OK, been doing some testing.
It appears that my old discone antenna is bad, or its location in the attic is poor. Either way I have placed a mobile magnet antenna to my galvanized roof (different roof), and reception is great. I also added the RS FM trap, the airport and marine channels are coming in better than before.
Do antennas give out?
I do have a Scantenna ready to put up, but I need help and I wont have a helper until later this year.
Antennas mounted in attics usually do not go bad. I guess loading coils could dry up and crack from the heat but you have a discone and they are pretty simple.
I'd suspect the coax at the scanner end has broken inside or it is like you said, the discone is just not in a good location.
I assume you mentioned a different roof to say that the roof your discone is under is not galvanized or metal roofing! That would sure kill most signals from reaching the discone in the attic.
Coax usually last a long time as well especially if it is in an attic and not exposed to rain.
The end that connects to your scanner, do you disconnect that or move it around much? That can cause a break in the inner conductor or outer shield which will cause problems. If you are using large coax like LMR400 or RG8, many will place a coupler at the scanner end and then attach a short 5 foot or so length of smaller coax so it does not place strain on the large coax or the scanner. The loss is minimal so that is the way to go if you are running large diameter coax from your antenna to your scanner. Put a short jumper coax of say RG58 not more then a few feet long at the indoor end.
In your case, you could end the large coax at the FM Trap and then use smaller stuff from the trap to the scanner. You really don't want a lot of weight hanging off the scanner especially a handheld one. If a mobile or base scanner, you can run the large stuff to the scanner but support it and the scanner somehow so neither can move when you operate the scanner and the weight of the coax does not bend down in back and eventually rip out of its connector or worse, rip the connector jack out of the scanner!
Is your discone used? If so, it could have spent several years outdoors but again, I think it is rare for a discone to fail. If it has all the rod elements, it should be fine.
What kind of coax are you using and how long to the discone?
You don't really want anything longer than maybe 25-30 feet if you are dealing with frequencies above 170 MHz. The loss in the smaller RG-58 type coax adds up very fast at UHF and higher frequencies so keep that stuff short.
If your coax run must be longer, try and use LMR400 from Times Microwave or Belden's 9913. I think you will find cheaper cost fittings for the LMR400 over the 9913. 9913 has a slightly larger center conductor diameter which prevents the use of most of the fittings made for RG8.
Of course if you do not have the needed tools to install the connectors on LMR400, I'd try and purchase it online with the length you need and the correct connectors already installed. Installing the connectors on the larger coax can be a pain in the rear if you have never done it plus some of the tools can cost more than the coax with fittings installed already!!
Many here also use a good quality RG-6 Quad Shield. It is 75 ohm though and the antennas and most scanners are designed for 50 ohms. Many will argue that it makes no difference but I've seen quad shield not bring in a signal that LMR400 will so I usually disagree with those thoughts and only use 50 ohm cable. If your intended signals are decent strength, then using RG6 quad shield is fine though and it sure is cheaper plus you can buy a kit with the compression connectors and the tool for compressing them to the cable really cheap. Most of that will be the typical F connector though like used for cable tv so you must still use adapters to get back to the antenna's connector and the BNC of the scanner. Each time you add an adapter, you loose about 0.5 dB of signal strength. If you loss adds to 3dB from adapters, you have now lost half your signal. Each 3dB of loss equals about half the original available signal. Same goes for gain, if you buy an antenna with 3dB of gain at the frequency you monitor, you have about doubled the signal strength. Discone's usually have 0 gain. That does not make them bad though especially if you are working with decent signal levels to begin with.
I'm glad to hear that the FM Trap seems to have helped.
I'm sure you tried adding the trap in the coax line from the discone.
At least you are making some progress!
