Re: [Q] Antenna Measurements

APPEAL TO NEC-LIST MEMBERS: The topic brought up by Wael Ellithy
seems to be of considerable general interest. I have listed my
companies interests and activities within the message and believe
there must be quite a few people out there with experience/problems
and solutions in some of the areas highlighted. It would be
beneficial to all if we can generate some more discussion on these
and related topics, since you will notice that most of this work is
done in close association with NEC2 and other modelling techniques.
I think the NEC-LIST will be a suitable forum to discuss many of
these issues.

Wael Ellithy,

Unfortunately I need even more information to give you semi decent
answers (I have tried to answer some questions as best I can at the
end of this message):

a) Are you measuring the performance of the antenna by itself (not
mounted on the aircraft)?.
b) If so, is it a monopole type antenna (s). ?
c) Typical size?
d) The normally important measurement parameters would be impedance
(VSWR), efficiency and directivity (or gain which combines both of
these). Are you interested in all three?
e) You clearly seem to need radiation pattern information
  - Do you need only azimuth cuts or three dimensional pattern?
  - Once again do you need to know pattern of the antenna by itself
or including the aircraft?

We have performed extensive measurements and modelling of aircraft
antennas in the frequency range 30-420MHz. These include:
- HF, VHF and UHF antenna principal plane
pattern measurements on aircraft scale models.
- Simulation of the above to get 3D patterns
- Measurement of aircraft (monopole) antennas on ground planes in
comparison to quarter wave monopoles. These we use to determine
antenna azimuth pattern variations, but most importantly the antenna
radiation efficiency.
- Measurement of aircraft antenna input impedance (VSWR) on pseudo
aircraft shapes (Curved surface with fin and wing-like appendages)
above to obtain 3-d patterns.
-Simulation of full link with real spherical earth propagation and
using 3D pattern to estimate the full link performance during typical
aircraft profiles (air-air, air to ground at high and low level
flight for instance).
-Measurements on real aircraft link performance using a defined
ground station and tracking radars to provide accurate aircraft
position information. Orientation was obtained from aircraft
instruments.

These studies were done on a variety of aircraft (C-130, DC3/Dakota, fighter
plane, attack and transport heli's and a turboprop trainer aircraft)

I should hence be able to give you some more detailed advice if you
can provide more detail. I shall try answering some of your questions
below, by making some assumptions.

> I have the following questions
> ------------------------------
> 1] When we measure the gain of an antenna at a given frequency do we
> measure the power at this frequency, or the power of the (modulated)
> signal? Is the noise level of the environment a factor?

The best means is normally to measure the insertion loss between
transmitting antenna and AUT using the network analyzer (and possibly
an in-line amplifier to give sufficient sensitivity. When comparing
this to a "known" antenna insertion loss (at same position and
condition as AUT you will theoretically obtain the gain of the AUT
relative to the known antenna PROVIDED that they are both well
matched to the receiving port of the network analyzer i.e. insertion
loss includes mismatch losses whereas the definition of gain normally
refers to directivity + radiation efficiency (dB). Possible causes of
error:
 - Reflections is the major cause of errors. We normally calibrate by
moving a probe around in the measurement volume and record the signal
strength to give an indication of the effect of standing waves
associated with reflections. A time gating option on the network
analyzer may also be used, provided that the measurement bandwidth is
large enough (broad band antennas a transmit and receive side.

 
> 2] Is there a measuring device (watt meter) to measure the direct power
> (with no amplifying) transmitted/received from/by the antenna? Is this a
> correct way? What else can measure the gain of an antenna from its
> terminals from a power point of view?

Discussed above: the network analyzer can be calibrated to the input
and output ports of the AUT and transmitting antenna which will then
ensure that cable loss etc is "calibrated out" from the measurement.
The network analyzer works in terms of comparing received to transmit
power.
 
> 3] Is lower SWR is the main measure of a `good` matching circuit? Is SWR is
> the ONLY indication of how well the antenna perform at a given frequncy?

It certainly is an indication of a "good" matching circuit.
Definitely no the only indication of how well the antenna would
perform. Radiation efficiency, as well as pattern characteristics
must also be considered.
 
> 4] I understand we have to eliminate reflections. Is there a standard
> description of how the site should look like?

There are many recommendations and some EMC specifications (CISPR and
others) give specifications for radiated emission measurements. For
serious development work I believe one should use all the guidelines
(viewed in conjunction with practical constraints) set up a site and
evaluate/calibrate the site. This will give you the best "measure" of
your site, provide error limits and if your design was really bad I
suppose will tell you that you need a different site.
 
> 5] Is comparing dipole-dipole then dipole-antenna a good way of measuring
> the performance of the antenna? Is this a `fair` comparison to ALL
> antennas?

In principle nothing wrong with that approach as long as the dipoles
are well defined and errected (no unbalanced currents and minimal
interaction between the dipoles and the supporting/surrounding
structures. We have rather used a log periodic dipole array (LPDA)
since it does not need to be tuned at each frequeny and its more
directional nature reduces off axis reflections. As "standard"
antenna we use a telescopic monopole which gets adjusted to resonance
at each frequency.
 
Regards and hope some of this is useful. I shall appreciate input
from other list members, because I have undoubtedly left out
important considerations. We have been and are continuing to learn
more as we do this type of work

Andre Fourie

e-mail: fourie_at_odie.ee.wits.ac.za
Tel: intl + 27 11 716 5386
Fax: intl + 27 11 339 4610
Address: Dr. APC Fourie, Dept Elec Eng, PO WITS, 2050, South Africa
Received on Fri Mar 22 1996 - 15:54:00 EST