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I have been asked innumerable times to 'reproduce' several topics I have discussed on Antenna's, Grounding of Antennas/radios, and in general neat 'tricks' involving 'rolling your own'.
If you'll pardon my typing AND spelling (lack of ) skills, I'll try to put as much into this "101" as I have time for. This a a highly UN technical approach to 'rolling your own' antennas. IF you want formulas and SWR charts there are shelves of books on the subject.
==============ZIP CORD ANTENNA ============================
Several "General Rules" to remember. Ignore them and you'll spend big bucks with little improvements in receiving ability. PLUS - We are only talking about Receiving Antennas.
Transmitting is an entirely different ball game.
-------------------------------------------------
SW (Below 30 HHZ)
A. The higher and the longer (the antenna), the better.
B. Resonance isn't everything
C. The Cheaper (you can build it) the better
D. Any old piece of Coax will do
F. Distance from Transmitter to Receiver is of no consequence.
E. SWR is only for Transmitting
F. Grounding is everything
G. Disconnect (it) from your radios at ANY sound of thunder.
H. The direction of the signal isn't all that important,
(besides it's not coming from where you'd expect)
I. An antenna is always resonant on MORE than one frequency
VHF (Above 50HMZ)
A. Bigger isn't always better
B. The higher off the ground (your antenna) the better
C. Resonance is everything
D. Signals leak out of coax like a sieve
in proportion to the frequency.
E. If you can't stand on top of your antenna and see where the
signal is 'tranmitting from', you probably can't receive it.
(disregarding trees and bushes)
F. The direction of the signal AND the antenna is everything
G. An antenna is always resonant on MORE than one frequency
For Brevity, Frequencies below 30MHZ will be referred to as "SW" and above 50MHZ as "VHF"
Personally I love SW. If you plan it right, the entire world is available at any given time of the day. AND, for the most part it is 'commercial free'. Lots depends on the Sun's position (ionization layers), and it's spots, but you can put a receiver just about anywhere in the world and pick up any OTHER part of the world within a 24 hour period. Shortwave signals bounce around the earth, basically 'unattenuated'. When you receive one, it's almost like 'line of sight' communications. Transmit power helps, but even 'peanut whistles' can get thru crystal clear if there isn't some 'bigger station' on the same frequency. (Pirate Radio for instance)
==============ZIP CORD ANTENNA ============================
If you travel a lot, you know that it isn't possible to carry everything with you, especially antennas. This little idea will give you both a 'packable' shortwave antenna, and have something that works better than the 'coathanger' we usually get with our portables.
Go to the hardware store and buy a 15-20 foot extension cord. Buy the type that is made of ZIP CORD - two parallel wires molded together. They call it zip cord cause you can easily 'unzip' the two wires from each other. CUT one of the two wires about on inch from the MALE END of the AC plug that normally goes into the wall. UNZIP the cut wire backwards away from the plug about 4-6 inches and again cut it off. This leaves just ONE wire connected to the plug, and the other 'insulated' or not electrically connected.
\
|--> ======||
| ||-----------------/ /------------
| || ------------/ /------------------ to radio <---\
|--> ======|| |
| / |
| ac plug |
| |
|______________ZERO - CONTINUITY WITH A Ohm Meter__________________|
Then cut OFF the RECEPTACLE on the other end. Zip back both wires until they are seperated and have about 1/2 foot of each wire dangling. THEN follow the wire that is connected to the PLUG upwards to the other end, and cut IT of where the two wires you just unzipped are just starting to be one piece. USUALLY zip cord has one side 'ribbed' and the other wire smooth so you should be able to tell which is which. Strip back the longer piece of wire on the 'bare end' about an inch or so to connect either an alligator clip or a BNC/SO-239 connector for your radio.
You want to be certain there is NO electrical connection between the plug end and the bared wire on the other end. One way is to connect an ohm meter. No matter WHICH of the (three) ends you touch, there should be NO CONTINUITY - Period. (see above)
IF the PLUG has one blade larger than another, you may need to grind them to be both the same size. This enables you to plug it into the ac outlet either way.
THE THEORY IS......The length of cable that is 'connected' by the molding acts as a capacitor. It will not pass ac current, but at radio frequencies, will enable transfer of signal from one ribbed piece to the other (almost like the wires were 'soldered togther').
Just connect the one end to your radio. Plug in the other end to the AC outlet. Depending on if the wire attached to the plug is on the Ground Side, or the HOT side, will determine how much of the 'wall wiring' is used as an antenna. Reversing the plug back and forth will let you determine which side is the 'best side'.
When you're traveling the whole thing will fit in a "zipLock" bag or a briefcase etc. Unroll it when your ready to listen.
have fun Jim Springer