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

On Sat, 2 Jan 2010 09:08:16 -0800 (PST), mr1956
wrote:

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?

Hi Curt,

To answer your last question first: you can use odd multiples (1, 3,
5, 7...) if your initial computation was correct. Consider, if your
computation contains errors, they, too, will multiply.

Computations aside, a receiver rarely needs exact matching when "close
is good enough." Two 50 Ohm antennas in parallel or series are not
seriously off. The mismatch loss would probably amount to less than 1
dB whereas your using two crossed antennas is in an effort to recover
from the loss of polarization mismatch that can exceed 20 to 30 dB.

As a practical matter, have you tried testing these antenna issues at
ground level? There are certainly complications that ground proximity
can introduce (reflection from ground being principle among them). As
a first pass approximation, however, you can come to some feeling for
many issues without having to loft anything into the air. Insofar as
polarization mismatch goes, ground's proximity would tend to dilute my
suggestion as ground and anything that is in proximity would tend to
offer many reflections some of which don't come anywhere near your
final application.

73's
Richard Clark, KB7QHC

On Jan 2, 12:45*pm, Richard Clark wrote:
On Sat, 2 Jan 2010 09:08:16 -0800 (PST), mr1956
wrote:

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?

Hi Curt,

To answer your last question first: you can use odd multiples (1, 3,
5, 7...) if your initial computation was correct. *Consider, if your
computation contains errors, they, too, will multiply.

Computations aside, a receiver rarely needs exact matching when "close
is good enough." *Two 50 Ohm antennas in parallel or series are not
seriously off. *The mismatch loss would probably amount to less than 1
dB whereas your using two crossed antennas is in an effort to recover
from the loss of polarization mismatch that can exceed 20 to 30 dB.

As a practical matter, have you tried testing these antenna issues at
ground level? *There are certainly complications that ground proximity
can introduce (reflection from ground being principle among them). *As
a first pass approximation, however, you can come to some feeling for
many issues without having to loft anything into the air. *Insofar as
polarization mismatch goes, ground's proximity would tend to dilute my
suggestion as ground and anything that is in proximity would tend to
offer many reflections some of which don't come anywhere near your
final application.

73's
Richard Clark, KB7QHC

Richard:

Yes I have tried testing one of the antennas at ground level but that
is not really representative of the reality. I suppose the bottom line
for me, is that how much of a loss and I looking at if I simply hook
these two 50 ohm antennas in parallel to a tee section, then on to the
receiver? The whole reason to have the two yagis to begin with is to
increase overall gain and insure that the polarization between the
transmitting and receiving antennas is good enough to maintain a link.

If I elect to do a multiple of 1/4 wave, then how does that work out
with the formula regarding frequency?

Thanks,

Curt Newport

On Sat, 2 Jan 2010 10:12:51 -0800 (PST), mr1956
wrote:

Yes I have tried testing one of the antennas at ground level but that
is not really representative of the reality. I suppose the bottom line
for me, is that how much of a loss and I looking at if I simply hook
these two 50 ohm antennas in parallel to a tee section, then on to the
receiver? The whole reason to have the two yagis to begin with is to
increase overall gain and insure that the polarization between the
transmitting and receiving antennas is good enough to maintain a link.

If I elect to do a multiple of 1/4 wave, then how does that work out
with the formula regarding frequency?

Hi Curt,

To your last question: frequency and wavelength are inversely related.
Compute at any frequency, convert to wavelength and use any odd
multiple of 1/4th wavelength. This, I presume, you are already are
familiar with. What I further presume is the basis of your question
is more oriented to the span of frequency involved
910 to 918 MHz; consequently, I am using 914 MHz as a baseline.
which is good as it is going to get.

As for using a T, as I posted earlier, the loss there (more for
transmitting than reception) would be less than 1dB. The additional
complexity of more lines, more connections, more components could
easily equal this OR you could find optimal results = more loss than
0dB, but less loss than something less than 1dB.

The difference between more than 0dB and less than 1dB is pretty slim.
If you feel you need to go there, you should be looking at working on
a path loss and power budget. I don't think you've offered that here
in the past. Two yagis set in cross polarity (at right angles) does
not increase "overall gain" except in the sense of insuring that all
polarizations are covered. In fact, given the random orientation, you
stand to lose "overall gain" on average when comparing one fixed yagi
to a known polarized source. In fact more than 1dB (probably 3).

