Paul I attach an email that Ian McPhee sent to this list last year. An
explanaation for the discrepancy. Jeff Farrow
From: aus-soaring-boun...@lists.internode.on.net
[mailto:aus-soaring-boun...@lists.internode.on.net] On Behalf Of Ian Mc Phee
Sent: Thursday, 10 December 2009 8:24 AM
To: Discussion of issues relating to Soaring in Australia.
Subject: [Aus-soaring] many OZFLARM comm (stick) aerials in use may well be the
wrong aerial (far too short)
Been troubled by some lack of performance in OZFLARMS for some time now and
only yesterday after talking to Mark Fisher realised many of the stick
communication aerials on OZFLARMS may well be wrong frequency. I do know 4 in
our club are wrong length and the fifth glider has one of those Mobile one long
range aerial which gives you green LEDs well beyong what the eye can see even 4
to 5 km!!!
My calculation for 900mhz is the 1/4 wave aerial 75 to 80mm long. At our club
4 the little white aerial say 40 mm long - far too short. It would seem the
little person in China threw all the say 900MHz and either 1800 or 2400MHz
aerials in the one box as they looks the same on the outside.
I urge as many people as possible this weekend pop the rubber cover off the
aerial and inspect the inside aerial length I am predicting 50% of the aerials
will be the wrong length - far too short. Job will take less than 1 minute. It
would be easy to add a little copper wire of the right length
While on Flarms etc I can say GPS (flat aerial) are best placed 25cm apart as
the GPSs can interfer with each other.
Also in motorglider if you are using the engine battery I would suggest fitting
a 22000uF capicator in the power lead near flarm
I will be very interested to hear the results of the aerial wore length
Ian McPhee
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06:13:00
----- Original Message -----
From: Paul Bart
To: Discussion of issues relating to Soaring in Australia. ; Chat DDSC
Sent: Sunday, April 17, 2011 12:31 PM
Subject: [Aus-soaring] Antena length for Oz Flarm
Hi
I have become concerned that the range at which my Flarm detects gliders is
not anywhere near specification. It often bypasses the green warning, going
straight to red. I understand that there are many factors in play, such as
shielding from the instrument panel, carbon etc., but I do not think that this
is the explanation in my case. So I thought I would invest in a better antenna,
but before I did that I had a look at the standard antenna provided. When I
removed the plastic covering I was surprised that the actual antenna was only
48 mm long. Yet the manual states, that the antenna at 1/4 wavelength should be
86 mm long. I also have the original Swiss Flarm (inherited from a glider
purchase from Europe) so I compared and its antenna is approximately 80 mm, but
as the cover is not removable, I cannot confirm the length of the actual
conductor.
Anyway I had a look at a few wavelength calculators on the web and given the
average frequency the Oz Flarm operates on is about 921MHz, the 1/4 wavelength
should be approximately 77mm and not the 86 indicated in the manual. I haste to
add here that I know next to nothing about antenna design, so a simple
application of the formula might not be correct.
In any case though, the length of the antenna on my Flarm is 48 mm, well
short of the manual stated 86 mm.
So I would like to ask those that know more then I on this topic
1) Is 48 mm the correct length (too long for 1/8 and too short for 1/4
wavelength)
2) If not, should it be 86mm as per manual, or 77mm as per calculated length
3) If mine is NOT an isolated case and the 45 mm length is not correct, are
we having a fleet of Flarms out there with much poorer range and hence
effectiveness that we should have.
Any help on the topic greatly appreciated
Cheers
Paul
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