Computer Aided Technologies
Since 1989, the recognized Leader in Computer Control
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.
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
============VHF ANTENNAS = THE SHORT COURSE================
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.
Lots of people like to 'roll their own', so here's a simple formula and construction idea for most antennas above 50 MHZ..
cost .....under $7.50 (not including coax of course)
Plan on making three stops on your "shopping trip".
FIRST go to the local RADIO/TV parts store and buy some Amphenol SO-239's. These are the chassis mount counterpart of the familiar PL-259 so many of us have used for CB's and VHF radios. The reason I recommend AMPHENOL is they use a bakelite type material that can withstand gobs of heat and abuse. Pick up a couple of PL-259's if your 'outside' coax does not use this type connector.
NEXT on your way home visit the local WELDING SUPPLY house. The carry solid brass welding rod in a variety of diameters. I prefer something between a 1/16 and 1/8 inch. These won't break you as they are 30" long and a handfull is probably under $5.00. Ask for solid brass, as it contains high copper content and accepts ROSIN CORE solder very well.
LASTLY, go by the hardware store and pick up a couple of expandable (screw type) hose clamps. Buy "wide range" clamps that will accomodate both a PL259 AND a mounting pipe for the antenna.
OK - get out the old Weller 440 (solder gun).
Here's the formula for 1/4 wave resonant antennas
2992/freq in MHZ EXAMPLE 2992 divided by 155.0 = 19.3 (inches)
You may think this isn't exactly correct, but the 2992 is compensated for "end effect" - which means - the traveling 'wave' tends to 'stretch' itself a bit beyond the end of the antenna (like a ghost image). The THINNER the wire, the more pronounced the effect, AND the narrower the bandwidth. This is why most TV antenna elements are THICK. It's not just the support thickness gives, it the bandwidth of the reception frequency and the reduced end effect.
so.....cut 5 pieces of identical length for the center frequency of the band desired, and add 1/4 inch for 'soldering'.
NOTE - do NOT use ACID CORE - Use ONLY ROSIN based solder. ACID solder will corrode in a few weeks and render your creation worthless.
The SO-239 has four holes, one in each corner. Bend at a right angle four of the brass wires, on one end, at about 1/4 inch. Place each of these into a hole, point straight out from the corner, and liberally solder till it flows in a nice smooth joint. This takes some heat, so if the center conductor of the SO239 melts, you didn't get AMPHENOL. When you're done
Next solder the remaining straight rod into the center conductor of the SO239. Let it cool naturally, and when you can touch it, position the antenna so the center conductor element is pointing upward, and gently bend each of the horizontal elements downward to about a 30 degree angle. The result will look like an umbrella hit by the wind. Apply some Nail Polish on the solder joints for a protective coat (corrosion protection).
Now go outside, FIRST connect your coax with the PL259 and then position the new antenna on the side/top of your mounting pole, and attach it with the stainless steel 'screw type' hose clamp.
that's it.
This is a 'simple' quarter wave ground plane. It has no "gain" but is very good for the band that it is cut for. Compared to the 'commercial stuff' you can't tell the difference.
IF you want to experiment here are some formulas for "other" type antennas. Remember ONE simple principle - the ground radials are ALWAYS 1/4 wave length, regardless of the 'wave length' of the receiving radial, AND they should always DROOP at 30-45 degrees.
5/8 wave formula 7480 / freq in MHZ (exhibits 3-4 db gain over a 1/4 wave)
1/2 wave formula 5980 / freq in MHZ (exhibits 2-3 db gain over a 1/4 wave)
1/4 wave formula 2992 / freq in MHZ (exhibits 0 db gain)
Want a GAIN antenna ???? try a COLINEAR. This is laid sideways because of how it can be drawn with text characters ) "Theoretically" you could stack these forever, but it would probably collapse. MOST commercial antennas use this very same principal, but enclose the vertical collinear radial inside of a supporting fiberglass tube or use a very rigid center conductor. Most use FOUR 1/4 elements and 3 'u-type stubs'. You've probably seen a LOT of these on 150 MHZ repeaters )
The below arrangement exhibits about 3 db gain over the same quarter wave. Add one more and your up to about 5 db. AND the bonus is....the total length acts as a 1/4 wave length at lower frequencies (simply "ignores" the stub).
