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Author Topic: Help with co-phasing wi fi yagi's.  (Read 2407 times)

Posts: 31

« on: March 19, 2007, 02:56:57 PM »

Hey guys,
I am having GREAT success with my new MFJ wi-fi yagi on my laptop. I was sick of poor performance with the in-built 801.2 wireless connection that I bought a 200 mw pcmi card (most laptops only have about 32 mw output!, and useless inbuilt antenna) that has an external antenna connection. With the $30 yagi I can pick up strong open signals almost anywhere, especially RV'ing, or around town with a magmount antenna on the car. The yagi is very small that I would like to buy another one and co-phase them together, so my question is best way to do it? I don't mean the physical mounting of the 2 yagi's, more so how should I have the coax configured, length for each section from T join etc (I'll use a T-joiner and L400 coax), and distance between the 2 antennas. They will be run in vertical position of coarse. If you use your laptop while travelling and would like another option than $$$ per hr, I highly recommend this setup, BUT would love to try the 2 yagi setup.
Hope someone can shed a few ideas!

Posts: 5483


« Reply #1 on: March 19, 2007, 08:00:53 PM »

2 yagi's buys you 3dB max, closer to 2dB once you build a phasing harness and add connectors.  Doesn't sound worth it to me.  You would likely get more improvement by raising the height of what you have versus adding a 2nd antenna.

Mark K5LXP
Albuquerque, NM

Posts: 17484

« Reply #2 on: March 20, 2007, 05:48:19 PM »

Two yagis side-by-side will narrow your horizontal pattern, making
aiming more critical.  Stacking one above the other will narrow the
vertical pattern instead which may be easier to use (unless the signal
is coming from overhead - which may be the case around tall buildings.)

The general approach for phasing ("co-phasing" is a CB term - from an
engineering perspective it has no meaning except that the person using
it doesn't know anything about the subject) is to connect the two antennas
to a common point using identical lengths of feedline.  For two 50 ohm
antennas and 50 ohm coax, this gives an impedance at the junction of
25 ohms.  The problem is how to match this impedance to your 50
ohm cable from there to the receiver.  There are several common methods:

first, use a quarter wavelength of 35 ohm coax as an impedance
transformer.  This will be around 4" of cable, depending on the velocity
factor.  You can approximate a 35 ohm cable using two pieces of 75
ohm coax in parallel (37.5 ohms) or make your own using brass or
copper tubing with the proper ratio of diameters.

second option:  step up the impedance of each antenna to 100 ohms
then connect them in parallel to get 50 ohms.  You can do this with a
quarter wavelength of 75 ohm coax in series with each antenna lead
right before it connects to the T.  Or, since the length could also be
some odd multiple of a quarter wavelength, you can often use 3/2 or
5/2 wave of 75 ohm coax to connect each antenna to the T (again,
correcting for the velocity factor of the cable.)

(Either of these options can easily be build on a printed circuit board
using microstripline elements.)

Third option:  design the antenna to have a 100 ohm feedpoint (like
some quads) and use 100 ohm coax to connect it to the T.  (More
likely one of the 92 ohm types.)

fourth option:  just connect the antennas together and don't worry
about the 2 : 1 SWR on the cable.  This is a tradeoff between the
additional losses in the coax due to SWR and the losses from all of the
connectors and joints necessary to build a matching section.  Certainly
it is the simplest.
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