view thinks are much different because the
full run of 40 ft is used as well for TXing so that the LMR400 with only
1.08 dB attenuation versus 1.2 dB for the LMR400UF is preferred.

In conclusion all considering for RX/TX conditions I would use:

LMR400UF between the antenna and the preamplifier and LMR400
between the preamplifier and RX/TX

For detailed information on this matter and how to perform calculations I
suggest to read the article:

"Receiver Noise Figure Sensitivity and Dinamic Range ,what the Numbar mean"
by James R. Fisk, W1DTY Ham Radio Magazine october 1975

Since the above article is very old and difficult to find I cand send a
zipped copy of it to everyone is interested.

Best 73" de

i8CVS Domenico

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