When you consider the reduction in the size of antennae when going
from dipoles et al to magnetic loops, I wonder why those
on wildlife safaris who have attached beacons to animals
use full size yagi-udas to track the animals?

Surely the same reduction in size should be possible
with mag loops for VHF?

On Sat, 8 Apr 2017, Gareth's Downstairs Computer wrote:

When you consider the reduction in the size of antennae when going
from dipoles et al to magnetic loops, I wonder why those
on wildlife safaris who have attached beacons to animals
use full size yagi-udas to track the animals?

Surely the same reduction in size should be possible
with mag loops for VHF?


Because the frequencies are high enough that a full antenna can be used.

The bigger/better the antenna, the more gain it has, useful for finding
those low power transmitters, and the more diectionality the antenna has,
which is really useful for tracking small animals.

A lot of direction finding in general has been at "low" frequencies like
the AM broadcast band (all those beacons, and of course broadcast stations
were once used for homing), and those used loopsticks. But low
frequencies don't work so well when you want a small transmitter and
antenna.

Loop antennas have been used for direction finding, but that seems well in
the past. The move has been to bigger antennas for the gain and
directionality.

Michael

On 4/8/2017 8:19 AM, Gareth's Downstairs Computer wrote:
When you consider the reduction in the size of antennae when going
from dipoles et al to magnetic loops, I wonder why those
on wildlife safaris who have attached beacons to animals
use full size yagi-udas to track the animals?

Surely the same reduction in size should be possible
with mag loops for VHF?

A magloop antenna has limited gain, but more importantly the direction
finding has to be done by pointing a null, losing the signal. So if
your signal is not strong to begin with the null is hard to use.

Still, the Yagi-Uda antenna design can pick up a much weaker signal
because of the significant gain. A magloop doesn't even have the gain
of a simple dipole.

--

Rick C

On Sat, 8 Apr 2017 13:19:53 +0100, Gareth's Downstairs Computer
wrote:

When you consider the reduction in the size of antennae when going
from dipoles et al to magnetic loops, I wonder why those
on wildlife safaris who have attached beacons to animals
use full size yagi-udas to track the animals?

Because the collar tags transmit at power levels in the low milliwatt
range. These days, the collars use energy harvesting power sources,
solar power, and supercap energy storage. Here's someone's detailed
masters thesis on the topic:
https://vtechworks.lib.vt.edu/bitstream/handle/10919/54022/Wu_Y_T_2015.pdf?sequence=1
When you're trying to track a very low power transmitter, that only
transmits a few times per day, you need all the antenna gain you can
get. The typical 3 element VHF yagis are not very directional, but
good enough to determine the general direction. When they get close,
they rely on visual sightings and not RDF.

Surely the same reduction in size should be possible
with mag loops for VHF?

VHF magnetic loops are certainly tiny but don't have much gain.

I've seen folding VHF cubical quads used along the shoreline, where
tracking distances tend to be longer than on land.

Photos:
https://www.google.com/search?q=wildlife+tracking+antennas&tbm=isch
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558