Thursday, April 18, 2019

Antenna coupling


Figure 1, Figuring out where the damn antennas are going to go.
Radios require antennas, and unless you're blessed with a decent acreage, lots of money and an understanding spouse, there's limited room to put them up. As things currently stand, I have a 6m-2m-70cm beam already up. I want to put up one or more wire antennas for HF use. I also need a receive-only antenna for my scanner(s).

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.
I've also considered mounting it on a 5' mast extension over the beam. It would be simple and I have the stuff to do it. Being a discone, it's omni-directional, so which way the beam points isn't an issue for it. But when looking into how far the two antennas needed to be separated, the subject of "antenna coupling" comes up.

Based on what I've found so far, this shit is dark magic it's a complicated subject. I'm still working on it, but so far here are the bullet points:
  • 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'm working on the assumption (oh-oh) that the only two antennas I need to be concerned with are the 6m-2m-70cm beam and the discone. 2 meters is uncomfortably close to the VHF public service bands.

I've found the following useful in this research:
And believe it or not, I've found something on a ham forum that I think is useful.
I like this concept. Doing so may show that the difference between the Real World and Theory World are enough to solve my problem for me. Unless it gets worse once I do the modeling.

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. :-)


1 comment:

Robert said...

Good gawd! Good luck.