5/8 vertical antenna design
The 5/8 wave antenna consists of a vertical radiator, a loading coil and colinear
plane at the base of the antenna. The best RF transmission system consists of matching impedence
across all sections. For maximum power transmission, 50 Ohm coax cables are
used to match the RF PA's output impedence. Since 5/8 wave length antenna is no
longer matching the transmission line impedence, a feeding coil must be added
between the antenna and the feedline so that the antenna can match the impedence
of the tranmission feedline..
The reason for 5/8 antenna
We all know the 1/2 and 1/4 wave length provide the perfect impedence match to the feedline.
Why would anyone want to go through all the trouble to build a 5/8 wave antenna, since an additional
matching coil is a must? The reason is higher gain. By reshaping
the RF energy in certain directions, high gain reached at the desired direction.
1/4 wavelength antenna almost double the gain of 1/2 wavelength antenna by reflecting
the energy toward bottom upwards. 5/8 antenna could have about 3.5dBi gain over a
1/2 wave length antenna (mounted same height) by shaping some energy from vertical direction to
the side.
The RF power wave reshaping help taking some of the RF power
emitting vertically to the horizontal radiation. A quater wave antenna's radiation pattern
like a dome. The 5/8 wave antenna takes some of the energy from its top and distributed to
the side, so that it covers more ground radius. That is the best reason for using the 5/8 antenna.
Antenna is a passive element. It does not produce more power. The gain in one direction is
by losing in other direction. If the gain to the horizontal direction is too much, the
energy field will be reshaped like a donut. The result would be the immediate surrounding
area of the antenna may have weaker signals.
The height of the 5/8 antenna
Antenna length depends on many surrounding factors. Because the capacitance between
the antenna elements to the ground and nearby struction could impact the actual
impedence of the antenna, even the same antenna installed in different location may
have slightly different impedence. To reduce the ground effect, 5/8 wavelength antenna
should be installed at least 1/2 wavelength above ground to 1.5 wavelength above ground.
For best result, adjust the length of the antenna to get the smallest SWR, or adjust
loading coil for the best matching impedence.
From many popular antenna design web sites, we can see most people using the standard
formula, 585/f (178.308/f for metric) MHz to calculate the vertical
element lengths. This formula is based on a theoritical 5/8 wave antenna build by a
wire. If the antenna is constructed by larger diameter material, its length will
be shorter, due to larger diameter material will have larger capacitance to the
ground. The actual antenna needs to be fine tuned either by adjusting
its length or by adjusting the loading coil. Power meter or SWR meter is helpful for
that fine tuning process.
High Power High Q Loading Coil Design
As we mentioned before, 5/8 wave antenna does not match the 50 Ohm transmission line impedence,
since its radiation elements not resonant to the RF frequency being transmitted. The result of
that will cause sizeable RF power being reflected back to power amplifier (PA) through the
transmittion line. That could cause power amplifier to be fried. To be able to
let maximum power transmitted by a 5/8 wave length antenna, avoid damage to the
equipment, a loading coil is must.
A perfect antenna has only radiation resistance, and zero reactance. So that all the RF
power will be flowed to the reactive near field. But in reality, there
are always unbalanced reactance in the antenna system. Reactance can be both inductive
and capacitive. Antenna radiation elements has capacitive reactance, and loading coil has
inductive reactance. Perfect tuned antenna will have matching capactive and inductive reactance
so that phase shifting can be even or desirable. To design loading coil, we also need to
consider how to increase the Q of the coil. The loading coil's Q is defined by the reactance of the inductor
divided by the resisteance. Many factors affects a coil's Q. A silver wire winding on good isolation materal
may produce a coil with Q ~= 250, an air based large diameter loading coil could have Q ~= 800. High Q loading
coil will reduce the power loss in the loading coil, but also make it tuned sharply reduce the
bandwidth the antenna being tuned on.
Radiation Resistance
We know that the maximum transmission can be reached if the PA output impedence matching
the transmisison line impedence and antenna system impedence. We assume the transmission
lines are pefect 50 Ohms. So if we check the SWR meter and see refelected power, we can
assume that is totally refected from antenna. Radiation resistence is calculated by
1580 * ( antenna height / wave length) * (antenna height / wave length)
For example, if you have set frequency as 100Mhz, which has wave length 3M, and the
vertical elements height from calculator above is 1.78M, your radiation resistence is
about 55 Ohms. Not too bad.
However, a 1/4 wave length antenna has 100% transmission current on the top, wihout
loading coil, the top of a 5/8 wave length antenna will only has about 60% of the
maiximum transmisison current. A matching loading coil will effectively make the
top of the 5/8 wave length antenna has 100% transmission current. With its longer
radiation element and RF radiation pattern reshaping, a 5/8 wave length antenna with proper
loading coil will have higher gain and far reaching radius.
AWG wire gauge and RF current limitation for coil
Coil design needs to consider the power handling of the wire.
