Tuesday, May 24, 2011

KD8JHJ Homebrew Vertical Antenna Update



Here is a peek inside the matchbox with the inductance coil in it's place.  The bottom end is not soldered to the PL-259 chassis mount connector yet.  If I have to shorten the coil a bit I can easily do so.  When I first removed the coil from the form the individual turns tightened up against each other like a spring or a slinky.  From past experience I learned that it is impossible to stretch out the coil by hand to precise spacing without deforming the soft copper wire.  After much consideration I decided to use a bead of epoxy resin to hold the coils apart at 1/8" intervals.  I am hoping that the resin does not cause any detrimental effects on the coil's ability to do it's job providing inductance.  The screw is now visible that penetrates through the back of the box and provides electrical contact to the antenna. 



The antenna lower section clamped in my Park Tools bicycle work stand.  While this stand works great for it's intended purpose I have also found it useful for many other tasks requiring a sturdy third hand.  One of my favorite non-bike uses for the Park stand is to hold an umbrella over the gas grill on rainy days.  It can also make an acceptable coat rack in a pinch.

Last night I measured and cut four radials from 12 gauge heavy duty copper stranded machine wire.  I crimped and soldered ring terminals to one end of each of the wires for easy and secure attachment to the wing nutted screws on the radial plate.  At the far ends I will use a polycarbonate insulator and short piece of cord terminating at a tent stake pounded into the ground.  I used this radial calculator to figure the length of the radial wires.  Entering 7.050 MHz as my target frequency the calculator showed 33.191 feet.  I cut the wires to 33 feet 3 inches. 

In the deployed position the matchbox will be between 5 and 8 feet above the ground.  The four radial wires will extend out from the antenna in the shape of a big X gradually sloping to about a foot off the ground.  In the following paragraph I will do my best to describe the purpose of this elevated radial system.

A major problem to overcome when using vertical antennas is ground losses.  To put it simply, the Earth loves to absorb Radio Frequency energy especially in the near field close to an antenna.  Because the radial wires are cut to 1/4 wavelength of the center of my operating frequency range they will act as little antennas themselves capturing some of the RF energy when I transmit and recycle it back into the antenna system before it is lost to earth.  I have learned that antennas are all about compromise and the name of the game is to minimize losses to maximize radiation efficiency.  Radial systems can be very large especially for the lower ham bands.  In my case for the 40 meter band I will need a square area 66 feet on a side.  Because my antenna is closer to 1/8 wavelength rather than 1/4 wave typically used for vertical radiators I have already compromised some efficiency so I feel a good radial system is essential.

The next stage will involve hooking up the antenna analyzer and sending some milliwatts up the feedline to see what happens.  In other words the the moment of truth is almost here.


2 comments:

  1. Those raised radials are going to work great. Rudy Severn's (N6LF) exhaustive analysis shows 4 raised radials to be as effective as a whole mess of buried radials. Weird parallels- I've always had a bike glued to my ass, won the cold war working on B-52H models, and pound brass from an Icom 718. Had an insulator collection when I was young too.
    I put this page right into my faves best of 73 de Tom, AB9NZ

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  2. Hi Tom,
    Yes I got the idea from Rudy's piece. Had to go back and look up it up in last years antenna issue. I thought we might be thinking of the same guy. Great article! I was convinced and the elevated radial system can be packed up to go portable with not too much trouble.

    My only question is between the two of us which one is the evil twin? Hi Hi

    73 Mike kd8jhj

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