"Roy Lewallen" wrote in message
...
There are a lot of factors that can potentially affect measurements. The
first and most obvious is impedance matching, which has to be done to
extract the maximum possible power from the antennas (assuming that this
is what you'll be doing when actually using the antenna) and delivering
a known amount of power to each. Another is to make sure the feedlines
aren't radiating or picking up signals, by using proper baluns, which
you've mentioned. Move the feedlines around and change their lengths,
making sure the results don't change. If they do, your baluns aren't
adequate. If you want quantitative measurements, you'll have to
carefully calibrate your signal strength meter at the power level
involved. Don't make assumptions about its linearity. Better yet would
be to carefully match the antennas to 50 ohms, then insert a step
attenuator in series with the Yagi in the 50 ohm environment and
increase the attenuation until it reads the same as the dipole. That
way, the FS meter linearity is immaterial (although the attenuator
accuracy is important). Then there's the possibility of signals radiated
off the back of the dipole being reflected from near or distant objects
which would affect the dipole's measured field strength more than it
would the Yagi's. The different widths of the forward lobes can also
cause unequal reflections. Although reflections can affect the forward
gain up to several dB, their impact on nulls or front/back measurements
is likely to be greater.

I'd do a couple of things. One is to build an NBS reference Yagi which
is easy to construct and has a well known and documented gain. It's also
easy to model. Modern modeling programs do very well with full size
Yagis. Measure this on your range and verify that the measurements agree
with its known properties. Another check would be to rotate the dipole
and see if its pattern is what it should be. And rotate the NBS Yagi and
verify that its pattern matches modeled results. Only after doing those
tests would I have reasonable confidence in other measurements made with
similar types of antennas.

Roy Lewallen, W7EL


I thought that NIST prefers a dipole antenna up to 1 GHz, and then a
waveguide-fed horn beyond that. I would think that a pyramidal horn would be
more predictable that a Yagi, and not unreasonably large, even for 400 MHz.
Plus, the construction is simpler, and the design is more robust over time
(i.e., it's easy to bend a Yagi element slightly, not enough to notice it,
but enough to shift performance).

Ed
WB6WSN

Mike,

Here is something to watch out for. I tried to measure the pattern for a 432
antenna. I had coax going from the shack to the transmit antenna, and
another coax going to the receive antenna. I measured complete garbage for
the pattern, until I realized that the receive antenna was picking up a
signal any time it was aimed at the transmitter COAX. I fixed the problem by
moving the signal source to be collocated with the transmit antenna.

One way to get accurate gain readings is to use the receive S meter and a
calibrated attenuator. For instance, if with the reference antenna you need
15db of attenuation to get an S9 reading, and with the target antenna you
need 27 db of attenuation to get S9, the gain of the target antenna is 27 -
15 = 12 db. Some S meters are more accurate for low signals; so, you might
want to use something like S3 for a reference.

Tam/WB2TT
"Peter" wrote in message
...
Hi all.

This may seem like a fairly basic question. But here we go!

I want to performance test a 436MHz high gain antenna. My plan is to
construct a simple dipole with a 1:1 balun for 436MHz as a reference
antenna
and construct another dipole with a 1:1 balun to receive the test signal,
measure it with a diode detector and a milli-amp meter (field strength
meter) at the shack. Do the calculation and have the antenna gain.
This seems to me to be fairly straight forward, but has anyone carried out
similar measurements and concur with the approach or are there are there
traps and pit falls that I need to be aware of. Or is their simply a
better
way?


Cheers

--
Peter Miles VK3YSF
Melbourne, Australia

Mike,

Here is something to watch out for. I tried to measure the pattern for a 432
antenna. I had coax going from the shack to the transmit antenna, and
another coax going to the receive antenna. I measured complete garbage for
the pattern, until I realized that the receive antenna was picking up a
signal any time it was aimed at the transmitter COAX. I fixed the problem by
moving the signal source to be collocated with the transmit antenna.

One way to get accurate gain readings is to use the receive S meter and a
calibrated attenuator. For instance, if with the reference antenna you need
15db of attenuation to get an S9 reading, and with the target antenna you
need 27 db of attenuation to get S9, the gain of the target antenna is 27 -
15 = 12 db. Some S meters are more accurate for low signals; so, you might
want to use something like S3 for a reference.

Tam/WB2TT
"Peter" wrote in message
...
Hi all.

This may seem like a fairly basic question. But here we go!

I want to performance test a 436MHz high gain antenna. My plan is to
construct a simple dipole with a 1:1 balun for 436MHz as a reference
antenna
and construct another dipole with a 1:1 balun to receive the test signal,
measure it with a diode detector and a milli-amp meter (field strength
meter) at the shack. Do the calculation and have the antenna gain.
This seems to me to be fairly straight forward, but has anyone carried out
similar measurements and concur with the approach or are there are there
traps and pit falls that I need to be aware of. Or is their simply a
better
way?


Cheers

--
Peter Miles VK3YSF
Melbourne, Australia

Perhaps. If I were going to measure a Yagi against a dipole, I'd first
try measuring a Yagi with known gain against a dipole. You might prefer
to measure a dipole relative to a dipole. I can't see what you'd learn
from it, but each to his own.

