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March 9th 04, 02:35 AM

In article ,
(Keith.fisher) writes:

Hello,
(Disclaimer - I am a novice electronics buff - just a few small
projects)
I have a small keychain remote used for advancing powerpoint slides.
It has an integrated laser pointer that is very useful in presenting
which is why I really want to get this to work.The remote is stated to
be 433 mhz. The reciever is a small box about the size of a pack of
cards.
Right now it has a range of about 40 feet (the product spec says 100')
I think it may have to do with the enviroment. We use it in a church
and there are several wierless mics ect.
I opened the recever and inside was a stiff wire antena bent into a
square about 1.5 inchs per side. It is soldered onto the board.
So I am wondering how I can increase the range of the
transmitter/recever. Can I just solder a longer antena?

I would advise against soldering anything into it. What you describe
is a typical "patch antenna" that uses part of the existing PCB as a
sort of "ground plane." The impedance of the patch antenna will be
matched to the input impedance of the receiver's first stage and is
not guaranteed to be in the 40 to 90 Ohms impedance of, say, an
almost quarter-wave wire typical to an HT.

Just soldering in an extra wire length MAY work but there is no way
of telling - without a lot of other information about the receiver - what
the effects will be. It MAY be better but, in all likelyhood, will be
worse.

The RF environment of any inside of any building is unpredictable
for anything but gross results. All sorts of conductive things can
affect the RF field causing in-phase reinforcement of wavefronts or
out-of-phase attenuation of wavefronts.

There's a very easy test to do with no more than a coat hanger wire
stuck in a small block of wood. [done by a friend of mine needing to
do a presentation in a larger room than originally scheduled, some
years ago, but also for projection purposes] Cut the wire about
12 1/2 to 13 inches and stick it into the wood block (the block is an
insulator). Position it somewhere between the transmitter and
receiver by trial and error.

This is the most simple form of "passive repeater," in this case a
half-wavelength resonant thing at 433 MHz. All it does is to mess
about with the already-mixed-up wavefronts in the room (fairly well
fixed by the building) such as to move a wavefront reinforcement
towards the receiver. There's no amplification with that, of course,
but it can cancel a deep attenuation at the receiver's location.

Does the room contain metal folding chairs? I've heard of those
affecting wireless devices such as microphones. The dimensions
of straight-line conductive objects in those chairs are close to a
full wavelength at that frequency and many would be expected to
change the overall wavefront pattern. If the effects are different when
the chairs are folded and stacked, that is proof that they do.

Passive repeaters (of a more elaborate kind) have been used in
"fringe areas" to improve reception, especially for a community that
is behind some moderate hills. Those are just a high-gain antenna
on one side of a hill connected by a feedline to another high-gain
antenna on the other side of the hill. It works well enough without
any electrical power supply of any kind.

A passive repeater Yagi antenna pair can be simple enough and
small enough at 433 MHz even if each Yagi has three lengths
(reflector, dipole, director) or just two (dipole and reflector). No
power needed, just a short coaxial cable beteen each Yagi's
dipoles. The Yagis should be at least a foot apart and preferrably
at right angles (or slightly greater) to each other. That can be put
at the side of the room or hall, one Yagi pointed towards the normal
location of the transmitter, the other pointed towards the receiver
location.

Such a Yagi can be built with #12 or #14 bare solid copper wire
common to household electrical wiring. The dipole would be 12 1/2
to 13 inches long, the reflector about 3/4 to 1 inch longer than the
dipole, the director element about 3/4 to 1 inch shorter than the
dipole element. Each element can be spaced 4 to 5 inches apart.
The elements could be mounted on-end, stuck in partway drill
holes made in a piece of 2 x 4. Exact dimensions aren't critical...
unless one wants to argue details to the end of time in newsgroups. :-)

If possible I would like it to be somewhat omni directinal.

A passive repeater setup isn't precisely omnidirectional but it does
allow a lot of freedom of movement in the general transmitter
location.

The product spec says it
is (is that why the square ant and not a long skinny one???)

No. The internal patch antenna is fixed in shape by dozens of other
factors. The fact that it is, in its largest dimension, smaller than a
quarter wavelength makes its pattern approach, but not reach, that
of an isotropic or point-source antenna.

Also while looking online I see places that sell 433 mhz ant's for
about 15$- They are made (I think) for handheld walki-talkies(??)
would somthing like this help me.

No. A quarter wave whip for 433 MHz can be made from #14
bare solid copper for pennies and is self-supporting...and can
be soldered readily for adapting it to an adapter. I would advise
trying to fuss with either receiver or transmitter's antenna unless
you have a lot of experience under your belt and have access to
UHF-range test equipment.

A number of experienced people at the manufacturer have already
spent a lot of time on design to get the product in that shape and
small size. It may not be optimum (chances are it is a mass of
compromises) and may not be the best in ALL environments, but
fooling with it can result in it not working at all well afterwards.

Len Anderson
retired (from regular hours) electronic engineer person

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