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

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?

You may want to use a small directional antenna on both ends. The ARRL
handbook gives a standard refferance gain antenna that you can use on the
end that you are going to replace with the antenna you are building. The
antenna is suspose to have a certain ammount of gain if constructed
exectally as given.
The directiona antennas will help eliminate some reflections. Also make
sure they are far enough apart to be out of the near field.

We just made a range dipole for 900 MHz. The balun we ordered from
Minicircuits (pay careful attention to pinouts) was a 1:1. It resonated
well once we took into account that the polyethelene mounting block
imparted a velocity factor which shortened the rods slightly.


Peter wrote:

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








--
Joe Leikhim K4SAT


"Jazz is not dead. It just smells funny." -F.Z.

Peter

You are right on track, that is exactly how measurements are conducted on
antenna ranges. Of course you will have to convert the linear voltage change
from your dipole reference to what is measured from that 'high gain' antenna
to dB's.

Enjoy, Hugh KF6HHS
Retired, now life moves at my pace.
please note spam filter

In message , Peter
writes
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?

I've done it on 1296MHz in a similar way. However I borrowed a
calibrated step attenuator and took the difference in readings between
the reference dipole and the antenna under test as the gain for the same
detector reading to avoid nonlinearities. You can also measure antenna
gain using sun noise on 70cm, but that's a bit trickier.

Brian GM4DIJ


--
Brian Howie

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

Peter wrote:
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

"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

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

Peter, VK3YSF wrote:
"Or is there simply a better way?"

The 0.7 mtr wavelength is an advantage. The separation between antennas
only needs to be a few mtrs to be "far field".

The tested antenna can be side by side and tip to tip with a reference
antenna, dipole or otherwise, of known gain, and the interaction should
be small. The antennas will be in each other`s nulls.

Reception or transmission from the tested and reference antennas may be
compared as reciprocity rules.

Of course, the same power or field strength must apply to the tested and
reference antennas for validity. Losses in associated systems must be
the same for the tested and reference antennas or any differences must
be accounted for.

Ther antenna sitings likely won`t correspond to free-space, so a
reference of 0 dBd or its equivalent of +2.2 dBi won`t be precise, but
the comparison between the "known" reference and tested antenna should
be valid if the antennas are operated in similar circumstances.

It isn`t necessary to build a field strength meter if a receiver with a
good signal strength indicator is available and a calibrated generator
is available. The off the air signal can be replaced with a signal from
the generator and its strength can be read from the generator`s
attenuator. I`ve done it countless times and it works well.

Best regards, Richard Harrison, KB5WZI