On Jan 2, 11:08*am, mr1956 wrote:
I have posted on here before and found the member's comments very
helpful so here it goes again.

I am trying to stack two Yagi antennas to form a ground station to
receive GPS data from an experimental rocket. The rocket portion of
the system has already been tested but now I have two antennas to
couple together to make up the receiving end. The particulars are as
follows:

I have two 11 db 900 MHz Yagis (Pacific Wireless), both of which have
50 ohm pigtails. I am trying to hook these together in a cross
polarized fashion and need to insert two 75 ohm impedance matching
cables so that I end up with 50 ohms at the cable end attached to the
input of my receiver. Basically, there will be the two antennas,
matching sections, a tee connector, then the single coax going to the
receiver.

I figure that 75 ohm RG-11 coax should work for this purpose and am
trying to calculate the correct length of these impedance matching
sections. One formula I have found online is as follows:

Length (feet) = 246 * VF / Frequency (MHz)

The transmitter sends RF via Frequency Hopping Spread Spectrum from
910 to 918 MHz; consequently, I am using 914 MHz as a baseline.

When I plug in the numbers, I get a length of about 2.1" for the
length of the 75 ohm matching sections. Needless to say, it will be
difficult if not impossible to do a 2.1" length of RG-11 cable with
two connectors. I have considered basically fabricating a "tee"
section out of two short lengths of the 75 ohm coax and RG-8 going to
the receiver (soldering the whole thing together), then connecting the
two Yagi antennas to that.

I suppose my question is this: Is there a longer multiple of
wavelength I can use for the impedance matching 75 ohm sections to
develop a more practical design for what I need? Or, am I stuck with
the 2.1" length due to the frequency?

Thanks,

Curt Newport

Sir
If you have the time to try, make a sphere about 4 Ft diameter of
aluminum window mesh. This gives you total coverage of all polarities
and you don't need a rotor as you have full sky coverage. 6 panes 2
foot wide stapled or riveted together will do the job. This is a
Faraday cage where you ground the outside and connect the inside to
your radio. Cost of alum window mesh is minimal and it does not take a
lot of time to make.Nothing ventured nothing gained! Hula hoops inside
will help to keep its shape.
I assume that the idea of total sky coverage with total polarity
coverage will provide you with worthwhile advantages over the route
you are contemplating making it worthwile to spend a day making one.