Choosing the Right Coax

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Choosing the Right Coax

• by Marty Woll N6VI
• ARES-Los Angeles

Amateur Radio . . . when all else fails
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Coaxial cables vary in

• size and weight
• loss characteristics
• durability and weather resistance
• flexibility and ease of handling
• impedance and velocity factor
• connector requirements

Amateur Radio . . . when all else fails
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General observations about coax

• Most ham gear is designed for 50 ohms
• loss goes down as diameter goes up
• loss goes up as frequency goes up
• larger coax can handle more power
• more flexible means somewhat lossier
• better coax improves both TX and RX

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How are coax losses measured?

• The decibel (dB) is a comparative measure.
• Typical spec is dB/100 ft. at xx MHz.
• dB 10 log P2 / P1
• 3 dB x2 10 dB x10
• -3 dB x1/2 -10 dB x1/10

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Some actual numbers RG-58/U

• on 80m 0.62 dB/100 13 loss
• on 10m 2.35 dB/100 42 loss
• on 2m 5.52 dB/100 72 loss
• on 70 cm 10.82 dB/100 92 loss ! ! !
• RG-58/U is a bad choice for long VHF runs

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Some actual numbers RG-8/U

• on 80m 0.28 dB/100 6 loss
• on 10m 1.00 dB/100 21 loss
• on 2m 2.40 dB/100 42 loss
• on 70 cm 4.78 dB/100 66 loss
• RG-8/U is a bit better for long VHF runs

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Some actual numbers LMR-400

• on 80m 0.25 dB/100 5 loss
• on 10m 0.70 dB/100 15 loss
• on 2m 1.50 dB/100 29 loss
• on 70 cm 2.90 dB/100 49 loss
• LMR-400 is a low-loss, RG-8-sized cable
• (but solid-cond. center makes it a bit stiffer)

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Some actual numbers LDF4-50

• on 80m 0.12 dB/100 3 loss
• on 10m 0.35 dB/100 08 loss
• on 2m 0.80 dB/100 17 loss
• on 70 cm 1.42 dB/100 28 loss
• Andrew LDF4-50 is a semi-rigid cable, sometimes
  referred to as Heliax.

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Some actual numbers LDF5-50

• on 80m 0.07 dB/100 lt2 loss
• on 10m 0.19 dB/100 04 loss
• on 2m 0.44 dB/100 10 loss
• on 70 cm 0.79 dB/100 17 loss
• Andrew LDF5-50 is commonly used in commercial VHF
  and UHF installations.

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On 80m, coax is not so critical

• RG-58/U 0.62 dB/100 13 loss
• RG-8/U 0.28 dB/100 06 loss
• LMR-400 0.25 dB/100 05 loss
• LDF4-50 0.12 dB/100 03 loss
• LDF5-50 0.07 dB/100 lt2 loss
• Not much benefit to big, expensive coax

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On 70 cm, coax is everything!

• RG-58/U 10.82 dB/100 92 loss
• RG-8/U 4.78 dB/100 66 loss
• LMR-400 2.90 dB/100 49 loss
• LDF4-50 1.42 dB/100 28 loss
• LDF5-50 0.79 dB/100 17 loss
• Nice return on your coax investment!

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Why not use Heliax all the time?

• It does have great performance, but . . .
• it wont roll up under your car seat
• it makes a lousy rotator loop
• it will pull the HT right out of your hand
• it costs more (but there are surplus deals)
• connectors cost more (but more deals)

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Suggested way to choose coax

• -Whatever the size, get good quality
• Use low-loss for long, fixed runs
• Use short, flexible jumpers at the ends
• Weatherproof the joints in fixed installations
• Use proprietary connectors if called for
• Read manufacturers instructions data

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Coax connector types PL-259

• UHF connector is a misnomer
• non-constant impedance raises SWR
• not inherently weatherproof
• lossy at VHF and UHF frequencies
• reasonably durable in the field
• right vs wrong way to install on the cable
• ubiquitous on commercial ham gear

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Coax connector types Type N

• Military grade- and for a reason
• good past 10,000 MHz (10 GHz)
• constant 50-ohm impedance
• weatherproof (but tape them up anyway)
• installs with hand tools minimal soldering

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Coax connector types BNC TNC

• similar to Type N but for smaller cable
• BNC uses bayonet and spring to join
• TNC uses threaded shell very rugged
• available in mil-clamp and crimp types
• BNCs fit many handheld radios
• single-hole patch panels and feed-throughs

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Coax connector types SMA

• good well into microwave region
• very small and lightweight
• uses threaded shell and coax center
• solders onto miniature hardline (UT-141)
• harder to install need special tools
• used on some mini-hts (e.g., VX-2R)
• not designed for frequent mate / unmate

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A tip for your Go-Kit Keep various adapters
between connector series, such as UHF-BNC) on hand
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Thank you!
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