|Figure 1, Figuring out where the damn antennas are going to go.|
I also need to keep in mind that I also need for a spot to mount a WeatherFlow weather station.
Without a tower (expressly forbidden by Mrs. Freeholder) and given the wooded nature of our back yard, the house itself is the logical place to mount all of them save for the HF wire(s). This also serves to keep the cabling short.
For the scanners, I'm going with a wideband discone antenna. It's relatively small and this one is relatively visually unobtrusive. I like that with my antennas, given that there are a lot of them around here.
But where does that discone go?
I've considered just getting a 20' TV mast and mounting it on the back of the house, but I can't find those any more. The longest I've found so far are 9', but you can stack them. I've found 10' non-stackable masts. I've also found a lot of telescoping masts of various lengths, but those cost more than my budget wants to allow. I'd like to use things I have on hand. That means the front runners for this method are:
- Use the two 9' lengths of 3/4 galvanized pipe I have laying around, connected with a coupling and mounting it on the back of the house. This would probably leave me a bit short of the needed height.
- Get another gable-end mount and put it at the opposite end of the house from the existing beam. That gable is 2 stories in the air. I have a ladder that should reach it, but my days of hanging on a ladder that far up are probably over. I'll likely need to rent a lift. There goes the budget.
- If you have more than one antenna, you need to consider the issue of antenna coupling. It becomes more important if you have multiple antennas connected to multiple transmitting radios at the same time, and even more important if the antennas are resonant on the same frequency or harmonics of the frequency in use. A lot of people don't take this issue into consideration, and some of them blow up equipment. It appears to be rare, but it happens.
- Antenna coupling causes two main problems: Desensitization and receiver overload. Of the two, the protection of your receiver(s) is much more important. You can physically damage a receiver when a very large amount of signal comes in through an antenna.
- Horizontal and vertical separation are important. Incredibly rough rule of thumb is that you need to be at least one wavelength away in all directions from the neighboring antennas but more is better. You may be able to get away with less than one wavelength as long as it isn't a harmonic of the wavelength. More separation is always better.
- Higher frequencies cause more trouble than lower frequencies.
- There are on-line calculators that will give you the necessary vertical and horizontal separation of your antennas. Unfortunately, these assume omnidirectional antennas.
- Unless you are operating at very high power, VHF/UHF and HF antennas exhibit little coupling as long as you keep then reasonably separated.
- There are no perfect solutions.
I've found the following useful in this research:
- Schneider Electric's "Antenna Horizontal & Vertical Separation". They recommend ~60db of separation. At 2m that's going to be +/- 800'. Doesn't seem doable to me.
- "Antenna to Antenna Isolation" on the Antenna Theory website. I particularly like this one for two reasons-the author is a professional in the field and he likes to get things down to the brass tacks.
- "How Close Can I Mount Two VHF/UHF Antennas"
- "Unwanted Antenna Coupling"
- "If it were me, I'd start by modeling the two antennas in EZNEC or another software program, driving one antenna and measuring the current at the feedpoint of the other. This will give you a starting point, and allow you to experiment with different placements, but the result probably will still need some derating due to coupling via other paths as well."
But after all that, what if there is still no clear answer to the question "Where do I mount the discone and the weather station?" Given that I've ruled out almost every possible location, I'm thinking it has to go on the side gable over the driveway, expense be hanged. That leaves the back gable as a place for the weather station, using a gable-mount mast mount and a TV antenna mast. Done correctly, this gives me the largest possible physical separation of the two antennas that are my main concern.
This solution will cause me some grief down the road if I ever want to mount another antenna, but I don't see any realistic options.
An idea I'm toying with is to keep every antenna that's not in use disconnected and grounded. Operate on one radio/frequency at a time. Scanner is disconnected when running 2m. That should pretty much cure the location problem since only a single antenna would be in active use at any given time, and it has the benefit of being budget friendly. It also addresses the issue of lighting, something that is always present. Even if it isn't a final solution, it gets me past this hump and gets the antennas up.
I should have gotten a simpler hobby. :-)