Amp = RF Power / PA Voltage. If your PA output stage using 12V
DC power supply, and it transmitting 25W RF power, you will need to use 18AGW
size magnet wire to be able to handle that RF current, based on the chart below. However,
if your PA output stage using 24V power supply, then the same coil made by 18
gauge wire can handle 50W RF power.
| AWG Gauge |
Diameter Inches |
Diameter mm |
Ohms per 1000 ft |
Ohms per km |
Maximum amps for RF power transmission |
| OOOO |
0.46 |
11.684 |
0.049 |
0.16072 |
302 |
| OOO |
0.4096 |
10.40384 |
0.0618 |
0.202704 |
239 |
| OO |
0.3648 |
9.26592 |
0.0779 |
0.255512 |
190 |
| 0 |
0.3249 |
8.25246 |
0.0983 |
0.322424 |
150 |
| 1 |
0.2893 |
7.34822 |
0.1239 |
0.406392 |
119 |
| 2 |
0.2576 |
6.54304 |
0.1563 |
0.512664 |
94 |
| 3 |
0.2294 |
5.82676 |
0.197 |
0.64616 |
75 |
| 4 |
0.2043 |
5.18922 |
0.2485 |
0.81508 |
60 |
| 5 |
0.1819 |
4.62026 |
0.3133 |
1.027624 |
47 |
| 6 |
0.162 |
4.1148 |
0.3951 |
1.295928 |
37 |
| 7 |
0.1443 |
3.66522 |
0.4982 |
1.634096 |
30 |
| 8 |
0.1285 |
3.2639 |
0.6282 |
2.060496 |
24 |
| 9 |
0.1144 |
2.90576 |
0.7921 |
2.598088 |
19 |
| 10 |
0.1019 |
2.58826 |
0.9989 |
3.276392 |
15 |
| 11 |
0.0907 |
2.30378 |
1.26 |
4.1328 |
12 |
| 12 |
0.0808 |
2.05232 |
1.588 |
5.20864 |
9.3 |
| 13 |
0.072 |
1.8288 |
2.003 |
6.56984 |
7.4 |
| 14 |
0.0641 |
1.62814 |
2.525 |
8.282 |
5.9 |
| 15 |
0.0571 |
1.45034 |
3.184 |
10.44352 |
4.7 |
| 16 |
0.0508 |
1.29032 |
4.016 |
13.17248 |
3.7 |
| 17 |
0.0453 |
1.15062 |
5.064 |
16.60992 |
2.9 |
| 18 |
0.0403 |
1.02362 |
6.385 |
20.9428 |
2.3 |
| 19 |
0.0359 |
0.91186 |
8.051 |
26.40728 |
1.8 |
| 20 |
0.032 |
0.8128 |
10.15 |
33.292 |
1.5 |
| 21 |
0.0285 |
0.7239 |
12.8 |
41.984 |
1.2 |
| 22 |
0.0254 |
0.64516 |
16.14 |
52.9392 |
0.92 |
| 23 |
0.0226 |
0.57404 |
20.36 |
66.7808 |
0.729 |
| 24 |
0.0201 |
0.51054 |
25.67 |
84.1976 |
0.577 |
| 25 |
0.0179 |
0.45466 |
32.37 |
106.1736 |
0.457 |
| 26 |
0.0159 |
0.40386 |
40.81 |
133.8568 |
0.361 |
| 27 |
0.0142 |
0.36068 |
51.47 |
168.8216 |
0.288 |
| 28 |
0.0126 |
0.32004 |
64.9 |
212.872 |
0.226 |
| 29 |
0.0113 |
0.28702 |
81.83 |
268.4024 |
0.182 |
| 30 |
0.01 |
0.254 |
103.2 |
338.496 |
0.142 |
| 31 |
0.0089 |
0.22606 |
130.1 |
426.728 |
0.113 |
| 32 |
0.008 |
0.2032 |
164.1 |
538.248 |
0.091 |
| Metric 2.0 |
0.00787 |
0.200 |
169.39 |
555.61 |
0.088 |
| 33 |
0.0071 |
0.18034 |
206.9 |
678.632 |
0.072 |
| Metric 1.8 |
0.00709 |
0.180 |
207.5 |
680.55 |
0.072 |
| 34 |
0.0063 |
0.16002 |
260.9 |
855.752 |
0.056 |
| Metric 1.6 |
0.0063 |
0.16002 |
260.9 |
855.752 |
0.056 |
| 35 |
0.0056 |
0.14224 |
329 |
1079.12 |
0.044 |
| Metric 1.4 |
.00551 |
.140 |
339 |
1114 |
0.043 |
| 36 |
0.005 |
0.127 |
414.8 |
1360 |
0.035 |
| Metric 1.25 |
.00492 |
0.125 |
428.2 |
1404 |
0.034 |
| 37 |
0.0045 |
0.1143 |
523.1 |
1715 |
0.0289 |
| Metric 1.12 |
.00441 |
0.112 |
533.8 |
1750 |
0.0277 |
| 38 |
0.004 |
0.1016 |
659.6 |
2163 |
0.0228 |
| Metric 1 |
.00394 |
0.1000 |
670.2 |
2198 |
0.0225 |
| 39 |
0.0035 |
0.0889 |
831.8 |
2728 |
0.0175 |
| 40 |
0.0031 |
0.07874 |
1049 |
3440 |
0.0137 |
|
