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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.
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.
For Brevity, Frequencies below 30MHZ will be referred to as "SW" and above 50MHZ as "VHF"
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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
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)
VHF (basically above 50MHZ) is line of sight. IF you can't "see" the aircraft warning light on the transmit tower, probably you can't receive them. Radio energy like light waves can be refracted. A classic example of this is to drive at night into the country, and 'see the headlights' of an approaching car before it comes over the hill. Water droplets in the air act as small lenses to refract what would ordinarily be considered an 'unbendable' radio wave. So effectively, you get another 20-25% coverage due to the 'air'. POWER of the transmit signal has a lot to do with it to, but only to a certain extent. True, buildings reflect signals, but of no real "usable" value.
Shortwave first ..... then the VHF stuff
OK so what does this have to do with Antennas ?? Well, if you've been listening (so far), you'll see that SW signals come in "from above" (bouncing). While VHF is flat line (like a light beam). So the approach on how to grab that signal out of the air is totally different. SW bounces around the earth between the sky's layers and the ground so many times, that it gets 'mixed up' in polarity. Fact is, there isn't much polarity TO SW. So... vertical or horizontal antennas in many cases work equally well. Plus unless you own an aluminum factory, you couldn't afford to put up a very long vertical antenna. But wire comes in many varieties from clothesline wire to Braided Copper, and can be stretched and wound from place to place.
Every radio depends on ONE basic method of getting the signal. The feedline. AND most radios prefer/use a 50 ohms input. Hence most Coax except for TV coax is 50 ohms. That makes a nice "match" and the radio is happy. But where most people fail is in the 'match' for the antenna.
We have 3 basic types of (SW) antennas
Resistance or "Impedance" at resonance
. End fed LONG wires 300 to 500 ohms
random wire, slopers etc
. Closed Loops 100 ohms
. Center/Middle Fed Long wires 50-75 ohms
(dipoles)
PLUS we have ONE generally 'ignored' effect on the antenna's "resistance"....the Ground Effect.
A given low frequency (7 MHZ for example) antenna mounted 20, 50 and 100 feet above ground will exhibit 3 quite radically different 'resistances'. The ground acts as a giant 'capacitor', and in effect lowers the antenna's feedpoint resistance. The lower the antenna's height and frequency, the more dramatic the resistance is lowered.
BALUNS are 'transformers' that can reduce the resistance of the feedpoint to what the coax likes, and therefore give a much more efficient transfer of signal. Baluns also prevent the signal from using the coax as part of the antenna. BUT, they sometimes cost almost as much as the antenna. Since we are only concerned with RECEIVE, while you might expect to put an antenna on a 100 ft tower and use a balun, you can probably place it much lower to the ground and not even need one. Not to mention needing a looooong ladder or stong legs. PLUS, since the SW signal is bouncing OUT of the sky, not from the Horizon, what difference does it make HOW high (it) is ?
I have covered the above breifly just to give a little focus to the rest of the 'projects'. So much for the 'basics'. Now... here's a few 'cheap projects' for shortwave antennas. I have my 'favorites', and they can be built out of just about ANYthing, from Steel Aircraft cable to Braided Copper, but my FAVORITE source or antenna wire is an industrial motor rewinding shop.
SUPPLIES
1. Look in the Yellow pages under "Electrical Motors REPAIR/REWINDING". Go pay them a visit, don't call or they'll think you're a 'crank'. Most of them are 'mom and pop' locals. The shops do not usually adhere to 'Cleanliness is next to Godliness" . When you burn off the windings in an oven the black ash doesn't just stay in the oven. So don't go in a suit and tie )
But, they have GOBS of wonderful high quality copper wire. I prefer about 15-16 GA. ALL the wire is insulated with Enamel or some high temperature insulation coating. This is excellent for preventing corrosion. Most shops keep reels and reels of 'scrap'. Since you need so much to do a job, and what is left over isn't enough to complete another, the leftovers end up stacked in a corner. The shop usually just sells this brand new wire back to the factory for "by the pound" prices. PERFECT. With a bit of "Creative Haggling", a small investment of $20-25 will net you 500 to 1000 feet of top grade antenna wire. Thats the price of one el-cheapo antenna (maybe 40ft) from radio shack.
