[time-nuts] Lowloss cable?

Charles P. Steinmetz charles_steinmetz at lavabit.com
Sun Jun 12 21:02:17 UTC 2011


Dick wrote:

>What's the best small diameter (<0.25") low loss coax?

How nuts do you want to get?  You shouldn't need anything better than 
a good RG-59 (75 ohms) for a 30' run.  For that, I'd probably use 
Belden 1426A if I were restricted to < 0.25".  Good foam RG-6 (also 
75 ohms) like Belden 1694A is better, if you can tolerate 7mm 
diameter (.275"), but at 30' the difference in attenuation will be 
less than 1 dB.

Note that GPS antenna manufacturers often specify (and supply) 75 ohm 
RG-59 instead of 50 ohm RG-58, notwithstanding that the antennas and 
GPS units are 50 ohms.  This is because the effects from the 
relatively small mismatch are more than outweighed by the typically 2 
dB/100 ft. loss advantage of RG-59 over RG-58.

At 1 GHz, the losses per 100' of some candidates are:

1/4" Heliax:  ~4.5 dB  (~5 dB at 1.5 GHz)
Belden 1694A:  ~6 dB  (foam RG-6; 7mm diameter; some brands worse)
RG-401:  ~7 dB  (1/4" semi-rigid coax)
Belden 7808A:  ~7.5 dB  (low loss RG-8X; some others much worse)
LMR-240:  ~8 dB  (low loss RG-8X)
Aircell 7:  ~8 dB  (note: this is 7mm, a bit larger than 1/4")
Belden 1426A:  ~8.5  (foam RG-59; some types/brands worse)
Aircell 5:  ~9.4 dB  (5mm OD)

Best regards,

Charles







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