Antenna Experiments &
Explanations
FCARC November 17, 2014
Al Woodhull N1AW

Hamstick antenna
Hamstick dipole mount.
Antenna in the living room.... 1st 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/ you can see all the sites available
to choose from.
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 1/10th 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