Transcript HF Mobile Vertical Design

HF Mobile Vertical Design
By Larry Benson, N7GY
• Vertical Antenna Theory
• Base vs Center Loaded
• Efficiency/Installation Techniques
• Design Example
. Program Demo
July, 2002
Obtained from Worldradio Books
HF Mobile Vertical Design
by Larry Benson N7GY
Vertical Theory
Fundamental Freq. of a ¼ wave vert. is the lowest freq. for which the reactance (j) is
zero at the feedpoint. This is the resonant freq. ¼ = 3 x 108 m x ¼ -- > Free Space
Freq.
Physical length = Free Space x K This makes the physical length shorter than free
space. Approximately 5% shorter due to capacitive and inductive effects of the
conductor, end effects and proximity.
Electrical length for ¼ wave antenna is defined as 90.
When resonant, /4 vert. is just a resistor (no reactance, i.e. no j):
Rt
What value? Rt = RR (Radiation Resistance) + Losses
Radiation resistance, RR,, is proportional to the length and inversely
proportional to the diameter of the antenna element.
For 40 meters (7.2mhz), /4 = 33.3 feet! (399.68”) using ½ inch diam.
conductor. Too long for vehicle!
Let’s use 8 ½ft (102in) vertical. This is short on 40meters.
This would look like this:
Rt
-jXc
We must tune it with an inductor because a short antenna is capacitive
and a long antenna is inductive. NO perfect inductor. Can’t use
superconductors yet! Wouldn’t want to anyway. Q too high.
An inductor has losses:
A. DC (small)
B. AC (skin effects & proximity)
Q is Freq. sensitive. Measured in Q = XL
RAC
Grounded Verticals
RG = 0 if perfect ground, but hard to come by.
Would need solid copper surface wavelengths long. Typical is 120
radials minimum of 1/4 long.
Soil conductivity & path is the problem.
Radiation resistance for a 1/4 vertical, ½ in diam. over perfect ground at 7.2 mhz is
about 32.
How about on a vehicle?
Example:
A 102 in. whip on 7.2mhz (1/16), Rad. Resistance is about 4 !
RG = 2 - 20 for vehicle, but for a ground mounted vertical RG = 0 - 40.
Tune by inductor, capacity hat & or make longer. 13 ½ feet is the limit.
Two cases explored:
Base Loaded
H = 102 in. (2.59m), F = 7.2mhz >  = 41.667m
Electrical height = (360) 2.59m = 22.378
41.667m
A = 22.378 = 11.189 degree-amps
2
RR = .01215 (11.198)2 = 1.52 for Base loaded.
Center Loaded
H1 + H2 = H , I1 = I0 cos11.189 = .98 I0
H1 = H2 = 51” (1.295m)
G1 = G2 = 11.189
A = 11.189 (1+.98) + 11.189 (.98) = 16.56 degree-amps
2
2
RR = .01215 (16.56)2 = 3.33 for center-loaded.
What does the coax on a vehicle see at the feedpoint for antenna
impedance?
Radiation Resistance? Will it see 3.3? 52?
Remember the ground resistance? RG = 5 Where is this seen?
Coax will see RR + RG Is this all? Not yet done.
Remember the coil. Is it lossless?
>> NO!
Calculate the Inductance for each case:
Base Loaded
-jXA = -j
Z0
Tan GV
where Z = 138log 2H
2
H = ave. height above ground
A = ave. radius of element
GV = elec. Height of ant.
H = 102+18 = 60
2
A = .862 = .4375
2
Z0 = 138log2(60) = 336.47
GV = 22.378 + 5% for end effects = 23.5
-jXA –j336.47 = -j773.828 , (XL = 2FL) L = 773.828
= 17.105uh
Tan23.5
2(7.2 x 106)
Calculate the losses for coil. Not easy!
If Q = XL and I measure coil Q = 250,
RT
Then RC = 773.828 = 3.09
250
Center Loaded
XL0 = jZ0 (cotan G1 – tan G2 ),
jXL0
G1 = G2 = 23.5 = 17.75
2
= j336.47 (cotan11.75 - tan11.75) = 1547.66, L = 34.21uh
Calculate the coil losses:
If Q is 250, then RC = 1547.66 = 6.19
250
So the antenna impedance is RT = RR + RG + RC
Base Loaded antenna impedance is 1.5 + 5 + 3.09 = 9.59, SWR = 52 = 5.4
9.59
Center Loaded ant. impedance is 3.33 + 5 + 6.19 = 14.52, SWR = 52 = 3.6
14.52
Don’t jump to conclusions regarding the SWR just yet. We will discuss this later.
Which is more efficient?
BASE Loaded: 1.52
x 100 = 15.8%
1.52+3.09+5
Center Loaded:
3.33
x 100 = 22.9%
3.33+6.19+5
We have some control over the coil Q and a little control over the ground losses, but not
much over the Rad. Resistance unless we add length or change antenna design.
Minimizing Losses
RG is a function of:
1. Car
A. Size and shape
B. Height off ground
2. Soil losses at frequency
3. Position of antenna on vehicle
if using same exact antenna.
4. Grounding methods in car
My truck RG is around 5, but varies over the ground it rides on. Salt flats = 4
To minimize ground losses:
1. Large gage ground strap to back of rig!
2. Additional ground straps
A. Underneath vehicle, body to frame.
B. Matching point, short leads
3. Conductive compound at screws & regular cleaning.
RC Losses
Better Coil Methods
1. Larger diameter coil
2. Use optimum gage of wire
3. Optimum wire turns spacing/coil shape/size
4. Minimal coil form material, low dielectric
constant
Capacity Hat
1. Circular is best for pattern, but straight wire works well too.
2. Placement on element (not too close to coil, at least a diam.)
Can be used to tune the antenna to other bands like 18mhz from 21mhz or 24mhz
from 28mhz.
Placement of antenna on Vehicle:
1.
Bumper > Least desirable
2.
Fender > better location
3.
Top > Best location for signal strength but hard to do.
Ground losses are effected. Example: Bumper = 6 ground loss
Fender or hood = 4
Top = 2.5
ANTENNA PLACEMENT at 7.2mhz
Ant. A (104”) IN = 39uH Ht = 46” Rg = 5 ohms Rr = 4 ohms Eff. = 25.6%
Ant. A (102”) IN = 39uH Ht = 18” Rg = 7 ohms Rr = 3.9 ohms Eff. = 22.3%
Ant. B (132”) IN = 34uH Ht = 18” Rg = 7 ohms Rr = 6.7 ohms Eff. = 35.3%
Corona
Need round smooth conductive bead or ball at high voltage end of antenna.
Cause is ionization of the air at high voltage point of antenna. This lengthens the
antenna and de-tunes it.
Also causes static noise in receiver.
Diameter of radiating element
Larger diameter of radiating element will give lower DC losses and gives the
added benefit of more bandwidth and longer effective length.
This broadband “UnUn” will match your antenna’s
low impedance (16). The author recommends to put
a small cap across to help with the match above
8mhz. Use a variable to find value.
Example of Antenna design
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
HF Mobil Vertical Program Demo
Quarter wave for frequency section
Auto Optimize placement
Changes to efficiency (Placement of coil , diam., Q)
Radiation Resistance vs Antenna Impedance
SWR – Low SWR when losses are high
Less Bandwidth (with high Q coils)
Coil Section Demo
a. Optimum Wire Gage
b. Shape factor
c. Length of coil vs form length
Coil Q design helps
Interactive screens between Antenna & Coil Sections
Printing