VHF/UHF Mobile Antenna System Decisions, decisions, decisions Mansfield Johnson Amateur Radio Service Sept. 4, 2015 by Bill Adams AF4B

Just the facts Efficiency is ratio of power radiated (or received) to power applied Maximum E and H fields occur with maximum RF current Maximum RF current flows when system impedances are matched 1:1 Antenna gain comes from shaping the electromagnetic field Short thick transmission lines have lower losses than long thin ones Reciprocity means performance is the same on transmit and receive Vehicle constraints force choices (compromise) Line of sight – you can see more from higher up It is difficult to get high efficiency on all bands of a multiband antenna

Efficiency Transceiver power out - Losses in feed line - Matching component losses - “Ground” losses Radiated power

Near ground level impedance of feed point Driven element lengthR Ω j Ω λ/ λ/21100 to to 0 5/8 λ≈ to -150 R total = R transmission Line + R ground + R radiated

Gain Isotropic, λ/4, λ/2 and 5λ/8 elements ¼ λ pattern on vehicle (w8ji.com) 5 / 8 λ pattern on vehicle (w8ji.com)

Comparison λ/4 and 5/8 λ

W8JI’s conclusions With a normal mid-size roof contour 5/8th wave gain is now -4.64dBi at 2 degrees elevation. Maximum gain is now up slightly to 3.2dBi. With a normal sloped roof shape and mid-car size ground, the 1/4 wave has -6.59dBi at 2 degrees TO. Maximum gain is 3.05dBi. 5/8th wave gain is now 1.95 dB over a 1/4 wave antenna at 2 degrees, but the peak gain is only.15dB better! The smaller the roof and/or the less centered the antenna, the less gain advantage for a 5/8th wave. Lower height roofs also have less gain difference, and shape of the roof greatly affects pattern and gain. The 2.85dB theoretical gain is only for a perfect lossless loading coil antenna over perfect infinite flat ground plane. In the real world the actual gain difference between a 1/4 wave antenna and a 5/8th wave can be anywhere from the same up to a maximum of 2 dB. The 5/8th wave is never really worse, explaining why people have no problems using them. This is true for any repeater antenna height.

Transmission line Cable Flex versionsLosses dB/100’, MHzdB Losses in 6’ 1:1 SWR, MHz Dielectric / Shield type RG-174 Belden 100, 146, 443PE / Single braid Cu RG-58A Belden 100, 146, 443PE / Single braid Cu RG-8X Belden 100, 146, 443FPE / Single brain Cu TMS 100, 146, / Foil + tinned Cu braid RG-8 WM 100, 146, / Foil + tinned Cu braid RG-8 Belden 100, 146, 443PE / Single braid Cu

Mounting location & vehicle size impact the pattern ( 03QST2mmounting.pdf ) 03QST2mmounting.pdf

obile-antenna-placement.html

Larson tech note on antenna placement n_antenna_products/technical_reference

Is the coverage significantly different? Coverage at 6’ above ground levelCoverage at 8’ above ground level

“Just tall – that’s all” Of course we are no longer mobile, but now vehicle portable.

How do you mount the antenna? NMO Requires a ¾” hole. SO 259 requires a 5/8” hole. 3/8 24 Ball mount requires holes! On glass – heard of passivated? Double sided 3M tape – ½ λ mandated due to lack of ground ¼ λ and 5 / 8 λ require ground plane Magmount requires iron, scratches surfaces, requires ½ λ antenna Garages and low clearance mandates folding design Driver side mounting preserves antenna!

NE5DL install of Diamond adhesive mount

Here’s what Gordo thinks about HF antennas

References things mobile, best reference I’ve found and Source of several graphics used in this presentation Mounting location impact on pattern NMO installation Overview of several antenna models Commercial antenna site