VHF/UHF Mobile Antenna System

Decisions, decisions, decisionsMansfield 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 length R Ω j Ω

λ/4 36 +21

λ/2 1100 to 1900 -5000 to 0

5/8 λ ≈ 50 -50 to -150

Rtotal = Rtransmission Line + Rground + Rradiated

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 http://www.w8ji.com/VHF%20mobile%20vertical.htm

• 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 versions Losses dB/100’, MHz dB Losses in 6’ 1:1 SWR, MHz

Dielectric / Shield type

RG-174 Belden 8216 8.4 @ 100, 34.0 @ 1000 0.6 @ 146, 1.2 @ 443 PE / Single braid Cu

RG-58A Belden 8259 5.4 @ 100, 22.8 @ 1000 0.4 @ 146, 0.8 @ 443 PE / Single braid Cu

RG-8X Belden 9258 3.1 @ 100, 14.0 @ 1000 0.25 @ 146, 0.49 @ 443 FPE / Single brain Cu

TMS LMR240UF 2.5 @ 100, 8.0 @ 1000 0.18 @ 146, 0.32@443 FPE / Foil + tinned Cu braid

RG-8 WM CQ106 1.8 @ 100, 5.3 @ 1000 0.1 @ 146, 0.19 @443 FPE / Foil + tinned Cu braid

RG-8 Belden 8237 1.9 @ 100, 7.4 @ 1000 0.1 @ 146, 0.27 @ 443 PE / Single braid Cu

Mounting location & vehicle size impact the pattern (http://w6nbc.com/articles/2012-03QST2mmounting.pdf )

Larson tech note on antenna placement

http://www.pulseelectronics.com/products/antennas/larsen_antenna_products/technical_reference

Is the coverage significantly different?

Coverage at 6’ above ground level Coverage 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

• http://www.k0bg.com/ All things mobile, best reference I’ve found• http://www.w8ji.com/VHF%20mobile%20vertical.htm and http://

www.w8ji.com/end-fed_vertical.htm Source of several graphics used in this presentation• http://w6nbc.com/articles/2012-03QST2mmounting.pdf Mounting location

impact on pattern• http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf • https://youtu.be/7gFNeW2Z1Xo NMO installation• https://youtu.be/u7YYaCgjfbs Overview of several antenna models• https://youtu.be/Fmm63d2Zyv4 Commercial antenna site