[amsat-bb] Preamp for a Mobile Application MM

MM ka1rrw at yahoo.com
Thu Apr 3 11:34:00 PST 2008

For Leo Mobile, I started with the biggest antenna
that will fit
on my car and in my garage and then staring working on
the coax.

I use a 2/440 splitter into 2 Mono band NMO magmounts.
This way I get Taller antennas and more gain than with
a Dual Bander (2/440).
2 meters is a 5/8 wave mono bander (Salesman gain = 4,
Dipole gain = 1).
440 is a stacked mono band collinear 1/2 wave stacked
over a 1/2 wave (Salesman gain = 6, Dipole gain = 3).

The Amp on 440 with built-in preamp was the last part
I installed.
Although, for the first 12 years of Leo Mobile I never
used and amp.
Even though I have an amp and preamp in my car, I
would not recommend
them to the casual user.  The maintenance is usually
high to keep them
running.  Stick with the Biggest antenna and best coax

Coax Tips for Low loss.

If you buy a new NMO Mag Mount, make sure you can find
a way to dissect
the mount and remove
the magnet, etc. so you can replace the coax cable.
On some Larsen, its easy to pop off the magnet to get
to the screws
hidden under the magnet.
You will need to replace teflon pad.

Here are some number I put together on coax loss for
the typical mobile
                dB Loss                 dB Loss
Type            Freq/Length     Max     Freq/Length   
Center          440     440     Pwr     900     900   
 Pwr     Size   

Feet            15      100             15      100

RG-58/U         1.5     9.9     105     2.2     14.5  
 60      .195"  

RG-58/A         1.5     9.9     105     2.2     14.5  
 60      .195"  

RG-8X           1.2     8.1     250     1.8     12.1  
 150     .240"  

LMR-240UF       0.5     3.2     380     0.7     4.7   
 260     .240"  

RG-58 comes in many flavors, No letter, A, C and U
U sometimes is a solid 20 AWG copper center.
A and C have a stranded center conductor. 
The center conductor size would be similar to solid,
but will have a
little more loss.

Most cars will need 15 to 17 feet of coax.  
RG-8X and LMR-240 have the same Outside diameter
however the LMR has a foil shield and half the loss of
The LMR-UF-240 version is the Ultra Flex flavor and
can bend easier.

>From this list of small car style coax, the LMR beats
them all, in
lowest loss for the size and highest transmitter power
I was planning on installing a 440 AMP in my car, but
after looking at
the coax, I realized that I would fry the RG-58 coax
at 100+ watts on

I dissected an old Larsen Magmount and replaced the
original RG-58U with
I have been running this configuration for over a year
with good results
and I
have not melted any coax.  There is also a nice added
benefit, in the
winter time 
during a long rag chew on 440, the 440 amp keeps the
passenger seat

For those of you interested in 900 mc, the benefits
are even greater.

900 mc
10 watts, in, 15 feet of LMR-240, = 7.7 watts out
10 watts, in, 15 feet of RG-58/U, = 5.8 watts out

Power reference

--- "Kent R. Frazier" <k5knt at amsat.org> wrote:

> Hi all,
> I am really enjoying the satellite aspect of the
> hobby and have enjoyed the
> contacts I have made with my Icom IC-2720H with a
> 19" whip antenna from my
> vehicle. I have seen talk recently of using a preamp
> to improve the
> reception. From what I've read and understand, it
> appears that a "switching"
> preamp is needed for a setup using only one antenna
> for Tx and Rx such as
> mine. Is this correct?
> It also appears that most of the preamps are mast
> mounted. Since this would
> not be an option for me can the preamp be put near
> the transceiver? Are
> there any issues that result of doing so?
> Lastly, what preamp(s) would you recommend for an
> application such as mine?
> Thanks,
> Kent
> -- 
> Kent R. Frazier, K5KNT
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. Opinions expressed are
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