Antenna Experiments & Explanations

FCARC November 17, 2014

Al Woodhull N1AW

 Hamstick antenna

Hamstick dipole mount.

Antenna in the living room.... 1 st try, good SWR at 14.076 MHz

Antenna in the living room.... rotated 90 degrees, not so good now, best SWR only 2.4 at 14.570 MHz

Lower is better – now SWR 2.0 at 14.160. Moral: you would really prefer to have your antenna up in a tree.

What can we expect – some websites DX Propagation: Maximum Usable Frequency predictions: http://www.spacew.com/www/realtime.php

Let's look a bit closer at that...

KC1KRS in ARRL SS SSB Nov 15-16 2014: 14 states on 10 meters

Sometimes you don't want DX, you want to make a more local contact. Critical frequency is what you want to know about...

http://www.spacew.com/www/fof2.html

Let's look at these sites in real time

Propagation forecasts are nice, but you can do more using Internet tools...

RBN (Reverse Beacon Network) is a network of listening posts around the world that scan entire bands and decode and report digital (including CW) signals heard.

http://www.reversebeacon.net

/ (Sorry, it doesn't work for voice modes, yet)

You can look for your own signal – the next slide is the result while testing indoor antennas at my house a few days ago.

After that we'll try it live... with some variations.

There is also a network of Software Defined Radios - the WebSDR. Here you can see a graphic display in real time of all the signals in a range of frequencies and you can select any one signal and hear how it sounds at the distant site

.

At http://www.websdr.org/ to choose from. you can see all the sites available My favorite is a site in New Jersey, I sometimes monitor our Snail Net by looking at this site: http://96.225.100.244:8902/ (Note he is using a dynamic DNS service – his address may change from time to time

Before going on to part 2, try these things live on web: (see whether 20 meters still open, maybe go to 40) On RBN, Look for my own signal as dx Look for W1AW/1 or W1AW/5 as dx Look at K1TTT as de On WebSDR – see if we can be seen in NJ or somewhere else not too far If not us look for something else – W1AW/* good bet

Part 2: Explanations... The magic of ¼ wavelength wires

The J-pole – why so popular?

J-pole antenna: a half-wave dipole and a tapped quarter wave stub.

You can make it out of copper pipe:

You can make it from twinlead

How it works: the voltage on a half-wave dipole is maximum at the ends. The current is lowest where voltage is highest and vice-versa. A quarter wave matching section transforms the high impedance at the bottom of the half wave to very low at the shorted end. By connecting the feed line at a tap on the matching section about 1/10 th of the way from the low impedance end a match to the relatively low feedline impedance is made.

Al's twinlead J-Pole

The Magic of the Quarter Wave

A simple quarter wave vertical needs a ground connection. A quarter wavelength radial is a virtual ground – the high impedance at the open end makes the other end exactly equivalent to a connection to ground.

The ground plane antenna would work OK with just one radial, but having more than one makes the directional pattern symmetrical.

How Long is a ¼ wavelength?

Quarter wave (ft) = 234 / freq (MHz) 3.5 MHz 67' 7.0 MHz 33' 10 MHz 23' 14 MHz 16' 18 MHz 13' 21 MHz 11' 25 MHz 9' 28 MHz 8' 50 MHz ~5' 146 MHz ~19” 440 MHz ~6.4”

Multi-band radials – this is how Butternut suggests you can make a multi-band radial for the HF6V multiband vertical antenna.

A partial explanation of how it works....

N1AW HF6V

N1AW HF6V Elevated radials

N1AW HF6V Elevate radials and feedpoint

The End

This Powerpoint file will be online at: http://www.fcarc.org/Files/AntennaTalk141116.ppt