nuckolls.bob(at)aeroelect
Guest
|
 Posted: Fri Jan 28, 2011 8:42 am Post subject: Unfortunate Antenna Instructions |
 |
|
I've been given a copy of S-H antenna installation
instructions for review. I have the following observations
to share with members of the List. The article has
been posted at:
http://tinyurl.com/4cwq4yy
ADJUSTING the ANTENNA CABLE LENGTH
The process described under this title is a salute
to an old hangar myth that antenna system performance
can be improved by cutting coax feed lines to (1) multiples
of half-wavelengths or (2) maximize measured field strength
on some particular frequency.
As we discussed last week, the ONLY time that observable
effects are affected by length of the transmission line
is when the SWR on that line is high. The most profound
effects for fiddling with length will be noted when SWR
is a lot higher than what is traditionally accepted as
optimum performance. 3:1 or better.
This myth has roots that go back a long way in aviation
history. I recall similar discussions back in the 60's.
I would challenge the publisher of any such document to
search any engineering text and/or any ARRL publication
on amateur built antennas. They will not find any similar
suggestion anywhere. The adequately designed antenna
presents a load to the feedline that has no observable,
deleterious effects on system performance.
An interesting feature of the S-H suggestion is that
even if there WERE a valid reason for crafting an even-
multiples-half-wave transmission line, the coax cuts
would NOT be multiples of the free-space wavelength used
to design the antenna.
Propagation velocity of energy down a transmission line
is SLOWER than in free space. Depending on how the coax is
made, the VELOCITY FACTOR will be some number near .66
This means that for a 1/4-wave, 24" antenna element length
a 1/2 wave in coax would NOT be 48" but 48 x .66 or
31.5 inches.
Unfortunately, the idea published in this document is not
only non-sense, it is poorly implemented based on bad science.
INSTALLING BNC CONNECTORS
Finally, the tri-axial cable called out was designed for
use in systems were special connectors maintain SEPARATED
shield integrity throughout the system. VERY expensive
and rare connectors. On the other hand, RG-58 has been
handily replaced throughout the TC aircraft world with
RG-400 or RG142 which are DOUBLE LAYER bi-axial feedlines
that accept the standard RG-58 connectors with no
fabrication acrobatics.
It's unfortunate that this publication has been circulated
to S-H customers for 14 years. It has no doubt generated a lot
of unnecessary no-value added taxation of their customer's
time, talents and resources.
Bob . . . [quote][b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