Again if 1 to 3 dB is important, other solutions need to be considered
and you really need a more elaborate analysis.

73's
Richard Clark, KB7QHC

Richard:

Your reply is noted and let me do some figuring and see what I come up
with.

I agree, that the cross-polarization of the two yagis is more critical
than the overall gain as this is the greatest unknown. In other words,
I have no idea as to the orientation of the transmitting antenna
relative to the receiving antenna, except during the boost and descent
phases. Those are fairly predictable because the rocket goes generally
straight up, arcs at apogee, then descends with the bottom end down on
the large drogue chute.

I have found a power divider made by Cushcraft but am wondering
whether or not it is worth the investment.

Regards,

Curt Newport



On Jan 2, 4:42*pm, Richard Clark wrote:
On Sat, 2 Jan 2010 10:12:51 -0800 (PST), mr1956
wrote:

Yes I have tried testing one of the antennas at ground level but that
is not really representative of the reality. I suppose the bottom line
for me, is that how much of a loss and I looking at if I simply hook
these two 50 ohm antennas in parallel to a tee section, then on to the
receiver? *The whole reason to have the two yagis to begin with is to
increase overall gain and insure that the polarization between the
transmitting and receiving antennas is good enough to maintain a link.

If I elect to do a multiple of 1/4 wave, then how does that work out
with the formula regarding frequency?

Hi Curt,

To your last question: frequency and wavelength are inversely related.
Compute at any frequency, convert to wavelength and use any odd
multiple of 1/4th wavelength. *This, I presume, you are already are
familiar with. *What I further presume is the basis of your question
is more oriented to the span of frequency involved910 to 918 MHz; consequently, I am using 914 MHz as a baseline.

which is good as it is going to get.

As for using a T, as I posted earlier, the loss there (more for
transmitting than reception) would be less than 1dB. *The additional
complexity of more lines, more connections, more components could
easily equal this OR you could find optimal results = more loss than
0dB, but less loss than something less than 1dB.

The difference between more than 0dB and less than 1dB is pretty slim.
If you feel you need to go there, you should be looking at working on
a path loss and power budget. *I don't think you've offered that here
in the past. *Two yagis set in cross polarity (at right angles) does
not increase "overall gain" except in the sense of insuring that all
polarizations are covered. *In fact, given the random orientation, you
stand to lose "overall gain" on average when comparing one fixed yagi
to a known polarized source. *In fact more than 1dB (probably 3).

Again if 1 to 3 dB is important, other solutions need to be considered
and you really need a more elaborate analysis.

73's
Richard Clark, KB7QHC

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.

"mr1956" ha scritto nel messaggio
...
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

The answer to your question is yes. You can use odd multiples of the calcuated
length (my calculation give 2.13" for PE-insulated cables like RG-11, or 2.25"
for teflon insulated cables. For foam-insulated cables, calculation must take
into account the actual cable velocity factor).

However, at 900 MHz, building the coax combiner you have described would be an
headache, due to the difficulty of precisely calibrating the coax pieces length
and to verify their performance.

For a receive-only system a solution you may consider is to use a small
preamplifiers connected to each antenna connector, either directly if feasible,
or using a very short piece of coax.

Doing so, the receive system signal-to-noise ratio would be set by the
preamplifiers and not influenced by any loss occurring after them. So you could
combine the two preamplifier output signals fully disregarding the impedance
issue. You must only make sure that the two coax lengths joining the
preamplifiers to the summation point are of identical length (whatever it is),
so that the two signals get summed in phase. You could even use small coaxial
cables (e.g. RG-58 or perhaps even RG-174 depending on length) that can be very
easily handled, as any attenuation after the preamplifiers will not influence
the receive system noise figure (within certain limits).

The above arrangement would be OK if using two antennas having the same
polarization. But reading your post it seems to me that this is not the case. As
a matter of fact, if I understood it correctly:

- you wish to mount one antenna on horizontal polarization and the other one on
vertical polarization
- your objective is to receive a linearly polarized signal with a randomly
slanted polarization plane.

