"John Crighton" wrote in message
...
On Mon, 28 Jul 2003 10:19:13 +0100, "Hans Summers"
wrote:


Back to your interference problem. Is your operating
channel frequency smack in the middle of the 40 Mhz band?
If so, maybe you could try a different set of crystals, so
that you operate as far away from everyone else as
possible. Or simply just borrow a different set of crystals
in case there is a weird mix going on, just to eliminate
that possibility.

We've recently got wise to that one and I've ordered a pair of xtals
from the *last* channel of the band. That's what we'll be running with
come the last week in August, when we're due up for the next filming.


Not necessarily the solution, I got bitten by that one once some 9 or 10
years ago when I used to fly radio controlled aircraft (in the UK on
35MHz).
It bothered me when someone else at the flying field had the same crystal
as
I did and I had to wait for them to finish flying before I could fly. So
bought another pair of crystals, the highest I could find, if I recall
that
was channel 83 (35.230MHz). Everything was fine for a while but a few
weeks
later I crashed after losing radio contact with my plane.

A little investigation (in between gluing the aircraft bits back
together)
found the cause. A channel vs frequency listing, compared with the MHz
printed on the crystal case revealed that the receivers were single
conversion superhets with 110KHz IF. Channel separation was 10KHz.
Clearly
with that setup, image rejection is practically negligible. So someone
transmitting on channel 61, 220KHz away, interferes with channel 83.
After
that I went back to my crystals on 76 smack in the middle of everyone
elses,
learnt to be patient if someone was already using the channel, and had no
more problems ;-)

Hans G0UPL
http://www.HansSummers.com


Hello Hans,
what brand/make was that R/C set that you had?

It was a Futaba, I don't recall which one. Very old and battered but worked
great. I've been away from aero-modelling for 9 years or so and don't know
what's new now - but at that time, the latest control sets were dual
conversion (avoiding that image problem) and PCM (Pulse Code Modulation)
compared to the old PPM like mine (Pulse Position Modulation). It always
seemed to me (perhaps somewhat subjectively) that the PCM tranceivers, which
were supposed to be more reliable, were in fact more prone to interference.
In the presence of interference the flyer seemed to have no control,
presumably because the incoming code was scrambled and the receiver couldn't
make head or tail of it. With the old fashioned PPM sets it seemed that
often even if interference was causing your servos to twitch a little you
still had some degree of control over it, often enough to turn it round and
bring it in closer whereupon full control would return and permit a hasty
landing.


You have reminded me of a silly incident at my model
flying club decades ago. One of the wealthier club
members was having all sorts of trouble with his
model, engine and radio gear so he flung heaps of
money at a ready made, ready to fly model with an
expensive four stroke engine, and a new expensive
all singing and dancing radio control set. First day
out with the new model and everything is going well
for him, he was doing stunts all over the sky.

My models were el cheapo sticks and tissue construction,
I couldn't afford nice covering film/material. I was
more interested in home built radio control gear. Radio
assisted free flight was more my style.
When the fuel ran out in my models I didn't mind
walking a few hundred yards if necessary to retrieve
my model. So I am off for a long walk with my transmitter
still switched on as one would. (The receiver gets
switched off first then the transmitter.)

The rich guy is stunting around and decides to buzz
me at low level about 20 feet above the ground.
His model flies over me and then nose dives into
the ground near by. I could here his servos twitching
away as I walked past the wreckage.
After I retrieved my model and switched off my
transmitter, I stopped by the little gathering at the
wreck site. The rich guy was operating his servos OK
and scratching his head. " I spend thousands on
my model and that ******* John Crighton comes
here every weekend with 50 dollars worth of homebuilt
junk and flies. It just isn't fair." "Moan...grumble..moan."
I didn't try to explain that his receiver got swamped. His
mates put the crash down to pilot error at low level, and
that was that.

Ha Ha, similar story here. I was a student at the time, had very little
money. Serves that rich fellow right for flying so low near you, sounds
dangerous. I used to have a Hi-Boy 4-channel trainer with 0.40 cu inch
2-stroke OS engine. When it came to me it had spent at least a decade in a
damp garage and required an almost complete fuselage rebuild to clear the
rot. Quite likely therefore that by the time I finally flew it, the original
dimensions weren't adhered to any longer. A particular weakness seemed to be
the nosewheel which was endlessly breaking up during my bad landings.
Solution to that one was just to remove it and fly the thing as a
taildragger.

The OS40 engine worked a treat, perhaps slightly overpowering the model: I
used to be able to take off and fly vertically immediately like the jet
fighters at air shows.

Had SO many crashes with that plane, and came to love it - just glued it
back together every time. In the end it was probably more glue than balsa
wood. A number of times I had to carry the pieces home in a plastic shopping
bag. By the following Saturday it was glued back together and generally
caused jaws at the flying field to drop. No-one could believe anything with
that much glue in it and such bad aerodynamics could actually fly. The wings
(balsa sheet covering polystyrene) had snapped 4 times and been repaired by
application of a fibre-glass bandage, adding significant weight of course.
It actually flew quite well too, all the aerobatics no problem and I
particularly used to like slow low level inverted fly bys at 10 ft or less.

Happy days ;-)

Hans
http://www.HansSummers.com


Fun and games, eh! :-)
Regards,
John Crighton
Sydney