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L-Band uplink antennas



I have tried two antennas for L-Band uplink so far, a 45el loop yagi with
wideband feed (G3JVL constructed) and a 5ft dish with classic disk/dipole
feed.  I have not been able to run a side-by-side comparison through AO-40,
but the dish outperforms the yagi on sun noise measurements at 1296 and is
a noticeably better tropo antenna.

On AO-40 over the last week I have been active on most available orbits
using L-Band uplink exclusively.  I have used the loop yagi most of the
time owing to convenience, and have needed 15-30W of tx power to get a
level of -10dBMB on the downlink.  At less favourable squint angles I have
not been able to achieve this level on the downlink, but have still made a
number of good QSOs.  On the one occasion when I used the 5ft dish, tx
power needed to be backed off about 5dB to achieve the same downlink level.
 The conclusion is, the dish is significantly better than the loop yagi.
(Just for grins, on this occasion I kepf backing off the uplink until I
could just detect myself on the downlink and ended up with needing only 5mW
in the shack).

Re minimum sizes of dish antennas, most textbooks refer to the 10
wavelength rule, ie the dish diameter needs to be at leat 10 wavelengths
for the dish to work properly.  This is probably true if the dish needs to
have absolute maximum efficiency or some minimum sidelobe specification,
but for amateur use we can certainly get good results with dishes smaller
than 10 wavelengths.  I have heard 5 wavelengths mentioned as a "minimum",
and indeed many stations have used 4ft dishes on 23cm for years with
excellent results.  My 5ft dish has about the right gain for its size and a
very clean sidelobe patters, much better than the yagi in fact.  So I
believe it is working well, and will give me some margin to be able to
maintain a good downlink signal at less optimum squint angles.


Charlie G3WDG

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