In message <memo.911093@cix.compulink.co.uk>, Graham Clarke
<gemin@cix.compulink.co.uk> writes
>Speaking with my HAM hat on, to trim the tail dipole correctly you should
>contact the local radio amateur club. Many of them will have VSWR meters
>(which is wot you measure and optimise) and some will have them combined
>with tunable signal generators. The kit is hand-held portable size so can
>readily be brought to your site. The popular 2 meter band is only tens
>of Mhz. above the airband, so these devices are easily set up for
>optimising at mid-airband (taken as 120Mhz usually).
Don't expect a perfect match across such a wide bandwith you might be
quite horrified with the standing wave voltage at the band edges and it
depends just where the meter is inserted in the line. I prefer to use a
field strength meter and trim for best level at midband.
Geometric mean of 118-137Mhz=127.1456 or more simply 127.15 assuming you
are trimming a communications antenna or 112.900 if you are trimming a
navigation antenna. NB It's one of the reasons 121.50 was chosen as the
distress frequency.
>
>Interestingly it would not seem to be vital to use horizontal
>polarisation for VOR -after all the dual Icoms still use the same "rubber
>duck" (unless there is something devilish clever inside !) No doubt it's
>not so effective that way, but I have never seen a comparison.
>Polarisation tends to lost along low angle paths anyway for various
>reasons and might even pick up a circular component, so there would not
>even be a null plane when it reaches you.
Some VORs are circular polarised at source so they will only be 3dB down
whether the antenna is vertical or horizontal. Also rubber covered
helicals have odd polarisations so maybe that's why Icoms work equally
well with either linear polarisation.
I think the reason why "proper" nav antennas are swept back is to create
a forward biased cardoid radiation pattern. Based on the assumption that
those ahead will be getting stronger in signal level and the most useful
fixing is with two or three at right angles. I would imagine they will
be quite a few dB down for VORs immediately aft the aircraft compared
with those ahead and abeam. A vertical will "see" them all equally
though a tail mounted unit will suffer some blocking by the fuselage,
engine and occupants. Maybe a belly mounted crossed dipole made from
tape with 0.25 wave phasing section to give circular polarisation might
be an answer. The main problem with antennas embedded in epoxy/glass is
they can be very lossy compared with free space wire antennas.
--
Graham E Laucht
|