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915 MHz Repeater

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Jsears
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915 MHz Repeater

Post by Jsears »

I have a Weatherwise 1080 weather station that operates on 915MHz. I sometimes lose connection between the sending unit and the console. I suspect that it may be due to an obstruction between the two. I know that the logical fix would be to move the sending units to a better location. However, due to certain factors, that is really not an option for me.

I am wondering if it is possible to use some sort of repeater that would get the signal to my console. If anyone has done this, or knows of a way to do this, I would certainly appreciate any help that you can offer.

Thanking you in advance..

Jim Sears
Jeffersonville, Ohio
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Re: 915 MHz Repeater

Post by AllyCat »

Hi Jim,

The straightforward solution would be to improve/increase the transmitter antenna/power, but that is not easy with these Fine Offset (manufactuer) transmitters and it may not be "legal" under the FCC regulations.

So the solution I would investigate is to improve the receiver/antenna. The Console is easy to open up and the fitted antenna is very primitive (a piece of wire). Also, I believe the receiver is a simple module which might be replaced by a "better" one. But even this requires a reasonable amount of technical knowledge.

The problem with a "repeater" is that it cannot transmit on the same frequency that it's receiving (at the same time). It needs to transmit on another frequency and/or at a different time. Apart from the need to change the receiver frequency (i.e. to a different module) there are few available frequencies that are legal. More normally, a repeater would receive the data, decode and "clean it up" (remove noise and errors) and then recode and retransmit, perhaps some seconds later. That is of course specific to the station characteristics and I'm not aware that anyone has done it.

However, it certainly would be possible (with sufficient technical knlowledge) but there still could be a problem with "starting" the system: When power is applied to the Console it "looks" for a transmitter signal and after finding one, only switches on its receiver for a brief period every 48 seconds afterwards (when the next signal is due) to save power when running on battery. It is probable that it would (at least sometimes) still see the direct (not repeated) signal first and then only continue looking for that one. It might be possible to send the retransmission with very little delay (to fit in the same "time window") but the receiver could then get "confused" by the two signals. An altenative would be to transmit the data after a long delay of somewhat less than 48 seconds.

Cheers, Alan.
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Re: 915 MHz Repeater

Post by N0BGS »

You could build a simple dipole antenna on the receiving side first and see if things improved.

I'm not familiar with the FO, but if the antenna is indeed just a piece of wire a resonant dipole would be much better.

At 915 MHz each leg of the dipole would be 3.06 inches--let's just call it 3.0 inches. :)

SO:

Code: Select all

________  ________
   3"        3"
If the antenna on the sensor array is vertically oriented you'll want to have the dipole vertically oriented as well. If horizontal, then horizontal. In other words, the same antenna orientation on both ends of the circuit.

If the current antenna is just a "piece of wire" it should be at least be a 1/4 wavelength long piece or wire. That would be 3 inches. If you don't want to mess with the dipole idea measure the existing antenna and confirm that it is indeed 3 inches long. If it isn't, make it so.

Kurt
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Re: 915 MHz Repeater

Post by AllyCat »

Hi,
N0BGS wrote: it should be at least be a 1/4 wavelength long piece or wire.
Yes that's generally very good advice above, except that the above (in bold) may be misleading. Theory says that the resonance (or "tuning") of a monopole/dipole reduces very considerably for lengths even only slightly longer than a quarter-wave. It's an example where "bigger is not better" so one should err on the short side. Also, the "bandwidth" is wider (i.e. its tolerance to minor errors in the length) if the diameter of the conductor is larger, so a "rod" is better than "wire".

A problem with the dipole is where to connect the second wire/rod. Strictly it should be connected via a "Balun" (balanced to unbalanced signal converter) but often it is just connected to an "Earth" as near as possible to the (monopole) antenna connection. Also, it could be worth trying a "folded dipole" as used in many "Yagi" antennas for TV and Rdaio. Take a length of thick wire just less than a full wavelength (say 11 inches) and fold it like a wire coathanger (with the "hook" cut off).

I don't know the exact detail of the 915 MHZ (USA) Console receivers because we use 434 or 868 MHz over here. A 1/4 wavelength at 434 MHz is more than 6 inches so the wire has to be bent around inside the enclosure. See the "red wire" in the "The inside of.." sticky thread to see some examples and confirm that we are talking about the correct (or a similar) station !

