Friday, November 16, 2012

A single band 20m 5 element Wire Beam

A single band 20m 5 element Wire Beam will be discussed.

I have constructed a 20 meter wire beam on top of my garage.
My garage is 55 feet long about 35 feet wide. It points East-West.
I had a great idea that a full 5 element 20 meter beam in a fixed direction might reach Europe!
Some day when the Sun Spots are helping the Ionisphere. First we have to design the 5 element beam. For this layout i wanted as many elements as i could get on top of the roof. I tried the 5 and 6 element beams on 20m to see which would fit.
The 5 wire elements seems to have just enough room to play with the spacing and tuning.
I used two programs to find a starting point for a 5 wire beam. Most beams have round tubing as the elements.
The larger the diameter of the elments the wider the usable bandwidth will be where ever you center the beam null.
I use the WA7RAI and RAI Enterprises, Inc. program to help desing the beam. Public URL to WA7RAI design software.

I then turned to trial version of EZNEC software. This had 20 node limit but you can do many antenna within that limit. Public URL to EZNEC software.

Let us look at the layout of the wire 5 element beam. 

Here is the picture of the 10 fiberglass poles holding the ends of the 5 wire elements. 

Here are the steps i used to tune the driven element.
I first put up the driven element as a diople and attached 50 feet of LM400 coax cable.
I then used the MFJ_269 antenna tuner checker to see where the SWR was best.
The 20 meter band goes from 14.000 mhz at the CW end up to 14.170 mhz where the phone part stars and ends at 14.350 mhz.
The dipole setup showed these SWR settings:

Frequency : SWR : Loop-back wire length
13.500 : 2 : 1.5"
13.610 : 1.5 : 3.0"
13.980 : 1.2 : 12.0"
14.260 : 1.0 : 17.0" 49 ohm impedance
17.730 : 1.2 : 6.0"

So, the best resoance was about in the middle of the 20m band at 14.260 mhz. This is where we would like the final 5 elements to resonate.

The addition of a Reflector Element behind the Driven Element will change the electrical behavior of the best SWR.
I installed the Reflector wire and checked the SWR of the driven element and note that the SWR at 14.260 mhz changed to an SWR of 1.3 and a Xs impedance of 37 ohms. We will need to retune the length of the driven elements to bring it back to null. The best SWR moved from 14.260 mhz down to 14.086 mhz with the installation of the Reflector only.
So, note that if you have two wires close to each other they will change the SWR.

Now, i put up all 3 Director Elements to see how they effect the SWR of the driven element.
The best SWR now moved from 14.086 to 14.320 mhz.
This is too high in the 20m band.
So, we will have to tune the driven element by changing the wire lengths (via the loop back wires).

I had to lengthen the driven wire elements by 3" on each side to pull the best SWR back down near 14.200 mhz.
An effect that is not shown here is the height of the antenna above the ground.
I was only able to get the 10 poles to about 26 feet above ground. So, the garage and earth may effect the performace greatly.

Using EZNEC now we looked at the radiation pattern of the 5 wire beam at 10feet, 20feet and 40feet above the ground. The higher the better.

One last thing to add for this expirement.
The Ionospheric skip is what controls the longest possible contacts in ham radio.
The longest skip on any day or night depends on the wavelength of the radiation and the antenna take off angle of the radiation.
Public URL Article on Coax Cables of all types.

Next you have to choose the COAX cable type that will feed the antenna.
The coax needs to be selected based on the 50 ohm or 75 ohm inpedance output that your transmitter will be using.
Then choose the coax for the least losses for the band frequency.

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