Saturday, August 6, 2011

Stealthy Radio Antenna.. the Gutter!

There's a lot of great things about being a HAM radio operator.
I've served the public during sporting evens, and emergencies.
I've had a lot of fun talking to people all over the world using all manners of methods of communication.
I have to say though, that thing that I like most about it is that HAM radio, no matter what aspect of the hobby you enjoy the most, is a very hands on hobby. My wife can tell you I'm always plottin' something to get another antenna in the trees, or saving for a radio to build.  Some places and people are not friendly to my hobby though.  There's entire communities that force people to live antenna free.  I refuse to own propery in such a place.  I do, however sympathize with people who do live in deed restricted, covenant restricted communites, and do my best to help others get on the air despite, or in spite of whatever restrictions they face.

A buddy of mine once lived in an apartment, and had just got his General Ticket, so he had access to the HF bands.  Apartments are notoriously antenna unfriendly, so I helped him set up his gutters as an antenna.  Let me tell you how we did it. 
CAVEAT!
If you decide to do something similar, use caution! This is for power not to exceed 100 watts.  First of all, you should never use more power than you need to communicate. Secondly, RF can and does start fires, it's how microwave ovens warm your Hotpockets.  Plus, even at 100 watts, my buddy would sometimes experience rf in his shack, or interference with the TV and telephone.  The simplest solution to this is to turn the power down.

  
So with that in mind, let's get our minds on the gutters.

Notice, I said, on the gutters, not in the gutter.

Gutters, if they are made of metal, can carry RF, and they can do it reasonably well, if some things are kept in mind. They basically function as a random length wire antenna, although if you have the space for it, and do a little planning and experimentation, you could possibly create a system resonant on a particular frequency. Here are the rules about using METAL gutters as antennas:

1. ELECTRICALLY connect the pieces. Most gutter pieces are either friction fitted, or connected together via a small screw. This is a mechanical connection, and is not good enough for RF. RF needs to see the pieces as one piece electrically, so there needs to be a low resistance connection between the pieces. As it stands, they are loosely electrically connected joints, and can actually act as product detectors, meaning they they are small bits of radios themselves! This results in all kinds of noise and interference on your received signal. Fortunately this is easy to fix, but it does take work. This is also good information for you to store away when you want to install a mobile radio, as you will want to electrically connect the various body panels on your vehicle.

To connect gutter pieces you will need:

  • self tapping screws (many)
  • A machine screw
  • 2 washers (for machine screw)
  • locking nut (for machine screw)
  • sandpaper
  • copper bearing conductive grease.
  • A coax pigtail, one end with an SO-239 connector, the other should have the braid and center conductor seperated for about 6-10 inches.
  • ohm meter
Sand down the contacting surfaces for each section of gutter or downspot. Make sure you can fit them together tightly, but before you do, put some conducting grease between the contacting surfaces. Secure the pieces together with at least two self tapping screws. Check the electrical continuity between the two sections of gutter/downspout with an ohm meter. There will probably be some resistance, but there shouldn't be much, and if things are wiggled a little bit, the resistance shouldn't jump around. The machine screw, washers and nut is for the feedpoint. I've always fed gutter antennas from the bottom of a downspout using a coax pigtail, one end with an SO-239 UHF connector (Same as what's on the back of most HAM/CB radios) , center of the coax going to the gutter, and braid going to a nearby grounding point of some kind, either a ground rod (worst case scenario), Radials (better), or groundrod with radials (Best option!). Sand the area, outside and inside, where you want to feed the antenna, making sure you remove all the paint on an area a little bigger than your washer. Put one washer on the inside of the downspout with the nut, and one on the outside between the screw head and the downspout. Before you snug everything up, wrap your feedline center conductor around the screw a couple of times between the washer and the downspout. Give the washers, screw, and downspout a litte conductive grease, and tighten everything up.

  
2. Create a good ground plane. Unless you live near the ocean, your ground plane stinks. Shooting a ground rod into the ground near the spout where you will be feeding your antenna is one thing you can do. It will at least help give you some lightening safety. Hooking some radials up to that rod would be even better. Hook up at least 4, and space them an equidistance apart out in the front. Make sure they are at least a quarter-wavelength long on your lowest frequency of operation. Because that can be 66' for frequencies in the 3.5 MHz range, you may need to get creative on where they go, and how they sprawl out. I use a hand operated sidewalk edger to make a small slit in the ground, that can be used to bury the ground radial. You'll need to find some similar way of hiding the radials. When you connect the ground braid from the coax pigtail to the ground system of the antenna, use some more of the conductive grease.

  
3. Did you use the conductive grease? If not go back and use it! Water makes copper corrode quicker, and copper braid readily absorbs water. Corroded copper does not conduct RF well. Because gutters are designed to carry water, it's important to try and make the connections as waterproof as possible. The conductive grease helps do that, and it helps make the electrical connection better. USING REGULAR GREASE IS NOT RECOMMENDED! regular grease will act as an electrical insulator. That's the opposite of what we want to do.

4. Run as short a line as possible from the antenna's pigtail to an antenna matching device. You may or maynot need a balun. The antenna matching device should be grounded to the same point as the antenna, although if you can't do that, don't worry. Just get it grounded.

That's how you make a gutter system into an antenna. At my buddy's apartment, we stealthily connected the pieces of a downspout together electrically, and it wasn't easy. Fortunately, the downspout could be accessed from a stairwell. He had a great time on HF with 100 watts and less, but had a better time with a stealth longwire. That's for another post though...


EDIT:  Links for other downspout antennas will go here as people get them to me!
WB3GCK's antenna
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