January 2012
I live where cellular reception is poor, and I depend on a cellular modem, amplifier, and Yagi antenna (Wilson Electronics 301111) mounted on our chimney for an Internet connection. In the summer it works pretty well, but in the rainy winter, the signal deteriorates significantly and our connection speed drops. If we're using the fireplace, and the weather dries out for a few days, the signal improves. However, if it's rainy but we're not using our fireplace for a few days, the signal also improves.
My best guess as to the reason is that the combination of smoke from our fireplace and moisture from rain or dew are coating the antenna with a film that is conductive enough to create a partial short across elements of the antenna, causing some attenuation of the signal. The antenna is mounted about six or seven feet above the chimney cap, and it's not really practical to mount it higher without going to a much more complicated mast setup with guy wires.
I'd like to know if I can spray some insulating coating on the Yagi antenna that will prevent the film contacting the conductive parts of the antenna, but that also will not in itself attenuate the signal. Something like WD-40 might work for a while — causing water to just run off — but I want something more permanent, like maybe liquid tape that hardens into a rubbery coating. I just don't know if that type of coating might in itself attenuate the signal.
Does anybody have any advice on how I could seal my antenna without attenuating the signal by an appreciable amount? I'm also open to other ideas, except to go to a different connection technology (DSL isn't available, T1 is too expensive, satellite is worse than what we've got now).
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Hi, Gary! I am a ham (amateur radio operator) and here is what I know about Yagi antennas (actually Yagi-Udi antennas, but that's a story for a different time).
I would advise against WD40, as it will almost certainly mess up the impedance where it matters (at the driven element). But first, a bit about Yagis.
A yagi antenna is constructed such that the center point of all the elements (other than the driven element) is grounded to the mast. So shorting the elements to the mast is done at the factory for all but the driven element.
The driven element is the one that connects (usually via some sort of matching network) to the coax, and my bet is that THAT is where you need to focus your attention.
Also, there are affects on the antenna from 'nearby' objects, including 'ground' (or anything horizontal below (or even above) the line of sight of the antenna). If the 'nearby' objects are enough wavelengths away from the antenna and not in the 'line of sight' then their impact should be minimal.
However, 'ground effects' can be helpful or hurtful, even out to multiple wavelengths away from the antenna. This is where 'higher is better' comes from. By the way, the higher the antenna is above 'ground' the better the antenna works. However, if you mount an antenna higher above the earth, but your rooftop is in the line of sight to your other antenna, your 'ground level' is the roof, not the ground you walk on! (Especially at the frequencies we are talking about here). So, all other things being equal, it is better to put the antenna on the side of the house closest to the antenna you are 'talking to'.
For more information, I recommend the ARRL Antenna Book.
Anyway, back to the question:
There are a few places where you are likely to lose signal strength:
1 - At the end of the coax, or inside the end of the coax. If water gets into the coax, it will change the characteristics of the coax such that you WILL lose (lots of) signal strength. If the coax terminates in a connector of some kind, then you want to protect the connector from water. If the coax does not terminate in a connector, then does it end such that the wire angles DOWN to the end of the coax or up? If up, then you have a great place for water to get in to the coax. If possible, orient the coax such that water on the coax will not travel toward the open end of the coax. In either case, protect the end of the coax, and all connections, with something non-'conductive' at RF (at microwave, to be more precise). Either some sort of silicone sealer, or use a plastic margarine tub or similar (assuming the antenna is far enough from the chimney to avoid melting the plastic!). Before using a sealant, make sure it is not conductive at microwaves (PVC is notorious for being conductive there). If in doubt, one partial test is to place some of the material, along with a cup of water, in a microwave and cook for a while (start with a few seconds, if the material in question does not get hot, go for longer). If the material will 'bother' your antenna then it will likely get hot (or at least warm) in the microwave. Do this same experiment with your margarine tub also!