If so, combining the two antennas using identical pieces of coax, you would
obtain a system polarized on a 45-deg slanted plane, that would not help in
receiving randomly polarized signals (signals that are orthogonal, or nearly so,
to the antenna polarization plane would be strongly attenuated). You should
instead aim at obtaining a circular polarization which causes a steady 3 dB loss
independently of the signal polarization plane.

To do that you must introduce an extra 90-degree phase shift by adding a
quarter-wavelength 50-ohm section to one of the two indentical-length 75-ohm
coax pieces.

All this in theory. In practice I doubt that, at 900-MHz, a system like that
would behave precisely as expected.

Regards.

Tony I0JX, Rome Italy

This is all good information and I can appreciate the difficulty of
creating the 75 ohm matchings sections due to the precision involved.
I have however found a power divider for this frequency that should do
what I need with respect to stacking the two yagis.

That is probably the easiest solution in this case.

C. Newport


On Jan 3, 10:13*am, "Antonio Vernucci" wrote:
"mr1956" ha scritto nel ...





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

The answer to your question is yes. You can use odd multiples of the calcuated
length (my calculation give 2.13" for PE-insulated cables like RG-11, or 2.25"
for teflon insulated cables. For foam-insulated cables, calculation must take
into account the actual cable velocity factor).

However, at 900 MHz, building the coax combiner you have described would be an
headache, due to the difficulty of precisely calibrating the coax pieces length
and to verify their performance.

For a receive-only system a solution you may consider is to use a small
preamplifiers connected to each antenna connector, either directly if feasible,
or using a very short piece of coax.

Doing so, the receive system signal-to-noise ratio would be set by the
preamplifiers and not influenced by any loss occurring after them. So you could
combine the two preamplifier output signals fully disregarding the impedance
issue. You must only make sure that the two coax lengths joining the
preamplifiers to the summation point are of identical length (whatever it is),
so that the two signals get summed in phase. You could even use small coaxial
cables (e.g. RG-58 or perhaps even RG-174 depending on length) that can be very
easily handled, as any attenuation after the preamplifiers will not influence
the receive system noise figure (within certain limits).

The above arrangement would be OK if using two antennas having the same
polarization. But reading your post it seems to me that this is not the case. As
a matter of fact, if I understood it correctly:

- you wish to mount one antenna on horizontal polarization and the other one on
vertical polarization
- your objective is to receive a linearly polarized signal with a randomly
slanted polarization plane.

If so, combining the two antennas using identical pieces of coax, you would
obtain a system polarized on a 45-deg slanted plane, that would not help in
receiving randomly polarized signals (signals that are orthogonal, or nearly so,
to the antenna polarization plane would be strongly attenuated). You should
instead aim at obtaining a circular polarization which causes a steady 3 dB loss
independently of the signal polarization plane.

To do that you must introduce an extra 90-degree phase shift by adding a
quarter-wavelength 50-ohm section to one of the two indentical-length 75-ohm
coax pieces.

All this in theory. In practice I doubt that, at 900-MHz, a system like that
would behave precisely as expected.

Regards.

Tony I0JX, Rome Italy

Antonio wrote:

Combining the two antennas using identical pieces of coax, you would
obtain a system polarized on a 45-deg slanted plane, that would not help
in receiving randomly polarized signals (signals that are orthogonal, or
nearly so, to the antenna polarization plane would be strongly
attenuated). You should instead aim at obtaining a circular polarization
which causes a steady 3 dB loss independently of the signal polarization
plane.

To do that you must introduce an extra 90-degree phase shift by adding a
quarter-wavelength 50-ohm section to one of the two indentical-length
75-ohm coax pieces.


Instead of this, why not use identical feedline lengths into a Tee and
position one cross polarised antenna 0.25WL in front of the other, to obtain
the 90-deg phase shift?

David, VK3BDK

Instead of this, why not use identical feedline lengths into a Tee and
position one cross polarised antenna 0.25WL in front of the other, to obtain
the 90-deg phase shift?

David, VK3BDK

Doing as you propose is possible, and would produce the same circular
polarization on the antenna boresight.

But moving away from the boresight, the circularity of polarization would
degrade (into elliptical polarization) faster than if instead using a
quarter-wavelength delay on one of the two feedlines.

73

Tony I0JX