THIS IS ILLUSTRATED SIDEWAYS, BUT MOUNT AS A VERTICAL ANTENNA!
\
\
| 1/4 wave 1/2 wave (length)
feed point ---> 0 ------------------- --------------------------
50 ohms | | |
/ | |
/ | | 1/2 wave length
| | folded in a "U"
- (1/4 each side)
Here's a way to buld a cheap but effective antenna:
THIS will cost you all of 4 clams.
1. Cut a few pieces of wire to 1/2 wave length
(add 1 inch for each end)
2. Bend it in half.
3. put each end through a small drilled hole in the end of a piece of 3/4"
PVC pipe. leave 1 inch sticking out.
PVC pipe is ablut 3-4 inches long.
4. Holding the BENT end, start winding it onto the PVC you wind so the wire
spirals inward to the center This creates a coil that is wound to nullify
"inductance". Tape or glue the loose "bent end" to the pipe so it doesn't unwind..
5. Cut lengths of wire as shown in the above diagram for the RADIALS of the Colinear.
6. Solder the coil or coils (instead of) the stubs as shown.
Add as many pairs of coils AND 1/'2 wave radials as practical
7. Solder on a SO-239 to the feedpoint - add radials as shown above
the radiols are 1/4 wave length.
8. Get a bow and arrow - put a AA battery on the arrow head
. Tie MONO Filament to arrow - Shoot OVER the top of tall pine tree.
9. Pick up the arrow and tie on the REAL rope. Hoist it UP the tree - coax and all
Hre's a ''Dirt Heap'' multiband quarter wave ground plane.
(Sideways view)
It can be made out of 1/16th dia. brass welding rod and
one SO-239 Coax connector.
\
\
| | -------- 1/4 wave 850 mhz
feed point ---> 0 ------------------- 1/4 wave 455 mhz
50 ohms | |--------------------------------------- 1/4 wave 155 mhz
/
/
Solder them ALL to one feedpoint Bend them a bit at the bottom
so the vierticals don't touch.
Make each of the ground radials as long as the longest vertical antenna.
LAST hint, by using the 5/8 and 1/2 formulas, you can quickly calculate
resonances at different frequencies for the SAME antenna length.
You'll quickly find that you have a 'multi frequency' antenna. Especially
using a UHF type collinear arrangement. With a LOT of math you can design
an antenna that's resonant on just about ALL the frequencies you listen to.
1/4 wave resonance at 220 mhz (13 inches)
1/2 wave = 5980 / 13 = 460 MHZ
5/8 wave = 7480 / 13 = 575 MHZ
7/8 wave = 10472 / 13 = 805 MHZ
COLLINEAR 1/4 wave resonance at 460 mhz (6.5 inches X 2 + 6.5 = 19.5 inches)
1/4 wave = 2992 / 19.5 = 153 MHZ (Pub Serv)
1/2 wave = 5980 / 19.5 = 306 MHZ (Air Force UHF)
5/8 wave = 7480 / 19.5 = 383 MHZ (not quite 450 mhz)
TELEVISON ANTENNAS
VHF/UHF TV Antennas are great multi-frequency antennas. In addition, many of them come
with 75 OHM coax connectors already included. AND 75 OHM coax for reception is
not that far off 50 ohm inputs to the radio and works just as well.
B U T . . . TV is Horizontally polorized.
and TWO-WAY signals are mostly VERTIALLY polarized. Just mounting a TV
antenna would not work well. SO - remove the Pole Mounting bracket and
redrill two holes on the MAST so that it mounts HORIZONTALLY !!.
Drill the holes a litte bit to the front or back of the original holes
and that will not weaken the mast's strength from the original design, but
Will now permit the TV antenna to be mounted for Vertical reception.
Run some 75 ohm coax, get a "F" to BNC adaptor if needed and - - - have fun
Jim Springer