Post by AD5DO on Mar 1, 2015 1:14:39 GMT -6
5 Band Ground Mounted Vertical
AD5DO
VERTICAL ELEMENTS
The vertical length (height) for the frequency and wavelength chosen will be for the center of the phone band. When calculating for a 20m antenna this will be 14.250MHz resulting in a length of 16’-5 1/16” for a ¼ wave antenna. When calculating for a 10m antenna this will be 28.400MHz resulting in a length of 16’- 5 ¾” for a ½ wave antenna. This does NOT take into consideration the type or size of horizontal reflectors used. The horizontal counterpoise’s width (dia.) and length will affect the overall resonance of the antenna so adjustments will need to be made to the vertical length to obtain the desired readings. For this antenna build we will use 3 sections of aluminum tubing 6 feet in length. We will use 2”, 1.875” and 1.750” dia. Tubing. These will be snug when they are telescoped together. We have 18 feet of tubing which allows enough length for them to be fitted together and for tuning. When the antenna is placed on the mounting mast, you will connect the center conductor of the coax to the bottom section of the antenna with a bolt through the bottom antenna element. You will need to weather proof this connection to protect your coax.
COUNTERPOISE
What is a counterpoise? A counterpoise is a network of wires or a metal screen, used as a substitute for an earth (ground) connection in a radio antenna system. It is used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance or other reasons. It usually consists of a network of horizontal wires, parallel to the ground, suspended or just below the ground under the antenna, connected to the transmitter’s coaxial ground wire. A counterpoise system is a must for this antenna to be effective. Think of the wire grid as a ground plane for your antenna. You can read as many books and get as many opinions as you want but here is the simple truth. This antenna will work with 4 radials or as many as 360 radials. Every antenna is a compromise of some type so put down as many radials as you want but for this antenna anything over 8 wires is just bragging. More is better, but how much of your yard do you want to use? Let’s keep it simple. Your radial wire should be 1 foot longer than your longest wave length. You can run it around obstacles or if necessary make a couple of them a little shorter or longer than the others. Then stake them to the ground and let the grass grow over them. Collect the loose ends at the base of the antenna and connect them to a common connector. This is where the coax shield will be attached. You will want to weather proof your connection to keep water from entering your coax and to stop corrosion.
MOUNT
When designing the antenna mount you must insulate the vertical radial of the antenna from earth ground. This can be done by using a non-conductive material that doesn’t absorb water (wood may be a non-conductive material but not when it is wet). How you anchor your mount will depend on your needs. Do you want the antenna to be portable or fixed? You can mount the mast in concrete or place the antenna on an insulator and support it with guide wires. If you use a secured mast there is no need for guide wires on this antenna.
Choke
The main purpose of an RF choke is very simple. This allows your antenna tuner to tune the 5 bands you built the antenna to do. It is used to prevent stray RF from traveling back down the surface of the coax shield conductor, and into the shack and your equipment. This problem can cause all kinds of troubles in the radio room it is safe to say that you don't want RF from the antenna getting into your home. There is one very simple way to stop it at the antenna where it belongs. You can purchase a choke or make your own by making several loops with your coax (5-7 loops 6”-8” in Dia.) Place your choke as close to the antenna as possible for maximum effect.
This antenna is very low profile, inexpensive and effective. Not a bad compromise. I hope you enjoy.
Ken Campbell AD5DO