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