Cheers, Alan.
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Re: 915 MHz Repeater

Post by N0BGS »

AllyCat wrote:Hi,
N0BGS wrote: it should be at least be a 1/4 wavelength long piece of wire.
Alan:

Exactly right. Thanks for clarifying what was clearly an unclear statement. :)

What I meant by "at least" was that the antenna should ideally be at a minimum a 1/4 wave antenna or another design with greater gain and performance--not that a single element should be longer than 3.06 inches. A comparable statement might be, "I'd like my 20 meter HF antenna to be at least a 10 element yagi on a 60-foot boom!" (I wish :D ) Hopefully, that explanation hasn't made things even more confusing!

I think a dipole could be constructed by soldering one leg to a common ground (earth) point inside the FO console and the other to the existing red wire connection point. I purposely stayed away from baluns, matching networks, skin effect, etc in hopes of not scaring off the OP. I fear we may have already done that. :roll:

I looked at the post you referenced re: the red wire. In the photos in looks like it could already be close to 3 inches long. It also looks like finding a ground point may be a bit of a challenge.

Jim, have we driven you off with all our antenna "geek-speak?"

--Kurt
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Jsears
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Re: 915 MHz Repeater

Post by Jsears »

N0BGS wrote:
AllyCat wrote:Hi,


Jim, have we driven you off with all our antenna "geek-speak?"

--Kurt
Not totally, but what you guys have been saying is way over my head I think. I didn't think it would be nearly this confusing or difficult to do. I might venture into trying to upgrade the antenna, but I am somewhat hesitant to take the console apart. It's well out of warranty anyway, so tampering with it is no big deal. I'd just hate to trash it by doing something wrong. It works perfectly when it can get a signal.

Thanks for your help and suggestions. If anyone thinks of anything else, I'd appreciate your comments.

Jim
Jsears
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Re: 915 MHz Repeater

Post by Jsears »

OK..So I decided what the hell.. I took the case apart on the console. The attached photo is what I found. The red wire (antenna) measures about 3-3/16" long as best as I can determine. Can someone give me some guidance on what I should try? (sketches would be wonderful if possible) I am not opposed to having the antenna on the outside of the case if that is what is needed. I have Googled dipole antennas, and everything that I have found seems to be very large scale contraptions that are not practical for my application. Thanks again..

[img][IMG]http://i167.photobucket.com/albums/u127 ... 2nov0k.jpg[/img][/img]
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Re: 915 MHz Repeater

Post by AllyCat »

Hi,

It would be better to attach the photo directly into this thread (or the "Inside..." sticky) on this forum in case somebody else is researching the same issue in a few years time.

Secondly, beware that if you remove all the screws around the outside of the circuit board (to see/access the other side) you may get a fright because the LCD is only a push contact with the PCB. It probably can be put back together again (and work) but better avoided if possible.

The design of the transmitter module is rather a concern. Most have only three connections/pins (plus the antenna wire) for "Power supply +" , "Earth" and "Data", but yours appears to have twelve ! I've seen that with an old 868 MHz version but thought that all recxent models used a three pin type (915 MHz is not a legal frequency over here). A better substitute 3-pin module might be found, but the 12 pins include a "data bus" so would have to be replaced by an identical (but unknown) transmitter chip.

So I think it's only possible to (hopefully) improve the antenna somewhat. Yes, dipoles are usually larger because they are used for lower frequencies (i.e. longer wavelengths). Theory says that your element(s) should be 3 inches long, but antenna design is very much a "black art". Changing the red wire to a thicker one of a slightly different length (theoretically shorter) might help, but the important thing is that it should face the transmitter "side on" (e.g. the wire horizontal if the transmitter is above).

A similar wire (for the dipole) needs to be connected to an Earth near to the monopole wire connection and head in the opposite direction. There will be Earth on at least one of those (12) pins but which one? If you have a multimeter then look for the lowest resistance (max a few ohms) between each pin and the battery negative contact (with batteries removed). If you don't have a meter then could you post a "macro" close-up photo of the transmitter module and we may be able to identify something.

Cheers, Alan.
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Re: 915 MHz Repeater

Post by N0BGS »

I've been thinking a bit more about this. After seeing the inside of the FO, I think a full wave loop might be a better way to go than the dipole. It would probably be easier to tie one end to ground (earth) and has greater physical receive area than the dipole.

At 915 MHz the loop would be 1.09 feet long or 13.18 inches (335 mm). You could route it around the inside edge of the case. A loop can be square, round, rectangular, triangular, etc, so you have a lot of leeway as far as the actual shape is concerned.

(Antenna geek-speak ahead...) Although the impedance would be around 100 ohms giving a VSWR of around 2:1, I think it would be worth a try and would probably work somewhat better than the dipole.

Like so:
FO_Edit.jpg
What do you think, Alan?

