A Small Coax for Field Use - RG-174/U

Steve Yates - AA5TB

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Last Update: November 04, 2009

I know this is kind of a silly page but I generated all of this stuff for myself so I thought I'd share it.

For portable QRP activities in the field I do not like to carry around heavy coax cable. RG-174/U is a very small coax cable that is well known for its high losses. Most hams would shy away from it. However, there are those who will even use zip cord or computer ribbon cable for a transmission line to minimize weight. I decided to investigate the losses of RG-174/U to see if it could be used in certain applications. I already use the stuff in my Mini Cooper for my VHF/UHF rig and have had very acceptable results. The length is only 8 feet in my Mini and the losses are about 0.8 dB at the 2 m band and 1.5 dB at the 70 cm band. Such loss figures are barely perceivable.

For low power use in the field I figured that as long as I keep the losses to 1 dB or less it should be acceptable. QRP operators are already 13 dB or more below a standard 100 W rig so what's 1 dB. Purest may say loss is more of an issue at low power but remember 1 dB is 1 dB, no matter what the power is. Also remember that for field use 1 dB (power loss) may equal 20 dB (weight loss)! It is the ERP (Effective Radiated Power) what is important. I have found that an ERP of 1 Watt is adequate for solid communications (ex., CW, PSK31) in most cases. If you start of with 5 Watts then you can afford a little loss for the sake of convenience.

Here are the lengths of RG-174/U at various ham bands that give 1 dB of loss when matched:

MHz     Feet     Meter
 1.9	88.310	 26.917
 3.6	63.994	 19.505
 5.4	52.143	 15.893
 7.1	45.399	 13.838
10.1	37.970	 11.573
14.1	32.046	  9.768
18.1	28.217	  8.600
21.1	26.091	  7.953
24.9	23.973	  7.307
28.1	22.533	  6.868
29.6	21.940	  6.687

Do not try to use RG-174/U for high power applications. It simply isn't large enough to dissipate much energy effectively and even at 1 dB of loss there may be many Watts of power converted to heat. If the SWR is high then the situation is even worse. This is yet another advantage of QRP operation.

The first chart below indicates the loss from 0.5 to 30 MHz for a 50 foot and 100 foot length of RG-174/U. The second chart shows what length of cable will have 1 dB for a given frequency. The data for these charts were derived from the pages of VK1OD.

RG-174/U Loss
RG-174/U Length at 1 dB Loss