2 - At the matching circuit. What sort of matching circuit is being used will determine how easily affected it will be by water and so forth, but generally the matching circuit wants to be dry. Some stay drier by design than others. In any case, the margarine tub from 1 could probably be placed over the entire driven element and coax end. Another possibility would be to build a 'roof' for the antenna, but if you use metal or anything affected by microwaves (see experiment in 1 above), then you need to keep it a reasonable number of wavelengths away from the antenna. Since we are talking microwaves, or at least 900MHz, then that isn't so bad as trying to build a roof for an HF antenna! If I remember correctly, 5 to 10 wavelengths away should be more than adequate. At this point, I should recommend the ARRL antenna book, as it has MUCH more info, including the frequency to wavelength formula and how far you should put the 'roof.' You might get away with 1 to 2 wavelengths away, IF the 'roof' is not resonant at the frequencies of your internet provider. This is where the ARRL Antenna Book is probably your best bet.
3 - At any point between the antenna and the receiver where water can enter the coax.
Since you say that the fireplace seems to exacerbate the issue, then you probably have a combination of soot/whatever plus water in the matching circuit, coax, or both.
Random ideas and thoughts:
Can you move the antenna further from the chimney?
Perhaps you need to shield the antenna from the smoke as well as from the rain.
Rather than moving the antenna higher, is there anywhere else where you still have line of sight to the cell tower (or whatever is the other end of your radio circuit)? In other words, if you could protect the water-sensitive areas of the antenna, AND move it to where it does not pick up 'stuff' from the chimney that would be a win-win. One concern is water dissolving stuff from the fire that then attacks and corrodes the antenna!
Given that we are working with at least 900MHz, as long as you have line of sight to the other antenna, anywhere on your roof should be almost as good as anywhere else. In fact, if you have a clearer shot 6 feet lower, then 6 feet lower is better, as long as you don't move your roof, (or neighbors roof), into the 'ground effect' area) So, if you can mount the antenna away from the fireplace and still get a good signal, that's probably better all the way around. In fact, as I say, start out with the side of the house closest to the other antenna.
Bottom line: weatherproof the coax, especially the ends; protect the matching circuit from excessive water; keep the exhaust from the chimney from mixing with water and depositing on the antenna (possibly by moving the antenna elsewhere).
Water combined chemicals in smoke from a fireplace could get into connectors and cause signal loss in Gary Rathburn's antenna system. He notes, "If it's rainy and we're not using our fireplace for a few days, the signal also improves." Perhaps the rainwater by itself flushes some soluble chemicals out of the connectors. It's unlikely water on the antenna or rain droplets cause a problem.
During a dry period, wrap exposed connectors with Coax-Seal or Scotch-brand "2228 Moisture Sealing Electrical Tape." Better yet, replace the connectors and the cable and then seal the new connectors. If moisture and chemicals from smoke got into a connector, they probably got into the cable too. Also, always create a drip loop to keep water from running down cables and into connectors.
If you try to "seal" the antenna, you can de-tune it and cause signal loss. Antennas are manufactured to work outdoors in bad weather.
Yes, fire makes carbon smoke a conductive. I'd try a plastic bag to see if it corrects it. Or, place a shield one foot under the antenna.
I suggest you try wrapping the elements of your Yagi antenna with PVC electrical tape. It might protect the antenna enough to provide the necessary improvement. As a young "shop gopher" in a two-way radio shop, I earned many hours of minimum wage wrapping two-way base station antennas with electrical tape. The owner of the shop insisted it reduced received noise and extended the life of the antenna.
It sounds like you have a faulty cable and/or cable connector at the antenna. I would replace the cable with Wilson 400 low-loss cable, and put a boot on the connection at the antenna to keep out moisture.
Additionally, the Wilson 301111 Yagi is quite directional; try aiming.
If it has been several years since you installed the antenna, maybe more cell towers have been built in your area.
I think you should look at the connectors. They may not be water tight!
Electrical duct butter, electrical duct seal, and coax seal can all be used to encapsulate the connector and socket area to seal it from water inclusion.
I’ve used Beam antennas up in the air for decades with good results.