--Kurt
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Re: 915 MHz Repeater

Post by AllyCat »

Hi,

Yes, well worth a try (if we can find the Earth pin). Or squeeze it more like a wire coathanger and you have a folded dipole. I believe that would have a higher impedance (Techspeak: normally it's the presence of numerous reflector/directors in a "Yagi" antenna that lowers the theoretical 300 ohms impedance down to around 75 ohms) but I can't say if that impedance would be better or worse.

In practice the 1 foot (~30 cms) loop will be a lot smaller than the red line shown on the diagram (the LCD and Console are quite large).

Cheers, Alan.
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Re: 915 MHz Repeater

Post by N0BGS »

AllyCat wrote:Hi,
In practice the 1 foot (~30 cms) loop will be a lot smaller than the red line shown on the diagram (the LCD and Console are quite large).
Right. In a square configuration each side would only be 3.30 inches--so smaller than in my diagram.

If the antenna impedance is 50 ohms--and I don't know that to be the case, but lets assume for the moment that it is--then I think a folded dipole will have too much of an impedance mismatch without some kind of matching network or balun. If the impedance is 50 ohms a folded dipole would give us a VSWR of 6:1 in this scenario. That's pretty high. <<--you can pretty much ignore that paragraph, Jim. ;)

The thing to do now is find the closest ground (earth) point to the existing antenna as Alan explained earlier.

--Kurt
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Re: 915 MHz Repeater

Post by Jsears »

Thanks again for all your help, guys. Sure wish one of you lived closer. :D
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Re: 915 MHz Repeater

Post by N0BGS »

Jsears wrote:Thanks again for all your help, guys. Sure wish one of you lived closer. :D
Jim:

You can do this! :) All you need is a low power soldering iron, a very small amount of electrical 60/40 solder, and an ohm meter--or a friend with those things. You can often get a multi-meter at Harbor Freight for nothing or next to nothing.

Set the meter to read ohms (resistance) or Ω. Touch the red and black leads together and look at the display. It will read 0.00. Once you've found 0 ohms with one meter test lead (doesn't matter which one for testing resistance) on any one of the negative(-) battery terminals and the other on one of those 12 pins that's where you'll solder one end of your new antenna. The other end goes to where the current antenna is.

Happy New Year!

--Kurt
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Re: 915 MHz Repeater

Post by AllyCat »

Hi,

OK, so I took the back off my old 868 MHz console to look at the receiver pins. Unfortuantely I couldn't take a useful photo because some time ago I had attached a ribbon cable onto most of the pins, with the thought that one day I might "hack" the data bus (but lost interest when I discovered that the more recent consoles use a simple 3-pin "analogue" receiver).

I think the Earth connection is the 4th pin up in the left column, but much more useful is that there's an Earth pad at the top-left corner, apparently marked with an inverted triangle (or perhaps it's a manufacturer's logo). I've marked the two points with yellow circles on the original photo (below). Simply soldering another 3 inch wire onto that earth (heading in the opposite direction to the original wire) to make a dipole would be the least drastic change. But you could solder in a new 13 inch loop as suggested, and then later cut out the middle section if the loop doesn't give any improvement.
FineOffsetRXgrounds.jpg
However, ensuring that the antenna "faces" towards the transmitter may give the greatest benefit. Note that most antennas of this type receive the greatest signal "side on". In particular, that applies to the loop antenna; looking through the coil like a "window" gives the direction of minimum reception. [Hint: Most simple antennas like these receive most strongly along two axes and have a "null" along the third axis. So it's possible to work out the "worst" direction from the symmetry of the antenna.]

Finally,, Kurt (and you) may not appreciate how difficult it is to do "trial and error" signal strength tests with these stations. The transmitter "broadcasts" for less than 100 ms after each 48 seconds of "silence" and worse, the Console doesn't report "loss of contact" ("--:--" on the LCD screen) until about eight consecutive messages have not been received. So even if you make the reception much worse (in the extreme by pulling the batteries out of the transmitter), it will still be more than 5 minutes before the Console tells you that it isn't working. :(

Cheers, Alan.
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Jsears
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Re: 915 MHz Repeater

Post by Jsears »

OK..I'm back. After all this time, I finally worked up the courage to look into trying the antenna thing. I got my meter and checked to make sure it zeroed by touching the leads together. But I couldn't get it to zero when I touched one lead to a neg battery terminal and ANY of the pins. The only way that it would zero is to touch the pins that Alleycat circled in yellow above together. Does that mean that they are both grounds, and that I can use either one of those for one end of the wire, with the other end soldered to the pin where the antenna is now?

Thanks again..
Jim
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