433 MHz antenna - help needed

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mikluhaf

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FL
Hi everyone,

Not really a radio antenna, but hopefully gurus can help me.

I'm not a radio/electronics guy (just a computer one), so my knowledge of antennas is pretty limited.
I have a 433 mhz data acquisition base unit with number of remote wireless temperature sensors. Couple of them are too far from the base + walls + something else. I opened base unit and found a single dipole receiver antenna inside (about 2.5" each side, not helical).

Here is what I'd like to do. Run a 6-10 ft coax cable through the roof and make a simple half-wave antenna (=vertical piece of 12 ga wire, 6.5" long). Roof is metal, so I can attach coax shield to it.

My question is. Before I completely break base unit apart, I'm wondering if this will improve my reception (at least there will be no wall). How long can coax be (standard old TV 75 om cable)? Do I need anything else, for example, to adjust its impedance (well, I don't have anything to measure it, anyway)? There are two connectors on the PCB, does it matter which one will be soldered to coax shield (suspect that it has be ground one, but how to tell)? Do I need to cut original dipole off, or I can leave it?

Any other suggestions (like using old TV antenna). Yagi antenna won't work because I have sensors everywhere, so it has to be omnidirectional.


Thanks in advance,
Mike
 

fineshot1

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Hi everyone,

Not really a radio antenna, but hopefully gurus can help me.

I'm not a radio/electronics guy (just a computer one), so my knowledge of antennas is pretty limited.
I have a 433 mhz data acquisition base unit with number of remote wireless temperature sensors. Couple of them are too far from the base + walls + something else. I opened base unit and found a single dipole receiver antenna inside (about 2.5" each side, not helical).

Here is what I'd like to do. Run a 6-10 ft coax cable through the roof and make a simple half-wave antenna (=vertical piece of 12 ga wire, 6.5" long). Roof is metal, so I can attach coax shield to it.

My question is. Before I completely break base unit apart, I'm wondering if this will improve my reception (at least there will be no wall). How long can coax be (standard old TV 75 om cable)? Do I need anything else, for example, to adjust its impedance (well, I don't have anything to measure it, anyway)? There are two connectors on the PCB, does it matter which one will be soldered to coax shield (suspect that it has be ground one, but how to tell)? Do I need to cut original dipole off, or I can leave it?

Any other suggestions (like using old TV antenna). Yagi antenna won't work because I have sensors everywhere, so it has to be omnidirectional.

Thanks in advance,
Mike

Hooking up an external antenna would make this device out of compliance with the FCC Part 15
Rules that it is supposed to be in compliance with. In addition this device could also cause
interference to other Part 15 and Non-Part 15 devices causing even more problems to others
such as amateur radio operators in that segment of the ham band. This is also an extremely
low power device so if you had too long a run of cable to an outside antenna it would probably
be a loosing situation for you - not likely to be worth the effort.
 
Joined
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Hooking up an external antenna would make this device out of compliance with the FCC Part 15
Rules that it is supposed to be in compliance with. In addition this device could also cause
interference to other Part 15 and Non-Part 15 devices causing even more problems to others
such as amateur radio operators in that segment of the ham band. This is also an extremely
low power device so if you had too long a run of cable to an outside antenna it would probably
be a loosing situation for you - not likely to be worth the effort.

The OP specified it was a receiver he wanted to modify by mounting an external antenna; he won't be fiddling with the remote sensor (transmitter) portion.
 

kb2vxa

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Basic laws of physics still apply. At those frequencies transmission line loss soars so you must use "high quality" cable and keep it short. The antenna end poses few problems (once you get the design right which you haven't) but the receiver end sure does. The simple statement of not knowing which land on the PCB is which OR even if it's an ungrounded balanced input tells me you're thinking way over your head. If you want to get the entire system up and running you must rethink the entire installation. Like they say in the TV commercial location location location. Clue: dipoles are bidirectional 90 degrees to the axis and don't forget to think in 3 dimensions.
 

fineshot1

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Messages
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The OP specified it was a receiver he wanted to modify by mounting an external antenna; he won't be fiddling with the remote sensor (transmitter) portion.

Ok - sorry I mis-understood. I would still not attempt the antenna on the roof idea due to the length of cable required. Probably better to try a home brew 1/4 wave ground plane or perhaps a HT ducky or similar very close to the receiver unit to keep the cable length to a minimum. With the flea power of those small remote tx units keeping the loss's (on the rx unit cable & antenna)down to an acceptable limit will be tough.
 

davidmc36

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Joined
Nov 14, 2004
Messages
1,861
Location
South East Ontario
Hi everyone,

Not really a radio antenna, but hopefully gurus can help me.

I'm not a radio/electronics guy (just a computer one), so my knowledge of antennas is pretty limited.
I have a 433 mhz data acquisition base unit with number of remote wireless temperature sensors. Couple of them are too far from the base + walls + something else. I opened base unit and found a single dipole receiver antenna inside (about 2.5" each side, not helical).

Here is what I'd like to do. Run a 6-10 ft coax cable through the roof and make a simple half-wave antenna (=vertical piece of 12 ga wire, 6.5" long). Roof is metal, so I can attach coax shield to it.

My question is. Before I completely break base unit apart, I'm wondering if this will improve my reception (at least there will be no wall). How long can coax be (standard old TV 75 om cable)? Do I need anything else, for example, to adjust its impedance (well, I don't have anything to measure it, anyway)? There are two connectors on the PCB, does it matter which one will be soldered to coax shield (suspect that it has be ground one, but how to tell)? Do I need to cut original dipole off, or I can leave it?

Any other suggestions (like using old TV antenna). Yagi antenna won't work because I have sensors everywhere, so it has to be omnidirectional.


Thanks in advance,
Mike

Have you tried turning the transmitters 90 degrees? I have a wind speed/temp/humidity/rainfall unit and the reciever kept losing signal after a few minutes. I would go and reset it and it would happen all over again. I went out and rotated the transmitter on its round mount 90 degrees and it has been rock solid for about a year now. Have not even needed to replace the batteries.

Or how is the antenna mounted in the reciever? Could you rotate it to get the best compromise of alignment with the transmitters?
 

mancow

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Oh fooey... It's not freaking rocket science... why can't we all just tell the guy to desolder the internal antenna connections, hook up a portion of good coax 75 or 50 ohm and an antenna as he already mentioned. It will work fine. Must everything be over analyzed?
 
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