Roy Lewallen, W7EL

Ed Price wrote:

I thought that NIST prefers a dipole antenna up to 1 GHz, and then a
waveguide-fed horn beyond that. I would think that a pyramidal horn would be
more predictable that a Yagi, and not unreasonably large, even for 400 MHz.
Plus, the construction is simpler, and the design is more robust over time
(i.e., it's easy to bend a Yagi element slightly, not enough to notice it,
but enough to shift performance).

Ed
WB6WSN

Perhaps. If I were going to measure a Yagi against a dipole, I'd first
try measuring a Yagi with known gain against a dipole. You might prefer
to measure a dipole relative to a dipole. I can't see what you'd learn
from it, but each to his own.

Roy Lewallen, W7EL

Ed Price wrote:

I thought that NIST prefers a dipole antenna up to 1 GHz, and then a
waveguide-fed horn beyond that. I would think that a pyramidal horn would be
more predictable that a Yagi, and not unreasonably large, even for 400 MHz.
Plus, the construction is simpler, and the design is more robust over time
(i.e., it's easy to bend a Yagi element slightly, not enough to notice it,
but enough to shift performance).

Ed
WB6WSN

"Roy Lewallen" wrote in message
...
There are a lot of factors that can potentially ...............

Roy,

Does one have to worry about elevation? Generally the gain at 0 degrees
elevation is very low. Does this apply when the two antennas are facing each
other in plain line of sight? should he raise the transmit antenna to say,
15 degrees above the receiving antennas?

Tam/WB2TT

"Roy Lewallen" wrote in message
...
There are a lot of factors that can potentially ...............

Roy,

Does one have to worry about elevation? Generally the gain at 0 degrees
elevation is very low. Does this apply when the two antennas are facing each
other in plain line of sight? should he raise the transmit antenna to say,
15 degrees above the receiving antennas?

Tam/WB2TT

The gain at low elevation angles is low only if you're distant from the
antenna, and ground reflection is taking place -- EZNEC's far field
analysis assumptions don't apply to a typical antenna measurement setup.
Low far field gain at low angles is due to interference between the
direct radiation and radiation reflected from the ground. This can and
does occur when measuring antennas which are relatively close, but not
to the extent you see in idealized far field modeling results, where the
antennas are effectively an infinite distance apart. I don't have a
great deal of real experience in measuring antennas, but believe that
the common practice is to try to put the antennas high and relatively
close together. It shouldn't be difficult to get a pretty good idea of
the amount of signal cancellation you'd get for a given height and
spacing, by setting up and solving a pretty simple geometry/trigonometry
problem. Or you could model the two antennas over ground at their
proposed positions, with a source in the transmitting antenna and a
matched load in the receiving antenna, and look at the power delivered
to a load as the height and spacing are changed.

Fortunately, dipoles and moderate size Yagis have similar elevation
patterns for at least moderate angles relative to straight-on. That
means that the percentage of signal directed toward the ground and
therefore arriving at the receive antenna as a reflection will be about
the same for a reference dipole as for a test Yagi. So whatever
attenuation occurs, it should be about the same for the two antennas.
This wouldn't hold, though, for a very long Yagi or for high-angle
reflections from antennas which are high and close together. It's one of
the factors that make really good antenna measurements tricky.

Roy Lewallen, W7EL


Tarmo Tammaru wrote:
"Roy Lewallen" wrote in message
...

There are a lot of factors that can potentially ...............


Roy,

Does one have to worry about elevation? Generally the gain at 0 degrees
elevation is very low. Does this apply when the two antennas are facing each
other in plain line of sight? should he raise the transmit antenna to say,
15 degrees above the receiving antennas?

Tam/WB2TT

The gain at low elevation angles is low only if you're distant from the
antenna, and ground reflection is taking place -- EZNEC's far field
analysis assumptions don't apply to a typical antenna measurement setup.
Low far field gain at low angles is due to interference between the
direct radiation and radiation reflected from the ground. This can and
does occur when measuring antennas which are relatively close, but not
to the extent you see in idealized far field modeling results, where the
antennas are effectively an infinite distance apart. I don't have a
great deal of real experience in measuring antennas, but believe that
the common practice is to try to put the antennas high and relatively
close together. It shouldn't be difficult to get a pretty good idea of
the amount of signal cancellation you'd get for a given height and
spacing, by setting up and solving a pretty simple geometry/trigonometry
problem. Or you could model the two antennas over ground at their
proposed positions, with a source in the transmitting antenna and a
matched load in the receiving antenna, and look at the power delivered
to a load as the height and spacing are changed.

Fortunately, dipoles and moderate size Yagis have similar elevation
patterns for at least moderate angles relative to straight-on. That
means that the percentage of signal directed toward the ground and
therefore arriving at the receive antenna as a reflection will be about
the same for a reference dipole as for a test Yagi. So whatever
attenuation occurs, it should be about the same for the two antennas.
This wouldn't hold, though, for a very long Yagi or for high-angle
reflections from antennas which are high and close together. It's one of
the factors that make really good antenna measurements tricky.

Roy Lewallen, W7EL


Tarmo Tammaru wrote:
"Roy Lewallen" wrote in message
...

There are a lot of factors that can potentially ...............


Roy,

Does one have to worry about elevation? Generally the gain at 0 degrees
elevation is very low. Does this apply when the two antennas are facing each
other in plain line of sight? should he raise the transmit antenna to say,
15 degrees above the receiving antennas?

Tam/WB2TT

Thanks

Tam