Posted: 8/27/2007 10:22:01 AM EDT
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For a variety of reason, I decided to install a 20 meter Hamstick at my house. Last night, I discovered that my coax wasn't long enough to reach the roof so I elected to put it in the attic for now. This didn't work out so well and I will describe. First, I made 4 radial wires and cut them to exactly 16'5" which should center it at 14.250. I attached these to the base and laid them out in my livingroom. I then attached my antenna analyzer and set about trimming the whip to center at the same 14.250. I got it so the SWR is about 1.2 at that point but doesn't go above 1.5 between 14.150 to 14.350. So far, so good, right? I then too the assembly up to the attic and bolted the thing to a 2x4 roof brace. I strung the radials out in 4 different directions. They are not contacting the walls nor any metal. I went back to the kitchen and fired up the radio. The first thing I noticed was the incredible amount of noise. I was getting a noise floor of S5 to S7 with this antenna. I switched to my roof mounted loop and had only an S2 noise floor. Hmmm... not good. I then turned the dial with the loop antenna until I found a good signal. I then switched to the Hamstick and it wasn't nearly as loud. It was a weak signal pushing through a lot of noise. Hmmmm... not good. The radio I'm using is the 746 Pro which has an internal tuner. I turned off the tuner and checked the SWR and it was off the scale. I then attached my analyzer to see what was happening. Here's where it gets really strange. As expected, the SWR was maxxed out until I started turning the frequency dial. I found the dip at about 14.300. The strange part is that the dip only brought it down to about a 4:1 SWR. I'm using 65 feet of low loss, RG 213U feed line. The antenna isn't contacting anything. Noise is high, signals low and not naturally resonant anywhere. It can be tuned with the tuner, but that's pretty dramatic. My guess is that there's something in my attic that is causing this. The inside of my siding has a reflective finish that looks like an aluminum foil. I'm thinking that is reflecting signals inside the attic and probably shielding it from what I'm trying to hear. There are also metal seams between pieces of siding so maybe one of the radials is too close to that. Until I can get this thing outside, I'm not giving up. I think it could make a good low profile antenna at my townhouse and would be good for DX contacts. As it is, its useless. |
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A vertical will inherently pickup more noise since most urban noise is vertically polarized... Second, you have way to much metal going on in your attic to even try putting the antenna there. You should really try to get as much separation as possible from anything metal. Including your reflective barrier on your roof. A short vertical is not very forgiving in regards to bandwidth so you will need to carefully trim the radials to give the match where you want it... Get it outside, away from as much metal as possible and try again... BTW...if you find the noise too high for your liking, build a magnetic loop for receive and transmit on the hamstick...many modern transceivers have receive only antenna connections on the back of the rig. |
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I had a similar issue when attempting to install a dipole in a condominium. Turns out that underneath that stucco was chicken wire. Played havoc with the antenna. On another note, I've found that you can jumper two different sections of coax together (without any noticeable attenuation) with a female-to-female connector in order to get that extra bit of reach you may need from time to time; saving you from having to purchase a longer single run of coax. |