NEXT, go by your local TV Parts Supply house and pick up 10 or so goose egg insulators. Porcalines are high, but the plastic ones do just fine. Get some SO-239 (50 ohm "UHF" chassis connectors), one per antenna project, and 50ft or so of RG58U (solid wire center) coax.
Now here are the formulas for the 3 types of antennas
LONG WIRE 1/4 WAVE END FED 234/FREQ IN MHZ = FEET TIP TO TIP
DIPOLE 1/2 WAVE CENTER FED 468/FREQ IN MHZ = FEET TIP TO TIP
FULL WAVE LOOP 1005/FRQ IN MHZ = FEET TIP TO TIP
7 MHZ example
LONG WIRE 1/4 WAVE END FED 234/7.0 = feet
DIPOLE 1/2 WAVE CENTER FED 468/7.0 = feet
FULL WAVE LOOP 1005/7.0 = feet
A dipole is simply two 1/4 wave antennas "fed in the middle"
It's hard to draw a full wave loop in text but the other two look like this
0 0-------------------------------(end fed long wire antenna)
|
-----
--- < -- earth ground
-
------------------------------0 0-------------------------------
dipole fed at center
------------------------------------------------------------------
/ \
| |
| |
| |
| |
| |
| | |
\------------------------------0 0-----------------------------------/
full wave loop
But wait, it is NO coincidence the Ham bands are (for instance) 80 - 40 - 20 - 10 "meters"
Each is exactly half of the next - This was designed so that antennas (and other equipment) could be easily designed for operation in several bands and have 'resonance'. With ONE antenna cut long enough to listen on 80 meters, you can also get 'resonance' at all the others. The Resistance" of the antenna input will change. But all you have to do is 'match that resistance' to the receiver with a "Antenna Tuner"
Taking this a step further, we can 'parallel' several antennas together, and each will then be resonant on it's own frequency and not "bother" the others.
so we could conceivably have FOUR ( 80 - 40 - 20 -10)
meter receive antennas and tie all the feedpoints together to ONE 'coax'.
--------------------------------------0 0--------------------------------------
------------------------------0 0-------------------------------
---------------------0 0---------------------
------------0 0------------
NOW...... if we can do this with negligable interference with dipoles, why not full wave loops ??? Fact is.... you can.... and they're my favorite. I have put up a 80 meter AND a 40 Meter full wave loop, one inside the other, and used it for tranmitting - didn't even need the Auto Tuner on my Kenwood. I used the autotuner, and found that I got pretty good coverage on OTHER ham bands as well !
WHY loops, well for one they have very low noise as compared to dipoles. Most man made interference is VERTICALLY polorized. Line noise is the worst offender. T magnetic waves of 'noise' on horizontal wires are like a bunch of donuts, staggered down the line. The donuts are on the wire 'vertically, and so is the transmitted noise. Car ignition seems to be the same. While Most VHF signals ARE transmitted vertically, it is mainly because the car antennas are vertical. That makes them non directional and easy to mount.
FULL WAVE LOOP
Get out your roll of wire, and onto the loose end slid about 8 or so of the goose egg insulators, one right behind the other. NOW, pick the spot in your property where the coax will be connected and twist the loose end to something (to hold it firm). Then start going around your yard and at each extreme of your yard, attach one of the goose eggs to a tree, telephone pole, whatever. I tie a one or two foot piece of small diameter nylon cord to each egg and nail the other end into the 'tree'. Go around your property until the length you have picked (above formula) is expended, AND the return point to where you started is reached. (Takes a bit of 'calculation').
Now just solder the two antenna ends to either end of a SO239 - put an anchor tie on one of the SO-239's holes to the house or whatever, and screw in the coax. Bingo, instant Shortwave.
have fun Jim Springer