Antenna for Parallax RF Set

BJJohnson

I purchased the Parallax RF transmitter (27980) and receiver set (27981). I am not getting anywhere close to the 500' range.· I was thinking about switching the 1.5" antennas that comes with the set with 6" 433mhz antennas from digi key. Has anyone tried to increase the range by increasing the antenna size, if so does it work and how much more range will it pickup?·· Thanks Bob

SRLM

Antenna design is critical to the operation. A longer antenna will not increase the range. Instead, make sure that you're range is line of sight, that you're powering it with the appropriate voltage, and that the antennas are parallel.

Moskog

Well, I removed the little rubber antenna on top of the transmitter and replaced with an old car antenna, based on the 450MHz phone system that they used some time ago. I put the transmitter in a shielded box and added a bnc antenna connector to be able to connect the antenna cable. The original mobile antenna was probably a 1/4 wave antenna, about 166 mm, so that was again replaced with a 1/2 wave antenna made out of thin copper wire, trimmed for 433MHz, some 346 mm long. I've got a much better range for the signal even though the receiver remainds with its original rubber antenna.
First I put the antenna on top of the garage roof. That was not a good idea as the transmitter seems to be very senitive to static electricity. I killed two transmitters due to heavy hail showers last winter. So I decided to put the antenna down under the cealing instead and so far my transmitter still works despite some thunderstorms lately.
Attaced is one image of the antenna and the other show the 433 transmitter in the metal candy box.

KjellO


Post Edited (Moskog) : 5/29/2009 7:33:51 AM GMT

Leon

A 1/2 wave antenna like that won't work very well; if you want to use a 1/2 wave antenna you should use a dipole. It will work a lot better than a 1/4 wave, but is directional. Get the ARRL Handbook for Radio Amateurs if you want to learn about RF and antenna design.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Alan Bradford

HI,

Is that a tin (Steel) roof over your antenna?
If so it also acts like an attenuator and will reduce range.
If the transmitter is talking to one receiver then a good directional antenna will improve the range. No sense radiating power in directions away from the receiver.
Look at Yagi antennas in the ARRL Handbook. If you dont want to buy one maybe any of your Ham Radio friends may have one to look at.



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Alan Bradford ·N1YMQ

Plasma Technologies
Canaan NH 03741
www.plasmatechnologies.com

John Bond

Strange....
I’ve been playing with the same Links chips that Parallax used to fit to their transceivers (different frequencies though) and I get a lot more than double the 3000 feet specified for the IC. I’ve got reliable comms over 2.5Km (7500 feet) but I live in the African Veldt far away from any electrical or wireless interference so I assume the RF noise “floor” round here is exceptionally low.
Antellas are critical... Even 10mm shorter or longer makes a difference. Visit the Links site and read the datasheet on
http://www.linxtechnologies.com/Support/Application-Notes/
AN-00501 - Understanding Antenna Specifications and Operation. This ·gives all the dope for the·antenna on the parallax module.
The receiver also comes with a signal strength port. You should be able to measure the difference between when you’re transmitting and when not (the RF noise floor) and see roughly how efficient different antennas are.
But I warn you, RF is a black art. RF engineers are more confusing that the African witchdoctors. They talk of signals leaking and corners being too sharp, They tell you some RF law then say “but it doesn’t always apply”. ·Sometimes things work, sometimes they don’t. That’s why it’s usually wiser to buy an RF module complete with antenna rather that spending hours/days/months/years/lifetimes fiddling…
The tin should help so don't throw it away just yet (see the comments in the Links·notes on ground planes). Remember that each antenna is frequency specific so it won't help to use a ham antenna. There are Yagi's for 433 Mhz and·using one·on the receiver should·give you a good improvement, using one on the transmitter·could make the signal too strong to comply with FCC which would then make your device illegal in the USA.
If you do come up with a revolutionary antenna, patent it first then let us know.
Kind regards from Kwa Dukuza in South Africa
·
John Bond



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Post Edited (John Bond) : 5/29/2009 12:06:13 PM GMT

Moskog

Leon, sure a simple dipole would be much better and I been thinking that way too, but so far the range is good enough with this 1/2 wave pin, so·too save time I made it simple like that, this time.

Alan, no its not a steel or metal roof, its a kind of old fasion roof material including asbestos and other stuff, I don't think it affect radio·waves much. And in my case I cant use an antenna that is too directional because this transmitter is talking to two recievers in two very different directions. So this solution works for me but I'm sure a different solution, maybe a yagi as John mention, would fit better for Bob starting this thread.



KjellO




Post Edited (Moskog) : 5/29/2009 1:26:57 PM GMT

Leon

A 1/4 wave with four radials at 45 degrees should work much better than your 1/2 wave. The latter has a very high impedance at the feed point, and really requires a matching network. A 5/8 antenna, on the other hand, doesn't need a matching network.

If you think about it, an end-fed 1/2 wave antenna is really an end-fed dipole, but it obviously has to be properly matched to the feeder to be effective.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 5/29/2009 2:08:12 PM GMT

Moskog

I don't mean to hijack this thread by making Bob's problem be my problem but the discussion here·should be very useful for him anyway.

Leon, I do appreciate your advices on antenna design, guess I will try a 1/4 wave pin with radials instead.

So, in Bob's case, to help increase the range of his transmitter, he could solder a 50 ohm coax (like RG58 or so) to the TX module, or use a connector, and then connect a 1/4 wave antenna at the other end of the cable.

I found a calculator for such antennas here:

http://www.csgnetwork.com/antennagpcalc.html

I also found a description on how to make a simple antenna here:

http://www.mattstuff.nq.nu/antennas/quarter-wave.html

I do think the output impedance of the TX module is obout 50 ohm, the center core of the coax should be connected to the square where the original antenna is mounted, the shield should be connected to the ground or minus connector. At the other end the center core should be connected to the vertical pin, the shield to the four radials.

Leon

The radials (1/4 wavelength long) should point downwards at 45 degrees, to give about 50 ohms impedance. Easiest way to make it is to use a UHF or BNC bulkhead connector and solder them to the mounting holes. Then solder the antenna itself to the connector, pointing up, with the cable plug connecting to the bottom of the socket. It can be made more robust by mounting everything on a small Al plate, drilling holes for the connector and the radials. A long time ago I built a 144 MHz antenna for amateur packet radio like that, it worked very well.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 5/29/2009 4:12:14 PM GMT