[VHFcontesting] VHF/UHF cable installation?

Stu Benner w3stu at myactv.net
Fri May 4 18:16:32 EDT 2007


First we must know on what frequencies you'll be operating. Based on your
qrz.com description of your station equipment, I'll assume that you're
interested in 50 MHz through 432 MHz.

Your best choice for connectors will certainly be type N. They are
controlled impedance, low loss, handle the power levels of amateur stations,
readily available, economical, and (in my opinion) easiest to install. Just
because the connector has a diameter smaller than the cable, you are not
"compressing the energy." The characteristic impedance of the cable and
connector are both 50 ohms (i.e., there is no impedance mismatch between the
two that would result in loss of forward power). Every element that you
place in a transmission path (e.g., a connector or cable) has an associated
loss which is most conveniently measured in decibels (dB). If you're going
to be doing weak signal work and are not familiar with working with dB, it's
time to do some web searches and some studying. When designing a system,
I'll usually assume a loss of about 0.05 dB for a mated pair N connectors
for 50 MHz through 144 MHz and about 0.1 dB per mated pair at 222 MHz and
432 MHz. This assumes that the connectors are properly installed.

You're lucky to have available some 1/2" Heliax. You'll typically find a lot
of 1/2" and 7/8" in a typical weak signal station. The short answer about
1-5/8" Heliax is: costly, difficult to handle, not needed for the bands in

As for the transmission line, here are some numbers you can use to estimate
your losses. These are represent the loss for a cable that is 150 ft. long.
You can scale proportionally for longer or shorter lengths:
Frequency (MHz)  LDF4-50A (1/2" Heliax)   LMR-400 
  50.1                           0.73 dB                                1.35
144.2                           1.24 dB                                2.24
222.1                           1.57 dB                                2.71
432.1                           2.24 dB                                3.96
Take all of the pieces of your proposed system (from the radio to the
antenna feedpoint) and add up the total losses (a spreadsheet works nicely
as a "what if" tool) and you'll soon get a feel for what's acceptable and
what is not. For my systems, I try to design for a total path loss of less
than 1 dB on each band. That's quite conservative even for weak signal work;
2 dB or so is a reasonable goal. There are ways to "work around" using very
low loss cable, but I won't try to go into that in a quick e-mail. Just go
to the ARRL Technical Information page
(http://www.arrl.org/tis/tismenu.html), RF Cafe http://www.rfcafe.com, or
search the web to learn about receive preamplifiers, noise figure, coaxial
cables, connectors and the like.

Stu Benner

-----Original Message-----
From: vhfcontesting-bounces at contesting.com
[mailto:vhfcontesting-bounces at contesting.com] On Behalf Of steve d
Sent: Thursday, May 03, 2007 6:46 PM
To: vhfcontesting at contesting.com
Subject: [VHFcontesting] VHF/UHF cable installation?

Hi all, I have a question regarding installing cable for VHF/UHF.
  What I want to do is wire up my cabin in EN39 for coax and rotor  control.
How I want to do it is run cables from the inside wall of the  cabin through
the wall, into the porch, and through the outside porch  wall. Then, connect
my cables on to those inside to the rig and outside  to the antennas. 
  I have 1/2" heliax LDF4-50A. The cable runs will be under 150'. Will it
be OK to run that through the cabin as I described with N females on  both
ends? Or will the attenuation through the connections not be worth  it? What
size cable would you reccomend? What connector? Obviously if I  go to
something like 1-5/8" through the cabin and put an N connector on  there
then I am compressing the energy through the cable to come out a  small
connector and into a decent size cable on the other end. 
  Im no pro when it comes to these connections, but I want to have the
least attenuation I can get without having it be a pain in the butt and
  Steve, KC8QVO
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