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

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

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

Thanks for all the information it has been of great assistance.

Cheers

Peter

--
Peter Miles VK3YSF
Melbourne, Australia


http://members.optushome.com.au/vk3ysf/main.htm