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 10 8 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): R t What value? R t = R R (Radiation Resistance) + Losses Radiation resistance, R R,, 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: R t -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 = X L R AC

Grounded Verticals R G = 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  ! R G =  for vehicle, but for a ground mounted vertical R G = . 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 > = m Electrical height = (360  ) 2.59m =  m A =  = degree-amps 2 R R = (11.198) 2 = 1.52  for Base loaded.

Center Loaded H1 + H2 = H, I 1 = I 0 cos =.98 I 0 H1 = H2 = 51” (1.295m) G1 = G2 =  A = (1+.98) (.98) = degree-amps 2 2 R R = (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? R G = 5  Where is this seen? Coax will see R R + R G Is this all? Not yet done. Remember the coil. Is it lossless? >> NO!

Calculate the Inductance for each case: Base Loaded -jX A = -j Z 0 where Z = 138log 2H Tan G V 2 H = ave. height above ground H = = 60A = ave. radius of element 2 G V = elec. Height of ant. A =.862 = Z 0 = 138log2(60) =  G V =  + 5% for end effects = 23.5  -jX A –j = -j , (X L = 2  FL) L = = uh Tan23.5  2  (7.2 x 10 6 ) Calculate the losses for coil. Not easy! If Q = X L and I measure coil Q = 250, R T Then R C = = 3.09  250

Center Loaded X L0 = jZ 0 (cotan G 1 – tan G 2 ), G 1 = G 2 = 23.5  =  2 jX L0 = j (cotan11.75  - tan11.75  ) = , L = 34.21uh Calculate the coil losses: If Q is 250, then R C =  = 6.19  250

So the antenna impedance is R T = R R + R G + R C Base Loaded antenna impedance is 1.5  + 5   = 9.59 , SWR = 52 = Center Loaded ant. impedance is 3.33  + 5   = 14.52, SWR = 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% Center Loaded: 3.33 x 100 = 22.9% 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 R G 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 R G 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.

R C 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

HF Mobil Vertical Program Demo 1.Quarter wave for frequency section 2.Auto Optimize placement 3.Changes to efficiency (Placement of coil, diam., Q) 4.Radiation Resistance vs Antenna Impedance 5.SWR – Low SWR when losses are high 6.Less Bandwidth (with high Q coils) 7.Coil Section Demo a. Optimum Wire Gage b. Shape factor c. Length of coil vs form length 8. Coil Q design helps 9.Interactive screens between Antenna & Coil Sections 10. Printing